Talk Page



Electrical maintenance requires a basic understanding of electrical theory and electrical safety.

7.3.1 Contacts Interlocks Overloads Timer

Basic Electrical Theory

For a full understanding of electrical theory, Please click the titled link above or refer to the Basic Electrical Theory article under the Industry Fundamentals category.

One of the most important theories in electricity is Ohm’s law. It basically defines how electricity will behave in electrical circuits. This is not to say that there are no other laws that define behaviors in more complex electrical circuits. For the time being however, we will consider Ohm’s law only. The equation is:

'''E = I x R'''

Putting some values to this equation, if a circuit has a resistance of 25 ohms (O) and a potential of 125 volts, the current that is drawn in this circuit would be 5 amps. Schematically, the circuit looks like the one shown in '''Figure 1'''. This basic circuit will give you some understanding for the next subject that we will discuss: electric shock.

'''Figure 1: Simple Electrical Circuit'''

Effects of Electrical Shock

An important concern when working near electrical equipment is the effects of electrical shock on the human body. When we understand what these effects are, we can examine the other electrical hazards found on the job.

Most people are aware that the principal danger from electricity is that of electrocution, but few really understand just how minute a quantity of electric energy is required for electrocution. Actually, the current drawn by a 7 -watt, 120-volt lamp passed from hand to hand or from hand to foot is enough to cause fatal electrocution. Just as it is current and not voltage that heats a wire, it is current that causes physiological damage.

Different values of 60-hertz alternating current (AC) and their effects on an average human are listed in Table 1. In short, any current of 15 mA or more may be fatal, those between 100 mA and 4 amperes are probably fatal due to heart failure, and those above 5 amperes may be fatal from severe burns. It is a fact, however, that shocks in this last current range are statistically less dangerous than those in the 100 mA to 4 amperes range. In view of the wide diversity of injuries derived from contact with electric energy, it is only logical that there must be minimum exposure to energized parts to prevent electric shock or electrocution.


Ohm’s law may be used to determine the current, which is the value of interest, with the human body serving as the resistive element of the circuit. The E of Ohm’s law is the voltage of the system itself. The R, which is the variable, is actually the controlling factor. Essentially, it is the human skin, along with such factors as area of contact, tightness of contact, dryness of the skin, and the presence of any cuts, abrasions, or blisters that causes the resistance to vary. Except for the skin, human resistance is about 250 ohms per arm or leg, and 100 to 500 ohms from shoulder to shoulder or hip. The more muscular the person, the lower their body’s resistance. Skinny arms or legs and those made up principally of fat have higher resistance. Bone too has a high level of resistance. Table 2 shows the range of human resistance variations. The total human circuit resistance is, of course, the sum of the two contact resistances and the internal body resistance. Use Table 3 to compare human resistance to various materials.

Troubleshooting Procedure

When given a problem, for instance the lights being out, the fan not working, an outlet not working, or another electrical problem, there are a few simple steps to determine what is wrong, if anything, and what needs to be done to correct it. The first step is to never overlook the obvious. Is the switch off, the bulb blown, or the circuit breaker tripped Many times, something simple will be the answer to the problem.

If the answer is not immediately apparent, then we begin the troubleshooting process. First, find the electrical prints for the circuit involved. As we showed in the beginning of this class, blueprints provide a road map in locating and isolating circuits. They also identify the location(s) of the various pieces of equipment associated with a circuit.

The next step is to determine if the piece of equipment is getting any power to it. If it is, then the equipment is defective and needs to be repaired or replaced.

If there is no power at the equipment, the source of the break in the circuit must be determined. Look for any physical damage to the conduit or connection boxes associated with the circuit. If those are OK, determine where the approximate middle of the circuit is and its closest connection box using the prints. At the box, check and see if any power is present at the box. If yes, go to the next connection box going toward the equipment. If no power is there, go the next connection box going back toward the power supply. Continue doing this until the break can be isolated.

Once the break is found, the circuit must be de-energized to make the repairs. Repairs should never be made on live electrical circuits.

If it is a break at the splice, then you can re-splice, re-insulate, and restore the circuit. If it is a break between two connection boxes, a new section of cable must be pulled.

For a maintenance mechanic, troubleshooting is a method of finding the cause of a problem and correcting it. This is a very important job because the entire facility’s operation may depend on the troubleshooter’s ability to solve the problem. Some guidelines for good troubleshooting are:

  • Use a clear and logical approach
  • Work quickly
  • Work efficiently
  • Work economically

Determining Circuit Parameters

When we are making repairs or adding additional equipment to an existing circuit, there are several things that must be taken into consideration:

What is the present loading on the circuit

What is the appropriate wire size for this circuit load, including the additional loading

Is the circuit protection adequate with the new addition

Most of this information can be found in the National Electrical Code 2002 (NEC®) and the Electrical Code of the City of New York.

Circuit Loading

The loading on the existing circuit can be determined by making measurements and by looking at the installed design capabilities on the electrical prints. Measurements can be done using a clamp-on ammeter.

Determining Wire Size

According to NEC®; Article 210.19, the rating of a branch circuit shall have an ampacity of not less than the maximum load to be served. Where a branch circuit supplies continuous loads or any combination of continuous and non-continuous loads, the minimum branch circuit conductor size before the application of any adjustment or correction factors shall have an allowable ampacity not less than the non-continuous load plus 125% of the continuous load.

Determining Circuit Protection

In both the NEC® and the NYC Electrical Codes, the sections and articles mentioned are not the only ones that may apply. The sections and articles reference other sections and articles that must be used for specific applications.

Replacing a 120-Volt Outlet

Open and tag the power supply circuit for the outlet being replaced.

Using the VOM as a voltmeter, check the voltmeter on a known live circuit to ensure that the meter is working properly.

Using the voltmeter, check that the outlet to be replaced is de-energized.

If the outlet shows that it is de-energized, check the voltmeter on a known live circuit to ensure that the meter is still working properly and did not fail when you checked the de-energized outlet.

Remove the cover plate and outlet from the box.

Replace with new outlet, using care to restore the wires to their original positions.

Replace the cover plate.

Reenergize the circuit and test the outlet for proper voltage.

Installing Three-Way Switches

In the following description, there are two diagrams shown. Figure 2A) shows a three-way switch circuit where the supply goes to the switch first using a raceway; in Figure 2(B), the same circuit is shown using cable instead.

Figure  2: Three-Way Switch Circuit (Supply to Switch)

The three-way switch has a common terminal (darker) and two traveler terminals (brass). There are no "ON" or "OFF" positions on the switch. The color red is used as the switch return, but any color other than white, gray, or green could also be used. The travelers should be a different color than the switch return, e.g., a pair of blue wires, a pair of yellow wires, etc.

When using cable to wire three-way switches, the NEC 200.7(C)(2) permits the use of conductors with white, gray, or three continuous white stripes as the switch return only if the insulation is permanently identified by painting or another effective means at each location where the conductor is visible and accessible. Figure 3(A) shows a three-way circuit with the supply going to the light fixture first using a raceway; in (B), the same circuit is shown using cable.

Figure 3: Three-Way Switch Circuit (Supply to Light)

Incandescent Lighting

This type of light accounts for more than half of all the lamps sold in the US. These lamps come in a variety of voltages, wattages, sizes, shapes, and base types. Some of the older types of lamps are no longer manufactured due to legislation enacted in 1994 and have been replaced by more efficient types. As with all equipment that is to be replaced, lighting must be replaced with like models or a suitable substitute or a dangerous condition may result.

In an incandescent lamp, the filament (tungsten) is heated to a high temperature. The resulting glow produced when current is passed through this low-resistance wire is what produces a light. To aid in the longevity of the light, the air is removed and replaced with an inert gas such as argon.

These lamps have the lowest efficiency, as they normally are operated at lower temperatures to increase the longevity of the lamp.

Fluorescent Lighting

The NEC considers fluorescent lighting to be electric discharge lighting®. The lamp (see '''Figure 12''') is a glass tube with contacts at each end. There is a phosphor coating on the inside of the glass that enables the light to be seen. The glass tube contains an inert gas and a small amount of mercury.

The fluorescent lamp normally requires a ballast to start the lamp. The older types ('''Figure 13a''') have a starter, which is a plug-in type can that allows the cathodes to heat up, allowing the lamp to fire. The newer lamps use rapid-start ballasts and do not require the use of a starter. There are two main types of ballasts: high output and very high output.

There are two types of starterless lamps: rapid start ('''Figure 13b''') and instant start ('''Figure 13c'''). The instant-start kind requires the use of single-pin lamps. The instant-start lamps do not require preheating the cathode, and the ballast has a higher output voltage than that of the rapid start.

'''Figure 13: Ballast Circuits'''

A special ballast is used where the lamps are subjected to low temperatures (<50°C), such as might occur outdoors. These are marked with the minimum operating temperature. The ballast generates heat and, according to the National Fire Protection Association, is the second leading cause of electrical fires in the United States. Underwriters Laboratories (UL) has developed a standard for thermally protected ballasts, designated Class P. These ballasts have a sensor that detects the internal temperature of the ballast and opens the ballast circuit if the temperature reaches 194°F (90°C).

Due to the possibility of the thermal protection failing in the close position, which can result in overheating and fire, the circuit should be protected with inline fuses to isolate the defective ballast.

Compact fluorescent lamps are the newest type and have a life span of ten times that of an incandescent lamp with about three times the light output per watt. The base of the lamp fits directly into an incandescent lamp base.

Replacing Ballast in Fluorescent Lighting Fixture

Hanging fluorescent light fixtures are an inexpensive source of overhead light for garages, basements, etc., but they rely on a component called a ballast in order to function. When it goes bad, the light is useless. Replace the bad ballast, and the fixture will be good as new.

Safety Precautions

  • Ensure power is de-energized.

Tools Required

  • Screwdriver
  • Wire stripper
  • Wire cutter
  • Wire nuts
  • Fluorescent ballast


'''Determine the problem.'''

  • If the bulbs in a fluorescent fixture are flickering, first make sure the ends of the bulbs are making a good connection.
  • Try replacing one or more of the bulbs with a spare new bulb; sometimes one bad bulb will affect the performance of others in the same fixture.
  • If the bulbs are good and they are still flickering, the problem is probably in the ballast.

'''Remove the old ballast.'''

  • Before beginning work, be sure to unplug the fixture or turn the power off.
  • Carefully remove the cover and bulbs ('''Figure 14B'''). It is a good idea to wear safety goggles.
  • Remove the inner cover to get to the ballast, which is a small black box. '''(Figure 14C)'''.
  • Use a circuit tester on the black and white wires to make sure there is no current. '''(Figure 14D)'''.
  • Disconnect/separate the wires coming from the ballast and the "feed line" wires '''(Figure 14E)'''.
  • With wire cutters, cut the other two sets of wires going to the ballast''' (Figure 14F)'''.
  • Loosen any nuts or screws holding the old ballast in place and remove the ballast.
  • Take the old ballast with you to the electrical supply store to ensure purchase of the same size replacement.

'''Install the new ballast.'''

  • The new ballast will have the same assortment of colored wires as the old.
  • Be sure to place the new ballast in the proper direction '''(G)'''.
  • The colored wires will match up if it’s installed properly.
  • Mount the new ballast onto the fixture using the same fasteners as before '''(Figure 14H)'''.

'''Connect the new wiring to the existing wiring.'''

  • Reconnect the new wiring to the existing wires, matching color to color.
  • If there are two sets of wires of a given color, it doesn’t matter how they are attached. Just be sure to connect blue to blue, red to red, etc.
  • To connect the wires, strip a little of the colored insulation off each wire '''(Figure 14I)'''.
  • Twist the bare ends of each pair of wires together and secure them with a wire nut''' (Figure 14J)'''.
  • Repeat this process for all of the wires until complete.
  • Check to make sure all of the wiring connections are snug.

'''Finish the project.'''

  • Tuck the wires into the middle of the fixture.
  • Replace the cover over the ballast, covering the wiring.
  • Replace the bulbs and turn the power back on.

'''Note:''' Ensure that the wires are not pinched when restoring the cover.

'''Figure 14: Replace Ballast in a Fluorescent Lighting Fixture'''

High-Intensity Discharge Lamps

There are three types of lamps in this category: mercury, metal halide, and sodium. The light is produced in an arc tube inside a glass bulb. The lamp will continue to produce light if the outside bulb is broken.

Broken bulbs should be removed from service as soon as possible. With the outer bulb broken, harmful ultraviolet light is produced.

Mercury and metal halide light produce a strong blue-content light, while sodium lamps produce a light that is orange in color.

Alternating Current Motors

Alternating current (AC) motors can be divided into two major types: single- phase and polyphase motors. Single-phase motors normally are limited to fractional horsepower ratings up to 5 horsepower. They are commonly used to power such things as fans, small pumps, appliances, and other devices not requiring a great amount of power. Single-phase motors are not likely to be connected to complicated motor control circuitry and will not be discussed in this text.

Polyphase motors comprise the majority of motors needed to drive large machinery such as pumps, large fans, and compressors found in industrial facilities. These motors have several advantages over single-phase motors in that they do not require a separate winding or other device to start the motor, they have relatively high starting torque, and they have good speed regulation for most applications.

There are two classes of polyphase motors: ''induction'' and ''synchronous''. The rotor of a synchronous motor revolves at synchronous speed, or the speed of the rotating magnetic field in the stator. The rotor of an induction motor rotates at a speed somewhat less than synchronous speed. The differences in rotor speed are due to differences in construction and operation.

Induction Motors

Three phase squirrel cage induction motors ('''Figure 15''') are perhaps the most commonly used motors in industrial applications. They are relatively small for their given horsepower and have good speed regulation under varying load conditions. They are simple in construction and rugged, so they cost little to manufacture. The induction motor has a rotor that is not connected to any external sources of power. It derives its name from the fact that the AC voltages are induced in the rotor due to the rotating magnetic field of the stator.

'''Figure 15: AC Induction Motor'''

Reversing Rotation

The direction of rotation of a three-phase induction motor can be readily reversed. The motor will rotate in the opposite direction if any two of the three incoming leads are reversed, as shown in '''Figure 16'''.

'''Figure 16: Reversing Rotation'''

Wound-Rotor Motors

The rotor of a wound-rotor motor is wound with insulated windings similar to the stator windings. This three phase winding is wye connected with the ends of each phase winding being connected to three slip rings. Connected to the rotor circuit through the slip rings is a wye connected variable resistance. '''Figure 17''' shows the circuit of a wound-rotor motor.

'''Figure 17: Wound-Rotor Motor'''

Direct Current Motors

The operation of DC motors is based on the same principles as is the operation of AC motors. A current-carrying conductor placed in a magnetic field perpendicular to the lines of flux will tend to move in a direction perpendicular to the magnetic lines of flux.

Stated simply, DC motors rotate because of the two magnetic fields interacting with each other. The armature of the DC motor acts like an electromagnet when current flows through its coils. Since the armature (rotor) is located within the magnetic field of the field poles (stator poles), these two fluxes will interact. Like poles will repel each other, and unlike poles will attract one another. The armature of a DC motor has windings on it that are connected to commutator segments. '''Figure 18''' shows a DC motor field structure and armature assembly.

'''Figure 18: DC Motor Construction'''

Primary Control and Pilot Control Devices

All components used in motor control circuits may be classed as either ''primary control'' devices or ''pilot control'' devices. A primary control device is one that connects the load to the line whether it is manual or automatic, such as a motor starter. Pilot control devices are those that control or modulate the primary control devices. Pilot devices are such things as pushbuttons, float switches, pressure switches, and thermostats.

An example, shown in '''Figure 19''', would be a magnetic contactor controlled by a toggle switch used to energize and de-energize the contactor, or M coil. To start the motor, the toggle switch is switched ON, which energizes the contactor coil, closing the main line contacts, which energizes the motor. Switching the toggle switch OFF de-energizes the contactor coil, opens the main line contacts, and de-energizes the motor.

'''Figure 19: Basic Motor Control Circuit'''

In this example, the contactor, because it connects the motor, or load, to the line, would be classed as a primary control device. The toggle switch does not connect the load to the line but is used to energize and de-energize the coil of the starter. Therefore, it would be classed as a pilot control device.

For any given controller, generally there are two primary control devices used. These are the disconnecting means, or circuit breaker (usually a manual device), and the magnetic contactor. There may be many pilot devices used in parallel and series combinations to control the start and stop function performed by the primary control device. The overload relays, for instance, included in the motor starter are actually pilot devices used to control the primary device whenever the motor is overloaded.

Pilot devices vary greatly with their function and intended use. From manual switches to automatic control devices, pilot devices are what make a motor controller adaptable to fit a multitude of applications.


Symbols 1 and 2 in '''Figure 20''' represent electrical contact devices. They may represent line contacts on a starter, a limit switch or relay, or any other type of control device that has electrical contacts. Recall that circuit diagrams are shown in their de-energized or standard reference position. Therefore, Symbol 1 is a normally open (NO) contact and Symbol 2 is a normally closed (NC) contact.

'''Figure 20: Typical Motor Controller Symbols'''

The designation "a" or "b" associated with a set of contacts is used to identify the state of the contacts (open or closed) in reference to the main operating device. An "a" contact will normally be closed when its associated main contacts are closed. These "a" contacts will open when the associated main contacts are opened. On the other hand, "b" contacts will normally be open when the associated main contacts are closed, thus operating opposite of "a" contacts. Remember, however, that not all electrical drawings will indicate the state of contacts with this designator. If not, the drawing notes should annotate whether the circuit is in the energized or de-energized state. This "a" and "b" notation holds true for auxiliary contactors and relays, as well as the main contactor.

'''Note''': All circuits are shown in the de-energized, or standard, reference position unless stated otherwise.

Two other terms often used in conjunction with contacts (either relay contacts or switch contacts) are ''make'' and ''break''. When a contact goes closed, it is said to "make," and when the contact opens, it is said to "break."

Pushbutton Switches

Symbols 3 and 4 of Figure 20 represent manually operated pushbutton switches with normally open and normally closed contacts, respectively. This spring returned type switch will return to its normal position when released by the operator. Because the switch returns to its original position and its contacts are only closed or open for the moment (however long) the switch is pushed, these contacts are referred to as ''momentary contacts''.

Toggle Switches

Symbols 5 and 6 of Figure 20 represent manual contacts of a toggle type of switch. Symbol 5 contacts are normally open and Symbol 6 contacts are normally closed. This type of switch has maintaining contacts; that is, once switched to a different position, the switch will stay in that position. When found in a circuit diagram, the switch positions should be labeled as OFF or ON, FAST or SLOW, or some other appropriate labeling. Symbol 7 of '''Figure 73''' is a toggle switch of the single pole double throw (SPDT) type, where one contact is normally open and the other normally closed.

When moving one handle or pushbutton operates more than one set of contacts, they generally are connected by dotted lines, as in symbols 8 and 9. The dotted lines represent any form of mechanical linkage that will make the two contacts operate together. One other method that is used frequently to show pushbuttons that have two sets of contacts is shown in symbols 10 and 11. Symbol 10 has two normally open contacts and symbol 11 has one normally open and one normally closed contact.

Switches can be designed to operate in one of two ways. The first, and most common, is referred to as a ''break before make'' contact arrangement. In an arrangement of this type, one set of contacts opens before the next set of contacts closes.

The second arrangement is referred to as ''make before break''. In this arrangement, shorting, or bridging, of the first and second sets of contacts occurs for a short period during the contact transfer when the switch is being switched from one position to the next,. This arrangement is used when it is necessary to ensure continuity of power to a circuit during switching.

Indicating Lights

Symbol 12 of Figure 20 is a pilot or indicating light, which is indicated chiefly by the short lines radiating out from the center circle. Normally, the letter in the circle designates the color of the light, such as RL for red or GL for green.

Coils, Relays, and Contactors

Symbols 13 and 14 of Figure 20 represent coils. They may indicate a relay coil or a main line contactor. Relays and contactors are electromagnetic devices in the sense that magnetic forces are produced when electric currents are passed through coils of wire; in response to such forces, contacts are closed or opened by the motion of plungers or pivoted armatures.

Symbols 15 and 16 are discussed later in this text.

As defined by the National Electrical Manufacturers Association (NEMA), a ''relay'' is "a device that is operated by a variation in the conditions of one electric circuit to affect the operation of other devices in the same or another electric circuit." A ''contactor'', on the other hand, is "a device for repeatedly establishing and interrupting an electric power circuit." It is important to recognize the difference between the two, noting particularly that the relay, serving a secondary role, causes other devices to function, whereas the contactor is the primary unit, doing its work in the main power circuit.

'''Figure 21''' is a drawing showing the basic construction of a relay. Note the relay coil and coil terminals.

'''Figure 21: Relay Construction'''

This relay has one set of NO (C1) and one set of NC (C2) contacts. The path for current flow on each set of contacts is through the movable contacts to the common terminal. In the de-energized state, contacts C2 are closed and contacts C1 are open. When the coil is energized, the coil attracts the movable contacts to close contacts C1 and open contacts C2.

'''Figure 22''' shows a contactor assembly drawing. Note the solenoid assembly, moving armature and contact assembly, and the stationary contacts with terminal connections for line and load wiring. Also, note from the picture that the contacts are normally encased to protect the contact assemblies.

'''Figure 22: Contactor Construction'''

Contactor assemblies are frequently made with main contacts that serve to connect and disconnect the main power circuit and any auxiliary contacts (both normally open and normally closed) that operate with the main contacts. The auxiliary contacts are then available for use in the control circuit of the original machine or another machine.

Auxiliary contacts are frequently used to "seal in" a coil. Sealing in provides a parallel path for current flow to keep a coil energized after the original path of current flow is interrupted. Auxiliary contacts of the coil being "sealed in" are commonly used to complete the parallel path for current, but this is not always the case. '''Figure 25''' shows a simple control circuit using a magnetic contactor to illustrate "sealing in."

Manual Motor Starter

The manual motor starter, Class 2510 type M and T, Series A, is used to apply voltage to start a motor. A pushbutton or toggle operates the contacts. '''Figure 23''' shows the starter, and the description of each part is in '''Table 4'''.

Use only copper wire on device power and control terminals. To inspect the contact, loosen the two captive screws (A in Figure 23) holding the contact actuator mechanism to the contact block. Lift the contact actuator to expose the contacts. Discoloration and slight pitting do not harm silver alloy contacts. Do not file the contacts; this wastes material. Replace the contacts only when they are worn thin. To replace the contacts, loosen the two captive screws and remove the contact actuator. Remove the movable contacts (1 in Figure 23) from the yoke bar by compressing the contact spring and sliding the contact sideways. Change the contact springs before inserting the new movable contacts. Remove the stationary contact by loosening the contact mounting screws (B and C in Figure 23). Reassemble the contacts in reverse order.

Thermal units must be installed and the device reset before the starter will operate. Install thermal units so the type designation can be read and the pawl is above the ratchet wheel. Continued overcurrent through the thermal unit raises the temperature, melting the alloy in its solder port, allowing the ratchet wheel to rotate. This releases the overload pawl assembly, allowing the toggle to retract the contacts. This action also centers the pushbutton or toggle operator, indicating that the starter is tripped.

One internal interlock (3 in Figure 23), either normally open (NO) or normally closed (NC), can be added to all manual starters. It occupies either the upper right-hand or left-hand corner of the device and is field-installable.

The starter can be locked in the OFF position by lifting the metal tab with the lock symbol on the contact actuator and placing a padlock through the hole in this tab. In the ON position, withdrawing the lock tab opens the contacts of the device.

Provide branch circuit overcurrent protection for each contactor or starter in accordance with the National Electric Code® and local electrical codes.

Magnetic Coil Control Circuits

Although the power circuit can be single-phase or three-phase, the control circuit to the magnetic coil is always single-phase. The control circuit includes:

The magnetic coil.

The contacts of the overload relay assembly.

A momentary or maintained contact pilot device, such as a pushbutton, pressure, temperature, liquid level, or limit switch, or a PLC signal.

Relay contacts or timers taking the place of pilot devices.

An auxiliary contact on the starter designed as the holding-circuit interlock. This may be required in certain control schemes.

'''Figure 24''' shows a size 1 starter control circuit.

'''Figure 24: Size 1 Starter Control Circuit'''

Magnet Coil Data

Published charts list identification numbers, ratings, and operating characteristics of magnetic coils. These charts list the rated voltage and the coil volt-amperes for both inrush and sealed conditions. AC magnetic coils, in general, are designed to operate on line voltages fluctuating as much as 15% below and 10% above nominal rating. DC coils have a corresponding limit of 20% below and 10% above nominal rating.

Holding Circuit Interlocks

The holding circuit interlock is a normally open (NO) auxiliary contact provided on standard magnetic starters and contactors. It closes when the coil is energized to form a holding circuit for the starter after the Start button has been released. These typically are mounted on the upper left portion of magnetic contactors.

Auxiliary contacts are frequently used to "seal in" a coil. Sealing in provides a parallel path for current flow to keep a coil energized after the original path of current flow is interrupted. Auxiliary contacts of the coil being "sealed in" are commonly used to complete the parallel path for current, but this is not always the case. '''Figure 25''' shows a simple control circuit using a magnetic contactor to illustrate "sealing in." The starting sequence for Figure 25 is stated below.

'''Figure 25: Simple Control Circuit'''

''Starting sequence'': When the Start button is pushed, the M coil is energized, which will ''make'' the Ma contacts, thus keeping the M coil energized when the Start button is released. The M coil is said to be sealed in. The M coil also closes the M contacts, energizing the motor. The "M" designation used here is frequently used in control circuits to designate the main contactor that controls the switching of line power to the device being controlled. Multifunction controllers frequently do not use ''M'', but rather use more specific designations, such as ''F'' or ''R'' for forward and reverse.


Control circuits frequently control more than one contactor, such as in a two- speed motor control circuit or a control circuit for controlling the direction (forward or reverse) on a motor. In many situations such as this, equipment damage could result if both contactors were closed at the same time.

Two methods are used to provide interlocking to prevent this from occurring. First is the electrical interlock. A "b" contact from each contactor is in series with the operating coil from the other contactor. If contactor A were energized, its open "b" contact would prevent energizing contactor B. The opposite would be true if contactor B was energized.

The second method employed is the mechanical interlock. To accomplish this, the two contactors are physically mounted side by side in the control box. A mechanical linkage prevents both contactors from being closed at the same time. If one contactor was closed and something occurred to energize the other contactor, the coil would be energized, but motion of the contact assembly would be physically blocked.


Symbol 15, shown previously in Figure 20, represents the heating element of an overload relay. Overload relays are devices found on all motor controllers in one form or another. The current that a motor draws while running is directly proportional to the load on the motor. An overload condition, whether caused by a mechanical or an electrical fault, will result in increased current flow.

Overload protection is achieved in almost all controllers by placing heating elements in series with the motor leads on polyphase motors. These heater elements activate electrical contacts, which open the coil circuit when used on magnetic controllers. When used on manual starters or controllers, the heating elements release a mechanical trip to drop out the line contacts. Older controllers use two overloads, while newer units are required to have three overloads in accordance with changes in the National Electrical Code®.

The overload relay is sensitive to the percentage of overload; therefore, a small overload will take some time to trip the relay, whereas a heavy overload will cause an almost instantaneous opening of the circuit. The overload relay does not give short-circuit protection, however. It is quite possible that, under short- circuit conditions, the relay might hold long enough to allow considerable damage to the motor and other equipment.

Short-circuit protection is provided by installing either a fused disconnect or a circuit breaker ahead of the motor in the main feeder lines.

There are three types of overload relays in general use today. The first uses a metal with a low melting point that holds a ratchet assembly, as shown in '''Figure 26'''.

'''Figure 26: Melting Pot Relay for Thermal Overload'''

When the metal is heated beyond the melting point, the ratchet releases, causing a set of contacts to open in the control circuit and open the main line contactor.

The second type of overload device, shown in '''Figure 27''', uses a bimetallic element. The bimetallic element is made of two different metals bonded together. When heated, the metals expand at different rates and the element bends. The resultant motion releases a trip mechanism, which opens contacts in the control circuit, and the main line contactor trips open.

'''Figure 27: Bimetallic Relay for Thermal Overload'''

'''Figure 28''' shows the third type of overload relay: the magnetic type. A magnetic trip element uses an electromagnet in series with the circuit load. With normal current, the electromagnet is not affected. As load current increases above the setpoint, the relay opens a set of contacts in the control circuit and the main line contactor trips open.

'''Figure 28: Magnetic Relay for Overloads'''

Overload relays must be reset after each trip, either automatically or manually. The automatic reset type should not be used except on equipment that is so designed that there can be no danger to life or equipment from the restarting of the motor. After the overload relay has been tripped, it requires a little time to cool, meaning there is some delay before resetting can be accomplished.

Factors that determine the overload relay thermal units or overload heaters needed are:

The motor full-load current

The type of motor

The possible difference in ambient temperature between the motor and the controller

Motors with the same speed and horsepower do not necessarily have the same full-load current. Refer to the motor nameplate for full-load current, not to published charts. Charts tend to show averages of normal full-load currents. The full-load current of a specific motor may be different. Selection tables usually are based on continuous-duty motors with a service factor of 1.15 operating under normal conditions.

Rotary Switches

The last symbol in Figure 20, symbol 16, is a rotary selector switch. A rotary switch is a multi-contact switch with the contacts arranged in a full or partial circle. Instead of a pushbutton or toggle, the mechanism used to select the contact moves in a circular motion and must be turned.

Rotary switches can be manual or automatic switches. An automobile distributor and the ignition switch on a motor vehicle are examples of rotary switches (see '''Figure 29'''). Some rotary switches are made with several layers, or levels. This arrangement makes possible the control of several circuits with a single switch.

'''Figure 29: Rotary Switch'''

Switches can be either automatic or manual. A manual switch is a switch that is turned on or off by an operator. Examples of common manual switches, such as the toggle, pushbutton, or rotary switches already covered, are a light switch, a dryer start button, and a TV channel selector switch. Each of these requires operator action to initiate a change in a control system.

Automatic Switches

An automatic switch is a switch that is controlled by a mechanical or electrical device. You do not have to turn an automatic switch on or off. Two examples of automatic switches are a thermostat and the distributor in a motor vehicle. The thermostat will turn a furnace or air conditioner on or off by responding to the temperature in a room. The distributor electrically turns on the spark plug circuit at the proper time by responding to the mechanical rotation of a shaft. Even the switch that turns on the light in a refrigerator when the door is opened is an automatic switch.

Automatic switches are not always as simple as the examples given above. Limit switches, which sense some limit such as fluid level, mechanical movement, pressure, or an electrical quantity, are automatic switches that are sometimes quite complicated.

Any switch that will turn a circuit on or off without operator action is an automatic switch. '''Figure 30''' shows the symbols for various commonly used automatic switches.

'''Figure 30: Symbols of Various Automatic Switches'''

Float Switches

Symbols 1 and 2 of '''Figure 30''' represent normally open and normally closed liquid level or float switches. Float switches take many forms in their physical or mechanical construction. They consist of one or more sets of contacts normally open or normally closed operated by a mechanical linkage. Many float switch units, as well as other pilot devices, employ a mercury switch in place of metallic contacts. The simplest mechanical arrangement for a float switch, shown in '''Figure 31''', would be a pivoted arm having the contacts fastened to one end and a float suspended from the other end.

'''Figure 31: Float Switch'''

As the water level rises, it would lift the float, thus moving the contact end of the level downward and either making or breaking the contact, depending on whether the stationary contact were mounted above or below the arm. If a single pole, double throw action of the contacts were desirable, then one stationary contact could be mounted above and one below the center of the arm. If the float were all the way up, it would make the lower set of contacts, and if the float were all the way down it would make the upper set of contacts.

Pressure Switches

Symbols 3 and 4 of Figure 30 represent normally open and normally closed vacuum, or pressure, switches. This mechanical motion is used to operate one or more sets of contacts. '''Figure 32''' shows a typical pressure switch design using a bellows as the pressure-sensing element. Two other common sensing elements used are the diaphragm and the bourdon tube. The type of detector is determined by the system requirements. Most devices of this type have a means to adjust the setpoint of the sensing device.

'''Figure 32: Pressure Switch (Bellows Type)'''

Symbols 5 and 6 of Figure 30 represent temperature-activated switches, more commonly called ''thermostats''. Many different types of thermostats are available that employ different methods of sensing temperature. The two most common are bellows and bimetallic strips. As in the pressure switch, the mechanical motion of the sensing element is used to operate a set of contacts. A typical thermostat is shown in '''Figure 33'''.

'''Figure 33: Thermostat (Bellows Type)'''

Symbols 7 and 8 of '''Figure 30''' represent flow switches that are used to sense the flow of liquid, air, or some other gas through a pipe or duct and transform this flow or lack of flow into the opening or closing of a set of contacts.

One type of flow switch, shown in '''Figure 34''', uses a pivoted arm having contacts on one end and a paddle, or flag, on the other end. The end with the paddle is inserted into the pipe so that the flow of liquid or gas causes a lever to move and open or close the contacts.

'''Figure 34: Flow Switch'''


Symbols 9, 10, 11, and 12 of '''Figure 30''' represent timer contacts that are normally operated by a timing relay. This type of relay and contact arrangement provides two important advantages of automatically controlled circuits: sequencing and delaying events in a control system. Many types of timing relays are available that can be adjusted to give time delays of as little as a fraction of a second to as much as several minutes. Moreover, extremely long time delays, up to several hours, are possible with timing relays that are motor driven. Since most industrial control systems do not run through unattended repetitive cycles, the timing relays for such installations generally are non-cyclical. Timers are used to separate events in a control starting sequence that occur instantaneously from those that are delayed. Instantaneous events are those that occur as soon as the start circuit is initiated, with the only delay being the time it takes coils to operate or the contacts to open or close. Delayed events are those that have some type of controlled delay provided by a pilot device.

Common designs are pneumatic, dashpot, and motor-controlled timers. Motor-controlled timers generally are used for operations that repeat themselves, such as traffic signal controllers and sequentially operated motor starting circuits. A simple motor timer found in many homes is used to control the wash cycles of automatic washing machines.

A dashpot timer, shown in '''Figure 35''', consists of a plunger that, when the coil of the timer is energized, moves slowly through a bath of oil and closes a contact at the end of its stroke. The dashpot usually is provided with a bypass near its upper limit of travel so that the contact is permitted to close with a snap action. Snap action allows quick-closing contacts to minimize arcing during the closing cycle. In addition, a valve is included during construction to allow the oil to flow freely as the plunger falls when the relay is de-energized. Many of these relays also have an adjustment that can be varied to change the time delay.

'''Figure 35: Time Delay Relay (Dashpot Type)'''

Another popular timer is the pneumatic timer. It uses the restricted airflow across a diaphragm to create the time delay. The airflow usually passes through an adjustable orifice so that the time delay is adjustable.

The time delay of a timing relay can be applied when the relay is energized or when it is de-energized. Symbols 9 and 10 shown in Figure 30 represent timer contacts that have timed closing after energization (TCAE) and timed opening after energization (TOAE) respectively. Symbols 11 and 12 also shown in Figure 30 represent timer contacts that have timed opening after de-energization (TOAD) and timed closing after de-energization (TCAD) respectively. Some timers may be equipped not only with contacts that are delayed, but also with contacts that operate instantaneously.

Limit Switches

Symbols 13, 14, 15, and 16 presented earlier in Figure 30 represent direct actuated limit switches. Limit switches use an arm, lever, or roller protruding from the switch that will be bumped or pushed by some piece of moving equipment. This movement is then used to operate a set of contacts. Limit switches vary widely in size and design. There are large, rugged devices for heavy industrial use, such as that shown in '''Figure 36''', and smaller more accurate and precise units that use microswitches that can operate on very minute movements of the operating lever.

'''Figure 36: Limit Switch'''

Symbols 13 and 14 of Figure 30 show limit switches in their normally open or closed condition, and symbol 15 represents a normally open limit switch that is held closed; symbol 16 represents the opposite.

Foot Switches

Symbols 17 and 18 of '''Figure 30''' represent foot switches. Switches of this type are often used in applications that require the machine or process cycle to be started at a time when the operator hands are both engaged in loading or handling the materials. Foot-operated switches are frequently employed for such purposes as punch presses, drill presses, and sewing machines. Foot switches are actually limit switches enclosed in a convenient and rugged casing for foot operation. They are available in a variety of contact arrangements such as single pole double throw, two pole double throw, or other arrangements to suit a specific need. '''Figure 37''' shows a typical foot switch.

'''Figure 37: Foot Switch'''

Overhead Door Controls Maintenance

This section is a brief review of the power hoist that controls the overhead doors you will encounter in the building maintenance arena.

PowerMaster Model MG Hoist Operator

This section describes the PowerMaster model MG gear reduced hoist operator that controls the overhead doors (see '''Figure 38''').

'''Figure 38: PowerMaster Model MG Gear Reduced Hoist Operator'''

Wall Mounting

1. Using the center of the door drive shaft as a reference point, locate four mounting holes

2. The operator should be mounted on the wall using 3/8 inch through bolts for a secure mounting. If wall is of such construction as to prohibit the use of through bolts, bolts and shields of sufficient size may be used.

3. Install the driven sprocket on the door shaft.

Bracket or Shelf Mounting

The MG operator may be mounted on either the right or left side of the door, either above or below the door drive shaft.

Install mounting shelf in desired position.

Bolt operator to mounting shelf.

Install driven sprocket on door shaft.

Align driven sprocket with drive sprocket on operator.

Lock sprockets in place with their set screws. (Note: If no keyway exists in door shaft, drill a 5/16-inch hole through sprocket hub and door shaft, insert a 5/16-inch roll pin through hole, and insert 3/16-inch roll pin through center of 5/16-inch roll pin for more secure fastening.)

Connect the sprockets with the drive chain, shorten to proper length, using chain tool. Lock chain together with chain connector provided.

Adjust chain so that there is no more than 1/4-inch of slack when chain is depressed between sprockets.

Tighten all mounting bolts securely.

Connecting to Power Supply

Consult local electrical codes before proceeding with permanent installation.

Exercise caution when operating machine. The drive chain and limit drive chain are exposed; and, when turning, could cause injury. Wiring diagrams can be found inside electrical enclosure cover. Connect operator to properly grounded supply (green colored slotted head screw is supplied in control box), and install control stations as required.


On three phase units, make certain that operator rotates in correct direction. If direction is wrong, limit switches will not function, and damage will occur. It is recommended that door be moved manually to a mid position before turning on power, so that it may be stopped before damage occurs, if rotation is incorrect. If direction is wrong, reverse any two of the three incoming power supply leads to correct rotation.

After wiring has been checked and operator is running properly, limits will probably need re adjusting (see Limit Adjustment).

Where operators with wiring types that use constant pressure on the CLOSE button are used, the control station must be located so that the operation of the door may be observed by the person operating the door.

A separate 11V electrical circuit is provided for the connection of a safety edge device. Any such device that uses a normally open contact may be connected to terminals #8 and #9 on the low-voltage terminal block. When the safety device is actuated (when the door comes in contact with an object during downward travel), the circuit will cause the motor to step and reverse the door to the fully open position. In addition, there is a cut off limit switch in the circuit that will de activate the safety device during the last few inches of downward door travel.

NOTE: Mount control station and warning label within sight of doorway.

Manual Operation

To engage manual operating chain:

Pull down and hold the disconnect chain.

While holding the disconnect chain, move manual operating chain to assure complete engagement of manual drive gears.

Lock disconnect chain in place using locking device.

Interlock switch in operator cuts off power to motor when manual chain is engaged.

Release of disconnect chain will re engage operator for electrical operation.


All bearings throughout the MG operator are oil impregnated or sealed, lubricated for life, anti friction bearings. The motor is factory lubricated, and requires no additional lubrication. A few drops of oil should be applied periodically to the moving parts of the disconnect device.

Limit Adjustment


Before operating door, set grooved nuts approximately 1 inch from limit switches on each side.

IMPORTANT: Be sure door is in the midway position.


Open door electrically.

If door travels in correct direction, and stops at open limit proceed to step 4.

If door travels in the correct direction, and nears the end of door travel, but does not stop, STOP DOOR! Check to see if open limit has been tripped. If it has not been tripped, go to step 4.

If open limit switch has been tripped, but did not stop door, consult the factory technician.

If door travels in wrong direction, STOP DOOR! Go to step 3.

If direction of travel is wrong:

On three phase machines only: Turn off main power and swap power connection on L1 and L2 repeat limit switch testing.

On single phase machines only: If rotation is incorrect, and limit switches are inoperative, check that door controls are connected correctly. If controls are correct, consult the factory technician.

Under no circumstances should control station wiring be altered if rotation is incorrect. To do so will cause some control functions to be inoperative, which may result in personal injury, or damage to door and/or operator.

Depress pressure plate to move groove nuts to desired location: to stop door earlier: Move nut closer to limit switch plunger. To stop door later, back nut away from limit switch plunger. When making fine adjustments, turn nut only 1/4 to 1/2 turn at a time, and run door again electrically. After setting nuts in desired position, make certain that the groove is engaged by the pressure plate.

Repeat procedure for close limit adjustment.

Figure 39: Overhead Door Operator Field Wiring Connections

Figure 40: Overhead Door Operator Wiring Diagram

Routine Inspection and Maintenance

Basic maintenance tools include a flashlight, an air hose or vacuum cleaner, and a small brush. Debris, dirt, and dust can be spotted and removed from contacts and other areas of a motor control center. Light rust and dirt can be vacuumed out. Vacuuming is preferable to an air hose in that an air hose may contain water and/or simply redistribute the dirt to other areas of the enclosure. You should never file or use any abrasive on the pole faces of magnets. This could misalign the precise fit between the armature and the core and result in heating and noise production.

Dirt and dust on the pole faces should be removed since they produce an excessive air gap, which causes the equipment to hum as the AC changes the magnetic flux of the core.

Terminal connections should be checked to ensure they are tight. This ensures a low-resistance path for current flow. Deterioration of a loose terminal connection occurs at a rapidly increasing rate because the heat resulting from increased resistance of loose connections causes corrosion and greater resistance in the joint until the connection is subjected to a resistance that is too high and the circuit burns.

Oil should never be used on the moving parts of motor starters unless specifically suggested by an individual manufacturer. Oil forms a film over magnet pole faces, attracts dirt, and increases air gaps between the pole faces and air armatures. This could result in excessive noise. Another cause of excessive noise is broken shading coils. These are not always obvious to the naked eye and sometimes may require the use of a magnifying glass if the noise cannot be attributed to anything else. Improper alignment is another possible cause of excessive noise production.

Electrical surges, abrasion from vibration and door openings, as well as other factors can result in insulation breakdown to ground either in the coil or in the control circuit. This can cause low holding power on the contacts, erratic operation, failure to operate at all, or even hazardous voltage conditions if equipment grounds are not in good condition.

To test the insulation, disconnect the starter from the line at a visible disconnect and measure resistances from both power and control circuits to ground with a megger.

If the control circuit is supplied without a control power transformer, a single test to ground from one of the power source leads supplying both motor and control circuits would produce an adequate check of both circuits. However, if this test shows low resistance, it will be necessary to isolate the two sections to locate the trouble.

Improperly sized overload elements of any of the several types available could cause unnecessary tripping of the contactor and shutdown of the motor if the elements are undersized. It could result in motor burnout if the elements are oversized. Ratings of overload elements should always be checked during periodic maintenance to ensure they exactly match the requirements of the motor.

To meet NEC® safety requirements, motor starters should always be connected in combination with fuses and disconnect switches or a circuit breaker on the source side. The starter will remain closed if a fuse blows on one phase leg that isn't used for the control circuit, and the motor will run as a single-phase motor. It is often said that a single-phasing motor cannot be restarted; however, there is one instance in which it may be restarted. That is, if any other motors are running on the circuit beyond the blown fuse.

This situation contributes to or results in a burned-out motor or seized bearings if the load is of any appreciable size due to over-saturation of the iron core, excessive hysteresis, eddy currents, and large ampere flows in the unfaulted phase. Checking fuses for continuity and proper rating should always be an important part of any maintenance procedure.

Contact Wear and Replacement

Contactors are subject to both mechanical and electrical wear during their operation. In most cases, the mechanical wear is insignificant. The erosion of the contacts is due to electrical wear. During arcing, material from each contact is vaporized and blown away from the useful contacting surface.

Critical examination of the appearance of the contact surfaces and measurement of the remaining contact over travel will give the user the information required to get the maximum contact life. '''Figure 41''' shows both new and used contacts.

'''Figure 41: Contact Surfaces'''

Circuit Troubleshooting

Although most controllers are designed with reliability and long life as primary design factors, it is inevitable that trouble will occur. Sometimes, when a controller fails to operate or signs of trouble occur (heat, smoke, smell of burning insulation) the cause of the problem can be quickly identified and corrected. On other occasions, however, locating the cause of the problem will involve investigation that is more detailed and analysis to identify and correct the problem.

The secret to efficient and accurate troubleshooting lies in determining the section of the circuit that contains the trouble component and then selecting the proper component to be checked. This can only be accomplished by efficient and accurate circuit analysis, not by trial and error, extended wire tracing, or indiscriminate checking of components at random.

In this section, we will assume that the troubleshooting is being done on an existing system that was functioning properly but for causes unknown has failed.

If the circuit was a new installation that failed to work on the first attempt, the troubleshooting procedure would include analyzing the circuit design to ensure it was designed properly to achieve the desired effect and possibly to verify that the system was properly installed.

When considering troubleshooting an existing circuit, we generally can eliminate the possibility of improper connections. If the circuit had been improperly wired it would not have operated originally.

The first step in troubleshooting an existing circuit that has developed trouble is to understand that circuit and to understand the operation of the machine it controls. On complex circuits, time generally does not allow the troubleshooter to study the complete circuit in-depth to determine how it operates. With the help of the operator, however, you can quickly analyze and identify the various parts of the control circuit on the schematic. The operator should also be able to help you determine how much of the machine is operating. You can also depend on the operator to help locate and identify controller components and pilot devices that may be hidden by the machine or located at a remote location.

One concept that should be understood is that a control circuit is made up of two things: contacts, which make and break the circuit, and coils, or other pilot devices, which operate these contacts. If the contacts close and open as they should, then the proper voltages will be applied or disconnected from the coils. If this is true, then the malfunction must lie in the coil itself. If the contacts do not operate properly, then the trouble must be in the contacts, in the wire that carries this current from the contact to the coil, or in the device that operates the contacts.

The second step is to follow the machine through its cycles until it reaches the point where it does not function properly. Having determined this point, you can analyze the circuit, starting with the section that does not operate. A careful check of this circuit and locating the components involved in this section of the circuit generally will lead you to the source of the trouble you are seeking. The malfunction of some control component must be the cause of the circuit failure. To effectively troubleshoot a control circuit, the troubleshooter must first ensure he has the proper tools available. The basic tools that are needed are the multimeter and clamp-on ammeter. The multimeter will be used for measuring voltage and resistance. The ammeter can be used to measure current flow to the load.

'''Figure 42''' is used to show how a voltmeter can be effectively used to isolate a fault to a specific portion of a control circuit.

'''Figure 42: Using Voltmeter to Troubleshoot'''

Suppose the operator reports that the motor does not start when the Start button is pushed. You should begin your troubleshooting by pushing the Start button and listening for the holding coil to operate. In most controllers, you should be able to hear the main contactor operating. If the contactor does not operate, your troubleshooting should be directed at the control circuit. If the contactor does operate and the motor is not running, the controller should be shut off to prevent damaging the motor. The motor should then be checked to see it if has seized, has an open winding, or has some mechanical or electrical malfunction.

In this situation, the contactor does not appear to be operating and an electrical failure is suspected in the power circuit. You must first check the line voltage and fuses, as shown in Figure 42. Place the voltmeter probes on the hot side of the line fuses as shown at position A. A line voltage reading tells you that your voltmeter is operational and that you have voltage to the source side of the line fuses L1-L2. You should also check between L1-L3 and L2-L3. To check the fuse in line 1, place the voltmeter across the line fuse as shown at position B between L1 L2. A voltage reading shows a good fuse in L1. Likewise, check the other two fuses between L1 L3 and L2 L3. A no voltage reading would show a faulty fuse.

If the line fuses check good, then check the voltage between terminals T1 T2, T2 T3, and T1 T3. The controller is faulty if there are no voltmeter readings on all three of the terminal pairs. You would then proceed to check the power contacts, overloads, and lead connections within the controller. However, if voltage is indicated at all three terminals, then the trouble is in either the motor or the lines leading to the motor.

If the failure is suspected to be in the control circuit and the overload devices have been reset, the following voltage checks are typical of those that could be made to identify the faulty component if the main contacts are not closing (holding relay not energizing):

Check for power available at L1, L2, and L3, as previously instructed.

Place the voltmeter probes at points C and D shown in Figure 42. There should be a voltage reading when the Stop switch is closed and zero voltage when the Stop switch is open. These conditions would indicate a good Stop switch. If there was zero voltage in both cases, it could indicate a blown fuse.

Next, check the voltage between points C and E. If you get a no voltage reading when the Start switch is open and a voltage reading when the Start switch is closed, then the Start switch is good.

Place the voltmeter probes at C and F. A voltage reading with the Start button closed would indicate a good OL1 but would also indicate an open OL2, OL3, relay coil, or connection to line 3.

Place the voltmeter probes at points C and G and close the Start switch. A no voltage reading locates the trouble in the control circuit; the OL2 is faulty.

Place the voltmeter probes at points C and H and close the Start switch. A no voltage reading locates the trouble in the control circuit; the OL3 is faulty.

If it is certain that the overload is not overheated and causing the OL3 contact to open, the overload should be replaced.

This example used a simple circuit to demonstrate the technique of using a voltmeter to isolate a problem in a control circuit. The student should be aware, however, that most control circuits are not this simple. There may exist parallel paths that make it difficult to use a single voltage measurement to make a sound conclusion about whether a component is faulty or good. Sometimes it is necessary to disconnect a circuit at various points to take a voltage or resistance reading (resistance readings must be taken with the circuit de-energized).

Care must be taken to check all parts of a de-energized circuit because control circuits frequently have cross-connected interlocks that bring power into the controller from another source.

Common Problems

The following list is a brief summary of some common problems and their causes. There can be many variations to the faults listed; as you gain experience, you will find that there will be other faults that could be added to the list.

Main Contactor Energizes but Motor Does not Start

Overloads tripped, open overload heater, or bad connections.

Main contacts not making completely: this can be caused by sticking or binding of the movable parts of the contactor, or one or more of the contacts may be worn, pitted, dirty, or burned.

Broken or loose line connections.

On reduced voltage starters, in-line resistance devices could be open.

Main Contactor Does not Close when Energized

Low voltage supplied to the coil: either a dirty Start button contact or a poor connection in the circuit could cause this.

Open coil.

Short-circuited coil.

Contactor Closes and then Opens when Start Button is Released

Auxiliary contacts that act as maintaining contacts are either not closing or have a poor connection.

Interlock not making up: some control circuits have interlocks that are overridden at the start. If the interlock does not make, completing the maintaining circuit, the contactor will open when the start button is released.

Main Fuse Blows or Breaker Trips When the Start Button is Pushed

Grounded or shorted main power lines or motor windings.

Motor is seized.

Control Power Fuse Blows

Grounded control wire or device.

Short-circuited control wiring or device.

Main Contactor Does Not Energize

Control power not available.

A broken wire or a pilot device not functioning properly could cause an open in the starting circuit.

Many manufacturers furnish troubleshooting charts in their technical manuals. The troubleshooter should not overlook the use of such aids in trying to find a fault.

Identifying a Faulty Component

Assume that you have been troubleshooting a circuit and have now located the section of the control circuit that seems to be causing the trouble. The first step is to locate the components involved in this part of the circuit. There must be a coil or a relay and a contactor or some other device that is energized by this section of the control. The machine should be run through its sequence to determine if this coil is receiving energy.

If the contactor or relay does not close as it should, the circuit should be disconnected ('''CAUTION''': Circuit must be locked, tagged, and tried) and the wires removed from the coil of the relay or contactor so that a voltage check can be taken. Connect a voltmeter across the wires that were connected to the coil and again energize the circuit, operating the control sequence up to this point. If the voltmeter indicates a proper voltage is applied, then the trouble most likely is in the windings of the coil itself.

If it is suspected that the coil is at fault, disconnect the power from the circuit ('''CAUTION''': Circuit must be locked, tagged, and tried) and, with an ohmmeter, check the resistance of the coil, which should be very low on a DC resistance check. If the coil is burned out, you will receive a high resistance reading or a reading of infinity on the ohmmeter, indicating that the coil needs to be replaced. Do not depend on the coil smelling burned or showing any visible evidence of being burned out, since this is not always the case.

Suppose that our voltage check shows that the voltage did not reach the coil when it should have in the sequence of operation of the control circuit. This would indicate that some contact is not closing when it should, thus preventing the circuit to the coil from energizing. A careful study of this section of the control circuit should show us what contact must close to energize this coil. You must now locate the components that contain these contacts and again operate the machine through its sequence, observing the operation of the relay, limit switch, float switch, and pressure switch, or any other device that contains these contacts, to determine whether it operates mechanically as it should. If this component does operate mechanically, it indicates two possibilities. The first and most likely is that the contacts involved are not properly closing and are making a poor connection, which prevents them passing current to the coil as they should. Another possibility is an open circuit due to a broken or burned wire. Generally, this is the least likely cause although, in an industrial atmosphere, the possibility of a remote pilot device or its associated wiring being damaged cannot be readily ruled out. After identifying and correcting the problem, the circuit should be reconnected and tested for proper operation.

Troubleshooting for a Specific Fault

To make the troubleshooting guidelines presented above clearer, we shall now consider a circuit and determine the probable cause of some troubles that we shall assume to have occurred in it. '''Figure 43''' shows the primary control components of a chilled water air conditioning compressor.

'''Figure 43: Air Conditioning Compressor Controller'''

The components, as shown in Figure 43, are as follows:

Coil CR is a control relay.

Coil M1 is the starter for the chilled water pump.

Coil M2 is the starter for the condenser water pump.

Coil M3 is the starter for the oil pump on the compressor.

Coil M4 is the compressor motor starter.

The contact identified as T is a thermostat that senses the temperature of the chilled-water return. Its function is to start the condenser water pump when this temperature reaches a predetermined high level.

The contact identified as PS1 is an oil pressure switch whose function is to stop the compressor should the oil pump fail and also prevent from starting before the proper oil pressure has been obtained.

The contact identified as FS1 is a flow switch in the chilled water piping system.Its function is to prevent the compressor from running unless there is a sufficient flow of chilled water.

The contact identified as FS2 is a flow switch in the condenser water piping system.

Suppose now that you are called in to troubleshoot this circuit. The first step should be to determine from the operator what trouble he is having with the circuit. Suppose he tells you that the condenser water pump does not start as it should. From a study of the diagram, we can assume that the section of the circuit for the control relay is functioning properly, that contact CR2 closes, and that the chilled water pump runs as it should. Something must be wrong in the third line of our schematic diagram.

The next step is to check the overload relays and determine that they were not tripped. Having done this, the next step would be to start the system again by pressing the Start button and observing M2 to see if the contactor is picking up. In this case, it is not.

The next step would be to determine if the M1 and T contacts are shut. The M1 contact can be visually inspected inside the controller to determine if it is closed. Although visual inspection is not positive indication of a complete circuit (there could be a high-resistance connection across a closed contact) it is a good first check.

The next component to check is the temperature contact. Determining the setting of this thermostat and the actual water temperature will indicate whether it should be open or closed. We are assuming that, through the shutdown of the machine, the water temperature has increased to a point that demands that these contacts be closed. The position of the contacts can be verified by visual inspection or by removing power from the system, ('''CAUTION''': Circuit must be locked, tagged, and tried) lifting the leads to the temperature contacts, and using an ohmmeter to verify that the contacts are closed. In this case, the contact’s position has been verified. They are closed, and the leads have been reconnected.

Having checked the overloads, the M1 contact, and the T contact, the next item to check is the M2 coil. It is reasonable to suspect that the coil is open. This suspicion could be confirmed with the use of a voltmeter. With the circuit energized, after the Start button had been pushed, place the voltmeter probes at positions 1 and 2, 1 and 3, and 2 and 3. A no voltage reading at 1 and 2 with a voltage reading between 1 and 3 and between 2 and 3 would confirm that M2 is open. To check the coil further, lockout and tag the controller power supply, disconnect the wires from coil M2, and use an ohmmeter to determine if the coil is open or not.

Before replacing the coil, the starter should be examined for proper mechanical operation to ensure that there were not any mechanical faults that caused the coil to fail.

Using Figure 43 again, let us start with a different malfunction. The operator reports that everything seemed to work except the compressor. The initial troubleshooting step would be to energize the circuit and watch it sequencing through the steps to determine where it failed. We would see that the control relay operates, M1 operates to start the chilled water pump, M2 operates to start the condenser water pump, and M3 operates to start the compressor oil pump. Here the sequence stops, and you would note that the M4 contactor did not move and apparently was not energized.

Examining the circuit for M4, we find that we have a contact on the oil pump starter that could cause trouble. We have a pressure switch and two flow switches that might also be the source of trouble. Again, we must determine which of these components is not functioning. If these components are readily accessible, a visual inspection of each of them may immediately disclose the trouble. If they are inaccessible, however, a good procedure to follow is to remove power from the circuit ('''CAUTION:''' circuit must be locked, tagged, and tried). Then, disconnect the wires from the starter coil, connect a voltmeter to the coil, and energize and operate the control circuit to determine if voltage is reaching the coil, thus eliminating the possibility of trouble being in the coil itself.

Another option would be to lift the leads at points 4 and 5, restart the system, and, using an ohmmeter, verify continuity across the pilot device contacts M3, PS1, FS21, and FS1. If the circuit is open, then each contact could be checked individually to determine which one is open. After determining which contact is at fault, the procedure would be to physically inspect the device and determine what actually caused the device to fail. The problem could be calibration, component failure or damage, or the parameter being measured being out of the proper band to activate the pilot device.

While this procedure may seem oversimplified as you are guided through the diagram on a supposed troubleshooting job, it is the basis upon which good troubleshooting practice is laid. No matter how complex the control circuit is, it can be separated into simple branches such as we have illustrated. The efficient troubleshooter will narrow his trouble down to one of these simple branches of even a very complex circuit so that the actual process of locating the troublesome component will be as simple as outlined here.

The most important rule in troubleshooting is to ''change only one thing at a time''. If you find a set of contacts that you suspect is not properly functioning, correct this trouble and try the circuit again before changing anything else.

If you find a coil you suspect to be burned or otherwise causing trouble, repair or replace it and try the circuit again before attempting any other changes. One method used to test a circuit once a fault has been identified is to bypass that portion of the circuit if possible. This is easily done in the case of contacts that can be jumped, but it is not so easily done with relays. You must ensure that any actions of this sort will not bypass a safety function, which could result in damage to the system.

One of the most common mistakes of troubleshooters is to change or correct several supposed troubles at one time before trying the circuit for operation. Quite frequently, several changes made at one time may introduce more trouble than you had originally. This should be made a cardinal law in your work as a troubleshooter. It is very seldom that several parts of a machine would wear out at the same instant. Therefore, even though the overall condition of the control components may be poor, it remains probable that only one component has failed completely.

Confined Space

Quite often, you may have to work inside of a tank or vessel. This workspace will most probably be defined as a "confined space" and any worker entering this space must take extra precaution.

A ''confined space'' is defined as "any situation where a person’s head or body crosses the plane of an opening" that meets any one of the following criteria:

Limited openings for entry or exit.

Unfavorable natural ventilation which could contain or produce flammable, toxic, or oxygen deficient atmospheres.

Spaces or areas not intended or designed for continuous personnel occupancy.

Examples include storage tanks, sumps, tank trucks, pits, trenches, as well as large piping and ducts. '''Figure 44''' shows a confined space warning sign.

'''Figure 43: Confined Space Warning Sign'''

The safety hazards associated with confined spaces include the following:

Structural failure of the confined space.

Improper shoring.

Discharge of high pressure, air, gas, water, or chemicals into the confined space.

Inadequate oxygen to support respiration.

Explosive atmospheres.

Noise in excess of acceptable levels.

Inadequate lighting.

Specific rules for entering a confined space vary slightly from facility to facility, but most include the following precautions as a minimum:

Testing of the atmosphere inside the confined space for adequate oxygen content.

Testing of the atmosphere inside the confined space for explosive content.

Safety inspection of the shoring and support provided by the confined space.

Issuance of a confined space entry permit with a predetermined time limit for occupancy and an expiration date for the permit printed on it. This permit is usually posted at the entrance to the confined space.

Some facilities require a safety guard to be stationed at the entrance to any confined space to ensure the proper requirements and safety precautions are being met, as well as to act as a lifeguard for those personnel working within the confined space.


Safety is of paramount importance! When troubleshooting or taking voltage readings, safety glasses and gloves must be worn. Below 150 volts, leather gloves are required. Above 150 volts, rubber low-voltage gloves are worn inside the leather gloves. Before doing any work on the inside of a controller, the circuit should be de-energized, locked, and tagged. The incoming line leads should be tested to ensure that there is no longer any power available.