TEST INSTRUMENTS AND MEASURING DEVICES
Measurement of current being supplied to or from a component is measured by an ammeter.
The ammeter measures electric current. It may be calibrated in amperes, milliamperes, or microamperes. In order to measure current, the ammeter must be placed in series with the circuit to be tested (Figure 1).
Figure 1 Ammeter
When an ammeter is placed in series with a circuit, it will increase the resistance of that circuit by an amount equal to the internal resistance of the meter Rm. Equation (14-3) is the mathematical representation of the current without the meter installed.
Equation (14-4) is the mathematical representation of the current with the meter installed in the circuit.
The accuracy of the ammeter KA is the ratio of the current when the meter is in the circuit, Iw, to the current with the meter out of the circuit, Io. Equation (14-5) is the mathematical representation for solving for the accuracy of the ammeter (KA).
By substitution laws, Equation (14-6) is a mathematical representation of the accuracy using circuit resistance.
The percent loading error is that percent of error due to loading effects that result from the added resistance of the meter. Equation (14-7) is a mathematical representation of the percent loading error.
A second error which occurs in an ammeter is calibration error. Calibration error is an error that occurs due to inaccurately marked meter faces. Typical values of calibration error in terms of full scale current are about 3 percent.
Example: An ammeter, with a 10 mA full scale deflection and an internal resistance of 400 Ω, is placed in a circuit with a 20 V power source and a 2 KΩ resistor (Figure 2).
Figure 2 Ammeter Accuracy
An ammeter with a full scale Im can be shunted with a resistor RSH in order to measure currents in excess of Im. The reason for shunting an ammeter is to extend the range of the ammeter and, thereby, measure currents higher than the original full scale value.
By Kirchhoffs current law,
Since the voltage across the shunt must be equal to the voltage across the ammeter, shunt resistance is calculated as follows:
Therefore, the input resistance of a shunted ammeter is related to the meter and shunt resistance. Equation (14-8) is a mathematical representation of this relationship.
NOTE: When computing accuracy for a shunted ammeter, use R1m in place of Rm.
Equation (14-9) is a mathematical representation of the relationship between input voltage and current to the ammeter and the value of input resistance.
Example: An ammeter, with a 100 Ω meter resistance and a full scale deflection current of 4 mA, is to be shunted to measure currents from 1 to 20 mA.
A circuit breaker is a device that is used to completely disconnect a circuit when any abnormal condition exists. The circuit breaker can be designed to actuate under any undesirable condition.
The purpose of a circuit breaker is to break the circuit and stop the current flow when the current exceeds a predetermined value without causing damage to the circuit or the circuit breaker. Circuit breakers are commonly used in place of fuses and sometimes eliminate the need for a switch. A circuit breaker differs from a fuse in that it "trips" to break the circuit and may be reset, while a fuse melts and must be replaced. Air circuit breakers (ACBs) are breakers where the interruption of the breaker contacts takes place in an air environment. Oil circuit breakers (OCBs) use oil to quench the arc when the breaker contacts open.
A low-voltage circuit breaker is one which is suited for circuits rated at 600 volts or lower. One of the most commonly used low-voltage air circuit breakers is the molded case circuit breaker (Figure 3).
Figure 3 Molded Case Circuit Breaker
A cutaway view of the molded case circuit breaker is shown in Figure 4.
Figure 4 Cutaway View of Molded Case Circuit Breaker
A circuit can be connected or disconnected using a circuit breaker by manually moving the operating handle to the ON or OFF position. All breakers, with the exception of very small ones, have a linkage between the operating handle and contacts that allows a quick make (quick break contact action) regardless of how fast the operating handle is moved. The handle is also designed so that it cannot be held shut on a short circuit or overload condition. If the circuit breaker opens under one of these conditions, the handle will go to the trip-free position. The trip-free position is midway between the ON and OFF positions and cannot be re-shut until the handle is pushed to the OFF position and reset.
A circuit breaker will automatically trip when the current through it exceeds a pre-determined value. In lower current ratings, automatic tripping of the circuit breaker is accomplished by use of thermal tripping devices. Thermal trip elements consist of a bimetallic element that can be calibrated so that the heat from normal current through it does not cause it to deflect. An abnormally high current, which could be caused by a short circuit or overload condition, will cause the element to deflect and trip the linkage that holds the circuit breaker shut. The circuit breaker will then be opened by spring action. This bimetallic element, which is responsive to the heat produced by current flowing through it, has an inverse-time characteristic. If an extremely high current is developed, the circuit breaker will be tripped very rapidly.
For moderate overload currents, it will operate more slowly. Molded case breakers with much larger current ratings also have a magnetic trip element to supplement the thermal trip element.
The magnetic unit utilizes the magnetic force that surrounds the conductor to operate the circuit breaker tripping linkage.
When the separable contacts of an air circuit breaker are opened, an arc develops between the two contacts. Different manufacturers use many designs and arrangements of contacts and their surrounding chambers. The most common design places the moving contacts inside of an arc chute. The construction of this arc chute allows the arc formed as the contacts open to draw out into the arc chute. When the arc is drawn into the arc chute, it is divided into small segments and quenched. This action extinguishes the arc rapidly, which minimizes the chance of a fire and also minimizes damage to the breaker contacts.
Molded case circuit breakers come in a wide range of sizes and current ratings. There are six frame sizes available: 100, 225, 400, 600, 800, and 2,000 amps. The size, contact rating, and current interrupting ratings are the same for all circuit breakers of a given frame size. The continuous current rating of a breaker is governed by the trip element rating. The range of voltage available is from 120 to 600 volts, and interrupting capacity ranges as high as 100,000 amps.
Much larger air circuit breakers are used in large commercial and industrial distribution systems.
These circuit breakers are available in much higher continuous current and interrupting ratings than the molded case circuit breaker. Breakers of this type have current ratings as high as 4,000 amps, and interrupting ratings as high as 150,000 amps.
Most large air circuit breakers use a closing device, known as a "stored energy mechanism," for fast, positive closing action. Energy is stored by compressing large powerful coil springs that are attached to the contact assembly of a circuit breaker. Once these springs are compressed, the latch may be operated to release the springs, and spring pressure will shut the circuit breaker.
Circuit breaker closing springs may be compressed manually or by means of a small electric motor. This type of circuit breaker can be classified as either a manually- or electrically-operated circuit breaker.
When a large air circuit breaker is closed, the operating mechanism is latched. As the circuit breaker is closed, a set of tripping springs, or coils, are compressed, and the circuit breaker may then be tripped by means of a trip latch. The trip latch mechanism may be operated either manually or remotely by means of a solenoid trip coil.
As previously stated, circuit breakers may be operated either manually or electrically.
Electrically-operated circuit breakers are used when circuit breakers are to be operated at frequent intervals or when remote operation is required.
When the electrically-operated stored energy circuit breaker is tripped, the spring is recharged by the spring charging motor so that the breaker is ready for the next closing operation. The manually-operated circuit breaker closing springs are normally compressed by a hand crank just prior to operation of the breaker. Figure 6 shows a large air circuit breaker which is classified as a manually-operated stored energy circuit breaker. The closing springs are compressed by pulling downward on the large operating handle on the front of the breaker. Closing this circuit breaker is accomplished manually by depressing the small closing lever. Tripping this circuit breaker is done by means of the tripping lever, located at the bottom front of the breaker.
Figure 5 Large Air Circuit Breaker
High-voltage circuit breakers (including breakers rated at intermediate voltage) are used for service on circuits with voltage ratings higher than 600 volts. Standard voltage ratings for these circuit breakers are from 4,160 to 765,000 volts and three-phase interrupting ratings of 50,000 to 50,000,000 kVA.
In the early stages of electrical system development, the major portion of high-voltage circuit breakers were oil circuit breakers. However, magnetic and compressed-air type air circuit breakers have been developed and are in use today.
The magnetic air circuit breaker is rated up to 750,000 kVA at 13,800 volts. This type of circuit breaker interrupts in air between two separable contacts with the aid of magnetic blowout coils.
As the current-carrying contacts separate during a fault condition, the arc is drawn out horizontally and transferred to a set of arcing contacts. Simultaneously, the blowout coil provides a magnetic field to draw the arc upward into the arc chutes. The arc, aided by the blowout coil magnetic field and thermal effects, accelerates upward into the arc chute, where it is elongated and divided into many small segments.
The construction of this type of circuit breaker is similar to that of a large air circuit breaker used for low-voltage applications, except that they are all electrically operated.
Compressed-air circuit breakers, or air-blast circuit breakers, depend on a stream of compressed air directed toward the separable contacts of the breaker to interrupt the arc formed when the breaker is opened. Air-blast circuit breakers have recently been developed for use in extra high-voltage applications with standard ratings up to 765,000 volts.
Oil circuit breakers (OCBs) are circuit breakers that have their contacts immersed in oil. Current interruption takes place in oil which cools the arc developed and thereby quenches the arc. The poles of small oil circuit breakers can be placed in one oil tank; however, the large high-voltage circuit breakers have each pole in a separate oil tank. The oil tanks in oil circuit breakers are normally sealed. The electrical connections between the contacts and external circuits are made through porcelain bushings.
As we have discussed, circuit breakers may be remotely operated. In order to operate the breakers from a remote location, there must be an electrical control circuit incorporated.
Figure 6 shows a simple control circuit for a remotely-operated breaker.
Control power is supplied by an AC source and then rectified to DC. The major components of a simple control circuit are: the rectifier unit, the closing relay, the closing coil, the tripping coil, the auxiliary contacts, and the circuit breaker control switch.
Figure 6 Simple Circuit Breaker Control Circuit-Breaker Open
To close the remotely-operated circuit breaker, turn the circuit breaker control switch to the close position. This provides a complete path through the closing relay (CR) and energizes the closing relay. The closing relay shuts an auxiliary contact, which energizes the closing coil (CC), which, in turn, shuts the circuit breaker, as shown in Figure 8. The breaker latches in the closed position. Once the breaker is shut, the "b" contact associated with the closing relay opens, de-energizing the closing relay and, thereby, the closing coil. When the breaker closes, the "a" contact also closes, which enables the trip circuit for manual or automatic trips of the breaker.
The circuit breaker control switch may now be released and will automatically return to the neutral position.
To open the circuit breaker, turn the circuit breaker control switch to the trip position. This action energizes the trip coil (TC), which acts directly on the circuit breaker to release the latching mechanism that holds the circuit breaker closed.
When the latching mechanism is released, the circuit breaker will open, opening the "a" contact for the tripping coil and de-energizing the tripping coil. Also, when the circuit breaker opens, the "b" contact will close, thereby setting up the circuit breaker to be remotely closed using the closing relay, when desired. The circuit breaker control switch may now be released.
Figure 7 Simple Circuit Breaker Control Circuit - Breaker Closed
As you can see from Figure 6 or 7, the circuit breaker control circuit can be designed so that any one of a number of protective features may be incorporated. The three most commonly-used automatic trip features for a circuit breaker are overcurrent (as discussed previously), underfrequency, and undervoltage. If any one of the conditions exists while the circuit breaker is closed, it will close its associated contact and energize the tripping coil, which, in turn, will trip the circuit breaker.
There are three basic meter movements utilized in electrical meters: DArsonval, electrodynamometer, and the moving iron vane. Some meter movements can be used for both AC or DC measurements, but in general, each meter movement is best suited for a particular type.
The most commonly used sensing mechanism used in DC ammeters, voltmeters, and ohm meters is a current-sensing device called a D'Arsonval meter movement (Figure 8). The D'Arsonval movement is a DC moving coil-type movement in which an electromagnetic core is suspended between the poles of a permanent magnet.
Figure 8 D'Arsonval Meter Movement
The current measured is directed through the coils of the electromagnet so that the magnetic field produced by the current opposes the field of the permanent magnet and causes rotation of the core. The core is restrained by springs so that the needle will deflect or move in proportion to the current intensity. The more current applied to the core, the stronger the opposing field, and the larger the deflection, up to the limit of the current capacity of the coil. When the current is interrupted, the opposing field collapses, and the needle is returned to zero by the restraining springs. The limit of the current that can be applied to this type movement is usually less than one milliampere.
A common variation of the D'Arsonval movement is the Weston movement, which uses essentially the same principle built to a more rugged construction by employing jeweled supports for the core and employing a heavier winding in the electromagnet. Remember that the D'Arsonval movement is a DC device and can only measure DC current or AC current rectified to DC.
The electrodynamometer movement (Figure 9) has the same basic operating principle as the D'Arsonval meter movement, except that the permanent magnet is replaced by fixed coils. The moving coil and pointer, which are attached to the coil, are suspended between and connected in series with the two field coils. The two field coils and moving coil are connected in series such that the same current flows through each coil.
Figure 9 Electrodynamometer Movement
Current flow through the three coils in either direction causes a magnetic field to be produced between the field coils. The same current flow through the moving coil causes it to act as a magnet exerting a force against the spring. If the current is reversed, the field polarity and the polarity of the moving coil reverse, and the force continues in the same direction. Due to this characteristic of the electrodynamometer movement, it can be used in both AC and DC systems to measure current. Some voltmeters and ammeters use the electrodynamometer. However, its most important use is in the watt meter, which will be discussed later in this article.
The moving iron vane movement (Figure 10) can be used to measure both AC current and voltage. By changing the meter scale calibration, the movement can be used to measure DC current and voltage. The moving iron vane meter operates on the principle of magnetic repulsion between like poles. The measured current flows through a field coil which produces a magnetic field proportional to the magnitude of current. Suspended in this field are two iron vanes attached to a pointer. The two iron vanes consist of one fixed and one moveable vane. The magnetic field produced by the current flow magnetizes the two iron vanes with the same polarity regardless of the direction of current through the coil. Since like poles repel one another, the moving iron vane pulls away from the fixed vane and moves the meter pointer. This motion exerts a force against a spring. The distance the moving iron vane will travel against the spring depends on the strength of the magnetic field. The strength of the magnetic field depends on the magnitude of current flow.
Figure 10 Moving Iron Vane Meter Movement
As stated previously, this type of meter movement may also be used to measure voltage. When this type of movement is used to measure voltage, the field coil consists of many turns of fine wire used to generate a strong magnetic field with only a small current flow.
Motor controllers range from a simple toggle switch to a complex system using solenoids, relays, and timers. The basic function of a motor controller is to control and protect the operation of a motor.
Motor controllers range from a simple toggle switch to a complex system using solenoids, relays, and timers. The basic function of a motor controller is to control and protect the operation of a motor. This includes starting and stopping the motor, and protecting the motor from overcurrent, undervoltage, and overheating conditions that would cause damage to the motor. There are two basic categories of motor controllers: the manual controller and the magnetic controller.
A manual controller, illustrated by Figure 11, is a controller whose contact assembly is operated by mechanical linkage from a toggle-type handle or a pushbutton arrangement. The controller is operated by hand.
The manual controller is provided with thermal and direct-acting overload units to protect the motor from overload conditions. The manual controller is basically an "ON-OFF" switch with overload protection.
Manual controllers are normally used on small loads such as machine tools, fans, blowers, pumps, and compressors. These types of controllers are simple, and they provide quiet operation. The contacts are closed simply by moving the handle to the "ON" position or pushing the START button. They will remain closed until the handle is moved to the "OFF" position or the STOP button is pushed.
The contacts will also open if the thermal overload trips.
Manual controllers do NOT provide low voltage protection or low voltage release. When power fails, the manual controller contacts remain closed, and the motor will restart when power is restored. This feature is highly desirable for small loads because operator action is not needed to restart the small loads in a facility; however, it is undesirable for larger loads because it could cause a hazard to equipment and personnel.
Figure 11 Single Phase Manual Controller
A large percentage of controller applications require that the controller be operated from a remote location or operate automatically in response to control signals. As discussed, manual controllers cannot provide this type of control; therefore, magnetic controllers are necessary.
Basic operations using a magnetic controller, such as the closing of switches or contacts, are performed by magnetic contactors. A magnetic controller is one that will automatically perform all operations in the proper sequence after the closure of a master switch. The master switch (for example, float switch, pressure switch, or thermostat) is frequently operated automatically. But in some cases, such as pushbuttons, drum switches, or knife switches, the master switch is manually operated. Figure 12 shows a typical magnetic controller and its component parts.
Figure 12 Single Phase Manual Controller
A magnetic contactor (Figure 13) is a device operated by an electromagnet.
The magnetic contactor consists of an electromagnet and a movable iron armature on which movable and stationary contacts are mounted. When there is no current flow through the electromagnetic coil, the armature is held away by a spring. When the coil is energized, the electromagnet attracts the armature and closes the electrical contacts.
Overload devices are incorporated into magnetic controllers. These overload devices protect the motor from overcurrent conditions that would be extremely harmful. There are many types and forms of overload devices. The following types of overload devices are commonly used in motor-control equipment.
The thermal overload device is shown in Figure 12.
Figure 13 Magnetic Contactor Assembly
Within the two basic categories of motor controllers, there are three major types of AC across-the-line controllers in use today. There are low-voltage protection (LVP), low-voltage release (LVR), and low-voltage release effect (LVRE) controllers.
The main purpose of an LVP controller is to de-energize the motor in a low voltage condition and keep it from re-starting automatically upon return of normal voltage (Figure 14).
Figure 14 LVP Controller
LVP Controller Operation:
The purpose of the LVR controller is to de-energize the motor in a low voltage condition and restart the motor when normal voltage is restored. This type of controller (Figure 15) is used primarily on small and/or critical loads (e.g., cooling water pumps required for safety-related equipment).
Figure 15 LVR Controller
LVR Controller Operation:
The LVRE controller maintains the motor across the line at all times. This type of controller is of the manual variety and is found mostly on small loads that must start automatically upon restoration of voltage (Figure 14). An LVRE controller may or may not contain overloads. If overloads are used, they will be placed in the lines to the load.
Figure 16 LVRE Controller
The motor controllers that have been discussed are very basic. There are many automatic control functions that can be incorporated into these types of controllers, but they are beyond the scope of this text.
The resistance of a wire or a circuit is measured by an ohmmeter. An ohmmeter aids the troubleshooter in determining if a ground or a short exists in a circuit.
The ohmmeter is an instrument used to determine resistance. A simple ohmmeter (Figure 17) consists of a battery, a meter movement calibrated in ohms, and a variable resistor. Ohmmeters are connected to a component which is removed from the circuit as illustrated in Figure 9. The reason for removing the component is that measurement of current through the component determines the resistance. If the component remains in the circuit, and a parallel path exists in the circuit, the current will flow in the path of least resistance and give an erroneous reading.
Figure 17 Simple Ohm Meter Circuit
Ro, in Figure 17, is an adjustable resistor whose purpose is to zero the ohmmeter and correct for battery aging. It is also a current-limiting resistor which includes the meter resistance Rm. Zeroing the ohmmeter is accomplished by shorting the ohmmeter terminals a b and adjusting
Ro to give full-scale deflection.
Equation (14-10) is the mathematical representation for determining full-scale deflection meter current.
When the unknown resistance Rx is connected across the ohmmeter terminals, the current is measured by calculating the total series resistance and applying Equation (14-10). Equation (14-11) is the mathematical representation of this concept.
An easy way to determine ohmmeter deflection is by use of a deflection factor (D). Deflection factor is the ratio of circuit current to meter current. Equation (14-12) is the mathematical representation of the deflection factor.
The current through the circuit can be determined by solving for I. Equation (14-13) is the mathematical representation of this relationship.
To solve for Rx using Equations (14-10) through (14-13), the relationship between deflection factor and the meter resistance to the unknown resistance can be shown. Equation (14-14) is the mathematical representation of this relationship.
If half-scale deflection occurs, then Rx = Ro, so that the value of Ro is marked at mid-scale on the ohmmeter face.
Example 1: An ohmmeter has a meter movement with a 100 µA full-scale deflection. The open circuit voltage at terminals a b is 24 V. The ohmmeter is zeroed and then an unknown resistance Rx is measured, which produces quarter-scale deflection. Find Rx.
Therefore, quarter scale deflection of this ohmmeter face would read 720 ΩK.
Example 2: An ohmmeter with Ro = 30 Ω, and full scale current Im = 300 μA. Find I with: 1) 0 Ω, 2) 5 Ω, 3) 10 Ω, 4) 15 Ω, and 5) 1 MΩ resistors across the meter terminal.
First, the deflection factor for each resistor must be found.
Then find I by using:
Figure 18 Ohm Meter Scale
Other measuring devices are used to aid operators in determining the electric plant conditions at a facility, such as the amp-hour meter, power factor meter, ground detector, and synchroscope.
The amp-hour meter registers amp-hours and is an integrating meter similar to the watt-hour meter used to measure electricity usage in a home. Typical amp-hour meters are digital indicators similar to the odometer used in automobiles. The amp-hour meter is a direct current meter that will register in either direction depending on the direction of current flow. For example, starting from a given reading, it will register the amount of discharge of a battery; when the battery is placed on charge, it will operate in the opposite direction, returning once again to its starting point. When this point is reached, the battery has received a charge equal to the discharge, and the charge is stopped. It is normally desired to give a battery a 10% overcharge. This is accomplished by designing the amp-hour meter to run 10% slow in the charge direction. These meters are subject to inaccuracies and cannot record the internal losses of a battery. They attempt to follow the charge and discharge, but inherently do not indicate the correct state of charge. Similar to an ammeter, the amp-hour meter is connected in series. Although the amp-hour meters were used quite extensively in the past, they have been largely superseded by the voltage-time method of control.
A power factor meter is a type of electrodynamometer movement when it is made with two movable coils set at right angles to each other. The method of connection of this type of power factor meter, in a 3ÃƒÂÃ¢â‚¬Â circuit, is shown in Figure 14. The two stationary coils, S and S1, are connected in series in Phase B. Coils M and M1 are mounted on a common shaft, which is free to move without restraint or control springs. These coils are connected with their series resistors from Phase B to Phase A and from Phase B to Phase C. At a power factor of unity, one potential coil current leads and one lags the current in Phase B by 30Ãƒâ€šÃ‚Â° thus, the coils are balanced in the position shown in Figure 19. A change in power factor will cause the current of one potential coil to become more in phase and the other potential coil to be more out of phase with the current in Phase B, so that the moving element and pointer take a new position of balance to show the new power factor.
Figure 19 3ÃƒÂÃ¢â‚¬Â Power Factor Meter Schematic
The ground detector is an instrument which is used to detect conductor insulation resistance to ground. An ohm meter, or a series of lights, can be used to detect the insulation strength of an ungrounded distribution system. Most power distribution systems in use today are of the grounded variety; however, some ungrounded systems still exist.
In the ohm meter method (Figure 20), a DC voltage is applied to the conductor. If a leakage path exists between the conductor insulator and ground, a current will flow through the ground to the ohm meter proportional to the insulation resistance of the conductor.
Figure 20 Simple Ohm Meter Ground Detector
In the ground detector lamp method (Figure 21), a set of three lamps connected through transformers to the system is used. To check for grounds, the switch is closed and the brilliance of the lamps is observed. If the lamps are equally bright, no ground exists and all the lamps receive the same voltage. If one lamp is dark, and the other two lamps are brighter, the phase with the darkened lamp is grounded. In this case, the primary winding of the transformer is shorted to ground and receives no voltage.
Figure 21 Ground Detector Lamp Circuit
A synchroscope indicates when two AC generators are in the correct phase relation for connecting in parallel and shows whether the incoming generator is running faster or slower than the on-line generator. The synchroscope consists of a two-phase stator. The two stator windings are at right angles to one another, and by means of a phase-splitting network, the current in one phase leads the current of the other phase by 90°, thereby generating a rotating magnetic field. The stator windings are connected to the incoming generator, and a polarizing coil is connected to the running generator.
The rotating element is unrestrained and is free to rotate through 360°. It consists of two iron vanes mounted in opposite directions on a shaft, one at the top and one at the bottom, and magnetized by the polarizing coil.
If the frequencies of the incoming and running generators are different, the synchroscope will rotate at a speed corresponding to the difference. It is designed so that if incoming frequency is higher than running frequency, it will rotate in the clockwise direction; if incoming frequency is less than running frequency, it will rotate in the counterclockwise direction. When the synchroscope indicates 0o phase difference, the pointer is at the "12 oclock" position and the two AC generators are in phase.
Power Plant facilities rely on dependable electrical distribution systems to provide power to key vital equipment. Knowledge of the basic electrical power distribution system and its components will help the operator understand the importance of electrical power distribution systems.
Commercial or utility power is electrical power that is provided by commercial generating systems to the facility.
Diesel power is power generated by a diesel-driven generator. Diesel-driven generators are the most economical and practical source of "standby power."
A single, or one-line diagram of a distribution system is a simple and easy-to-read diagram showing power supplies, loads, and major components in the distribution system (Figure 22).
Figure 22 Ground Detector Lamp Circuit
Failure-free power is accomplished by providing vital equipment with automatic switching between two or more power supplies so that interruption of power is minimized.
Neutral grounding in electrical distribution systems helps prevent accidents to personnel and damage to property caused by:
If some point on the circuit is grounded (in this case neutral ground), lightning striking the wires will be conducted into the ground, and breakdown between the primary and secondary windings of a transformer will cause the primary transformer fuses to blow. Another advantage of neutral grounding is that it reduces the amount of insulation required for high-voltage transmission lines.
Voltage in distribution systems is classified into three groups: high voltage, intermediate voltage, and low voltage. High voltage is voltage that is above 15,000 volts, intermediate voltage is voltage between 15,000 volts and 600 volts, and low voltage is voltage at 600 volts or less.
Protective relays are designed to cause the prompt removal of any part of a power system that might cause damage or interfere with the effective and continuous operation of the rest of the system. Protective relays are aided in this task by circuit breakers that are capable of disconnecting faulty components or subsystems.
Protective relays can be used for types of protection other than short circuit or overcurrent. The relays can be designed to protect generating equipment and electrical circuits from any undesirable condition, such as undervoltage, underfrequency, or interlocking system lineups.
There are only two operating principles for protective relays: (1) electromagnetic attraction and (2) electromagnetic induction. Electromagnetic attraction relays operate by a plunger being drawn up into a solenoid or an armature that is attracted to the poles of an electromagnet. This type of relay can be actuated by either DC or AC systems. Electromagnetic induction relays operate on the induction motor principle whereby torque is developed by induction in a rotor. This type of relay can be used only in AC circuits.
A separate zone of protection is provided around each system element (Figure 23). Any failure that may occur within a given zone will cause the tripping or opening of all circuit breakers within that zone. For failures that occur within a region where two protective zones overlap, more breakers will be tripped than are necessary to disconnect the faulty component; however, if there were no overlap of protective zones, a fault in a region between the two zones would result in no protective action at all. Therefore, it is desirable for protective zone overlap to ensure the maximum system protection.
Figure 23 Protective Relaying Zones
A fuse is a device that protects a circuit from an overcurrent condition only. It has a fusible link directly heated and destroyed by the current passing through it. A fuse contains a current-carrying element sized so that the heat generated by the flow of normal current through it does not cause it to melt the element; however, when an overcurrent or short-circuit current flows through the fuse, the fusible link will melt and open the circuit. There are several types of fuses in use (Figure 24).
Figure 24 Types of Fuses
The plug fuse is a fuse that consists of a zinc or alloy strip, a fusible element enclosed in porcelain or Pyrex housing, and a screw base. This type of fuse is normally used on circuits rated at 125 V or less to ground and has a maximum continuous current-carrying capacity of 30 amps.
The cartridge fuse is constructed with a zinc or alloy fusible element enclosed in a cylindrical fiber tube with the element ends attached to a metallic contact piece at the ends of the tube. This type of fuse is normally used on circuits rated at either 250 volts or 600 volts and has a maximum continuous current-carrying capacity of 600 amps.
The multimeter can be used as an ammeter, an ohm meter, or a voltmeter.
Meggers® are used to measure insulation resistance.
The multimeter is a portable single instrument capable of measuring various electrical values including voltage, resistance, and current. The volt-ohm-milliammeter (VOM) is the most commonly used multimeter. The typical VOM has a meter movement with a full scale current of 50 µA, or a sensitivity of 20 KΩ/V, when used as a DC voltmeter. A single meter movement is used to measure current, AC and DC voltage, and resistance. Range switches are usually provided for scale selection (e.g., 0-1V, 0-10V, etc).
The megger® is a portable instrument used to measure insulation resistance. It consists of a hand-driven DC generator and a direct reading ohm meter. A simplified circuit diagram of the instrument is shown in Figure 25.
Figure 25 Simple Megger Circuit Diagram
Coil A is wound in a manner to produce a clockwise torque on the moving element. With the terminals marked "line" and "earth" shorted, giving a zero resistance, the current flow through the Coil A is sufficient to produce enough torque to overcome the torque of Coil B. The pointer then moves to the extreme clockwise position, which is marked as zero resistance. Resistance (R1) will protect Coil A from excessive current flow in this condition.
When an unknown resistance is connected across the test terminals, line and earth, the opposing torques of Coils A and B balance each other so that the instrument pointer comes to rest at some point on the scale. The scale is calibrated such that the pointer directly indicates the value of resistance being measured.
Voltmeters are used extensively in industry where the surveillance of input and/or output voltages is vital for plant operation.
A simple DC voltmeter can be constructed by placing a resistor (RS), called a multiplier, in series with the ammeter meter movement, and marking the meter face to read voltage (Figure 26). Voltmeters are connected in parallel with the load (RL) being measured.
Figure 26 Simple DC Voltmeter
When constructing a voltmeter, the resistance of the multiplier must be determined to measure the desired voltage. Equation (14-1) is a mathematical representation of the voltmeterÃƒÆ’Ã†â€™?ÃƒÆ’Ã¢â‚¬Å¡Ãƒâ€šÃ‚Â¢??s multiplier resistance.
Example: A 2 mA meter movement with internal resistance of 25 ohms is to be constructed as a voltmeter.
What value must the series resistance be to measure full scale voltage of 100 volts?
When a voltmeter is connected in a circuit, the voltmeter will draw current from that circuit. This current causes a voltage drop across the resistance of the meter, which is subtracted from the voltage being measured by the meter. This reduction in voltage is known as the loading effect and can have a serious effect on measurement accuracy, especially for low current circuits.
The accuracy of a voltmeter (Kv) is defined as the ratio of measured voltage when the meter is in the circuit (Vw) to the voltage measured with the meter out of the circuit. Equation (14-2) is a mathematical representation of the accuracy of a voltmeter, or true voltage (Vo).
Meter accuracy can also be determined by comparing the relationship between the input and circuit resistances using OhmÃƒÆ’Ã†â€™?ÃƒÆ’Ã¢â‚¬Å¡Ãƒâ€šÃ‚Â¢??s Law as described below.
Example: A voltmeter in the 100 volt range with a sensitivity of 40 KΩ/V is to measure the voltage across terminals ab (Figure 27).
Figure 27 Measuring Circuit Voltage
'Wattmeters are used to determine DC power or real AC power delivered to the load.''
The wattmeter is an instrument which measures DC power or true AC power. The wattmeter uses fixed coils to indicate current, while the movable coil indicates voltage. Coils LI1 and LI2 are the fixed coils in series with one another and serve as an ammeter. The two I terminals are connected in series with the load. The movable coil Lv, and its multiplier resistor Rs, are used as a voltmeter, with the V terminals connected in parallel with the load. The meter deflection is proportional to the VI, which is power.
Wattmeters are rated in terms of their maximum current, voltage, and power. All of these ratings must be observed to prevent damage to the meter.
Equation (14-15) is the mathematical representation of calculating power in a DC circuit.
Equation (14-16) is the mathematical representation for calculating power in an AC circuit.
Total power in a 3ÃƒÆ’Ã…Â½Ãƒâ€¹Ã…â€œ?circuit is the sum of the powers of the separate phases. The total power could be measured by placing a wattmeter in each phase (Figure 28); however, this method is not feasible since it is often impossible to break into the phases of a delta load. It also may not be feasible for the Y load, since the neutral point to which the wattmeters must be connected is not always accessible.
Normally, only two wattmeters are used in making 3ÃƒÆ’Ã‚ÂÃƒÂ¢Ã¢â€šÂ¬Ã‚Â power measurements (Figure 29).
In balanced 3ÃƒÆ’Ã‚ÂÃƒÂ¢Ã¢â€šÂ¬Ã‚Â systems, with any power factor, total power is calculated by adding the A and B phase powers. Equation (14-17) is the mathematical representation for calculating total power (PT).
Other measuring devices are used to aid operators in determining the electric plant conditions at a facility, such as the amp-hour meter, power factor meter, ground detector, and synchroscope.
The amp-hour meter registers ampere-hours and is an integrating meter similar to the watt-hour meter used to measure electricity usage in a home. Typical amp-hour meters are digital indicators similar to the odometer used in automobiles. The amp-hour meter is a direct current meter that will register in either direction depending on the direction of current flow. For example, starting from a given reading, it will register the amount of discharge of a battery; when the battery is placed on charge, it will operate in the opposite direction, returning once again to its starting point. When this point is reached, the battery has received a charge equal to the discharge, and the charge is stopped. It is normally desired to give a battery a 10% overcharge.
This is accomplished by designing the amp-hour meter to run 10% slow in the charge direction. These meters are subject to inaccuracies and cannot record the internal losses of a battery. They attempt to follow the charge and discharge, but inherently do not indicate the correct state of charge. Similar to an ammeter, the amp-hour meter is connected in series. Although the amp-hour meters were used quite extensively in the past, they have been largely superseded by the voltage-time method of control.
A power factor meter is a type of electrodynamometer movement when it is made with two movable coils set at right angles to each other. The method of connection of this type of power factor meter, in a 3ÃƒÆ’Ã‚ÂÃƒÂ¢Ã¢â€šÂ¬Ã‚Â circuit, is shown in Figure 30. The two stationary coils, S and S1, are connected in series in Phase B. Coils M and M1 are mounted on a common shaft, which is free to move without restraint or control springs. These coils are connected with their series resistors from Phase B to Phase A and from Phase B to Phase C. At a power factor of unity, one potential coil current leads and one lags the current in Phase B by 30ÃƒÆ’Ã¢â‚¬Å¡Ãƒâ€šÃ‚Â° thus, the coils are balanced in the position shown in Figure 14. A change in power factor will cause the current of one potential coil to become more in phase and the other potential coil to be more out of phase with the current in Phase B, so that the moving element and pointer take a new position of balance to show the new power factor.
The ground detector is an instrument which is used to detect conductor insulation resistance to ground. An ohm meter, or a series of lights, can be used to detect the insulation strength of an ungrounded distribution system. Most power distribution systems in use today are of the grounded variety; however, some ungrounded systems still exist.
In the ohm meter method (Figure 31), a DC voltage is applied to the conductor. If a leakage path exists between the conductor insulator and ground, a current will flow through the ground to the ohm meter proportional to the insulation resistance of the conductor.
In the ground detector lamp method (Figure 32), a set of three lamps connected through transformers to the system is used. To check for grounds, the switch is closed and the brilliance of the lamps is observed. If the lamps are equally bright, no ground exists and all the lamps receive the same voltage. If one lamp is dark, and the other two lamps are brighter, the phase in which the darkened lamp is in is grounded. In this case, the primary winding of the transformer is shorted to ground and receives no voltage.
Figure 32 Ground Detector Lamp Circuit
A synchroscope indicates when two AC generators are in the correct phase relation for connecting in parallel and shows whether the incoming generator is running faster or slower than the on-line generator. The synchroscope consists of a two-phase stator. The two stator windings are at right angles to one another, and by means of a phase-splitting network, the current in one phase leads the current of the other phase by 90Ãƒâ€šÃ‚Â°, thereby generating a rotating magnetic field. The stator windings are connected to the incoming generator, and a polarizing coil is connected to the running generator.
The rotating element is unrestrained and is free to rotate through 360Ãƒâ€šÃ‚Â°. It consists of two iron vanes mounted in opposite directions on a shaft, one at the top and one at the bottom, and magnetized by the polarizing coil.
If the frequencies of the incoming and running generators are different, the synchroscope will rotate at a speed corresponding to the difference. It is designed so that if incoming frequency is higher than running frequency, it will rotate in the clockwise direction; if incoming frequency is less than running frequency, it will rotate in the counterclockwise direction. When the synchroscope indicates 0Ãƒâ€šÃ‚Â° phase difference, the pointer is at the "12 o'clock" position and the two AC generators are in phase.
Industrial facilities rely on standardized wiring schemes to provide both single-phase and three-phase power distribution systems and protective grounds to insure safe operation.
Many advisory boards exist to insure the standardization of electrical installations in accordance with accepted designs and safe practices. The Institute of Electrical and Electronics Engineers (IEEE) and the American National Standards Institute (ANSI) are two advisory boards that have published numerous standards. These standards are utilized by the Department of Energy and the nuclear industry. However, for a day-to-day practical guide for noncritical installations, the recognized guide is the National Electrical Code Handbook (NEC), published by the National Fire Protection Association and endorsed by ANSI. The NEC Handbook is the primary source of much of the material presented in this chapter and may serve as a ready reference for specific questions not covered in this fundamental discussion.
To understand wiring schemes used in power distribution systems, you must be familiar with the following terms.
The source of single-phase (1φ) power in all facilities is by generation from a single-phase generator or by utilization of one phase of a three-phase (3φ) power source. Basically, each phase of the 3φ distribution system is a single-phase generator electrically spaced 120 degrees from the other two; therefore, a 3φ power source is convenient and practical to use as a source of single-phase power.
Single-phase loads can be connected to three-phase systems utilizing two methods. The diagram shown in Figure 33 illustrates these connections.
Figure 33 Three-Phase To Single-Phase Connections
The first scheme (Figure 33A) provides for the connection of the load from a phase leg to any ground point and is referred to as a phase-to-ground scheme. The remaining scheme (Figure 33B) connects the single-phase load between any two legs of the three-phase source and is referred to as a phase-to-phase connection. The choice of schemes, phase-to phase or phase-to-ground, allows several voltage options depending on whether the source three-phase system is a delta or wye configuration. This will be discussed in the three-phase segment of this chapter.
The only approved method of wiring single-phase power is the scheme commonly referred to as the 3-wire, single-phase Edison system. The illustration in Figure 34 depicts the use of a center-tapped transformer, with the center tap grounded, providing half voltage (120 V) connections on either side or full voltage (240 V) across both sides.
Figure 34 3-Wire Edison Scheme
The physical connections to the transformer secondary involve two insulated conductors and one bare conductor. If the conductor is a current-carrying leg or neutral leg, the conductor will be insulated. The remaining uninsulated conductor will serve as a safety ground and will be bonded to the ground point of the system. In all cases, 3 wires will be presented to the load terminals, and the safety ground will be bonded to each junction box, or device, in the distribution system. In the case of half voltage (120 V) use, the intended path of the current is from the supply leg through the load and back to the source on the neutral leg. No current would be carried on the ground unless a fault occurred in the system, in which case the current would flow safely to ground.
In the full voltage system (240V), the insulated conductors are connected across the full winding of the transformer, and the uninsulated conductor is again bonded to the grounded center tap. In a balanced system, all currents will flow on the insulated conductors, and the grounded neutral will carry no current, acting only in a ground capacity. In the case of either an unbalanced load or a fault in the system, the bare conductor will carry current, but the potential will remain at zero volts because it is tied to the ground point. As in the case of the half voltage system, the uninsulated conductor will be bonded to each device in the system for safety.
Unlike the single-phase wiring scheme that must make a provision for a neutral leg and separate ground, the three-phase system needs neither a separate neutral nor a ground to operate safely. However, to prevent any unsafe condition, all 3- and 4-wire, three-phase systems can include an effective ground path. As with the previous single-phase discussion, only the secondary side of the transformer and its connected load need to be studied.
The simplest three-phase system is the 3-wire Delta configuration, normally used for transmission of power in the intermediate voltage class from approximately 15,000 volts to 600 volts. The diagram in Figure 35 depicts the two methods of connecting the Delta secondary.
Figure 35 3-Wire Three-Phase Delta Scheme
The upper diagram depicts the ungrounded Delta, normally confined to protected environments such as fully enclosed ducts or overhead transmission lines that cannot breached without extraordinary means. Each conductorÃƒÆ’Ã†â€™Ãƒâ€šÃ‚Â¢??s ground voltage is equal to the full phase voltage of the system.
The lower diagram shows a ground point affixed to one corner of the Delta, which effectively lowers one phaseÃƒÆ’Ã†â€™Ãƒâ€šÃ‚Â¢??s voltage reference to ground to zero, but retains a phase-to-phase voltage potential. The corner-grounded phase acts in much the same way as the grounded neutral of the single-phase Edison system, carrying current and maintaining ground potential.
The corner-grounded Delta system has an obvious economy in wiring costs, and the grounded phase can be used to physically protect the other two phases from accidental grounding or lightning strikes in outdoor settings. This system is rarely used for low voltage (under 600 V), however, because of the absence of a safety ground required by many facilities for circuits involving potential worker contact.
The 4-wire, three-phase Delta system combines the ungrounded Delta discussed above for three-phase loads with the convenience of the Edison system for single-phase loads. As depicted in the example illustration in Figure 36, one side of the Delta has a grounded-neutral conductor connected to a center tap winding on one phase.
Figure 36 4-Wire Delta System
The single-phase voltage on each side of the half-tap is one-half the voltage available in the normal phase-to-phase relationship. This provides the same half- or full-voltage arrangement seen in the normal Edison scheme with a grounded neutral. Notice also that the legs coming from the corners of the Delta would have a normal ungrounded appearance if it were not for the center tap of one phase. Thus, at any given location in the system, either three-phase power at full voltage or single-phase power with half or full voltage is equally possible. However, there are several strict precautions that must be observed in the operation of this system. First, all loads must be carefully balanced on both the single-phase and three-phase legs. Second, because the voltage between one leg and the grounded neutral is considerably higher than the rest of the single-phase system, a measurement between the neutral and the phase must be taken to identify the "high leg," or "bastard voltage." Last, the "high leg" is never used as a single-phase source because no ground or grounded neutral exists for this circuit.
Until now, the voltage, the phase voltage, and the ground voltage of the three-phase systems have been equal, with the one exception of one phase of the corner-grounded Delta. The Wye system has completely different voltage characteristics from the Delta system. In the Wye system, the ground voltage or voltage available from phase to ground is the phase voltage divided by 1.73.
In Figure 37, an example of the Wye system, or center-grounded Wye as it is commonly referred to, extends three current-carrying insulated conductors and an insulated grounded neutral to the loads. Depending on the selection of conductors, one of the following is available: a reduced voltage single phase between a phase leg and the neutral; a full-voltage single-phase circuit between any two phase legs; or a full-voltage three-phase power. Again, some precautions must be taken when balancing the single-phase loads in the system. The full load ampacity of the neutral must be sized to 1.73 times the highest phase ampacity. This is done to avoid either an over-current condition if a fault is present or the operation of single-phase loads at reduced voltage if the loads become severely unbalanced by accidental interruption.
Figure 37 4-Wire, Three-Phase Wye System
As with all other grounded systems, bonds are established between the grounded neutral and all components of the system. This system is recognized as the safest possible multi-purpose distribution system for low voltage and is commonly seen in the 208/120 volt range in many facilities.