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This article introduces you to the different types of electrical measuring instruments and test sets used in a substation. The purpose of this article is to enable you to recognize and understand the functions of different instruments. Later, you will learn how to actually use and read these instruments on the job.

2.1.1 Accuracy
8.0.2 Operation Maintenance
8.0.9 Operation
8.0.12 Operation

Types of Test Equipment

This first section introduces multimeters, voltage testers, continuity testers.


Figure 1: Digital Multimeter
Let us begin by explaining what a DMM (digital multimeter) is. A DMM is simply an electronic tape measure for making electrical measurements. It may have any number of special features, but mainly a DMM measures volts, ohms, and amperes.

Resolution, Digits, and Counts

Resolution refers to how fine a measurement a meter can make. By knowing the resolution of a meter, you can determine if it is possible to see a small change in the measured signal. You would not buy a ruler marked in 1-inch (or 1 cm) segments if you had to measure down to a 1/4 inch (or 1 mm). For example, if the DMM has a resolution of 1 mV on the 4 V range, it is possible to see a change of 1 mV (1/1000 of a volt) while reading 1 V. The terms digits and counts are used to describe a meters resolution. DMMs are grouped by the number of counts or digits they display.

A 3 1⁄2 digit meter can display three full digits ranging from 0 to 9, and one "half" digit, which displays only a 1 or is left blank. A 3 1⁄2 digit meter will display up to 1,999 counts of resolution. A 4 1⁄2 digit meter can display up to 19,999. It is more precise to describe a meter by counts of resolution than by digits. Todays 3 1⁄2 digit meters may have enhanced resolutions of up to 3,200, 4,000, or 6,000 counts. For certain measurements, 3,200-count meters offer better resolution. For example, a 1,999 count meter would not be able to measure down to 1/10 V if you were measuring 200 V or more. However, a 3,200-count meter would display a tenth of a volt up to 320 V. This is the same resolution as a more expensive 20,000-count meter until the 320 V reading is exceeded.


Accuracy is the largest allowable error that will occur under specific operating conditions. In other words, it is an indication of how close the DMMs displayed measurement is to the actual value of the signal being measured. Accuracy for a DMM usually is expressed as a percent of reading. An accuracy of 1% of reading means that for a displayed reading of 100 V, the actual value of the voltage could be anywhere between 99 V and 101 V. A range of digits may also be added to the basic accuracy specification. This indicates how many counts the digit to the extreme right of the display may vary. So the preceding accuracy example might be stated as (1%+2). Therefore, for a display reading of 100 V, the actual voltage would be between 98.8 V and 101.2 V.

Analog meter specifications are determined by the error at full scale, not at the displayed reading. Typical accuracy for an analog meter is 2% or 3% of full scale. At 1/10 of full scale, these become 20 or 30% of reading. Typical basic accuracy for a DMM is between (0.7%+1) and (0.1%+1) of reading or better.

DC and AC Voltage

One of the most basic tasks of a DMM is measuring voltage. A typical DC voltage source is a battery, like the one used in your car. AC voltage usually is created by a generator. The wall outlets in your home are common sources of AC voltage. Some devices convert AC to DC. For example, electronic equipment, such as TVs, stereos, VCRs, and computers, that you plug into an AC wall outlet use devices called rectifiers to convert the AC voltage to a DC voltage. This DC voltage powers the electronic circuits in these devices. Testing for proper supply voltage usually is the first step when troubleshooting a circuit. If there is no voltage present, or if the voltage is too high or too low, then there is a problem that should be corrected before further investigation occurs.

The waveforms associated with AC voltages can be either sinusoidal or non-sinusoidal (sawtooth, square, ripple, etc.). Quality DMMs display the "RMS" (root mean square) value of these voltage waveforms. The RMS value is the effective, or equivalent, DC value of the AC voltage. Most DMMs are "average responding," meaning accurate RMS readings are present if the AC voltage signal is a pure sine wave. Average responding meters are not capable of measuring non-sinusoidal signals accurately. Non-sinusoidal signals are accurately measured using DMMs designated as "true-RMS" up to the DMMs specified crest factor. Crest factor is the ratio of a signals peak-to-RMS value. It is 1.414 for a pure sine wave, but is often much higher for a rectifier current pulse, for example. As a result, an average responding meter will often read much lower than the actual RMS value.

A DMMs ability to measure AC voltage can be limited by the frequency of the signal. Many DMMs can accurately measure AC voltages with frequencies from 50 Hz to 500 Hz, but a DMMs AC measurement bandwidth may be hundreds of kilohertz wide. Such a meter may read a higher value because it is "seeing" more of a complex AC signal. DMM accuracy specifications for AC voltage and AC current should state the frequency range and its accuracy level.


Resistance is measured in ohms (Ω). Resistance values can vary greatly from a few milliohms (mΩ) for contact resistance to billions of ohms for insulators. Most DMMs measure down to 0.1 Ω, and some measure as high as 300 MΩ (300,000,000 Ω). Infinite resistance (for an open circuit) is read as "OL" on the meter display, which means the resistance is greater than the meter can measure. Resistance measurements must be made with the circuit power off; otherwise the meter or circuit could be damaged. Some DMMs provide protection in the ohms mode in case of accidental contact with voltages. The level of protection may vary greatly among different DMM models though. For accurate low-resistance measurements, the test lead resistance must be subtracted from the total resistance measured. Typical test lead resistance is between 0.2 Ω and 0.5 Ω. If the test lead resistance is greater than 1 Ω, the test leads should be replaced. If the DMM supplies less than 0.6 V DC test voltage for measuring resistance, it will be able to measure the value of resistors that are isolated in a circuit by diodes or semiconductor junctions. This often allows testing resistors to be tested on a circuit board without being unsoldered.


Continuity is a quick go/no-go resistance test that distinguishes between an open and a closed circuit. A DMM with a continuity beeper allows many continuity tests to be completed easily and quickly. The meter beeps when it detects a closed circuit, and the level of resistance required to trigger the beeper varies among DMM models.

Diode Test

A diode is similar to an electronic switch. It can be turned on if the voltage is over a certain level, generally about 0.6 V for a silicon diode, and it allows current to flow in one direction. When checking a diode or transistor junctions condition, an analog VOM might give a variety of readings and can drive currents up to 50 mA through the junction (See Table 1). Some DMMs have a diode test mode. That mode measures and displays the actual voltage drop across a junction. A silicon junction should have a voltage drop less than 0.7 V in the forward direction, and an open circuit should be present in the reverse direction.

Measuring Current

Current measurements are different from other DMM measurements. Those taken with the DMM alone require placing the meter in series with the circuit being measured. This means opening the circuit and using the DMM test leads to complete the circuit. Then, all the circuit current flows through the DMMs circuitry. A current probe can be used to indirectly measure DMM current. The probe clamps around the outside of the conductor to avoid opening the circuit and connecting the DMM in series.

Input Protection

A common mistake made when measuring voltage is to leave the test leads plugged into the current input jacks and attempt a voltage measurement. This causes a direct short across the source voltage through a low-value resistor inside the DMM, called a current shunt. A high current flows through the DMM, and if it is not adequately protected, this current can cause extreme damage to both the DMM and the circuit, and possibly causing injury to the operator. Extremely high fault currents can occur if industrial high-voltage circuits are involved (>240 V). A DMM should therefore have current input fuse protection with a capacity high enough for the circuit being measured. Meters without fuse protection in the current inputs should not be used on high-energy electrical circuits (>240 V AC). Those DMMs that use fuses should have sufficient capacity to clear a high-energy fault. The voltage rating of the meters fuses should be greater than the maximum voltage you expect to measure. For example, a 20 A, 250 V fuse may not be able to clear a fault inside the meter when the meter is across a 480 V circuit, but a 20 A, 600 V fuse can clear its fault on a 480 V circuit.

Voltage Testers

Voltage testers (Figure 2) are used to verify that low-voltage circuits are de-energized and safe to work on. A voltage tester does not measure voltage, but rather detects the presence of voltage. Just touch the tip to a control wire, conductor, or outlet. When it glows red, you know there is voltage present. A voltage tester can be used on 120 to 600 V AC and DC circuits.

Figure 2: Voltage Tester

Walk onto any site where electricians are working, and you will probably see some voltage testers. These handy devices which often fit in a shirt pocket are popular because they quickly give an indication of voltage presence. That makes them very handy for general voltage checks, especially when working with control systems. Thus, voltage testers are popular with electricians. Yet, these devices are not all the same. The differences show up in safety, reliability, and convenience.

If you were to look at all the voltage testers on the market and note their differences, you would quickly see that they divide into two general categories: solenoid-based testers and electronic testers. Solenoid-based testers have a long tradition; they were the first voltage testers available and are still widely in use today.

When voltage passes some threshold, the tester will indicate that a voltage is present. Below that threshold, the tester will not indicate a voltage at all. The thresholds are markedly different between the two categories of testers, and that fact carries important implications for safety and convenience. Let us compare voltage testers in these two categories more closely so you can draw your own conclusions about what to have in your toolbox or what to clip to your pocket.

Solenoid-Based Voltage Testers

These devices operate, as their name implies, on solenoid principles. A solenoid depends on the movement of a ferrite core (commonly referred to as the "slug") in response to the energization and de-energization of an electromagnetic coil. The indication function of these testers depends on a spring that drives a mechanical pointer. The spring restrains the slug, which slides to one end of its chamber or the other, depending on whether the coil has enough energy to cause the slug to overcome the opposing force of the spring. The amount of energy required restricts the sensitivity of solenoid-based testers. Useful solenoid testers need to measure voltages up to 600 V or more. However, having the ability to measure higher voltages limits their capability to detect voltages below about 100 V due to the poor dynamic range of the magnetics, an unfortunate weakness of solenoid testers. Try using one on 24 V or 48 V control circuits and you may as well be using a stick of wood.

An important concern with solenoid-based testers is their relatively low input impedance. They may be rated for 10 kilohms at the upper end, but they often come as low as 1 kilohm. Applying Ohms law, you can see solenoid-based testers can easily make their presence felt in a circuit as loads and subsequently interfere with the operation of that circuit. The relatively high current draw of solenoid-based testers means significantly more heat enough that the testers can quickly overheat, even to the point of destruction (Figure 21) if the voltage is measured a little too long. In fact, you must allow for cooldowns (on the order of half a minute) as you take readings with solenoid-based testers. If your PLC goes down and the plant manager is screaming about production being lost forever, you are at the mercy of this limitation of the tester.

Figure 3: Solenoid-Based Tester After Overheating
Solenoid-based testers generally are unable to comply with IEC 1010 due to excessive current draw, poor dielectric withstand performance, and impulse destruction due to transients originating from the mains. This is one reason many companies forbid the use of voltage testers on anything but 120 V control circuits, while some forbid them altogether. In a moment, we will look at reasons to reconsider those restrictions.

This high current in solenoid-based testers has another downside. Applying Ohms law to the low-impedance solenoid-based tester shows that you can easily carry a lethal current through the tester. Wearing insulated gloves can eliminate the shock hazard, but you still will be breaking a circuit under load each time you use that tester and you will be risking an arc hazard each and every time. Yes, there are riskier things you can do than use a solenoid-based tester. However, there are also safer things you can do, such as use an electronic voltage tester instead.

Electronic Voltage Testers

The first noticeable advantage of electronic voltage testers is their compact size relative to their old-technology counterparts. Thus, they are easier to carry around. However, that advantage pales beside the significant advantages that come from the far higher input impedance of electronic voltage testers. Some of these have input impedances of one megohm about 100 times that of the best solenoid-based testers. Even at the low end, you are looking at 20 kiloohms. Simply apply Ohms law, and the advantages become clear. You are going to be dealing with far less input current. That means more safety. It also means less time, if any, waiting for the instrument to cool between readings. They work at lower voltages and typically carry an IEC rating. They also make troubleshooting a wider range of problems safer and faster (Figure 22).

Figure 4: Electronic Voltage Tester
This higher impedance has a downside though: an electronic tester might indicate voltage on a non-energized conductor.

This can happen when one conductor induces a voltage in another conductor parallel to it. This voltage indication can be a disadvantage by showing a false positive. Then again, it can work to your advantage. For one thing, it will not lull you into a false sense of safety that an energized conductor is de-energized. Suppose the solenoid-based tester does not show you the 80 V sitting on that wire, and you grab it. It is better to be too safe than not safe enough.

Another issue that arises in reference to induced voltages is the fact that the same technology used in the electronic testers also dominates todays control systems. We seldom ask our HVAC or industrial control systems to open and close electromechanical coils to operate control logic. Instead, we ask them to provide signals that electronic devices will interpret as the presence or absence of voltage, which is essentially what electronic voltage testers do. Doesnt it make sense to use a tester that matches the control system you are using?

Other Considerations

Differences occur not just in the testers themselves. Nor is the solenoid vs. electronic issue the only one to resolve. Several other considerations are important for your safety and job performance.

One mistake people make with test equipment is trying to save money on leads and probes. This can be a very expensive "cost-savings." Cheap and poorly constructed input accessories are prone to failure of the materials that are supposed to protect you. Remember that you often hold accessories in your hand while conducting measurements. If a failure occurs, the result could be extremely hazardous to you.

It is important to maintain quality throughout your measurement tool kit. Choose accessories that are suitable for industrial work, keep an eye on them for abrasions and other damage that eventually will occur with use. This way, you will never have to worry about the failure of a test lead or probe, or the consequences thereof. Look for the IEC rating (e.g., CAT II) (Figure 23) on your tester, and buy leads and other accessories that meet or exceed that rating.

Figure 5: Lead Rating

Additional features can make your tester a bit more useful. However, these may add weight and cost. If these features are important to you though, consider them when buying a voltage tester.

Continuity Testers

Continuity testers are used to verify and check wire continuity. In the substation, continuity testers are used to verify that a circuit is complete and the wiring is not broken. Testers are also used to verify that the correct lead is connected.

The continuity tester consists of a battery-powered buzzer and two leads. When the two leads are connected, a complete circuit is made and the buzzer will sound. If the circuit is not complete, the buzzer will not sound. Another type of continuity tester is a flashlight with two leads attached. When a complete circuit is made, the flashlight illuminates. A multimeter is another type if continuity meter. Figure 6 shows two types of continuity testers.

Figure 6: Continuity Testers

Continuity testers are used only on de-energized circuits. They are not to be used on energized circuits as damage to the continuity tester and possible injury may result.

Low-Resistance Ohmmeters

Low-resistance ohmmeters (Figure 7) have high accuracy levels and can measure resistance down to milli (10-3) or micro (10-6) ohms. Low-resistance ohmmeters are used for measuring how well a devices contacts and connections fit together.

Figure 7: Digital Low-Resistance Ohmmeter (DLRO)
The resistance of electrical contacts or connections should be very low so that the current flows without arcing or burning. Low-resistance ohmmeters are used to check the contact resistance of:

  • Circuit breakers
  • Tap changers
  • Disconnect switches
  • Circuit switches

There are two types of low-resistance ohmmeters used: the microohmmeter and milliohmmeter. The microohmmeter is an older style of meter with a dial-type meter that measures resistance in microohms (10-6ohms). The milliohmmeter is a newer style electronic meter with a digital readout. It measures resistance in milliohms (10-3ohms).


Ammeters (Figure 8) are used for measuring AC current in various circuits throughout the substation. The AC current measurement for a circuit provides the necessary info to measure the equipment load and indicates whether the equipment is overloaded. Ammeters are used to check the AC light and power boards and on motor loading.

Figure 8: Clamp-On Ammeters
The ammeters used in most substations are clamp-on ammeters. Unlike multimeters, a clamp-on ammeter does not have to be connected in the circuit to measure current. Instead, the jaws of the ammeter open to fit around the conductor and are closed, or "clamped," around the conductor. The ammeter measures the circuits current by measuring the conductors magnetic field. Most clamp-on ammeters measure AC current, while a special type of clamp-on ammeter called a Hall Effects ammeter can measure both AC and DC currents.

High-Voltage Detectors

A high-voltage detector is used to prove that a high-voltage circuit is de-energized before grounds are applied to the circuit. The tic tracer shown in Figure 9 emits an audible signal when in the presence of an electric field. It must be used with an insulated pole with a universal mounting head and the proper PPE.

Figure 9: High-Voltage Detector (Tic Tracer)


Thermography is gaining wide acceptance in industry. It is used to compare the infrared emissions from a machines baseline data. This data can then be used to ascertain, for a given load, if the temperature of the winding is abnormally high or if there is a hot spot associated with the machines bearings. This normally is tested with the equipment drawing in excess of 40% load. Baseline data is not always needed. If you have similar pieces of equipment under equivalent load conditions, then a simple comparison between the two units can be performed. Any significant changes in the infrared spectrum would indicate the presence of a problem. Typically, the problem is a high-resistance connection or a cleanliness problem.

Hand-held thermal imagers, like the one shown in Figure 10, collect heat signatures from a range of motors from 1,000 down to 5 hp. A thermal imager is good for spot checks, to determine if motors and associated panels and controls are operating when they are too hot, and troubleshooting, to track down the specific failed component. It can also check for phase imbalances, bad connections, and abnormal heating on the electrical supply.

Figure 10: Thermal Imager
A motors heat signature will tell you a lot about its quality and condition. If a motor is overheating, the windings will rapidly deteriorate. In fact, every increase of 10C on a motors windings above its designed operating temperature cuts the life of its windingsinsulation by 50%, even if the overheating is only temporary.

If a temperature reading in the middle of a motor housing comes up abnormally high, a thermal image of the motor should be taken to determine more precisely what is producing the high temperature: windings, bearings, or couplings.

These are the three primary causes for abnormal thermal patterns; the majority of other causes produce high temperatures and typically are the result of a high-resistance contact surface, usually either a connection or a switch contact. These normally appear warmest at the spot of highest resistance, with patterns cooling off as distance from the spot increases. The thermal image in Figure 11 shows a classic pattern in the center phase connection on the line-side of the breaker; note how the conductor cools off at the top of the image.

Load imbalances, whether normal or out of specification, appear equally warm throughout the phase or part of the circuit that is undersized or overloaded. Harmonic imbalances create a similar pattern. If the entire conductor is warm, it could be undersized or overloaded. The rating and actual load should be checked to determine which is the cause.

Failed components typically look cooler than ones that are functioning normally. The most common example is probably a blown fuse. In a motor circuit, this can result in a single-phase condition and, potentially, costly damage to the motor.

Figure 11: Thermal Image of a Hot Connection

Figure 11 shows a thermal image of a drive cabinet with hot connections on both the A and the B phases. The exact cause cannot be determined solely from the image, although we can tell it could be a load or balance issue.

Figure 12: Thermal Image Bearing Problem

Figure 12 depicts a warm bearing (or seal) on a pump. Clearly, the access is tight but the bearing and its housing can still be compared.

Figure 13 Left

Figure 13 Right

Figure 13: Thermal Image Heat Transfer

Figure 13 Left depicts another bearing problem where heat is transferred into the coupling on the right side.

Figure 13 Right, on the other hand, shows the motor itself heating up due to reduced airflow or misalignment.


A digital multimeter is one of the best all-around pieces of electrical test equipment available to a technician; however, it does have its limitations. Problems measuring very high or low resistances, like those required for a motor winding to ground measurement, are common. For this type of measurement, a megger, shown in Figure 14, is used to provide the accurate measurement required.

Figure 14:Megger MJ159

Meggers come in all varieties. Some are single-voltage devices, like 500 V meggers, and others offer a range of test potentials, from 100 to 5,000 V. Some are manually operated, and others are battery or motor-operated.

The higher voltage applied by a megger allows a more accurate measurement of highly resistive items, such as insulation. According to Ohms law, current is equal to the applied voltage divided by the resistance. Since most digital multimeters use a 6 or 9 V (or smaller) power supply, it is possible to get an infinite reading, which indicates an open circuit, whereas a megger might give a reading of 500 MΩ. The difference in results is due to the higher voltage available in the megger.

Meggers should not be used to measure small resistances or electronic components for the same reason it is used for high resistive values. The voltage produced by a megger is far too high for most standard electronic components to sustain.

Safety Information

As with the digital multimeter, the safety warnings in the users manual should be read and followed. The safety warnings for the MJ159 megger are

  • The circuit under test must be switched off, de-energized, and isolated before any test connections are made.
  • Test leads; probes, and crocodile clips must be in good order, clean, and have no broken or cracked insulation.
  • Circuit connections must not be touched during a test.
  • Circuits must be discharged before disconnecting the test leads.
  • Replacement fuses must be of the correct type and rating.
  • When making a voltage measurement, the TEST button must not be pressed.
  • The instrument should not be used if any part of it is damaged.

Refer to Test Precautions for further explanations and precautions.

The MJ159 is powered by a low-voltage, hand-cranked, brushless AC generator that is connected, after rectification, to a DC-to-DC converter. The generator is designed to be easy to turn, even at full load. Nominal test voltages are 100, 250, 500, and 1,000 V, selectable by the user. This switch is located on the left front side of the megger.

This megger's insulation measuring range is 0.1 Ω to 2,000 MΩ. Automatic discharge for capacitive circuits under test is provided. The instrument also has a 5,000Ω range for testing electrical installations. The voltage measuring range is 0 to 600 V, and it is calibrated for AC voltage. This feature allows monitoring of decaying voltage following the testing of equipment possessing capacitance.

The needle is a day-glow orange, and the scales are white on a black scale face. The needle is moved using a moving coil with taut band suspension.

There are three shrouded terminal sockets labeled (-), (+), and (G) provided on the side of the meter for attachment of the test leads. These terminals are located just above the scale. Since the resistance of the test leads is included in the calibration of the meter, the test leads supplied by the manufacturer should be used. After the test leads have been connected to the instrument terminals, the carrying handle folds down neatly over them to protect the user.


Meggers must only be used by suitably trained and competent people.

The instruments can give an electric shock. Highly capacitive circuits (e.g., long cables) charged to several kilovolts could create a potentially lethal charge. Circuit connections must not be touched when testing.

Care must be taken to prevent capacitive circuits from becoming disconnected during a test, leaving the circuit in a charged state.

The instruments voltmeter and automatic discharge should be regarded as additional safety features and not substitutes for maintaining normal safe working practices.

Fuse replacements must be of the correct type and rating. Specifications are listed in the user's manual.

If any part of the instrument is damaged, it should not be used but returned to the manufacturer or an approved organization for repair.

Performance Checks

The instrument will operate in any position, but the specified accuracies assume the instrument is face-up on a firm, level surface. This is particularly true for the hand-cranked units due to the need to obtain a smooth, constant crank speed.

Without the test leads being connected to the instrument, but with the rotary selector switch set to the 1 kV range, the TEST button should be pressed and held down while turning the generator handle at>180 RPM. The meter pointer should remain over the ∞ (infinity) position on the scale, proving that there is no leakage from the instrument itself.

Connect one of the test leads to the+and the other to the-terminals on the side of the instrument case and ensure that their clips are not touching anything.

Press the TEST button again and keep it pressed while turning the generator handle at>180 RPM. The meters needle should be observed. The needle should rest over the "∞" (infinity) position on the scale. If it does not, the test leads may be faulty and should be inspected more closely for damage. Replace them, if necessary, with calibrated leads available as optional accessories.

Next, the test lead clips should be connected together. Press the Test button and turn the generator handle. The meter should read zero. If it indicates infinity or a high resistance value, the leads may be open circuited and should be inspected. Replace them if necessary. (Shorting the leads together and obtaining a zero reading shows that the instrument is working.)

Voltage Measurement

When not testing (i.e., when in standby mode), the instrument acts as a voltmeter (0 to 600 VAC). Therefore, as soon as the test leads are connected to the item under test, any AC voltage present will be immediately shown. The indication indicates that the item has not been de-energized completely. The instrument monitors circuit discharge when the "Test button is released following an insulation test on a capacitive item (e.g., a long cable). In this case, it is important to realize that the actual voltage (DC) is not given, but that the meter does indicate when the voltage has decayed to zero and when it is safe to remove the test leads. Note, however, that the instrument does not indicate the presence of negative DC voltage, so care must still be taken.

Resistance Measurement

With the leads connected to the instrument and having completed the preliminary checks:

1. Set the selector switch to the "Ω" position.

2. Connect the leads across the isolated circuit.

3. Press and hold the Test button and turn the generator handle at>180 RPM. The resistance is indicated on the "Ω" scale.

Guard Terminal

For basic insulation tests and where there is little possibility of surface leakage affecting the measurement, it is unnecessary to use the guard terminal. (For example, if the insulator is clean and any adverse current paths are unlikely.) However, if the instrument is used to test a cable that has the possibility of surface leakage due to the presence of dirt or moisture, the guard terminal needs to be used.

The guard terminal is at the same potential as the negative test terminal. Since the leakage resistance is in parallel with the resistance being measured, the use of the guard terminal causes the current flowing through surface leakage to be diverted from the measuring circuit.


The only user-replaceable parts on the instrument are the leads and the fuses internal to the meter. There are separate fuses for the resistance circuit and the insulation circuit.

To determine if the resistance fuse is blown:

1. Disconnect the test leads and set the rotary switch to the "Ω" position.

2. Press the Test button and keep it pressed while turning the generator handle.

3. The reading obtained should be beyond full scale.

4. If the reading is approximately zero on the resistance scale, the 500 mA fuse is blown and should be replaced.

To determine if the insulation fuse is blown:

1. Connect the test leads together and set the rotary switch to the "MΩ" position.

2. Press the "Test" button and keep it pressed while turning the generator handle.

3. The reading obtained should be zero on the insulation scale.

4. If the reading is ∞, the 7 A fuse has blown and should be replaced.

Megger Test

In general, all megger tests are used to determine a value or ratio for insulation resistance. In this section, the following types of megger tests will be discussed:

  • Short-time test
  • Time resistnce test
  • Dielectric absorption ratio

Short-Time Test

In the short-time test, the megger is connected across the insulation to be tested and then operated for a short period of time. Sixty seconds usually is recommended, but sometimes 30 seconds is used. The short-time test is also called a spot test and is recommended for comparison with previous records.

If a hand-cranked megohmmeter is used, the reading should be taken while it is cranked at rated speed. Readings should be taken at the end of the 30-second or 60-second periods even though the pointer is still climbing. Then, if all future tests are made using the same time period, a good comparison of the insulations trend can be made.

Timed Resistance Test

The timed resistance test is fairly independent of temperature and often gives conclusive information for good insulation (as compared to moist or contaminated insulation) without record of past tests. It is based on the absorption effects of good insulation compared to that of moist or contaminated insulation. The test is performed by taking successive readings at specific times and noting the differences in the readings. This method is sometimes referred to as the absorption test method.

Note that good insulation shows a continual increase in resistance over a period of 5 to 10 minutes. The absorption current discussed above is the cause of this. Good insulation shows this charge effect over a longer period of time than just the time required to charge the insulations capacitance. If the insulation contains much moisture or contaminants though, the absorption effect is masked by a high leakage current that stays at a fairly constant value, keeping the resistance reading low. Remember, R = E/I.

The timed resistance test also is of value because it is independent of equipment size. The increase in resistance for clean and dry insulation occurs in the same manner whether a motor is large or small; therefore, the readings of several motors can be compared, and standards for new ones can be established regardless of the motors' horsepower.

Dielectric Absorption Ratio

The ratio of two timed resistance readings (such as a 60-second reading divided by a 30-second reading) is called a dielectric absorption ratio, which is useful in recording information about insulation. The intended ratio used is a 10-minute reading divided by a 1-minute reading; the value obtained is called the Polarization Index.

With hand-cranked megger instruments, it is easier to run the test for only 60 seconds, taking the first reading at 30 seconds. If a power-operated megger is used, the Polarization Index can be obtained after the test is run for a full 10 minutes, with readings taken at 1 and 10 minutes. Table 2 shows the ratio values for each test length and the corresponding relative conditions of insulation that they indicate.

In some cases with motors, values that are 20% higher than shown here indicate a dry, brittle winding that will fail under shock or during starts. Forpreventive maintenance, the motor winding should be cleaned, treated, and dried to restore winding flexibility.




Insulation Testing Applications

Insulation resistance is not a stable quality. It may fluctuate considerably with weather conditions, and the equipment will still remain relatively safe. Usually, moisture only temporarily lowers insulation resistance, but acid or alkali fumes often permanently lower it.

Insulation resistance tests should be conducted regularly on a predetermined basis depending on the type, location, and importance of the equipment. Test results are important, and graphs are helpful in establishing trends in the insulations condition.

New Installation Checking

When equipment is installed, an initial insulation reading is taken. This first reading is sometimes called a spot reading. This reading provides the baseline data for future tests and is a rough guide used to determine the insulations quality. The 1 MΩ rule has been established as the lowest limit permitted for insulation resistance. The rule may be stated as follows: "Insulation resistance should be about 1 MΩ for each 1,000 V of operating voltage plus 1 MΩ, with a minimum of 1 MΩ."

Troubleshooting Tool

Troubleshooting methods are used to determine problems and find solutions. A megger cannot correct a problem, but it can aid in locating it. For example, suppose a crack develops in the insulation of a servo winding of a DC compound motor. The motor then becomes grounded and trips out on overcurrent. During troubleshooting, the series winding is meggered and found to have zero resistance to ground. In this example, the problem was quickly located with the use of a megger. Whether the problem is a crack in the insulation or dirt and moisture build-up, the megger can quickly and effectively locate and isolate the cause of the ground.

Predictive/Facility Preventive Maintenance

By taking and recording readings periodically, a better basis for judging the actual insulation condition can be obtained. Even though each individual reading may be above the minimum requirements, a consistent downward trend is an indication of impending trouble. Equally true, as long as the periodic readings are consistent, they may be acceptable, even if they are lower than the recommended minimum values.

Keeping accurate records is essential to a logical analysis of insulation condition. Insulation resistance tests should be made regularly and in the same manner, using comparable test equipment. These tests may be made on a monthly, semi-annual, or annual basis, as conditions demand. The resistance, wet- and dry-bulb temperatures, and date should be recorded.

The megohmmeter readings should be corrected to a base temperature, such as 25 C, and plotted on semilog paper for easy determination of any trends (as shown in Figure 15).

Figure 15: Megger Readings

Battery-Operated Megger

The major problem with the hand-cranked megger is maintaining a high enough crank speed for a long enough duration to induce the level of voltage required to complete the test. However, electronics have developed to the point where it is possible to use a small, portable, battery-operated megger like the one shown in Figure 16 for these tests.

Figure 16: Battery-Operated Megger
There are many different models and manufacturers of battery-operated meggers. The extensive use of electronics makes it possible to find models capable of almost any output voltage, even ones above 1,000 V using a 9 V DC battery source. The example picked for this text is a Megger BM121. It has a nominal test voltage of 500 V. It is almost identical to the Megger BM122, which has a nominal test voltage of 1,000 V.

This instrument is also equipped with an automatic discharge feature, which will automatically discharge capacitive circuits through the instruments internal resistance (360 kΩ) when the test button is released after an insulation test.

The circuit being tested must be switched off, de-energized, and isolated before any tests are made. If an external voltage 25 V AC or DC is detected, normal testing is inhibited and the display flashes a "V" warning. This feature does not function when either the instrument is turned off or the batteries are exhausted.

Safety Warnings

Safety warnings and precautions must be read and understood before the instrument is used. They must be observed during usage.


The circuit under test must be completely de-energized and isolated before test connections are made.

1. The test push button must not be held down while connecting the test leads.

2. During an insulation test, the circuit must not be touched.

3. After an insulation test, capacitive circuits must be allowed to discharge before disconnecting the test leads.

4. Test leads, probe, and crocodile clips must be in good order, clean, and with no broken or cracked insulation.

5. Replacement fuses must be of the correct size, type, and rating.


When briefly pressed and released, the instruments single control button switches the instrument on and selects the "Continuity" mode. The continuity test automatically activates when the test leads make contact. Pressing and holding the button selects "Insulation" mode. Figure 17 shows the front panel.

Figure 17: Battery-Operated Megger Front Panel

Preliminary Test Lead Check

1. Before each use of the instrument, visually inspect the test leads to confirm that their condition is good, with no damaged or broken insulation

2. Check the continuity of the test leads by firmly shorting the leads together and reading the test lead resistance measurement directly from the display.

Continuity Testing

1. Turn the instrument on by briefly pressing and releasing the test button.

2. Firmly connect the test leads together and note their combined resistance.

3. Connect the test leads across the circuit under test.

4. Take the reading directly from the displayed continuity range (maximum reading 99.9 Ω).

5. Subtract the test lead resistance value from the measured reading.

Insulation Testing

1. Turn on the instrument by briefly pressing and releasing the test button.

2. Firmly connect the test leads to the isolated item/circuit under test.

3. Press and hold the test button. The display will change to the MΩ range and show the insulation value. As an additional safety feature, the BM122 display flashes its "1,000 V" symbol before performing the test.

4. Release the test button before removing the test leads. This enables the instrument to discharge the circuit under test.


There is little routine maintenance associated with this instrument. Cleaning of the case can be accomplished using a damp cloth. The only items replaceable by the user are the leads, batteries, and fuse. If the instrument requires repairs or calibration beyond this level, it is suggested that it be returned to the manufacturer.

Battery Replacement

The rear cover must not be opened if the test leads are connected. To remove the rear cover, release the screw at the bottom of the cover and lift the cover up. To avoid the possibility of shock, do not press the test button or touch the fuse when changing the batteries.

Changing the Fuse

This instrument is equipped with one 500 mA (F) HBC, 10 kA min (32 mm by 6 mm) fuse. To check this fuse, open circuit the test leads and press the test button until a reading is obtained. Display of the fuse symbol or an error code indicates a ruptured fuse. Located under the rear cover, this fuse can be replaced by the user. The rear cover must not be opened if the test leads are connected. The replacement fuse must be of the correct type and rating. To avoid the possibility of shock, disconnect the battery before touching the fuse.

High-Voltage Megger

A high-voltage megger, shown in Figure 18, works on the same principle as a small, 500 V hand-held megger. Tests such as the spot test, dielectric absorption test, and time resistance test are calculated the same way. The difference is that a much higher test voltage is available. A DC kilovolt meter shows the applied test voltage.

Figure 18: High-Voltage Megger
The high-voltage megger has many different controls and indications:

Voltmeter and Controls

The voltmeter measures the DC output voltage in kilovolts, as indicated on the scale. The voltmeter indicator should be at zero when the unit is off. Minor adjustments may be made only when the HV control is off.

The raise voltage control is used to regulate the output voltage. Markings on the raise voltage control denote the percentage of the units maximum output voltage to which the voltage may be increased. The raise voltage control should be set at zero when not in use. It should always be returned to zero immediately upon completion of a test.

Megohmmeter and Controls

The megohmmeter is used to measure resistance in megohms, as indicated on the scale.

The multiplier control allows for resistance readings in several ranges when performing insulation testing. To obtain the proper reading from the megger, simply multiply the reading on the scale by the value indicator at the range selected. The result when multiplied by the bottom reading of the voltmeter scale will give the total resistance of the sample in megohms.

Example: In performing an insulation resistance test, the megohmmeter indication is 4 on the megohm scale, with the multiplier control set at x100. The proper reading is therefore 400 megohms. The lower scale on the voltmeter is 2.5, so the final reading is 2.5 x 400, or 1,000 MΩ.

The megohmmeter indicator should indicate zero when the unit is off. If not, minor adjustments may be made only when the HV control is off.

AC Power Controls

The AC power section of the control panel contains a toggle switch for the unit, a pilot light that glows when the unit is on, and an internal HV fuse. The fuse may be accessed for replacement by removing the front panel after the unit is turned off.

Guard/Ground/Positive Connections

There are three plug-in connection posts on the front panel labeled Guard, Ground, and Positive. Either the Guard or the Positive post is always connected to Ground by a jumper link when testing. An explanation of these two connection combinations is as follows:

A jumper link connected between the Guard and Ground posts: The resistance of the material between the high-voltage electrode and the low-voltage electrode is directly proportional to the applied voltage and inversely proportional to the resultant current flow. The voltage stress on the insulation is affected by fringing at its edges, resulting in two types of leakage current flowing. With a grounded Guard hookup, all leakage current flowing directly to ground will be bypassed around the current meter and, therefore, not observed. Only leakage current flowing along the insulator will be observed and accurately measured by the current meter.

A jumper link connected between the Ground and Positive posts - This front panel hookup connection will only be used when sensitive leakage current measurements are not necessary or when only flashover tests to ground are required. The leakage current paths are not separated in this configuration, so all leakage current is observed on the current meter.

  • The negative post provides the high voltage to the test sample.
  • The High-Voltage control has a Push-To-Test button that must be pressed before output voltage may be applied. For long tests, the Push-To-Test switch can be locked in by pushing in and turning it slightly to the right. The pilot light to the right of the AC Power switch is lit when the AC power is on.


For the initial megger setup, the following procedure should be used:

1. Select a location for the unit where the meters are placed at eye level to allow maximum accuracy in readings.

2. Set the Raise Voltage control to zero and check to ensure that the AC Power switch is in the "off" position.

3. Ground the case before connecting the input power. The Ground post on the control panel may be used also.

4. Plug the line cord into 115 V, 50/60 Hz outlet. If a two-prong adapter is used, be sure to ground the third wire.

5. Turn the AC Power switch on (the "up" position). Depress the Press-To-Test button to commence the test.

For performing resistance testing, the following procedure should be used:

1. Plug the line cord into a three-wire grounded-type receptacle. For a 115 V AC plug, plug a three-prong plug into a three-prong adapter. For a 220 V plug, connect the green wire to ground, the white wire to the B line, and the black wire to the A line.

2. Connect the red test lead to the Pos terminal and the shielded black test lead to the Neg re-entry.

3. Connect the Ground strap to the Guard terminal if the specimen under test is isolated from ground. Connect the Ground strap to the Pos terminal if the specimen under test is not isolated from ground. The red test lead will act as a return in this case.

4. Connect the black and red test clips to the specimen under test. Connect the red clip to the high side and the black clip to the low side.

5. Turn the AC Power switch on. Check that the AC Power indicator lights are on.

6. Select the lowest range on the Multiplier switch to start. The range must be increased step-by-step until the desired range is reached. This is done to protect meter movement.

7. Depress the Push-To-Test button.

8. Adjust the Raise Voltage control until the desired voltage level is indicated on the kV meter.

9. Select the proper Multiplier range.

10. Record the resistance reading as indicated on the meter.

11. To obtain the correct resistance value, the megohm reading must be multiplied by the correct factor, as indicated on the kV meter.

12. Always release the Push-To-Test button so that the test sample is shorted to ground for operator safety.

13. Turn the AC Power switch off. Note that the AC Power indicator is out.

14. Disconnect the test leads from the equipment under test.

Continuity Testing

Continuity testing measures the resistance of each phase winding. If there are shorted turns in the winding, there is a voltage unbalance in that machine. This test is also used to ensure that the terminal box connections are low, typically less than 1 Ω. If this value is greater than 1 Ω, the motor has a high-resistance connection.

Vibration Analysis

Many factors affect the vibration signature of rotating machinery. As normal degradation or problems occur in the motor or its associated pump, this vibration signature changes. Based on baseline data for your equipment, this change can be identified and further corrective action implemented. Furthermore, over the last couple of decades, there has been a significant amount of data recorded on each type of equipment in the field. This data band allows us to pinpoint, with tremendous accuracy, the actual reason for the change in the vibration signature. This is accomplished by analyzing the various frequencies emitted by the machine. For instance, a dirty impeller reflects a certain frequency range, whereas a bearing that is failing influences an entirely different range of frequencies.

DC High-Potential Testing

This test measures the ability of insulation to withstand a voltage stress. The DC high-potential test is similar to the insulation resistance test, the difference being that the insulation resistance test measures the opposition of current flow across the insulation and the DC high-potential test measures leakage current directly. The manufacturer for each piece of electrical equipment states maximum leakage current for a given high-potential test value. When this value is met or exceeded, it means that the insulation would most likely fail if a normal overvoltage transient within the design tolerance of the machines insulation were to occur.

Nondestructive High-Potential Testing

A high-potential test is used to obtain positive proof that the insulation of an electrical apparatus has sufficient dielectric strength to ride out overvoltages caused by switching and lightning surges and continue to provide useful service. However, this test should be applied only if the equipment has a measured insulation resistance at 40C equal to a minimum of 1 MΩ per 1,000 V of nameplate rating plus 1 MΩ. This measurement is made with a 500 V megohmmeter. The objective of non-destructive high-potential tests is either diagnostic or go/no-go determination. These are not breakdown tests, which are destructive in nature and designed to determine the voltage at which actual breakdown would occur.

The major area of application for high-potential testing is checking equipment that has been scheduled out-of-service for evaluation and repair. This test must not be made on vital equipment that, if damaged, would disrupt the operation of a plant or pose a safety hazard.

Unless otherwise specified by the manufacturer or testing agency, the following recommended maximum allowable test voltages may be used for a nondestructive high-potential test: 1.5 x rated line-to-line voltage for an AC test voltage and 1.7 x 1.5 x rated line-to-line voltage for an equivalent DC test voltage.

When making connections for a high-potential test, all wiring of the apparatus under test should be connected to the hot terminal of the tester, and the framework of the apparatus should be connected to the ground terminal.

Example: A 200 hp, 2,300 V, three-phase, 60 Hz induction motor is to be given a high-potential test. The maximum allowable test voltage is determined to be:

DC High-Potential Test

A DC high-potential test can provide a considerable amount of data useful for evaluating the condition of electrical insulation. When making this test, an initial voltage step of approximately one-third of the recommended maximum allowable test voltage is applied and held constant for 10 minutes. During this part of the test, the leakage current should be plotted against time at the end of each 1-minute timing interval.

Figure 19a shows current-time curves for the 10-minute constant voltage test. Good insulation exhibits a steady decrease in leakage current with time; bad insulation exhibits a rise rather than a decline in leakage current. Any rise in leakage current during this test is a signal to stop the test. Note that the polarization index can be calculated from this test data by dividing the leakage current after 1 minute by that obtained after 10 minutes:

After the initial 10-minute test, the DC voltage should be increased in 8 to 10 uniform steps, each 1 minute long, from the 30 percent value up to the maximum allowable DC test voltage. Each set of current and voltage measurements taken during this part of the test should be plotted immediately after the end of the particular 1-minute interval, even though the current may still be changing. The test should be stopped at the first indication of an upward bend or knee in the curve. A knee in the curve indicates the need for cleaning and drying, and if the test is not stopped, the leakage current may increase to a value that may damage the insulation. Figure 19b shows representative current-voltage curves for the step voltage test. A straight line is indicative of good insulation, whereas a knee indicates the need for reconditioning.

Figure 19: Current Time Curve
As a safety precaution, when using the step voltage test, the leakage-current relay should be adjusted to an initial setting approximately four times the steady leakage current obtained when 30 percent of the maximum allowable test voltage is applied. As increased increments of test voltage cause the leakage current to approach the trip setting of the overcurrent relay, the trip setting should be gradually increased. It is important that the relay setting not be too high, or a sudden failure may cause arcing and extensive damage to the insulation. In any event, the test should be stopped when a sudden increase in leakage current is observed.

Repeated diagnostic DC high-potential tests on an apparatus during reconditioning should always be made with the same terminal of the test equipment connected to the apparatus ground. A good rule to follow is to always connect the negative terminal to ground and the positive terminal to the apparatus wiring.

AC High-Potential Test

For some classes of electrical equipment, a strong case can be made for using AC for acceptance, proof, and maintenance testing. However, in high-voltage equipment, an AC test requires a relatively high current capability and, therefore, a high kVA rating test set. A high kVA rating means high cost and weight. For this reason, DC testing is the generally accepted method for many routine proof test applications. For example, a 5 mA current rating is more than adequate for proof testing the longest lengths of cable commonly encountered during a DC test. This amount of current is only required during the time required to charge the cable's capacitance. The continuous current required once the cable is charged is substantially lower. As a result, DC proof testers are much lighter and more economical than the equivalent AC sets.

High-voltage testing, whether AC or DC, represents a potential hazard to life, and every safety precaution recommended by the manufacturer or testing agency must be followed. The voltages used and the available currents are lethal, even though they are only being measured in milliamperes. Electricians rubber gloves must be worn when making these tests and the wiring of the apparatus must be discharged by grounding for 15 minutes or more after completing the test. When making the ground discharge connection, one end of the grounding wire must first be firmly connected to the framework of the apparatus, and then the other end connected to the wiring. At very high test voltages, e.g., 75,000 V, appreciable voltage recovery may occur when the grounding wire is removed, even though the winding has been connected to ground for more than 30 minutes. This voltage is caused by the release of energy as the molecules return slowly to their unstressed equilibrium positions.

Power Factor

The power factor test is used to determine the overall quality of the machines insulation. As the insulation degrades or gets wet, the power factor value increases. By comparing this data to a data bank, just as in the vibration analysis program discussed earlier, the machine can be graded in terms of its operating condition.

Test Equipment Safety

Selecting and using the correct piece of test equipment is as important as understanding the variety of test equipment in use. Many of these devices are multifunctional, where in times past the electrical worker had to have access to many more devices. One theme has remained constant though: that the use of the wrong piece of test equipment will place the electrical worker in a dangerous position.

Understanding Categories

Where safety is a concern, choosing a multimeter is like choosing a motorcycle helmet if you have a "ten-dollar" head, choose a "ten-dollar" helmet. If you value your head, get a safe helmet. The hazards of motorcycle riding are obvious, but what about multimeters? You might think that as long as you choose a multimeter with a high-enough voltage rating, you're safe. Voltage is voltage, after all.

Not exactly. Engineers who analyze multimeter safety often discover that failed units were subjected to a much higher voltage than the user thought he was measuring. There are the occasional accidents when the meter, rated for low voltages (1,000 V or less), was used to measure medium voltages, such as 4,160 V. Just as common, the knock-out blow had nothing to do with misuse; it was a momentary high-voltage spike or transient that hit the multimeter input without warning, as shown in Figure 20.

Figure 20: High-Voltage Spike
As distribution systems and loads become more complex, the possibility of transient overvoltages increases. Motors, capacitors, and power conversion equipment, such as variable speed drives, can be prime generators of spikes. Lightning strikes on outdoor transmission lines also cause extremely hazardous high-energy transients. If you are taking measurements on electrical systems, these transients are "invisible" and largely unavoidable hazards. They occur regularly on low-voltage power circuits, and can reach peak values of many thousands of volts. In these cases, you are dependent for protection on the safety margin already built into your meter. The voltage rating alone will not tell you how well that meter was designed to survive high transient impulses.

Early clues about the safety hazard posed by spikes came from applications involving measurements on the supply bus of electric commuter railroads. The nominal bus voltage was only 600 V, but multimeters rated at 1,000 V lasted only a few minutes when taking measurements while the train was operating. A close look revealed that the train stopping and starting generated 10,000-volt spikes. These transients had no mercy on early multimeter input circuits. The lessons learned through this investigation led to significant improvements in multimeter input protection circuits.

New Safety Standards

To protect you against transients, safety must be built into the test equipment. What performance specification should you look for, especially if you know that you could be working on high-energy circuits? The task of defining new safety standards for test equipment was recently addressed by the IEC (International Electrotechnical Commission). This organization develops international safety standards for electrical test equipment.

For a number of years, the industry used IEC 348 in designing equipment. That standard has been replaced by IEC 1010. While well-designed IEC 348 meters have been used for years by technicians and electricians, the fact is that meters designed to the new IEC 1010 standard offer a significantly higher level of safety. Let us see how this is accomplished.

Transient Protection

The real issue for multimeter circuit protection is not just the maximum steady-state voltage range, but a combination of the ability to withstand both steady-state voltage and transient overvoltages. Transient protection is vital. When transients ride on high-energy circuits, they tend to be more dangerous because these circuits can deliver large currents. If a transient causes an arc-over, the high current can sustain the arc, producing a plasma breakdown or explosion, which occurs when the surrounding air becomes ionized and conductive. The result is an arc blast, a disastrous event that causes more electrical injuries per year than the better known hazard of electric shock.

Overvoltage Installation Categories

The most important single concept to understand about the new standards is the Overvoltage Installation Category system. The new standard defines Categories I through IV, often abbreviated as CAT I, CAT II, etc. (see Figure 21). The division of a power distribution system into categories is based on the fact that a dangerous high-energy transient such as a lightning strike will be attenuated or dampened as it travels through the impedance (AC resistance) of the system. A higher CAT number refers to an electrical environment with higher power available and higher energy transients. Thus, a multimeter designed to a CAT III standard is resistant to much higher energy transients than one designed to CAT II standards.

Figure 21: Overvoltage Installation Categories

Within a category, a higher voltage rating denotes a higher transient withstand rating; e.g., a CAT III-1,000 V meter has superior protection compared to a CAT III-600 V rated meter. The real misunderstanding occurs if someone selects a CAT II-1,000 V rated meter thinking that it is superior to a CAT III-600 V meter.

Refers to the "origin of installation"; i.e., where low-voltage connection is made to utility power.

Includes electricity meters, primary overcurrent protection equipment, outside and service entrances, service drops from pole to building, the run between meter and panel, overhead line to a detached building, and an underground line to a well pump.


Three-phase distribution, including single-phase commercial lighting equipment used in fixed installations, such as switchgear and polyphase motors,. for the bus and feeder in industrial plants, feeders and short branch circuits, distribution panel devices, lighting systems in larger buildings, and appliance outlets with short connections to service


Single-phase receptacle connected loads

Appliance, portable tools, and other household and similar loads.

Used in outlet and long branch circuits, outlets at more than 10 meters (30 feet) from a CAT III source, amd outlets more that 20 meters (60 feet) from CAT IV source.



Used in protected electronic equipment, equipment connected to (source) circuits in which measures are taken to limit transient overvoltages to an appropriately low level.

Any high-voltage, low-energy source derived from a high winding resistance transformer, such as the high-voltage section of a copier.

As shown in Figure 21, a technician working on office equipment in a CAT I location could actually encounter DC voltages much higher than the power line AC voltages measured by the motor electrician in the CAT III location. Yet transients in CAT I electronic circuitry, whatever the voltage, are clearly a lesser threat because the energy available to an arc is quite limited. This does not mean that there is no electrical hazard present in CAT I or CAT II equipment, of course. It just means that the primary hazard is electric shock, not transients and arc blast.

To cite another example, an overhead line run from a house to a detached workshed might be only 120 V or 240 V, but it is still technically CAT IV. Why? Any outdoor conductor is subject to very high-energy lightning-related transients. Even conductors buried underground are CAT IV because, although they will not be directly struck by lightning, a lightning strike nearby can induce a transient due to the presence of high electromagnetic fields.

When it comes to Overvoltage Installation Categories, the rules of real estate apply: location is what really matters.

How can you tell if a meter is a genuine CAT III or CAT II meter? Unfortunately, it is not always easy.Beware of wording such as "designed to meet specifications." Designers plans are never a substitute for an actual independent test. Also it is possible for a manufacturer to self-certify that its meter is CAT II or CAT III without any independent verification. The IEC (International Electrotechnical Commission) develops and proposes standards, but it is not responsible for enforcing those standards.Independent testing is the key to safety compliance.

Look for the symbol and listing number of an independent testing lab, such as UL, CSA, TV (Figure 22), or another recognized approval agency. These symbols can only be used if the product successfully completed testing to the agencys standards, which are based on national/international standards. UL 3111, for example, is based on IEC 1010. In an imperfect world, that is the closest you can come to ensuring that the multimeter you choose was actually tested for safety.

Figure 22: Independent Testing Lab Symbols

What does the CE symbol indicate? A product is marked CE (Conformit Europenne) to indicate its conformance to certain essential requirements concerning health, safety, environment, and consumer protection established by the European Commission and mandated through the use of "directives." There are directives affecting many product types, and products from outside the European Union cannot be imported and sold there if they do not comply with applicable directives. Compliance with the directives can be achieved by proving conformance to a relevant technical standard, such as IEC 1010 for low-voltage products. Manufacturers are permitted to self-certify that they have met the standards, issue their own Declaration of Conformity, and mark the product "CE." The CE mark is not, therefore, a guarantee of independent testing.

Transients The Hidden Danger

This section will take a look at a worst-case scenario in which a technician is performing measurements on a live three-phase motor control circuit using a meter without the necessary safety precautions.

Here is what could happen:

A lightning strike (Figure 23) causes a transient on the power line, which in turn strikes an arc between the input terminals inside the meter. The circuits and components to prevent this event have just failed or were missing. Perhaps it was not a CAT III rated meter. Whatever the reason, the result is a direct short between the two measurement terminals through the meter and the test leads.

Figure 23: Lightning-Generated Arc
A high fault current, possibly measuring several thousands of amps, flows in the short circuit just created. This happens in thousandths of a second. When the arc forms inside the meter, a very high-pressure shock wave might cause a loud bang!very much like a gunshot or the backfire from a car. At the same instant, the tech sees bright blue arc flashes at the test lead tips and the fault currents superheat the probe tips, which start to burn away, drawing an arc from the contact point to the probe. The natural reaction is to pull back in order to break contact with the hot circuit. However, as the techs hands are pulled back, an arc is drawn from the motor terminal to each probe. If these two arcs join to form a single arc, there will be another direct phase-to-phase short, this time directly between the motor terminals.

This arc can have temperatures approaching 6,000C (10,000F), which is higher than the temperature of an oxy-acetylene cutting torch! As the arc grows, fed by available short-circuit current, it superheats the surrounding air. Both a shock blast and a plasma fireball are created. If the technician is lucky, the shock blast blows him away and removes him from the proximity of the arc; though injured, his life is saved. In the worst case, the victim is subjected to fatal burn injuries from the fierce heat of the arc or plasma blast.

In addition to using a multimeter rated for the appropriate Overvoltage Installation Category, anyone working on live power circuits should be protected with flame-resistant clothing, wear safety glasses or, better yet, a safety face shield, and use insulated gloves.

Transients are not the only source of possible short circuits and arc blast hazards. One of the most common misuses of handheld multimeters can cause a similar chain of events. Suppose a user is making current measurements on signal circuits (Figure 24). The procedure is to select the amps function, insert the leads in the mA or amps input terminals, open the circuit, and take a series measurement. In a series circuit, current is always the same. The input impedance of the amps circuit must be low enough so that it does not affect the series circuits current. The input impedance on the 10 A terminal of a Fluke meter is .01 Ω. Compare this with the input impedance on the voltage terminals of 10 MΩ (10,000,000 Ω).

Figure 24: Multimeter Setup Incorrect
If the test leads are left in the amps terminals and then accidentally connected across a voltage source, the low input impedance becomes a short circuit! It does not matter if the selector dial is turned to volts; the leads are still physically connected to a low-impedance circuit. That is why the amps terminals must be protected by fuses. Those fuses are the only things standing between an inconvenience - blown fuses - and a potential disaster.

Use only a multimeter with its amps inputs protected by high-energy fuses. Never replace a blown fuse with the wrong fuse. Use only the high-energy fuses specified by the manufacturer. These fuses are rated at a voltage and with a short-circuit interrupting capacity designed for your safety.

Overload Protection

Fuses protect against overcurrent. The high input impedance of the volts/ohms terminals ensures that an overcurrent condition is unlikely, so fuses are not necessary. Overvoltage protection, on the other hand, is required. It is provided by a protection circuit that clamps high voltages to an acceptable level. In addition, in the event of an overvoltage condition, a thermal protection circuit detects the overvoltage condition, protects the meter until the condition is removed, and then automatically returns to normal operation. The most common benefit is to protect the multimeter from overloads when it is in ohms mode. In this way, overload protection with automatic recovery is provided for all measurement functions as long as the leads are in the voltage input terminals.

Shortcuts to Understanding Categories

Here are some quick ways to apply the concept of categories to your everyday work:

The general rule of thumb is that the closer you are to the power source, the higher the category number, and the greater the potential danger from transients.

It also follows that the greater the short-circuit current available at a particular point, the higher the CAT number.

Another way of saying the same thing is the greater the source impedance, the lower the CAT number. Source impedance is simply the total impedance, including the impedance of the wiring, between the point where you are measuring and the power source. This impedance is what dampens transients.

Finally, if you have any experience with the application of TVSS (transient voltage surge suppression) devices, you understand that a TVSS device installed at a panel must have higher energy-handling capacity than one installed right at the computer. In CAT terminology, the panel board TVSS is a CAT III application, while the computer is a receptacle-connected load and, therefore, a CAT II installation.

As you can see, the concept of categories is not new and exotic. It is simply an extension of the same common-sense concepts that people who work with electricity professionally apply every day.

There is one scenario that sometimes confuses people trying to apply categories to real world applications. In a single piece of equipment, there is often more than one category. For example, in office equipment, from the 120 V/240 V side of the power supply back to the receptacle is CAT II. The electronic circuitry, on the other hand, is CAT I. In building control systems, such as lighting control panels, or industrial control equipment, such as programmable controllers, it is common to find electronic circuits (CAT I) and power circuits (CAT III) existing in close proximity.

What do you do in these situations? As in all real-world situations, use common sense. In this case, that means using the meter with the higher category rating. In fact, it is not realistic to expect people to be going through the category-defining process all the time. What is realistic, and highly recommended, is to select a multi-meter rated to the highest category in which it could possibly be used. In other words, it is always best to error on the side of safety.

Understanding Voltage-Withstand Ratings

IEC 1010 test procedures take into account three main criteria: steady-state voltage, peak impulse transient voltage, and source impedance. These three criteria together will tell you a multimeters true voltage-withstand value.

Table 5 can help us understand an instruments true voltage-withstand rating. Test values for 50 V/150 V/300 V are not included.

When is 600 V more than 1000 V?

Within a category, a higher "working voltage" (steady-state voltage) is associated with a higher transient, as would be expected. For example, a CAT III 600 V meter is tested with 6,000 V transients while a CAT III 1,000 V meter is tested with 8,000 V transients. So far, so good.

What is not as obvious is the difference between the 6,000 V transient for CAT III 600 V and the 6,000 V transient for CAT II 1,000 V. They are not the same. This is where the source impedance comes in. Ohms law (Amps = Volts/Ohms) tells us that the 2 Ω test source for CAT III has six times the current of the 1 Ω test source for CAT II.

The CAT III 600 V meter clearly offers superior transient protection compared to the CAT II 1,000 V meter, even though its so-called "voltage rating" could be perceived as being lower. It is the combination of the steady-state voltage (called the working voltage), and the category that determines the total voltage withstand rating of the test instrument, including the all-important transient voltage withstand rating.

A note on CAT IV: Test values and design standards for Category IV voltage testing are addressed in IEC 1010 second edition.

In addition to being tested to an actual overvoltage transient value, multimeters are required by IEC 1010 to have minimum "creepage" and "clearance" distances between internal components and circuit nodes. Creepage measures distance across a surface. Clearance measures distances through the air. The higher the category and working voltage level, the greater the internal spacing requirements are. One of the main differences between the old IEC 348 and IEC 1010 is the increased spacing requirements in the latter.

If you are faced with the task of replacing your multimeter, do one simple task before you start shopping: analyze the worst-case scenario of your job and determine what category your use or application fits into.

First choose a meter rated for the highest category you could be working in. Then, look for a multimeter with a voltage rating for that category matching your needs. While you are at it, do not forget the test leads. IEC 1010 applies to test leads too: they should be certified to a category and voltage as high as or higher than the meter. When it comes to your personal protection, do not let test leads be the weak link.

Special AC Rotating Machines

Normally, what comes to mind when discussing rotating AC machines is an AC induction motor; however, there are two other types of AC machines that are frequently found in an industrial environment. These AC machines are the synchronous motor/generator (shown in Figure 25) and the wound rotor motor (Figure 26). These devices have the same maintenance considerations as discussed earlier. However, these machines have additional considerations because their particular designs use carbon brushes and slip-rings.
Figure 25: Synchronous Motor

Figure 26: Wound Rotor Motor
The synchronous machine uses these components to create a DC magnetic field on the rotor. This allows for constant speed characteristics from no-load to full-load condition on the synchronous machine. This differs from an induction motor in that, from no-load to full-load, the induction motor slows down. The wound rotor motor, however, uses brushes and slip-rings to connect resistors to a specially designed three-phase rotor circuit. This design enables the wound rotor motor to vary in torque and speed for a given load condition.