Test equipment and experienced electricians are not always needed to locate problems. Anyone who sees a ground wire dangling beneath a lightning arrester might suspect a problem. Little skill is required to consider an electrical service problem as a possible reason for the lack of power in a building.

Arcing, loud noises, and charred or burned electrical equipment sometimes indicate electrical faults; however, hidden, noiseless circuit problems are much more common and usually much harder to locate.

The right test equipment and an electrician who knows how to use it are a valuable combination for solving electrical circuit problems.


1.0 Ammeters

2.0 Voltmeters

3.0 Line Voltage Indicators

4.0 Ohmmeters

5.0 Multimeters

6.0 Megohmmeters

7.0 Insulation Resistance Testers

1.0 Ammeters

A meter that measures the flow of electric current is a current meter. Current meters that measure current in amperes are called ammeters. The ammeter is connected in series with the circuit source and load. Panel-mounted ammeters, such as those used in power plants, are permanently wired into the circuit. Figure 2 shows two typical panel-mounted ammeters.

Figure 2 — Typical panel ammeters.

A clamp-on ammeter (Figure  3) is an exception to the rule requiring ammeters to be series-connected. The clamp-on ammeter consists in part of clamp-on transformer jaws that can be opened and placed around a conductor. The jaws are actually part of a laminated iron core. Around this core, inside the instrument enclosure is a coil winding that connects to the meter circuit. The complete core (including the jaws) and the coil winding are the core and secondary of a transformer. The conductor, carrying the current to be measured, is like a primary winding of a transformer. The transformer secondary is the source of power that drives the meter movement. The strength of the magnetic field surrounding the conductor determines the amount of secondary current. The amount of secondary current determines the indication of current being measured by the meter.

Figure 3 —Clamp-on ammeter.

Clamp-on ammeters have an adjustable scale. The function and range of the meter change as the scale changes.

To take a current measurement, turn the selector until the AMP scale you wish to use appears in the window. To take measurements of unknown amounts of current, rotate the scale to the highest amperage range. After taking the reading at the highest range, you may see that the amount of current is within the limits of a lower range. If so, change the scale to that lower range for a more accurate reading.

After choosing the scale you want, depress the handle to open the transformer jaws. Clamp the jaws around only one conductor. The split core must be free of any debris because it must close completely for an accurate reading.

To measure very low currents in a small flexible conductor, wrap the conductor one or more times around the clamp-on jaws of the meter. One loop will double the reading. Several loops will increase the reading even more. After taking the measurement, divide the reading by the appropriate number of loops to determine the actual current value.

The clamp-on ammeter is convenient and easy to use. To measure the current of a single-phase motor, for example, simply rotate the selector until the desired amp scale appears; clamp the jaws around one of the two motor conductors, and take the reading.

Some clamp-on instruments are capable of more than one function, for example, they are designed for use as an ohmmeter or a voltmeter when used with the appropriate adapter or test leads.


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2.0 Voltmeters

The meter component (or voltage indicator) of a voltmeter is actually a milliammeter, or micrometer. This instrument is series-connected to a resistor (called a voltage multiplier) to operate as a voltmeter. The series resistance must be appropriate for the range of voltage to be measured. The scale of an instrument designed for use as a voltmeter is calibrated (marked off) for voltage measurements.

Panel voltmeters are similar in appearance to the ammeters except for the calibration of the scale. Examples of typical panel voltmeters are shown in Figure 4. Voltmeters are connected across a circuit or voltage source to measure voltage. Panel-mounted voltmeters are permanently wired into the circuit in which they are to be used.

Figure 4 — Typical panel voltmeters.

Portable voltmeters are designed to measure one or more ranges of voltage. Those intended for measurement of more than one voltage range are provided with range selector switches. The range selector switch internally connects the appropriate multiplier resistor into the meter circuit for the range of voltage to be measured; for example, a voltmeter may be designed to use a 0-1 milliammeter as a voltage indicator. For each setting of the selector switch, a different multiplier resistor is connected into the meter circuit. For each selection, a particular resistor value is designed to limit the current through the milliammeter to a maximum of 1/1,000 of an ampere (1 milliampere) for a full-scale reading.

In a similar way, voltmeters designed to use a micrometer, for example, a 50- microampere meter, include multiplier resistors that limit the meter current to a maximum value of 50 microamperes. In this case, 50 microamperes are flowing through the meter for a full-scale deflection of the needle.

Voltmeters that use either a milliammeter or micrometer to indicate voltage have a scale calibrated to read directly in volts. The flow of current in either type of meter represents the electrical pressure (voltage) between two points in an electrical circuit; for example, the two points may be the hot (ungrounded) conductor and the neutral (grounded) conductor of a 125-volt circuit. In this case, the voltmeter is said to be connected across the line.


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3.0 Line Voltage Indicators

The line voltage indicator (Figure 5) is much more durable than most voltmeters for rough construction work. Its durability is mainly due to its simple design and construction. It has no delicate meter movement inside the case as do the analog meters previously mentioned. The two test leads are permanently connected to a solenoid coil inside the molded case.

Figure 5 — Line voltage indicator

An indicator, attached to the solenoid core, moves along a marked scale when the leads are connected across a voltage source. The movement of the core is resisted by a spring. The indicator comes to rest at a point along the scale that is determined by both the strength of the magnetic field around the solenoid and the pressure of the opposing spring. The strength of the magnetic field is in proportion to the amount of voltage being measured.


Do not use the line voltage indicator on voltages exceeding the capabilities of the indicator

In the center of the tester is a neon lamp indicator. The lamp is used to indicate whether the circuit being tested is AC or DC.

When the tester is operated on AC, it produces light during a portion of each half-cycle, and both lamp electrodes are alternately surrounded with a glow. The eye cannot follow the rapidly changing alternations, so both electrodes appear to be continually glowing from AC current. Two other indications of AC voltage are an audible hum and a noticeable vibration you can feel when the instrument is hand-held.

When the tester is operated on DC, light is produced continuously, but only the negative electrode glows; therefore, the tester will indicate polarity on DC circuits. Both the test probes and the glow lamp enclosure are colored red and black. If, while you are testing a DC circuit, the electrode of the glow lamp on the side colored black is glowing, this glow indicates the black probe of the tester is on the negative side of the circuit; likewise, the opposite electrode glows when the red probe of the tester is on the negative side of the circuit.

The neon lamp is not the only method used on line voltage indicators to indicate DC polarity; for example, the Wigginton voltage tester, manufactured by the Square D Company, uses a permanent magnet mounted on a rotating shaft. The ends of the magnet are colored red and black. The magnet is viewed from a transparent cap located on top of the tester. When the red portion of the magnet is up, the red test prod is positive. When the black portion of the magnet is up, the black prod is positive. Neither type of line voltage indicator vibrates when measuring DC.

Be certain to read and understand the instructions for the particular instrument you use. As you can see from the example of polarity indicators, because of variations in similar instruments, you could easily misunderstand an indication from one instrument when thinking of the instructions for another.

The line voltage indicator does not determine the exact amount of circuit voltage. That presents no problem for most of the work professional electricians do. As you become proficient in the use of the solenoid type of voltage indicator, you can tell approximately what the voltage is by the location of the indicator within a voltage range on the scale.


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4.0 Ohmmeters

You can determine the resistance of a component or circuit, in ohms, by using Ohm’s law. With the instruments we just discussed, you can find circuit current and voltage. From electrical theory you already know that voltage divided by amperage equals resistance. But the fastest method of determining resistance is by taking a resistance reading directly from an ohmmeter.

Figure 6 — A simple series ohmmeter circuit.

The simplest type of ohmmeter consists of a housing that includes a milliammeter, a battery, and a resistor connected in series, as shown in Figure 6. The ohmmeter is designed so that the resistor R1 limits the current though the milliammeter to a value that results in a full-scale deflection of the meter needle. The scale (Figure 7) is calibrated in ohms. By using several resistors, more than one battery, and a selector switch (to select one of the several resistors and batteries), you can make the ohmmeter include more than one resistance range.

Figure 7 — Typical scale of a series type of ohmmeter.

You may use a variable resistor in the meter circuit (R2 in Figure 6) to compensate for variations in battery voltage. Before using an ohmmeter for a precise resistance measurement, short the leads together and set the needle to zero by rotating the “zero ohms” (variable resistor) knob. The result is a full-scale reading at zero ohms.


Be certain not to place the ohmmeter leads across an energized circuit or a charged capacitor. Ignoring this rule will likely result in damage to the test equipment. Always turn off the power on a circuit to be tested before making continuity or resistance tests. Before you test with an ohmmeter, bleed any capacitors that are included in the circuits under test. Use extreme care in testing solid-state components and equipment with an ohmmeter. The voltage from the internal batteries of the ohmmeter will severely damage many solid-state components. Always turn an ohmmeter off after you have completed your test to lengthen the life of the batteries.

After you zero the meter, place the leads across the circuit or component under test. The resistance of the unknown resistor between the ohmmeter leads limits the current through the meter, resulting in less than a full-scale deflection of the needle. The resistance reading may then be taken from the point along the scale at which the needle comes to rest.

Accurate readings become progressively more difficult to take toward the high-resistance end of the scale. When the needle comes to rest at the high end of the scale and the ohmmeter has several resistance ranges, you may simply switch to a higher range for a reading closer to center scale. Read the resistance directly from the scale at the lowest range (for example, the R x 1 range on some ohmmeters). At the higher ranges multiply the reading by 100 or 10,000 (as on the R x 100 or R x 1,000 ranges). The higher resistance ranges in a multi-range ohmmeter use a higher voltage battery than do the lower ranges.

We will discuss multimeters (meters that perform more than one function) later in this section, but since we have already discussed the ammeter as a clamp-on ammeter, we will look at the same instrument as an ohmmeter. 

Figure 8 — Clamp-on ammeter with ohmmeter battery adapter.

To use the ammeter as an ohmmeter, plug a battery adapter into the jack on the side of the case (Figure 8). The battery in the adapter powers the ohmmeter function of this instrument. Use one of two test leads that may be plugged into the instrument (for voltage measurements) for the second lead of the ohmmeter. Plug this test lead into the jack marked “COMMON.” The ohmmeter scale is a fixed scale at the right side of the scale window opening. It is not part of the rotating scale mechanism. The rotating mechanism has no effect on the ohmmeter operation. The leads are applied to the circuit or component, and the reading is taken as with any ohmmeter.

The series type of ohmmeter is only one type of instrument used for resistance measurements, but it is common in the design of ohmmeters used by electricians.


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5.0 Multimeters

Up to this point, each of the instruments we have discussed, for the most part, performs only one function. The exception was the clamp-on ammeter/ohmmeter. In a similar way the analog meters and digital meters perform several (or multiple) functions and are therefore referred to as multimeters.

An analog instrument usually makes use of a needle to indicate a measured quantity on a scale. Digital meters indicate the quantity directly in figures. We will discuss both types here because you will use both types. 

Notice that each multimeter in Figure 9 (A, B, C, and D) consists of a case to enclose the indicating device, one or more functions and/or range switches, and internal circuitry and jacks for external connections.

Figure 9 — Typical multimeters
(analog types A and B and digital types C and D).

3.5.1 Voltage Measurements

Before plugging the test leads into the jacks, set the switches for the measurement. Let’s look at an example. You are about to measure the voltage at a standard wall outlet in an office. You already know from experience that the voltage should be in the area of 115 to 125 volts AC. You have one of two types of multimeters-an analog meter or a digital meter. Because you know the voltage to be tested, you would set the function switch to AC and the voltage to 250V. For the operation of the range and function switches on the particular meter, check the manufacture’s literature. What should you do if you have no idea what the voltage is? There are times when you should not get near the equipment; in this case, you should check with someone who knows (for example, a public works engineer or line crew supervisor). Check the highest range on your instrument. If you have a meter and know the voltage value should not exceed 1,000 volts AC, then set the range/function switch to 1,000 ACV. Plug the test leads into the appropriate jacks for the test you are about to perform. When you have red and black test leads, get into the habit of using the black lead with the common or - (negative) jack, even when measuring AC volts. For either analog meter, plug the red lead into the + (positive) jack. With either of the digital meters, use the jack marked “V-O” (volts-ohms).


The following sequence of steps is important for your safety. Stay alert and follow them carefully.

Connect the two test leads to the two conductors/terminals of the wall outlet while holding the insulated protectors on the test leads. Do not touch the probes or clips of the test leads. Take the reading. If you have the meter range switch at the highest setting and see that the voltage value is within a lower voltage range, set the range switch to the lower range that is still higher than the voltage reading you remember. When you take a reading at a higher range and switch to a lower range, the reading at the lower range will be more accurate. Be certain to read from the scale that matches the range setting of the switch, for example, when using the multimeter with the switch set to 300 AC VOLTS, read from the scale that has a maximum reading of 300 AC. Simply take the reading directly from either of the digital multimeters.


Always be alert when taking voltage or amperage measurements if it is necessary to move the meter. If the instrument is moved in a way that causes tension on the test leads, one or both leads may be pulled from the jack(s). The leads will be energized just as the circuit to which they are connected, and they can be dangerous.

The positions of the jacks may differ for a particular measurement, from one meter to another. Notice how the jacks are labeled on the instrument you use, and follow the instructions from the manufacturer of the instrument.

3.5.2 Amperage Measurements

It is possible that you will never use a multimeter for amperage measurements. Most multimeters are designed with quite low current ranges. The clamp-on ammeter (discussed earlier) is the most convenient portable instrument for measuring AC amperes.

3.5.3 Resistance Measurements

As mentioned earlier, ohmmeters have their own voltage source. This circumstance is also true of the ohmmeter function of multimeters. The size and number of batteries for different instruments vary. Usually one or more 1 1/2- to 9-volt batteries are used for resistance measurements.

As you must set up the meter to measure voltage accurately, so you must set it up for measuring resistance. If you are to measure a 120-ohms resistor, for example, set the selector switch to ohms at the appropriate range. For the analog instruments, set the switch to the R x 1 or x 1 as appropriate. Read the value from the ohms scale directly. For higher values of resistance like 1,500 ohms, for example, use the R x 100 or x 100 range. In this case, multiply the reading from the ohms scale by 100.

For critical resistance measurements, always touch the leads together and set the indicator needle to zero with the appropriate adjustment knob. Do not let the leads touch your fingers or anything else while you are zeroing the meter.

On multimeters, use the common – (negative) and + (positive) jacks for resistance measurements.

Be certain that there is no power on the circuit or component you are to test when measuring resistance. Be sure also to discharge any capacitors associated with the circuit or component to be tested before connecting the instrument to the circuit or component. For critical measurements, make sure that only the circuit or component you are to test touches the leads while you take the reading; otherwise, the reading may be inaccurate, especially on the higher resistance ranges.

Many times you will use the ohmmeter for continuity tests. All you will want to know is whether the circuit is complete or not. You do not have to zero the meter for noncritical continuity tests. You will touch the leads together to see where the needle comes to rest. If it stops at the same place when you place the leads across the circuit, you know the path has a low resistance. In other words you know there is continuity through the circuit.

Electricians also use other instruments for different types of resistance measurements. We will discuss these instruments next.


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6.0 Megohmmeters

The megohmmeter is a portable instrument consisting of an indicating ohmmeter and a source of DC voltage. The DC source can be a hand-cranked generator, a motor-driven generator, a battery-supplied power pack, or rectified DC.

The megohmmeter is commonly called a "megger" although Megger© is a registered trademark. The megger tester shown in Figure 10 is an example of a dual-operated megohmmeter that has both a hand cranked generator and a built-in line power supply in the same module.

Figure 10 — Typical megohmmeter tester.

Any one of the ohmmeters shown in Figure 9 will measure several megaohms. You may wonder why they are not called megohmmeters. What is the difference between the megger and the typical ohmmeter? Does not each of them have an indicator and a DC voltage source within the instrument enclosure? The megger is capable of applying a much higher value of DC voltage to the circuit or component under test than is the typical ohmmeter. Meggers that will supply a test potential of 500 volts are common in the Navy. The megger (Figure 10) is capable of several test voltages up to 1,000 volts, depending on the setting of the selector switch. Ohmmeters are generally designed to include batteries as voltage sources. These batteries apply approximately 1/2 to 9 volts to the circuit under test.

The megger is designed so that the needle floats freely until the generator is operated. When the generator is not operating, the needle may come to rest at any point on the scale. This characteristic is due to internal design, unlike that used in the typical ohmmeter.


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7.0 Insulation Resistance Testers

The megger is used to measure highinsulation resistance. The high resistance may be between windings of a transformer or motor or between the conductor in a cable and the conduit or sheath surrounding the cable (Figure 11).

Figure 11 — Typical megger test instrument hooked up to measure insulation resistance.

If the test leads connected to the line and earth terminals are open-circuited (as when they are not allowed to touch anything) and the hand-cranked generator is operated, the needle is deflected to infinity (Figure 12). “Infinity” means that the resistance is too high for the instrument to measure. The symbol for infinity on the scale of the megger (Figure 10) is similar to a horizontal figure eight. During a test, a reading at or near infinity means either that the insulation is in excellent shape or the test leads are not making contact with the component being tested.

Figure 12 — Typical indicating scale on the megger insulation tester.

If the test leads are connected to each other while the hand crank is turned, the pointer will deflect to zero, indicating no resistance between the test leads. A zero deflection in the above-mentioned test (Figure 11) can mean that the conductor under test is touching the sheath or conduit surrounding it. This deflection could also be an indication that the insulation is worn or broken somewhere close to the test point. Any reading near the low end of the scale may mean faulty or wet insulation.

The megger serves well as an insulation tester because of the high-test voltage it produces. The low voltage of an ohmmeter may not produce enough leakage current through poor insulation to cause the meter to indicate a problem even when one exists. But the relatively high voltage of the megger will likely cause enough leakage current to reveal an insulation problem by a lower than normal resistance indication on the meter scale.

How low is the resistance of bad insulation? How high must the insulation resistance reading be before you can be sure the insulation is good?

Here are some general observations (See Table 1) about how you can interpret periodic insulation resistance tests, and what you should do with the results.

Table 1 — Insulation resistance problems and fixes.

Condition What to Do
 Fair to high values and well maintained No cause for concern
Fair to high values, but showing a constant tendency towards lower values Locate and remedy the cause and check the downward trend
Low but well maintained Condition is probably all right, but the cause of the low values should be checked
So low as to be unsafe Clean, dry out, or otherwise raise the values before placing equipment in service (test wet equipment while drying it out)
Fair or high values, previously well maintained but showing sudden lowering Make tests at frequent intervals until the cause of low values is located and remedied or until the values become steady at a level that is lower but safe for operation or until values become so low that it is unsafe to keep the equipment in operation

.7.1 Short Time or Spot Reading Tests

Several test methods are commonly used to test insulation. We will discuss the short-time or spot-reading tests.

In this method, simply connect the megger across the insulation to be tested and operate it for a short, specific time period (60 seconds usually is recommended). As shown in Figure 13, you have picked a point (to take the reading) on a curve of increasing resistance values; quite often the value will be less for 30 seconds, more for 60 seconds. Bear in mind also that temperature and humidity, as well as condition of the insulation, affect your reading.

Figure 13 — Typical curve of insulation resistance (in megohms) with time.

If the apparatus you are testing has low capacitance, such as a short run of type NM cable (Romex), the spot reading test is all that is necessary; however, most equipment is capacitive, so your first spot reading on equipment in your work area, with no prior tests can be only a rough guide as to how “good” or “bad” the insulation is. For many years, maintenance personnel have used the 1-megohm rule to establish the allowable lower limit for insulation resistance. The rule may be stated thus: Insulation resistance should be approximately 1 megohm for each 1,000 volts of operating voltage with a minimum value of 1 megohm. For example, a motor rotated at 2,400 volts should have a minimum insulation resistance of 2.4 megohms. In practice, megohm readings normally are considerably above this minimum value in new equipment or when insulation is in good condition.

By taking readings periodically and recording them, you have a better basis for judging the actual insulation condition. Any persistent downward trend is usually fair warning of trouble ahead, even though the readings may be higher than the suggested minimum safe values. Equally true, as long as your periodic readings are consistent, they may be all right even though lower than the recommended minimum values.

3.7.2 Common Test Voltages

Commonly used DC test voltages for routine maintenance are as follows:

Table 2 — Common DC voltages used.

Equipment AC Rating DC Test Voltage
Up to 100 volts 100 and 250 volts
440 to 550 volts 500 and 1,000 volts
2,400 volts 1,000 to 2,500 volts or higher
4,160 volts and above 1,000 to 5,000 volts or higher



Use care in applying test voltage to the component to be tested. Do not use a high-test voltage on low-voltage equipment or components.

Do not exceed the commonly used test voltages mentioned above unless you are following the equipment manufacturer’s instructions to do so. On the other hand, a test voltage lower than the operating voltage of the component to be tested may not reveal a problem that the test should indicate. If the test voltage is too low, you may get no more than a resistance reading such as you would get with an ohmmeter.

3.7.3 Causes of Low Insulation Resistance Readings

Insulation resistance varies with the temperature. The effect of temperature depends on the type of insulation, the amount of moisture in and on the insulation surface, and the condition of the surface.

The amount of moisture in insulation has a great effect on its resistance. For meaningful results, tests of insulation resistance should be made under as nearly similar conditions as practical. Long cables can be exposed to a variety of conditions along the cable route at the same time. A comparison of readings may not indicate a change in insulation condition.

An accumulation of things like dust, dirt, and moisture can cause low-resistance readings. A motor stored or kept idle for a while may have to be cleaned and dried out before being installed and placed in service.

3.7.4 Record Keeping

Keep records where tests are performed periodically. The frequency of the tests should be based on the importance of the circuit. One test each year is usually adequate. Compare records of each circuit or component. Trends may indicate a future problem, and corrections may be made in time to prevent future problems in cables or components like motors or transformers.

3.7.5 Effects of Temperature

If you want to make reliable comparisons between readings, correct the readings to a base temperature, such as 20°C (68°F), or take all your readings at approximately the same temperature (usually not difficult to do). We will cover some general guidelines to temperature correction.

One rule of thumb is that for every 10°C (50°F) increase in temperature, halve the resistance; or for every 10°C (50°F) decrease, double the resistance; for example, a 2- megohm resistance at 20°C (68°F) reduces to 1/2 megohm at 40°C (104°F).

Each type of insulating material will have a different degree of resistance change with temperature variation. Factors have been developed, however, to simplify the correction of resistance values. Table 3 gives such factors for rotating equipment, transformers, and cable. Multiply the reading you get by the factor corresponding to the temperature (which you need to measure).

For example, assume you have a motor with Class A insulation and you get a reading of 3.0 megohms at a temperature (in the windings) of 131°F (55°C). From Table 3, read across at 131°F to the next column (for Class A) and obtain the factor 15.50. Your correct value of resistance is then

3.0 megohms X 15.50 = 46.5 megohms

Note that the resistance is 14.5 times greater at 68°F (20°C) than the reading taken at 131°F. The reference temperature for cable is given as 60°F (15.6°C), but the important point is to be consistent-correcting to the same base before making comparisons between readings.

Table 3 — Temperature Correction Factors (Corrected to 20°C for rotating equipment and transformers; 15.6°C for cable)

3.7.6 Effects of Humidity

 We mentioned in this section the marked effect of the presence of moisture in insulation upon resistance values. You might expect that increasing humidity (moisture content) in the surrounding (ambient) air could affect insulation resistance. And it can, to varying degrees.

If your equipment operates regularly above what is called the “dew-point” temperature (that is, the temperature at which the moisture vapor in air condenses as a liquid), the test reading normally will not be affected much by the humidity. This stability is true even if the equipment to be tested is idle, so long as its temperature is kept above the dew point. In making this point, we are assuming that the insulation surfaces are free of contaminants, such as certain lints and acids or salts that have the property of absorbing moisture (chemists call them "hygroscopic," or "deliquescent," materials). Their presence could unpredictably affect your readings; remove them before making tests.

In electrical equipment we are concerned primarily with the conditions on the exposed surfaces where moisture condenses and affects the overall resistance of the insulation. Studies show, however, that dew will form in the cracks and crevices of insulation before it is visibly evident on the surface. Dew-point measurements will give you a clue as to whether such invisible conditions may exist, altering the test results.

As a part of your maintenance records, make note at least of whether the surrounding air is dry or humid when the test is made and whether the temperature is above or below the ambient. When you test vital equipment, record the ambient wet- and dry bulb temperatures, from which dew point and percent relative or absolute humidity can be obtained.

3.7.7 Preparation of Apparatus for Test

Before interrupting any power, be certain to check with your seniors (crew leader, project chief, or engineering officer, as appropriate) so that they can make any necessary notification of the power outage. Critical circuits and systems may require several days or even weeks advance notice before authorization for a power outage may be granted. Take Out of Service

Shut down the apparatus you intend to work on. Open the switches to de-energize the apparatus. Disconnect it from other equipment and circuits, including neutral and protective (workmen’s temporary) ground connections. See the safety precautions that follow in this section. Test Inclusion Requirements

Inspect the installation carefully to determine just what equipment is connected and will be included in the test, especially if it is difficult or expensive to disconnect associated apparatus and circuits. Pay particular attention to conductors that lead away from the installation. That is important, because the more equipment that is included in a test, the lower the reading will be, and the true insulation resistance of the apparatus in question may be masked by that of the associated equipment.


Take care in making electrical insulation tests to avoid the danger of electric shock. Read and understand the manufacturer’s safety precautions before using any megohmmeter. As with the ohmmeter, never connect a megger to energized lines or apparatus. Never use a megger or its leads or accessories for any purpose not described in the manufacturer’s literature. If in doubt about any safety aspects of testing, ask for help.

3.7.8 Safety Precautions


Observe all safety rules when taking equipment out of service:

  • Block out disconnected switches.
  • Be sure equipment is not live.
  • Test for foreign or induced voltages.
  • Ensure that all equipment is and remains grounded, both equipment that you are working on and other related equipment.
  • Use rubber gloves when required.
  • Discharge capacitance fully.
  • Do not use the megger insulation tester in an explosive atmosphere

When you are working around high-voltage equipment, remember that because of proximity to energized high-voltage equipment, there is always a possibility of voltages being induced in the apparatus under test or lines to which it is connected; therefore, rather than removing a workmen’s ground to make a test, disconnect the apparatus, such as a transformer or circuit breaker, from the exposed bus or line, leaving the latter grounded. Use rubber gloves when connecting the test leads to the apparatus and when operating the megger. Apparatus Under Test Must Not be Live

If neutral or other ground connections have to be disconnected, make sure that they are not carrying current at the time and that when they are disconnected, no other equipment will lack protection normally provided by the ground.

Pay particular attention to conductors that lead away from the circuit being tested and make sure that they have been properly disconnected from any source of voltage. Shock Hazard from Test Voltage

Observe the voltage rating of the megger and regard it with appropriate caution. Large electrical equipment and cables usually have sufficient capacitance to store up a dangerous amount of energy from the test current. Be sure to discharge this capacitance after the test and before you handle the test leads. Discharge of Capacitance

It is very important that capacitance be discharged, both before and after an insulation resistance test. It should be discharged for a period about four times as long as test voltage was applied in a previous test.

Megger instruments are frequently equipped with discharge switches for this purpose. If no discharge position is provided, use a discharge stick. Leave high capacitive apparatus (for instance, capacitors, large windings, etc.) short circuited until you are ready to re-energize them. Explosion and Fire Hazard



Do NOT use the megger insulation tester in an explosive atmosphere.

So far as is known, there is no fire hazard in the normal use of a megger insulation tester. There is, however, a hazard when your test equipment is located in a flammable or explosive atmosphere. You may encounter slight sparking (1) when you are attaching the test leads to equipment in which the capacitance has not been completely discharged, (2) through the occurrence of arcing through or over faulty insulation during a test, and (3) during the discharge of capacitance following a test. Therefore:

Suggestions: For (1) and (3) in the above paragraph, arrange permanently installed grounding facilities and test leads to a point where instrument connections can be made in a safe atmosphere. For (2): Use low-voltage testing instruments or a series resistance. For (3): To allow time for capacitance discharge, do not disconnect the test leads for at least 30 to 60 seconds following a test.


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