Insulation starts to age as soon as it's made. As it ages, its insulating performance deteriorates. Any harsh installation environments, especially those with temperature extremes and/or chemical contamination, accelerates this process. This deterioration can result in dangerous conditions in power reliability and personnel safety. As such, it's important to identify this deterioration quickly so that corrective steps can be taken. One of the simplest tests and its required test instrument are not universally understood. To help eliminate this lack of understanding, let's discuss in detail Insulation Resistance (IR) testing and the megohmmeter.

Insulation testing components

Let's approach the subject by component.

The megohmmeter

A basic megohmmeter hook-up schematic is shown in Fig. 1 (above). The megohmmeter is similar to a multimeter, when the latter is in its ohmmeter function. There are differences, however.

First, the megohmmeter's output is much higher than that of a multimeter. Voltages of 100, 250, 500, 1,000, 2500, 5,000, and even 10,000V are used (Table 1 below). The most common voltages are 500V and 1,000V. Higher voltages are used to stress an insulation to a greater degree and thus obtain more accurate results.

 

Equipment AC Rating

DC Test Voltage

 Up to 100V

 100V and 250V

 440V to 550V

 500V and 1,000V

 2,400V

 1,000V to 2,000V and higher

 4,160V and above

 1,000V to 5,000V or higher

Table 1.Recommended test voltages for routine maintenance insulation-resistance tests of equipment rated to 4,160V and above.

 

Second, the range of a megohmmeter is in megohms, as its name implies, instead of ohms as in a multimeter.

Third, a megohmmeter has a relatively high internal resistance, making the instrument less hazardous to use in spite of the higher voltages.

Testing connections

A megohmmeter usually is equipped with three terminals. The "LINE" (or "L") terminal is the so-called "hot" terminal and is connected to the conductor whose insulation resistance you are measuring. Remember: These tests are performed with the circuit deenergized.

The "EARTH" (or "E") terminal is connected to the other side of the insulation, the ground conductor.

The "GUARD" (or "G") terminal provides a return circuit that bypasses the meter. For example, if you are measuring a circuit having a current that you do not want to include, you connect that part of the circuit to the "GUARD" terminal.

Figs. 2, 3, and 4 show connections for testing three common types of equipment. Fig. 2 shows a connection for testing a transformer bushing, without measuring the surface leakage. Only the current through the insulation is measured, since any surface current will be returned on the "GUARD" lead.

Various insulation tests

Basically, there are three different tests that can be done using a megohmmeter.

1) Insulation resistance (IR)

This is the simplest of the tests. After the required connections are made, you apply the test voltage for a period of one min. (The one-min interval is an industry practice that allows everyone to take the reading at the same time. In this way, comparison of readings will be of value because, although taken by different people, the test methods are consistent.) During this interval, the resistance should drop or remain relatively steady. Larger insulation systems will show a steady decrease, while smaller systems will remain steady because the capacitive and absorption currents drop to zero faster on smaller insulation systems. After one min, read and record the resistance value.

Note that IR is temperature sensitive. When the temperature goes up, IR goes down, and vice versa. Therefore, to compare new readings with previous readings, you need to correct the readings to some base temperature. Usually, 20°C or 40°C are used as comparison temperatures; tables are available for any correction. However, a common rule of thumb is that IR changes by a factor of two for each 10°C change.

For example, suppose we obtained an IR reading of 100 megohms with an insulation temperature of 30°C. The corrected IR (at 20°C) would be 100 megohms times 2, or 200 megohms.

Also note that acceptable values of IR will depend upon the equipment. Historically, field personnel have used the questionable standard of one megohm per kV plus one. The international Electrical Testing Assoc. (NETA) specification NETA MTS-1993, Maintenance Testing Specifications for Electrical Power Distribution Equipment and Systems, provides much more realistic and useful values.

Test results should be compared with previous readings and with readings taken for similar equipment. Any values below the NETA standard minimums or sudden departures from previous values should be investigated.

2) Dielectric absorption ratio

This test recognizes the fact that "good" insulation will show a gradually increasing IR after the test voltage is applied. After the connections are made, the test voltage is applied, and the IR is read at two different times: Usually either 30 and 60 sec, or 60 sec and 10 min. The later reading is divided by the earlier reading, the result being the dielectric absorption ratio. The 10 min./60 sec. ratio is called the polarization index (PI).

For example, let's assume we apply the megohmmeter as described earlier with the appropriate test voltage impressed. The one min. IR reading is 50 megohms, and the 10 min. IR reading is 125 megohms. Thus, the PI is 125 megohms divided by 50 megohms, or 2.5.

Various sources have tables of acceptable values of dielectric absorption ratios (see Table 2 below).

 

Insulation Condition

60/30-sec Ratio

10/1-min Ratio
(Polarization Index)

Dangerous

-

Less than 1

Questionable

1.0 to 1.25

1.0 to 2*

Good

1.4 to 1.6

2 to 4

Excellent

Above 1.6**

Above 4**

Table 2. Listing of conditions of insulation as indicated by Dielectric Absorption Ratios. These values must be considered tentative and relative, subject to experience with the time-resistance method over a period of time.

*These results would be satisfactory for equipment with very low capacitance, such as short runs of house wiring.

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

 

3) Step voltage test

This test is particularly useful in evaluating aged or damaged insulation not necessarily having moisture or contamination. A dual voltage test instrument is required here. After the connections are made, the IR test is done at a low voltage, say 500V. The test specimen then is discharged and the test is done again, this time at a higher voltage, say 2500V. If more than a 25% difference exists between the two IR readings, age deterioration or damaged insulation should be suspected.

 

SIDEBAR: Basic Theory

An equivalent circuit for electrical insulation is shown in Fig. 5 (right). The top terminal might be the center conductor of a power cable, and the bottom terminal, its shield. The current flowing through the cable's insulation would be that current noted as "total current" in the diagram. As you can see, the total current is equal to the sum of the "capacitive current" plus the "absorption current" plus the "leakage current."

Note that the total current is not the load current flowing through the system. Rather, it's the current that flows from the energized conductor through the insulation to ground.

Let's provide some basic definitions here.

Capacitive current. A capacitor is created when two conductors are separated by an insulator. This is the situation in a power system.

If a DC voltage is suddenly applied (closing the switch in Fig. 5), electrons will rush into the negative plate and be drawn from the positive plate. Initially, this current flow will be very large, but it will gradually reduce to a much smaller value, eventually approaching zero. The current labeled "capacitive charging current" in Fig. 6 (right) shows how this current varies with time after DC voltage is applied.

Leakage current. No insulation is perfect; even new insulation will have some leakage current, albeit small. This leakage current will increase as the insulation ages. It also will worsen when the insulation is wet or contaminated.

The "conduction or leakage current" shown in Fig. 6 is a graphical representation of leakage current. Notice that it starts at zero, and quickly increases to a final value of 10 microamps. This is the way that good insulation behaves. As insulation ages and deteriorates, however, two changes may occur in leakage current. One change may be that the final value of leakage current may increase and not level off. For example, instead of leveling off at 10 microamps, the final current may increase to 20 microamps. The other change may be that, instead of rising quickly to a final value and the leveling out, the leakage current simply may continue to increase. In this scenario, the insulation eventually will fail.

Absorption current. The charges that form on the plates of the capacitor attract charges of the opposite polarity in the insulation, causing these charges to move and, thus, drawing current. The largest charge motion occurs in the initial moments and then gradually tapers off to near zero. This current is called dielectric absorption, or just absorption current. A time plot of this current, labeled "absorption current," also is shown in Fig. 6.

Total current. The total current flowing in the circuit is equal to the sum of the components shown in Fig. 6. The total current flow, when a DC voltage is applied, starts at a relatively high value and then drops, settling at a value just slightly above the leakage current. In bad or deteriorated insulation, the total current will drop slowly, or may even increase.