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1. Insulation test: This is the most important test for troubleshooting of any
electrical equipment. Depending on the system, a suitable test voltage is
applied to check the insulation resistance between the live conductors and
earth.
2. Earth continuity test: For electrical equipment, continuity between the
exposed portion (metallic) of earthed equipment and the earth terminal is checked. Resistance value should be low. If resistance value is high, then it is
indicative of poor earthing.
3. Flash test: To check the insulation strength of cables, a high voltage (as
specified by the cable manufacture) is applied in the same way as for the
insulation test. This determines the withstand capability of the cable insulation.
4. Electronic earth test: Generally, for microprocessor-based or electronic based
sensitive devices, separate earthing is provided. This is called electronic
earth. The voltage between the electronic earth and the power earth should be
lower than 2 V.

Megger testing cables and auxiliary devices of a single-phase system

Figure 1

Disconnect P and N from the supply side, as well as from the other end.
Now we have isolated our test circuit, making it dead. Short P and N with a temporary
short link. Close switch and protection devices.
As shown in Figure 1, open motor terminals, so that the motor remains isolated from
the test circuit. Check resistance with the insulation tester between the neutral link and
earth. If the value shown in the meter is less than 1 M?, then there is a fault with either
the cable insulation or device terminals.

Megger testing cables and auxiliaries of a three-phase system

Figure 2

Disconnect L1, L2, and L3 from the supply side, as well as from the other end. This makes it a dead circuit.
Short L1, L2, and L3 terminals with a temporary link. Close the breaker device and
protection devices. As shown in figure 2, open motor terminals T1, T2, and T3, so
that the motor remains isolated from the test circuit.
Check resistance with insulation tester between each conductor and earth.
If the meter shows a low value less than 1 M?, there is a fault in either cable insulation
or device terminals.

Megger testing of motor

A pre-condition for megger testing of a motor is to isolate the motor from the supply totally. Take the megger value of a motor between each conductor and earth, as shown in figure, to check the earthing of the stator winding. This will help us to conclude on earthing status of the stator winding. Similarly, check for shorting between two windings by checking the megger value between two stator-winding terminals, as shown in figure below.

Thus, a low reading can identify insulation failure of any winding inside the motor.

Megger testing cables and auxiliary devices of a single-phase system

Disconnect P and N from the supply side, as well as from the other end.
Now we have isolated our test circuit, making it dead. Short P and N with a temporary short link. Close switch and protection devices.
(Refer figure below) Open motor terminals, so that the motor remains isolated from the test circuit. Check resistance with the insulation tester between the neutral link and earth. If the value shown in the meter is less than 1 M?, then there is a fault with either the cable insulation or device terminals.

Megger testing cables and auxiliaries of a three-phase system

Disconnect L1, L2, and L3 from the supply side, as well as from the other end. This makes it a dead circuit.
Short L1, L2, and L3 terminals with a temporary link. Close the breaker device and protection devices. Open motor terminals T1, T2, and T3, so that the motor remains isolated from the test circuit.
Check resistance with insulation tester between each conductor and earth.
If the meter shows a low value less than 1 M?, there is a fault in either cable insulation or device terminals.

Megger testing of motor

A pre-condition for megger testing of a motor is to isolate the motor from the supply totally. Take the megger value of a motor between each conductor and earth, as shown in figure above to check the earthing of the stator winding. This will help us to conclude on earthing status of the stator winding. Similarly, check for shorting between two windings by checking the megger value between two stator-winding terminals, as shown in figure below.

Thus, a low reading can identify insulation failure of any winding inside the motor.

To conduct an insulation-resistance test, perform the procedure listed below:

1. Check the insulation tester by shorting its test leads. It should show zero resistance. If test leads are kept open, it should show infinite resistance.
2. Isolate the section to be tested from the power supply.
3. Disconnect all lamps or electronic devices from the circuit to be tested.
4. Select the proper operating voltage for conducting the test, depending upon the rating of the system.
5. Check for connections while conducting tests so that only the section to be tested is included in the test.
6. There should not be any stray parallel leakage paths.
7. Check the instrument for pointer index or any other pre-adjustment necessary.
8. Test leads to be used should have good-quality insulation.
9. Before starting the test, insure that all the capacitors in the circuit are discharged by shorting their two leads together. Similarly, after the test ensure that they are in discharge condition. If this is not done they may give false readings.
10. Before touching cable ends after testing, discharge any energy that might have been stored in the cables during the test. This is most likely to occur in long runs of larger cables due to their capacitance.
11. Checking continuity of an earthing system requires the use of low-reading ohmmeters, which should be zero-adjusted before each test and calibrated on regular intervals.
12. Where the testing of the earth electrode resistance is required (i.e., the resistance between the electrode and the general mass of earth), one of the special types of earth-resistance testers must be used.