T & D - 10) Measurement of insulation resistance (IR) – Part 1

Introduction

The measurement of insulation resistance is a common routine test performed on all types of electrical wires and cables. As a production test, this test is often used as a customer acceptance test, with minimum insulation resistance per unit length often specified by the customer. The results obtained from IR Test are not intended to be useful in finding localized defects in the insulation as in a true HIPOT test, but rather give information on the quality of the bulk material used as the insulation.

Even when not required by the end customer, many wire and cable manufacturers use the insulation resistance test to track their insulation manufacturing processes, and spot developing problems before process variables drift outside of allowed limits.

Selection of IR Testers (Megger):

Insulation testers with test voltage of 500, 1000, 2500 and 5000 V are available. The recommended ratings of the insulation testers are given below:

Voltage Level	IR Tester
650V	500V DC
1.1KV	1KV DC
3.3KV	2.5KV DC
66Kv and Above	5KV DC

 Test Voltage for Meggering:

When AC Voltage is used, The Rule of Thumb is:
Test Voltage (A.C) = (2X Name Plate Voltage) +1000.

When DC Voltage is used (Most used in All Megger)
Test Voltage (D.C) = (2X Name Plate Voltage).

Equipment / Cable Rating	DC Test Voltage
24V To 50V	50V To 100V
50V To 100V	100V To 250V
100V To 240V	250V To 500V
440V To 550V	500V To 1000V
2400V	1000V To 2500V
4100V	1000V To 5000V

Measurement Range of Megger:

Test voltage	Measurement Range
250V DC	0MΩ to 250GΩ
500V DC	0MΩ to 500GΩ
1KV DC	0MΩ to 1TΩ
2.5KV DC	0MΩ to 2.5TΩ
5KV DC	0MΩ to 5TΩ

Precaution while Meggering

Before Meggering:

Make sure that all connections in the test circuit are tight. Test the megger before use, whether it gives INFINITY value when not connected, and ZERO when the two terminals are connected together and the handle is rotated.

During Meggering:

Make sure when testing for earth, that the far end of the conductor is not touching, otherwise the test will show faulty insulation when such is not actually the case.

Make sure that the earth used when testing for earth and open circuits is a good one otherwise the test will give wrong information. Spare conductors should not be meggered when other working conductors of the same cable are connected to the respective circuits.

After completion of cable Meggering:

• Ensure that all conductors have been reconnected properly.
• Test the functions of Points, Tracks & Signals connected through the cable for their correct response.
• In case of signals, aspect should be verified personally.
• In case of points, verify positions at site. Check whether any polarity of any feed taken through the cable has got earthed inadvertently.

Safety Requirements for Meggering:

• All equipment under test MUST be disconnected and isolated.
• Equipment should be discharged (shunted or shorted out) for at least as long as the test voltage was applied in order to be absolutely safe for the person conducting the test.
• Never use Megger in an explosive atmosphere.
• Make sure all switches are blocked out and cable ends marked properly for safety.
• Cable ends to be isolated shall be disconnected from the supply and protected from contact to supply, or ground, or accidental contact.
• Erection of safety barriers with warning signs, and an open communication channel between testing personnel.
• Do not megger when humidity is more than 70 %.
• Good Insulation: Megger reading increases first then remain constant.
• Bad Insulation: Megger reading increases first and then decreases.
• Expected IR value gets on Temp. 20 to 30 decree centigrade.
• If above temperature reduces by 10 degree centigrade, IR values will increased by two times.
• If above temperature increased by 70 degree centigrade IR values decreases by 700 times.

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How to use Megger

Meggers is equipped with three connection Line Terminal (L), Earth Terminal (E) and Guard Terminal (G).

Megger connections

Resistance is measured between the Line and Earth terminals, where current will travel through coil 1.The “Guard” terminal is provided for special testing situations where one resistance must be isolated from another. Let’s us check one situation where the insulation resistance is to be tested in a two-wire cable.

To measure insulation resistance from a conductor to the outside of the cable, we need to connect the “Line” lead of the megger to one of the conductors and connect the “Earth” lead of the megger to a wire wrapped around the sheath of the cable.

Megger configuration

In this configuration the Megger should read the resistance between one conductor and the outside sheath.

We want to measure Resistance between Conductor- 2 to sheaths but actually megger measure resistance in parallel with the series combination of conductor-to-conductor resistance (R_c1-c2) and the first conductor to the sheath (R_c1-s).

If we don’t care about this fact, we can proceed with the test as configured. If we desire to measure onlythe resistance between the second conductor and the sheath (R_c2-s), then we need to use the megger’s “Guard” terminal.

Megger - Connecting guard terminal

Connecting the “Guard” terminal to the first conductor places the two conductors at almost equal potential. With little or no voltage between them, the insulation resistance is nearly infinite, and thus there will be no current between the two conductors. Consequently, the Megger’s resistance indication will be based exclusively on the current through the second conductor’s insulation, through the cable sheath, and to the wire wrapped around, not the current leaking through the first conductor’s insulation.

The guard terminal (if fitted) acts as a shunt to remove the connected element from the measurement. In other words, it allows you to be selective in evaluating certain specific components in a large piece of electrical equipment. For example consider a two core cable with a sheath. As the diagram below shows there are three resistances to be considered.

Meggering wiring

If we measure between core B and sheath without a connection to the guard terminal some current will pass from B to A and from A to the sheath. Our measurement would be low. By connecting the guard terminal to A the two cable cores will be at very nearly the same potential and thus the shunting effect is eliminated.