Insulation resistance measurement (IR)
Defects in insulation
Insulation resistance measurement 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 a minimum insulation resistance per unit length often specified by the customer.
Megger MIT1020 10kV Insulation Resistance Testers are all designed specifically to assist the user in testing and maintaining high voltage equipment
Megger MIT1020 10kV Insulation Resistance Testers are all designed specifically to assist the user in testing and maintaining high voltage equipment
The results obtained by IR Test are not intended to be useful for looking for localized defects in the insulation as in a real HIPOT test, but rather to give information on the quality of the bulk material used as insulation.
Even when the end customer doesn’t need it, many wire and cable manufacturers use insulation resistance testing to track their insulation manufacturing processes and spot development issues before process variables run out. drift outside authorized limits.
Selection of IR testers (Megger):
Insulation testers with a test voltage of 500, 1000, 2500 and 5000 V are available. The recommended values of the insulation checkers are shown below:
Voltage level IR tester
Precaution in Meggering
Before chewing:
Make sure all test circuit connections are tight. Test the megger before use, if it gives INFINITE value when not connected, and ZERO when the two terminals are connected together and the handle is turned.
During the Meggering:
When testing for grounding, make sure that the far end of the conductor does not touch each other, otherwise the test will indicate faulty insulation if it does not.
Make sure the earth used when testing the earth and open circuits is good, otherwise the test will give wrong information. Spare conductors should not be combined when other working conductors of the same cable are connected to the respective circuits.
Make sure all conductors have been reconnected correctly.
Test the functions of the points, tracks and signals connected via the cable to know their correct response.
In case of signals, the appearance must be checked personally.
In case of points, check the positions on the site. Check whether the polarity of a power supply taken through the cable has been inadvertently grounded.
Safety requirements for méglage:
All equipment under test MUST be disconnected and isolated.
The equipment must be discharged (bypassed or shorted) at least as long as the test voltage has been applied in order to be absolutely safe for the person performing the test.
Never use Megger in an explosive atmosphere.
Make sure all switches are blocked and cable ends are properly marked for safety.
The ends of the cables to be insulated must be disconnected from the power supply and protected against any contact with the power supply, the earth or any accidental contact.
Installation of safety barriers with warning signs and open communication channel between test personnel
Do not digest when humidity is above 70%.
Good insulation: the reading in Megger increases first and then remains constant.
Bad insulation: Megger reading increases first, then decreases.
The expected IR value changes to Temp. 20 to 30 centigrade decrees.
If the temperature exceeds 10 ° C, the IR values increase twice.
If the temperature is above 70 degrees centigrade, the IR values decrease 700 times.
How to use Megger
Meggers is equipped with three line terminals (L), one earth terminal (E) and one guard terminal (G).
The resistance is measured between the Line and Earth terminals, where current will flow through coil 1. The “Guard” terminal is intended for special test situations where one resistance must be isolated from another. Now let’s see a situation where the insulation resistance needs to be tested in a two-wire cable.
Measure the insulation resistance of a conductor on the outside of the cable, we need to connect the “Line” wire from the Megger to one of the conductors and connect the “Earth” wire from the Megger to a wire wrapped around the sheath of the cable.
In this configuration, the Megger should read the resistance between a conductor and the outer jacket.
We want to measure the resistance between conductor 2 and the sheaths but we are actually measuring the resistance in parallel with the series combination of the resistance between conductors ( R c1-c2 ) and the first conductor of the sheath ( R c1-s ) .
If we don’t care, we can run the test as configured. If we want to measure only the resistance between the second conductor and the sheath ( R c2-s ), then we have to use the “ 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 almost infinite and therefore there will be no current between the two conductors. Therefore, the Megger’s resistance indication will be based exclusively on the current flowing through the insulation of the second conductor, the sheath of the cable, and the wire wrapped around it, and not the current flowing through the insulation of the first conductor.
The guard terminal (if present) 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 of a large electrical appliance. For example, consider a two-core cable with a sheath.
If we measure between core B and sheath without connection to the guard terminal the current will go from B to A and from A to the sheath. Our measure would be weak. By connecting the protection terminal to A, the two conductors of the cable will have almost the same potential, which will eliminate the bypass effect.
