How to do Insulation Resistance Test using Megger (IR test)

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.

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

For insulation testing, we need to disconnect from panel or equipment and keep them isolated from power supply. The wiring and cables need to test for each other ( phase to phase ) with a ground ( E ) cable. The Insulated Power Cable Engineers Association (IPCEA) provides the formula to determine minimum insulation resistance values.

  • R = K x Log 10 (D/d)

  • R =IR Value in MΩs per 1000 feet (305 meters) of cable.
  • K =Insulation material constant.( Varnished Cambric=2460, Thermoplastic Polyethlene=50000,Composite Polyethylene=30000)
    D =Outside diameter of conductor insulation for single conductor wire and cable
  • ( D = d + 2c + 2b diameter of single conductor cable )
    d – Diameter of conductor
    c – Thickness of conductor insulation
    b – Thickness of jacket insulation

HV test on new XLPE cable (As per ETSA Standard)

Application Test Voltage Min IR Value
New cables – Sheath 1KV DC 100 MΩ
New cables – Insulation 10KV DC 1000 MΩ
After repairs – Sheath 1KV DC 10 MΩ
After repairs – Insulation 5KV DC 1000MΩ

11kV and 33kV Cables between Cores and Earth (As per ETSA Standard)

Application Test Voltage Min IR Value
11KV New cables – Sheath 5KV DC 1000 MΩ
11KV After repairs – Sheath 5KV DC 100 MΩ
33KV no TF’s connected 5KV DC 1000 MΩ
33KV with TF’s connected. 5KV DC 15MΩ

 

insulation resistance test

IR Value Measurement (Conductors to conductor (Cross Insulation))

  • The first conductor for which cross insulation is being measured shall be connected to Line terminal of the megger. The remaining conductors looped together (with the help of crocodile clips) i. e. Conductor 2 and onwards, are connected to Earth terminal of megger. Conductors at the other end are left free.
  • Now rotate the handle of megger or press push button of megger. The reading of meter will show the cross Insulation between conductor 1 and rest of the conductors. Insulation reading shall be recorded.
  • Now connect next conductor to Line terminal of the megger & connect the remaining conductors to earth terminal of the megger and take measurements.

IR Value Measurement (Conductor to Earth Insulation)

  • Connect conductor under test to the Line terminal of the megger.
  • Connect earth terminal of the megger to the earth.
  • Rotate the handle of megger or press push button of megger. The reading of meter will show the insulation resistance of the conductors. Insulation reading shall be recorded after applying the test voltage for about a minute till a steady reading is obtained.

 IR Value Measurements:

  • If during periodical testing, insulation resistance of cable is found between 5 and 1 /km at buried temperature, the subject cable should be programmed for replacement.
  • If insulation resistance of the cable is found between 1000 and 100 /km, at buried temperature, the subject cable should be replaced urgently within a year.
  • If the insulation resistance of the cable is found less than 100 kilo ohm/km., the subject cable must be replaced immediately on emergency basis.

Help us improve. Rate this post

Leave a Reply

Your email address will not be published. Required fields are marked *

2017 @ All rights reserved by Electricalmastar.com