This article describes step by step procedure of testing cable with a megger. An electric cable is used to carry the electric current. The cable has two important parts- conductors and insulation material around the conductor. The conductor should have minimum resistance so that it can allow maximum current through it. Contrary to low conductor resistance value, cable insulation resistance must be very high so it does not allow current to pass from the conductor to the earth.
When the insulation of the cable fails, heavy fault current flows from the conductor to armor to the earth point. The armor of cable is held at earth’s potential. The reliability of the cable depends on the quality of the insulation used in the cable.
What is a Megger?
The test equipment used to test the cable insulation is known as megger. The megger is derived from megaohm because it measures insulation resistance in megaohm or even higher.
Pre-activities before taking the megger Test
The following activities must be performed before conducting a megger test on a cable.
- Switch OFF the circuit breaker, remove its control plug & rack out the breaker from the panel. Take the electrical work permit and apply the LOTO system on the panel.
- Open the HT Terminal cover of the feeder/motor/transformer.
- Discharge any accumulated static or capacitive charges on the cable by connecting an earthing rod between the cable conductor and the earth. All three conductors must be discharged following the same procedure.
- Remove both sides of cable connections from sending and receiving end feeder.
- Now, the cable is ready for the megger test.
Procedure for Megger test of cable
Step-1
Before starting the megger test of the cable you should first identify the cable and check its physical condition. The cable shown in the below picture is a 6.6 kV XLPE(E) cable
Step-2
Set the megger voltage for the phase voltage. In our case, we are testing a 6.6 kV cable, its phase voltage is √3 times of line voltage. Therefore its phase voltage is 1.732 X 6.6= 3.8kV=3600 Volts. Set the megger voltage to 3800 volts. Do not connect the cable to the megger equipment before setting the megger voltage.
Step-3– Cable Megger Test
Now, connect the RED lead of the megger to one of the conductors of the cable, we here designate it as R Phase, and BLACK lead to the shield point of the cable. Connect the Green Lead of the megger to the inner insulation of the cable. The green lead is connected to the guard terminal of the megger. The guard terminal connection improves reading accuracy. For high voltage cables, you will get more accurate results of the megger test on cable if you connect the guard terminal to the inner insulation of the cable through GREEN lead.
Step-4
Press the test button on Megger, it will feed 3800 volts between the conductor and shield of the cable. Wait for about 1 minute to settle the value of insulation resistance. Initially, it will show less megger value because of the cable capacitance. finally, the value will be steady and this value is the insulation resistance of the cable core under test.
The megger or insulation resistance value of one phase conductor is 79.5 GΩ.
Step-5- Cable Megger Test
Now, remove the red lead from the R Phase conductor and connect it to the second phase conductors ( Let it call Y Phase). Now repeat the same procedure as step-4 and note down the insulation resistance value. The Megger test value shows the insulation resistance of the R-phase conductor of the cable.
Step-6
Now, remove the red lead from the Y Phase conductor and connect it to the third phase conductor( Let it call B Phase). Now repeat the same procedure as step-4 and note down the insulation resistance value.
Step-7- Cable Megger Test
Remove the red and black wire from the phase conductor and shield of the cable. In step-5,6 & 7 we tested the cable insulation for all phases.
Now, we will test the cable for its phase-to-phase insulation. First, set the megger voltage to 6600 volts.
Step-8
Now connect the RED and BLACK leads of the megger to R and Y phases respectively. After that, start the test button. The megger will feed 6600 volts between the phases. Wait for about 1 minute to settle the value of insulation resistance. Initially, it will show less megger value because of the cable capacitance. finally, the value will be steady and this value shows the phase-to-phase insulation resistance value of the R and Y phases of the cable.
Step-9
Now, Remove the black lead from the Y phase and connect it B phase and repeat the same procedure of step-8 and note down the insulation resistance value. The megger value shows the insulation resistance value between R and B phases.
Step-10
Remove the red lead from the R phase and connect it B phase and repeat the same procedure of step-8 and note down the insulation resistance value. The megger value shows the insulation resistance value between Y and B phases.
This is all about how to test cable with a Megger.
Safety Consideration
The following safety measures must be ensured for taking a megger test of cable.
- Personal safety equipment like dielectric hand gloves, dielectric gloves, and arc flash shield is mandatory.
- Prior to starting the megger test, carry out pre-job planning and risk assessment.
- Obtain safe work and isolation permit with LOTO before carrying out the job.
- Ensure personnel performing megger tests are authorized to do the job.
