How to test a three-phase distribution box by using a megger?

The distribution box testing is very important and before doing this test we need to check the megger or insulation tester. In the merger we can see a red wire and a black wire connect the red wire to the megger’s line terminal and then connect the black wire to the megger’s earth terminal. Then connect the two-port tips (short it), so while doing this the megger should display a zero valve, and if not there is a problem. The testing of the distribution box must be done by experienced personnel otherwise it could create a lot of problems certain circuit components in the distribution box could be damaged. Mostly while doing the testing of the distribution box the meter coils are damaged so the testing must be conducted by an experienced person. Also while doing the testing certain devices in the distribution box should be turned off like an RCCB.

How to check if the megger is working or not?

We can check the megger by using a multimeter, select 250 or 500 by using the range selector knob in the megger. In the multimeter turn the range selector knob to DC and connect the probe of the multimeter to the megger’s probe. If the megger is in the 250 voltage range then the multimeter value should be similar to this range and If the megger is in the 500 range then the multimeter would show a similar voltage range in it. If the values are not similar then there is a problem in the megger and this problem could be because of the megger battery so check the megger’s battery and if required replace it. So after checking the megger we can do our distribution box testing.

Testing the distribution box

Before doing the testing of the distribution box we need to turn off the isolator and RCCB in the distribution box. We must also cut the phase and neutral to the distribution box before doing the testing. If we didn’t cut the phase and neutral while checking the distribution box circuit then the meter coil will be damaged and the meter won’t work.

The black probe in the probe must be connected with a clip and this clip should be connected to the earth of the distribution box. Select the 250 range in the megger at this time we can see OL in the display which means that open link. After that connect the red probe of the megger to the neutral bus bar then a value will be displayed in the megger and the value will be in Megha ohms and this shows the insulation resistance. If the megger valve is zero when connected to the bus bar then there is a short.

After that test, check the phase wire of the MCB which is the outgoing wire, place the red probe of the megger in the MCB’s output and we can see a certain value and this will be more than a thousand mega ohms. If the value is zero then there is a short. Now connect the megger’s red wire to the main neutral and the value will be zero because the earth and the neutral are ground.

Phase testing in the DB

Connect the earth probe in the megger to the neutral of the distribution box then short it by using the other probe and the value should be zero which shows that it is in good condition.

Check the phase by using the red probe in the megger and a value will be displayed in the megger’s display which means that the connection is good and if the value is zero then there is a complaint in the circuit. So if there is any short then we should find that line and remove the short.


post-graduate in Electronics & communication.

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