Why do you have voltage between neutral and ground.

Normally when you are called in to troubleshoot a trip or in my case, the complaint was that they receive a serious shock if any appliance with a metallic part is plugged into the power outlet (receptacle). 

Even when they plug in a laptop to the receptacle all screws on the laptop effectively become a naked wire charged with 220V although it doesn't damage the laptop. 

The first thing I did was to open up the power outlet and took voltage measurements of phase-to-neutral got 220v as expected, neutral-to-ground got 220v to my surprise, and phase-to-ground got 6v. 

 My first suspicion was that the electrician swapped phase and neutral wires, but I carried out further checks just to be sure I went to the distribution panel.

I powered off the panel and performed a continuity test between the neutral and ground and I got a beep. Now I switched off the MCB that feeds the circuit and performed another continuity test between the neutral and ground this time no reading.

To cut long story short this is the circuit type.



Ideally, we expect 0V between Neutral and Ground because the neutral point at the transformer is usually earthed putting neutral and ground at the same potential.  Although experience has shown me that there is usually potential of 0.2V to 2V between them.  

So in my case why did I get a get a huge amount of voltage between them? The simple answer is that somewhere within the circuit two wires had shorted. This short could be caused by a lot of factors which include Insulation failure, damp conduit or damp outlet wall box e.t.c

I separated the wires and performed a continuity test between Ground (G), Live (L) and Neutral (N) on each outlet wall box going GL, GN, LN and got a continuity reading between G and L. 

Looking at the circuit when the Live becomes shorted with the Earth, The earth wire effectively becomes a live wire this caused the 220V reading btw Earth and Neutral.

I also realized the facility wasn't properly grounded this is why the MCB on the distribution board didn't trip when the live wire came in contact with the ground wire,

I replaced the wires and everything worked OK.




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