AC Current in unconnected wires!

GingerLee

Hello,
My electrician had to disconnect the ground wire from mains distribution box a day ago. And he forgot to attach it back. I did not know it was un-attached.

Then in the evening, I touched my computer and got a shock. I used a screw driver type phase tester and checked the ground part of all the outlets in the house and they all were showing AC current.

I thought maybe there is mixup of live and ground wire, so I quickly disconnected all the devices and appliances. I switched off the live using MCB from the mains. Neutral was still connected. And now ground was not showing any AC current.



Later on I found, the electrician forgot to attach the ground in the mains distribution box.

This beats me. The ground wire was totally unconnected from both ends. I checked all the outlets one by one. But still whenever I switched on live, ground wire would show current too.

I checked live and ground using multimeter's continuity test, there is no short circuit.

Can someone please explain why an isolate ground wire is showing AC current ?

nsaspook

Fire the electrician! He should have checked his work before leaving. The stray current could be from insulation leakage or stray electrical fields capacitive coupling on the floating wire.

Jobrag

Did you test for current or voltage?

GingerLee

Thanks for telling about capacitive coupling. I think this explains it :)

I used a screw driver phase tester, so I guess I checked voltage. The lamp inside the screw driver would glow if its at higher voltage than person using it.

Jobrag

Do you have RCCBs fitted? If not, use the compensation you get from the electrician to fit them.

sophiecentaur

If you connect a DMM between the points where you got a shock and measure the actual current that can flow. It will probably be no more than a milliAmp or so. Not life threatening but your electrician should still sort it out as the next fault could kill someone.

NascentOxygen

You presumably contacted a metallic part of the computer or a peripheral, but were you also in contact with some other conductive path to ground?

GingerLee

After I connected the ground wire properly, there is no more shock. The current in the ground wire is not showing anything. Now its 0 mA.

I think so. I was barefoot on marbled floor. And I touched one of the screws of the metallic cabinet.

It was not working thats why I called the electrician in the first place. I test it every two months, as is written on the box. Installed a new one now :)

NascentOxygen

Hmm, I would not have expected a marble floor to be conductive at the low voltage in question.

But was the old RCCB actually faulty? If the system's ground wire was disconnected, as you say it was, then the TEST button (whose purpose I presume is to divert a small current to ground) cannot fulfill its function so even a properly functioning RCCB will not respond to the TEST button under these circumstances, it seems to me.

sophiecentaur

It all depends on how bad a shock you actually got. You will feel a small (1mA) shock that an RCD will ignore because it isn't considered lethal. If RDCs tripped with less than their specified current then they would always be going off and people wouldn't put up with the inconvenience.