CU voltage with main isolator open

[CBX]

Afternoon all

Tested CU today which had 2.5Vac L & E and L & N in load side of CU with main isolator open. All MCBs were open also. Load side L to supply side L was 50Vac or so.

When closing the MCBs the potential difference dropped to 0 and the load L to supply L went to a normal 245Vac.

Is this expected behaviour?

 

[PrenticeBoyofDerry]

If the main isolator was open you would expect a 0V reading, what instruments were you testing with and is it in calibration?

 

[CBX]

Thanks for the reply. That's what I expected too.

I'm using a digital miltimeter (Mastech MS8229). I know this is not a professional tool, though it has given me expected readings on everything else.

When closing the MCBs, the L & N pd is 0Vac and when all open L & N pd is 2.5Vac

 

[AndyPRK]

I noticed a similar thing the other day.
7v with no load connected.

 

[CBX]

Any idea why this is?

I don't think it is due to a faulty main isolator.

 

[ricicle]

If you are testing across the L on an open mainswitch then remember that with the MCBs open as well on the dead side the L terminal will just be floating so although isolated from the energised side it will not have a neutral or earth reference. This means that the voltage measured across the L terminals of the switch could be anything.
If you left the main switch open but closed all the MCBs then the dead side would reference to neutral through any connected loads so you would see 230/240v across the L terminals of the main switch.
The small measure voltages L/E or L/N could be caused by circulating currents in the main protective bonding conductors, combines with maybe some small N/E faults.

 

[CBX]

Thanks very much for that - that makes sense.