CU voltages querey

[Norv]

Testing a split load CU today that another Electrician had fitted. It was in a very small office. There is one ring with 6 sockets & 2 lighting circuits (1 for the kitchen & toilet & 1 for the main room)

All CU switches were off & the circuits were connected in the CU, but when I measured from supply Live to load Neutral on the main switch I got 139Volts

When I measured on the supply Live to the Neutral bar on the RCD side with the RCD on I got 113Volts.

If I removed 1 circuit at a time from the CU the voltage dropped until all circuits were removed & the voltage was zero.

Load live to load Neutral 0v

What do you think is going on here ?

 

[flyingsparks]

What Is the supply live and neutraul voltage with main switch off? Soounds like a broken supply neutraul. Is it fed from a submain?

 

[333rocky333]

low insulation resistance L to E or N to E, on one or more circuits, maybe

 

[VanSolo]

Any emergency lights on the lighting circuits? Sometimes get some odd voltages back from them when they aren't isolated from the circuit.

 

[Norv]

supply L & N voltages 246V with main sw off

Ill be looking tomorrow because I couldnt get access but there was someone on site that said the supply had its own meter & cutout & was fed through a 100A type C Circuit breaker. The run from the meter to the office is about 10Metres in 25mm SWA & then it terminates into a big rotary isolator switch & then 25mm tails into the CU.

Fully tested all circuits & they were OK, even got >299 on the IR tests

 

[RF Lighting]

I'm a bit confused here.

You tested 246V on the incomming terminals of the main switch.

You tested 0V on the outgoing terminals of the main switch.

You tested between the incomming live and the outgoing neutral and got 113V.

I presume all these tests were with the main switch open (off), and you were using a high impedence voltmeter?

Which result is unexpected?

 

[Norv]

You tested 246V on the incomming terminals of the main switch.

yes

You tested 0V on the outgoing terminals of the main switch.

yes

You tested between the incomming live and the outgoing neutral and got 113V.

yes

I presume all these tests were with the main switch open (off), and you were using a high impedence voltmeter?

yes

Which result is unexpected?

I didnt expect to get 113V. If this is normal then fair enough.

 

[333rocky333]

have you ever been on site with the electric board

Before they crack the fuse in, they use a drummond tester between the

supply and the load, with the main switch off, the neutral is open, so if it

lights, then the brighter the tester, the more chance the fuse will blow

when they put it in.

This is because the tester is in series with the load

Ideally there will be no light,but if any circuits are leaking to earth, then

the lamp will glow.

In proportion to the resistance,

on older installations more so, that is one of the reasons they use them testers.

I would not think doing it with a multimeter would be a good idea