Voltage in disconnected CU!?!

[sebmaster]

Hi,

I am trying to use an old storage heater radial to power a socket and fixed light/pump installation outdoors, I have got a 32A 30mA RCBO to replace the current fuse in the off peak CU. (I called the electric company and asked them to switch it over to full rate)

To find which circuit at the CU was my radial I was intending to short out the phase and neutral, then use a multimeter in the CU to detect the right cable.

Despite removing all the fuses and turning the CU off, being cautious, I measured the voltage accross the ends before shorting them, and found about 5v.

Very worried I went back to the CU and found this was common with every circuit in it. I checked and it is completely isolated from the meter. There is a difference of 5-7v between all phases and the neutral bar, and the earth in the CU. (It is connected to the earth block by the meter)

This isnt normal surely, does this mean there is some short or connection there shouldnt be somewhere in the house? Why is it affecting all the circuits?

Cheers!

 

[davelx]

You have other circuits in the house fed from another CU still live? In which case this is almost certainly capacitive coupling between cables and nothing to worry about.

 

[sebmaster]

Hi, Yes there are other circuits still live. Thanks alot Davelx! Ill get on now.

One other question while here, I intend to replace a 15A fuse with the 32A RCBO.

At the moment there are 7 fuses, 15A each, and the main breaker on the board is 100A.

What would be the best practice do you think:

1. Put in the RCBO and leave the others alone as the heaters are disconnected anyway.

2. Put in the RCBO and replace a few fuses with lower rated ones to bring the total back down to 100A

3. Disconnect the unused heater radials, remove the fuses and buy blanks, and label the circuits "Old Unused Heater Circuit".

 

[JohnD]

the cable you are using is very unlikely to be rated at 32A, so your RCBO is too high for it. It will have been sized for the 15A circuit. It is probably wired in 1.5mm or 2.5mm

Can you take it back and change it for a 16A RCBO?

What make is it?

As for total loading, your company fuse (main fuse) will not be over 100A, it may be 80A or 60A. It does not matter if you have a multiplicity of circuits in the CU whose MCBs added together exceed 100A, because you will not actually be using that much.

If you are sure you do not want to use the other old circuits for anything, it is better to disconnect them and fit blanks. But consider a dedicated freezer circuit, or an outdoor circuit, or an alarm circuit, or a washing machine or tumble drier circuit, or a boiler circuit.

 

[Space cat]

Something is not quite right here. If I read your post correctly, you have much the same voltage on the live wires of all the circuits with respect to a floating neutral - and also with respect to an earthed earth. That's a bit of a coincidence. Why are the live wires in all the circuits picking up similar amounts of leakage? Moreover, I would expect more voltage on the neutral because it's common to all circuits and can therefore collect more leakage current.

The only thing that would make sense would be if the voltage was on the neutral bar. Do you have any voltage between your isolated neutral bar and earth?

 

[sebmaster]

Hi JohnD, thanks, ill leave the others alone.

For the loading, the radial is wired in 4mm T&E, so that can serve a 75sq.m. area with a 32A breaker right?

The CU is a Hager.

I have however had to replace the RCBO with a 32A MCB because the RCBO was too big to fit the CU, so ill add a second unit at the summer house with RCD protection and make sure everything in the garden is taken off that.

Space Cat, the voltage between the disconnected netural and earth is 7.4v.

I also had a conversation with the electric company today, I was very sure that Economy 7 means that the whole CU will activate and deactivate at set times (12-7), but the guy I talked to seemed equally as sure that the CU would be connected all the time, and that the clock by the meter with the on-off tabs was just so between set times I was charged economy rates, so no tails or little time tabs have to be moved.

If this is the case that means theres something very wrong as im only getting 10-16v accross the tails in the CU, so they are sending someone out on the 18th to have a look. In the mean time im going to wait until 12:01 and measure again.

Wouldnt suppose anyone could shed any light on that?

 

[Space cat]

the voltage between the disconnected netural and earth is 7.4v.

OK, that makes sense now. Capacitive coupling it is.

 

[sebmaster]

Thanks Space Cat

Just waiting for eon to come over and change the meter tails now, from what I can figure out the installation here is an old one, done before they used pulses over the lines to switch between peak and off peak charging. I Really didnt think it was that old...

 

[Click-Sure]

Its not pulses over the lines. (unless theres something even newer I am not aware of)

Radio teleswitches are activated/deactivated by a signal broadcast on part of BBC Radio 4's Long Wave frequency.

Its near droitwich and you can see the twin masts from the Brimingham-Gloucester railway line and the M5.