RCBO

[DAJ]

As some of you may already know, I have recently installed a new 16 way consumer unit containing 6 x 6a, 5 x 32a and 1 x 40a RCBO's. 6 x light circuits (1 x outside), 5 x socket rings (1 x outside) and a cooker/hob outlet. It replaces 2 old fuse boxes, 1 x 4 way and 1 x 2 way (plus new circuits).

Most of the new CU is feeding new circuits but there are 5; 2 old 5a lighting and 2 old 30a socket, 1 x cooker, that have been simply removed from the old fuses in the old box (CU) and put into the new CU. That is to say the cables removed from the fuse block and N bar in the old box and put into the new RCBO (L & N).

Everything seems OK and is working absolutely fine. HOWEVER – today I completed the transition and the very last RCBO installed trips immediately. The neutral is located OK in the bar and the earth ‘fly’ is inserted OK. The RCBO is for an existing 6a lighting circuit which has been working fine for over 15 years. I simply transferred the cables as described earlier. The RCBO trips as soon as it’s switched ‘on’, even with no load on the circuit.

As the lights have operated fine for all these years, would I be correct in saying that the cause is probably a borrowed neutral from elsewhere? If this is the case, how can it be traced and rectified?

PLEASE no criticism and quotes from BR and/or part P, just some advise if you can help – thanks!

 

[Spark123]

Shared neutral on the landing light is a common problem, i.e. phase from downstairs and neutral from upstairs.
Failing that, a N-E issue with one of the lights.

 

[flameport]

Everything seems OK and is working absolutely fine.
...
has been working fine for over 15 years.
...
As the lights have operated fine for all these years

Lights and other circuits working does not mean they are wired properly or are safe to use.

The circuits will need to be tested to identify the faults.

 

[DAJ]

Shared neutral on the landing light is a common problem, i.e. phase from downstairs and neutral from upstairs.
Failing that, a N-E issue with one of the lights.

Thanks Spark123 - SPOT ON mate, that is the lighting circuit (landing light) the RCBO is on! I'll check it out tomorrow, thanks again!

 

[DAJ]

Everything seems OK and is working absolutely fine.
...
has been working fine for over 15 years.
...
As the lights have operated fine for all these years

Lights and other circuits working does not mean they are wired properly or are safe to use.

The circuits will need to be tested to identify the faults.

"The circuits will need to be tested to identify the faults" - thanks for the help mate!

 

[skenk]

Sorry but you will get criticism. You don't even know the right terminology for what you're talking about, you are asking questions that anyone installing new circuits and ESPECIALLY someone transferring old circuits into a new CU should already know the answers to.

You are wrong that the 'cause is probably a borrowed neutral', more likely to be a N/CPC fault that was always present and would have been found when you did your IR testing, what were your results for that circuit? . . . let me guess you didn't bother with that bit? Do you even know if the 'socket rings' are still actually rings? Did you test for earth continuity in the lighting circuit?

Please, correct me if i'm wrong, but judging by vague memories of your previous postings and your post above you are too inexperienced to safely carry out the work you have been doing, and that is why people will have a go.

 

[flameport]

Shared neutral on the landing light is a common problem

Common, but not the problem on this installation.

The RCBO trips as soon as it’s switched ‘on’, even with no load on the circuit.

and only 1 RCBO trips.

 

[DAJ]

Sorry but you will get criticism. You don't even know the right terminology for what you're talking about, you are asking questions that anyone installing new circuits and ESPECIALLY someone transferring old circuits into a new CU should already know the answers to.

You are wrong that the 'cause is probably a borrowed neutral', more likely to be a N/CPC fault that was always present and would have been found when you did your IR testing, what were your results for that circuit? . . . let me guess you didn't bother with that bit? Do you even know if the 'socket rings' are still actually rings? Did you test for earth continuity in the lighting circuit?

Please, correct me if i'm wrong, but judging by vague memories of your previous postings and your post above you are too inexperienced to safely carry out the work you have been doing, and that is why people will have a go.

OH DEAR - here we go! Blar, blar,blar!

 

[DAJ]

Shared neutral on the landing light is a common problem

Common, but not the problem on this installation.

The RCBO trips as soon as it’s switched ‘on’, even with no load on the circuit.

and only 1 RCBO trips.

GOOD POINT flameport! Maybe back to the drawing board!

 

[1john]

You need an insulation resistance tester to find this fault. You also need an earth fault loop impedance tester and a RCD tester and a continuity tester to make sure these circuits are safe.

 

[ericmark]

To test and do the job correctly you need expensive meters but a standard multimeter will find faults able to trip a RCBO with such ease.

Connecting a neutral and earth with no RCBO will not blow any fuses and will run although it does mean you could if a fault arises get a shock. Likely the fault has been there all the time and you have just highlighted an existing fault.

Years ago before too much was known about impedance and problems with EMC it was common to use two core cable between two way switches and link the line wire from another switch in same box. This was stopped as it causes hum on radios etc. However as people increased the amount of lights it was common to split the lighting supply and if one switch got its supply from one fuse and other off a different fuse a borrowed neutral (in real terms a borrowed line) situation accrued. The quick cure is to re-combine the two supplies so both from same fuse/MCB/RCBO. That does not cure the impedance problem and mains hum but does stop the RCBO tripping.

With 50 Hz impedance is not much of a problem. Normally it is with higher frequencies this becomes a problem. Talk to any CB'er and he will talk about SWR (Voltage standing wave ratio) and with 27 Mhz it is quite important. But even on 50 Hz it can cause mains hum when the line and neutral are not feed together. Only with DC can you use different routes for feed and return. If the feed and return go though different holes in a ferrous box this can also cause cables to heat up.

The more one learns about electric the more one realises how little one knows.

 

[DAJ]

You need an insulation resistance tester to find this fault. You also need an earth fault loop impedance tester and a RCD tester and a continuity tester to make sure these circuits are safe.

"Removed" ?

 

[DAJ]

To test and do the job correctly you need expensive meters but a standard multimeter will find faults able to trip a RCBO with such ease.

Connecting a neutral and earth with no RCBO will not blow any fuses and will run although it does mean you could if a fault arises get a shock. Likely the fault has been there all the time and you have just highlighted an existing fault.

Years ago before too much was known about impedance and problems with EMC it was common to use two core cable between two way switches and link the line wire from another switch in same box. This was stopped as it causes hum on radios etc. However as people increased the amount of lights it was common to split the lighting supply and if one switch got its supply from one fuse and other off a different fuse a borrowed neutral (in real terms a borrowed line) situation accrued. The quick cure is to re-combine the two supplies so both from same fuse/MCB/RCBO. That does not cure the impedance problem and mains hum but does stop the RCBO tripping.

With 50 Hz impedance is not much of a problem. Normally it is with higher frequencies this becomes a problem. Talk to any CB'er and he will talk about SWR (Voltage standing wave ratio) and with 27 Mhz it is quite important. But even on 50 Hz it can cause mains hum when the line and neutral are not feed together. Only with DC can you use different routes for feed and return. If the feed and return go though different holes in a ferrous box this can also cause cables to heat up.

The more one learns about electric the more one realises how little one knows.

Thanks for your time ericmark, GOOD reply! Everything does point to the "borrowed line", but wouldn't the RCBO trip on both circuits if that was the case? Spark123 pointed this out earlier.

 

[holmslaw]

..

 

[DAJ]

Everything does point to the "borrowed line", but wouldn't the RCBO trip on both circuits if that was the case?

If both of the 'borrowed' circuits are dead there will not be an imbalance. As soon as current flows then one or the other (maybe both) will trip.

I agree! But even when both circuits are "dead", a single RCBO, the one covering the suspected landing light, trips - regardless of the state of other circuits!