New consumer unit - diagnostics challenge

[SimonH2]

I imagine some of you may well spot the answer very quickly, but here goes ...

Background. A colleague is doing some work in his new (to him) house (which is still empty at the moment). One of the task is to fit a new consumer unit - which is something I would have done for him if it weren't for the LABC fees to notify.
At the weekend I took the old Wylex fuse board off, moved the base board a little, tidied up the cables (in trunking instead of grey stripy wallpaper), etc. To avoid any surprises, I did IR checks (all in excess of 10M both L-N and L+N to CPC) and loop resistance of the RFCs. So no surprises expected.

Friendly spark came to fit new CU on Tuesday, and all went well until he came to test the last few breakers (all RCBOs). Ccts 1 to 5 were all OK. Cct6 wouldn't set - breaker just tripped free immediately.
Ccts 7 & 8 were odd, the tester (Metrel ?) just beeped and displayed "PE" on the display.

The situation didn't change with load wiring disconnected from the breakers.

Consumer unit is this one. Ccts 1-4 are 32A RCBO supplying 3xRFC plus cooker, cct5 16A supplying immersion heater, and ccts 6-8 6A supplying lights.
Yes I know what some people think of these "budget" CUs, that's why I've got spares for the ones I've used in my properties But at those prices, it's not a big premium to go all RCBO.

So based on that, would anyone like to hazard a guess at what fault(s) were present ? Or what to check next ?

Yes, I do know what the faults were, and everything is all fine now.

PS - (especially for B-A-S), please skip the lectures.

 

[securespark]

So, are you telling us you've omitted RCD protection on some circuits?

 

[holmslaw]

..

 

[SimonH2]

So, are you telling us you've omitted RCD protection on some circuits?

No, all circuits are RCBO

Borrowed neutral.

No, but that was one of the first thoughts - not hard to mix the wires up.

 

[bernardgreen]

A live or neutral fault to earth via something other than the CPC.

 

[ColJack]

borrowed neutral is different to mixing them up at the board.. you have the upstairs landing lights connected to the L of one circuit and the N of another ( usually L from downstairs light and N from upstairs.. )..

load leaves on one RCBO but is never heard from again as far as that breaker is concerned.

and for those of us that have never used a metrel tester, what does the PE indicate?

 

[Alarm]

PE is the Line CPC reversed.
Or the test mode selected by pressing the test button 3 times.

 

[SimonH2]

A live or neutral fault to earth via something other than the CPC.

Only affects these two circuits, and with the load wiring disconnected from them.

borrowed neutral is different to mixing them up at the board.. you have the upstairs landing lights connected to the L of one circuit and the N of another ( usually L from downstairs light and N from upstairs.. )..

Ah yes, of course. But no - see above, wiring disconnected.

and for those of us that have never used a metrel tester, what does the PE indicate?

I've never used one either, the guy was showing me what it was doing as we went along.

PE is the Line CPC reversed.
Or the test mode selected by pressing the test button 3 times.

Thanks, I was wondering.
Yes, line-CPC reversed would sort-of make sense given the fault.

The guy doing the install was a bit thrown by the fault, and didn't seem to be heading towards doing any "more basic" checks. The error on the tester didn't seem to be giving much clue about the fault.
I grabbed my voltage indicator and did a couple of checks.

What do you think I checked, and what were the results ? This did find the problem - after some more head scratching.

 

[SimonH2]

Oh yes, I'll throw in that there were two distinct faults - cct 6 was different to ccts 7 & 8.

 

[JohnW2]

Oh yes, I'll throw in that there were two distinct faults - cct 6 was different to ccts 7 & 8.

Hmmm - from what you've said, it sounds as if RCBOs 6, 7 and 8 were probably connected to a different neutral bar from 1-5 - and one wonders what was feeding the 6/7/8 neutral bar (and what potential it was at).

Kind Regards, John

 

[securespark]

Seems like the board only has one N bar.

 

[Electrifying]

Do you know what's really annoying to electricians when a problem is posed - like this one?

We think along the lines of how we would do the job and what will have already been ruled out.

So, you see, you're talking about RCBOs not holding - therefore your onto 'live' testing...........which must mean that all your 'dead' test results were OK.

So,
Continuity of CPC must have been OK,
RFC conductor continuity must have been OK
IR must have been OK
Polarity (of all circuits), must have been OK

.....otherwise, you wouldn't energise, RIGHT??

So, I'm stumped - it can't be a neutral bar issue, because were talking RCBOs.
You say it's not a borrowed neutral and all loads are disconnected.

So go on - I give in??

 

[JohnW2]

Seems like the board only has one N bar.

So it does - I confess I hadn't looked at the pic!

On reflection, I think I must be mising something here. SimonH2 keeps telling us that the problem exists 'with the load wiring disconnected'. If he means that nothing is connected to the load side of the RCBOs in question, it's surely impossible that (unless the RCBO was faulty), one couldn't turn the RCBO on? I think I must be misunderstanding!

If he didn't mean that the load side of the RCBOs was disconnected, then I suppsoe one of the (two) faults could be a muddling up of which conductors had been connected to which of the three affected RCBOs (i.e. Ls and E's not paired properly).

Kind Regards, John.

 

[SimonH2]

Seems like the board only has one N bar.

It's not the only option in the range - they do all but the smaller sizes in split load versions. The neutral bar is split and has a link between the sections in the non-split boards.

Do you know what's really annoying to electricians when a problem is posed - like this one?

We think along the lines of how we would do the job and what will have already been ruled out.

So, you see, you're talking about RCBOs not holding - therefore your onto 'live' testing...........which must mean that all your 'dead' test results were OK.

So,
Continuity of CPC must have been OK,
RFC conductor continuity must have been OK
IR must have been OK
Polarity (of all circuits), must have been OK

.....otherwise, you wouldn't energise, RIGHT??

Right, which is why I mentioned testing done in advance. I'd done RFC continuity and IR tests in advance. The sparks did polarity and R1+R2 tests before connecting each group of circuits - which I confess I failed to mention.

So, I'm stumped - it can't be a neutral bar issue, because were talking RCBOs.

Why can't it be a neutral bar issue ? Not sure what is special about RCBOs vs MCBs in this respect - you still need a neutral on each circuit.

So go on - I give in??

It'll be clear soon. I can assure you it falls into the category where Homer would be shouting "Doh"

Cct6 was indeed a faulty breaker. Once swapped that was OK so ccts1-6 were OK, 7&8 weren't.

So, you've got your voltage tester in your hands. The breakers set and hold, the tester just beeps and says PE, and your voltage indicator doesn't show anything between live and neutral at the breaker load terminals. Where would you stick the probes next ? What would you expect to see ?

BTW - the reason I started the thread wasn't to irritate people, but because two of us "learned something new" the other day. It's probably not a new problem to a lot of you, but equally it may be new to some. I've found being on the receiving end of such can be useful in assessing your thought processes.

 

[Electrifying]

Why can't it be a neutral bar issue ? Not sure what is special about RCBOs vs MCBs in this respect - you still need a neutral on each circuit.

You can get away with one single neutral bar if all circuits are on RCBOs - so there wouldn't be an issue of connecting a circuit to the wrong neutral bar........like there can be with two RCDs covering a number of circuits each.

It'll be clear soon. I can assure you it falls into the category where Homer would be shouting "Doh"

Cct6 was indeed a faulty breaker. Once swapped that was OK so ccts1-6 were OK, 7&8 weren't.

So, you've got your voltage tester in your hands. The breakers set and hold, the tester just beeps and says PE, and your voltage indicator doesn't show anything between live and neutral at the breaker load terminals. Where would you stick the probes next ? What would you expect to see ?

I'm not following what you are testing for here?? Is it me? What test are you performing?

The only reason that I can think of that an RCBO would trip without a load (line conductor) connected to it would be a neutral/earth fault on that circuit - but the other circuits would have to be under load.

You're not being very clear, I'm afraid.