Consumer Unit Change

[Nickj]

Hi All,

I am tomorrow going to be changing a consumer unit ahead of my Elecsa assessment next week.

Just wanted to check that I'm not missing anything with regards to amendment 3.

I'm going to be using a BG consumer unit supplied with RCBOs.

It has a cut out at back for cable entry and so I am going to pass all cables through that using a grommet strip around the metal edges. Do I need to seal the entry hole? I believe not however I have heard arguments that it needs to be sealed. Passing all cables through the rear entry may require de-rating cables due to grouping however the cables enter the current board (3036's) through the rear. Can i therefore assume ratings are fine?

I am going to attach the consumer unit to the wall using dewalt wall dogs which require no anchor in wall meaning I don't have to worry about plastic wall plugs falling out in the event of a fire.


If the cables are too short (I sincerely hope they won't be) I'll use an adaptable box above the board to mount a din rail to extend wiring. Does this adaptable box need to be non-combustible too? (associated switch gear covers this possibly?)

Thanks in advance

Nick

 

[Nickj]

Went to look at the board change again today and redid all the tests before commencing with the change.

All tests ok except ring final circuit is quirky.

Continuity of ring final:
R1 0.4 ohms
Rn 0.4 ohms
R2 29.8 ohms !!!!

High resistance joint somewhere, taken all the sockets off and checked, couldn't find anything. no spurs in house and only 5 sockets down stairs and 3 sockets upstairs (small house).

Surely there isn't a junction box for such a small house????

I forgot to note the Zs but it was an acceptable value.

Should I go searching for the high resistance joint (lifting boards etc - the fun) or as it has an acceptable Zs is it ok to leave it? (assessor questions)

Thanks all

 

[NDG Elecs]

Could easily have a JB anywhere in any house unfortunately! I would not be too happy with R2 of that value. I would suspect it's a discontinuous CPC and the 30 ohm value was through parallel paths of boiler/gas hob and such like. I would split the RFC in half/quarter/etc and retest both legs until you have a rough area where the issue is. Maybe disconnect the CPCs from the boiler etc to see if parallel paths are the reason for the high reading.

Did you do a Zs at every socket/FCU/etc? Not sure what the assessor would say, but my view is that it should be sorted.

 

[westward10]

Agreed you cannot leave it.

 

[Briggs69]

Hopefully, you will have slack to re term, if not as westie said, extend in CU.

Agreed extend in cu with wagos

 

[Octopus]

They won't inspect the attic.

ha ha.

Not come across an "inspector" yet who wants to get messy.... I did suggest an inspection to an Elecsa chap once which would have meant a trip into a "nasty" loft - he was not interested! He looked at the CU instead!

 

[1Justin]

You can't leave 30 Ohms r2. But you really don't need to start splitting the ring to begin to locate the fault. Several ways to do similar, but why not connect L-CPC at CU on just one cable of the pair. Nip round the sockets with your MFT measuring L-CPC and find the one where the reading jumps up. Bingo (or at least nearly so - because it could be at a hidden joint before that point). Reversing it can help you home in on it from both sides if that helps.

 

[Nickj]

I stayed late to find the fault. it's the return leg to the consumer unit that tests 30 ohms. Rest of ring tests 0.76 which is pretty much bang on what you'd expect.

The return leg is a very short run to the consumer unit and can't think what could be causing it.

My plan is to run a new return leg in conduit tomorrow when it's light.

That should then make all circuits good to go for the board change which I'll also do tomorrow.

Hate using conduit but at same time don't want to be chasing out and lifting floorboards (mammoth task as house is pretty full with furniture).

 

[NDG Elecs]

You say it's a fairly short run with the fault on it. I know what you are saying re lifting board etc, but if the floor was carpeted and boards accessible, I would be tempted to have a quick look to see if there are any small cut boards that are obvious access points to underneath. There may well be a JB there and it could be a quicker fix than running a new section in trunking. If there is a great big wardrobe in the way then maybe not though! Sounds like you found it pretty quickly at least mate.
Out of curiosity are there any sockets etc on the faulty leg? Are the IR tests all clear? Is it downstairs? Just thinking of possible moisture causing an issue with the last question.

Good call by 1Justin.

 

[Nickj]

No sockets on the leg, the faulty leg has an ir of >2000MOhm l-n, a little less l-e ~ 1800MOhm.

The resistance on the faulty leg:
L-l <1 ohm (forget the result)
n-n <1 ohm
E-e ~30 ohm.

I'm thinking it must be a junction box somewhere although why anyone would use one for such a short leg is beyond me!

Won't take long to put trunking up for a new leg, there is a lot of furniture there and using a tape measure to estimate where I'd need to look there is a wardrobe right where I'd need to lift

Must say, I can't wait to finish this job! Hopefully to a standard the assessor will be pleased with

 

[stitch]

Let us know how you get on.

 

[Pete999]

Went to look at the board change again today and redid all the tests before commencing with the change.

All tests ok except ring final circuit is quirky.

Continuity of ring final:
R1 0.4 ohms
Rn 0.4 ohms
R2 29.8 ohms !!!!

High resistance joint somewhere, taken all the sockets off and checked, couldn't find anything. no spurs in house and only 5 sockets down stairs and 3 sockets upstairs (small house).

Surely there isn't a junction box for such a small house????

I forgot to note the Zs but it was an acceptable value.

Should I go searching for the high resistance joint (lifting boards etc - the fun) or as it has an acceptable Zs is it ok to leave it? (assessor questions)

Thanks all

Just a point how many times have you measured r2 could be an iffy connection on your MFT's leads

 

[Nickj]

Hi Pete, thanks for the input.

I checked it at different points of the cable, used a wire brush to clear any surface tarnish. I couldn't believe the result, sadly it seems real.

 

[Pete999]

Hi Pete, thanks for the input.

I checked it at different points of the cable, used a wire brush to clear any surface tarnish. I couldn't believe the result, sadly it seems real.

Are the croc clips making contact properly?

 

[telectrix]

if the fault is between the last socket and the CU, as there are not many sockets on the circuit, you could diss. the faulty section (at CU and at socket) and convert to a radial on a 20A MCB. just a though to save cost.

 

[Midwest]

Good luck Nick with your assessment, and you've have been given some good advice by members, especially fault finding!

As regards the fixings for the CU, I take NDG's point, but I do not think that relates to the fixing of CU's more to that of cable supports in escape routes. reg 421.1.201 was all to do with (so I'm told), remove a source of fuel (plastic CU) from the source of a fire. So sealing holes (intumescent), fixings etc don't relate.

As regards using a plastic enclosure above a CU to extend cables, I was advised by Elecsa Tech support a while back (wrongly IMO) that such an enclosure would not comply with the reg, and could not be used in such circumstance. Now that we've had some time, to allow for the dust to settle as it were, I have asked Elecsa for some clarification on the subject. Thought you should perhaps know this before your assessment, just in case. Having said that, when I had my recent assessment, the Elecsa Inspector raised his eyebrows when I told him of the advise given.

Personally, I would use MF joint boxes, in ceiling void above CU, if the cables weren't long enough to terminate in new CU or Ideal in-line splice connectors inside the CU. I would only do that for one or two circuits. Anymore, and I would consider part rewire.