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

 

[Risteard]

(associated switch gear covers this possibly?)

Given that an adaptable box and connectors aren't switchgear of any sort they certainly couldn't be a similar switchgear assembly.

 

[Pat H]

Metal not plastic CU I hope.

 

[Nickj]

Hi Pat, Yes it is indeed a metal consumer unit. Is there a need to seal the rear cable entry or is it ok not sealed?

If I need to extend the cabling does the adaptable box need to be metal too?

Can you see any other flaws in my plan of action?

Really nervous about assessment

 

[Pat H]

If the CU is flush with the wall then it should be fine. Regs state the penetration for top sides etc but not back.
If you need a box above then thats not a CCU so not metal but if you have an entry from that to the CCU then a fire in the CCU would spread up and out so I'd look at a seal between them or use a metal box.

 

[Nickj]

Thank you very much, I am sitting here questioning everything, including myself, I hate assessments

 

[westward10]

I personally would extend conductors within the CU to save another enclosure.

 

[7029 dave]

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

 

[Wilko]

Hi Nick,
Honestly I hadn't thought about plastic wall plugs failing in a fire. I'm hoping as they are covered by a non combustible shield (the CU) they will be fine. I am interested to hear other views ...

A few thoughts/reminders, hopefully helpful

All Labels attached
Periodic test (I tend to put 5 yrs), RCD test, circuit ID (ID the fire alarm), Main Switch, Circuit Details, the 2 versions wiring colour warning (if needed) ... could there be more?

Tails
Good neat job, using tails gland (doesn't have to be metal, I reckon) using internal clamp to secure them (if provided) and keep all insulation on right up to iso switch.

Testing
Allow time for testing the installation. The more thorough and complete you can be with your testing and the certificate the more confident you will feel during interview. And the less they can ping you.

Cheers.

 

[westward10]

Plastic wall plugs into believe are fine as the brickwork will act as a heatsink, well that is what I heard.
Me being fussy use 2.0 bore green/sleeving cos it looks much neater and fits all T&E up to 6.0.
Good luck with it all, don't doubt yourself they will not be trying to catch you out.

 

[Nickj]

If I extend within the consumer unit am I ok to use wagos where possible and then terminal blocks for the cooker cable? (wago don't go high enough unless I use their din mounting stuff).

Tails will be coming in through rear of CU as well (as they currently do). Looking at the picture of CU it has one large knockout at the back.

De-rating aside (I assume I'm fine not to consider de-rating as entry is same as current board?) it's ok for all cables, including tails to come through the one provided rear knock out without the use of glands etc? (I'll use a grommet strip to ensure cables can't be da,he'd from metal)

I have all the labels ready to go and I'm not too worried about the testing side of the assessment as so far not struggled with testing. It's the smaller regs stuff that I'm afraid I've missed.

 

[Pat H]

You can't assume the current grouping and derating is compliant.
I'd do some calcs myself. If unsure detate the breakers.

 

[westward10]

Not a lover of Wagos on a board change, crimps much neater but ensure the green/yellow sleeving in entered into the crimp.

 

[Nickj]

Pat, I was under the impression that as long as the circuits test safe they can be re-energised in a new consumer unit. I was under the impression that I needed to make sure that my work (the replacement of CU complied) and that I didn't need to ensure ratings were correct for the pre-existing circuits.

I didn't install the circuits and so it's difficult for me to ascertain how they've been installed...

I'm happy to be proven wrong here however and would like to see what others have to say?

 

[Nickj]

Westward how do you go about crimping solid core? I love crimping but have stopped crimping solid core due to it not being as strong a connection as I once thought :S

 

[Pat H]

I believe thats true. I just prefer to do some rough calcs. Normally possible to get a rough idea of runs and lengths. Zs of cables will give an indication as well.
If that looks ok then fine. I wouldn't sweat too much on the short local group into the CCU

 

[westward10]

Have you previously tested the installation.

 

[HandySparks]

... and then terminal blocks for the cooker cable? (wago don't go high enough unless I use their din mounting stuff).

Wago 773-173 Push Wire Connector 2.5mm - 6.0mm 3 Pole - https://www.tlc-direct.co.uk/Products/WA173.html

De-rating aside (I assume I'm fine not to consider de-rating as entry is same as current board?)

With de-rating due to grouping on domestic, I wouldn't disregard completely. However, generally the typical loading on circuits will be well within the capacity of the cable. Heavy loading, eg showers, will only be of short duration. Two exceptions might be if the place is lit entirely by dozens of 50W GU10s or for storage heaters and immersion heaters, which are pulling full load for an extended period of time.

Another indication is whether the cables show signs of overheating at the grouped location.

It's better to show that you've considered the situation and deem it acceptable, rather than ignored the issue completely.

 

[Pat H]

I'd agree. Do the maths do the visual inspection. Be happy you can show you've considered those elements.

 

[westward10]

Westward how do you go about crimping solid core? I love crimping but have stopped crimping solid core due to it not being as strong a connection as I once thought :S

I use crimps and Wagos, stick with what you are happy with. if you use Wagos tuck them behind the breakers to keep it tidy.

 

[Nickj]

I have previously tested it and all appears fine ~(so hoping for a straight forward swap - fingers crossed!) no signs of overheating currently and they are grouped together. I have not inspected attic as its cram packed full of items and I've been asked to avoid going up there if possible.

I'm a little wary of doing the maths in case it doesn't comply, the likelihood of there being insulation in the attic is high and once that and grouping de-rating are taken into consideration one may find it doesn't comply...what would you guys do then?

I'll tuck the wagos behind the breakers and make it as tidy as possible.

thanks to all so far, It's greatto be able to bounce ideas around, you're saving me from going crazy

 

[Pat H]

They won't inspect the attic. Say you checked what you could access and cables are clipped to joists so you are ok.
They probably are anyway.

 

[Nickj]

To pick up on what wilko was saying earlier regarding tails in a gland and internal clamp (if provided). Its fine if they are coming in with rest of cables to not use a gland? I'll secure cables with a clamp if it's provided (yet to pick it up so not sure whats in the box)

 

[westward10]

Don't worry about grouping or clamping, it is all rear entry. Undertake a neat job and you are halfway there. They will not ask you difficult questions as I said before they are not going there to catch you out.

 

[Nickj]

thank you very much! I'm generally neat and tidy with the installs and the testing should be spot on, hopefully i'll get through it

 

[HandySparks]

To pick up on what wilko was saying earlier regarding tails in a gland and internal clamp (if provided). Its fine if they are coming in with rest of cables to not use a gland? I'll secure cables with a clamp if it's provided (yet to pick it up so not sure whats in the box)

The question is: are the cables, and therefore the terminations liable to be disturbed? If not, no need for clamping.

 

[NDG Elecs]

Plastic wall plugs into believe are fine as the brickwork will act as a heatsink, well that is what I heard.

The link below makes interesting reading (if you have a spare 10 mins). There is a download at the bottom of the link.

Fire performance of cable supports - IET Electrical - http://electrical.------.org/wiring-matters/58/bre-report/index.cfm

 

[westward10]

thank you very much! I'm generally neat and tidy with the installs and the testing should be spot on, hopefully i'll get through it

Well good luck keep us posted.

 

[westward10]

The link below makes interesting reading (if you have a spare 10 mins). There is a download at the bottom of the link.

Fire performance of cable supports - IET Electrical - http://electrical.------.org/wiring-matters/58/bre-report/index.cfm

Interesting.

 

[Wilko]

The link below makes interesting reading (if you have a spare 10 mins). There is a download at the bottom of the link.

Fire performance of cable supports - IET Electrical - http://electrical.------.org/wiring-matters/58/bre-report/index.cfm

Thanks for this ref, and thanks also @Nickj - Wall-dogs it is !

 

[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.

 

[tony mc]

I know faults can be a real pain to find especially when you have been looking for some time!
You say the fault is the last leg or first leg to a S/O from the CU and a CPC fault.
I would be inclined to join L/N at one end and take a reading just to see if the maths matches up with the distance to the CU just in case its a way out reading and the cables run goes elswhere.
Just my opinion as I have come across strange cables runs with no logic in the past!

Good luck with your assessment!

 

[westward10]

I love fault finding, it is the bit before I find the fault which is an ---.

 

[Nickj]

Ok so decided to lift floorboards to satisfy my curiosity. Cable went to an unused and totally forgotten about socket in a cupboard in the hall!

There was a loose connection in that. tightened up and I'm now getting 0.95- 1 ohm.

One would expect it to be 0.7 ohms or so.

My question is, would you continue fault finding to find where the last 0.3 ish ohms are coming from or would you (and my assesssor) agree that as it's close to expected value it's fine?

 

[Octopus]

I would remove the "last" socket and test the Zs on each leg of the ring to see what the readings are like - then consider the options......

 

[Midwest]

The purists will argue you should. But you will have to round to each connection, undo & re-terminate. Your first assessment, your choice. Of course you could argue what you have done thus far has improved the installation, and you determination to locate the fault, might impress your inspector. Perhaps try the former, but you will need to draw the line somewhere, think your Zs' was good?