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

 

[Wilko]

Hi Nick, A few thoughts in addition to NDGs good points.
Meter tails item 7.7 - reg 521.5.1 says (in last part) 'does not preclude the use of an additional protective conductor ... to enter enclosure individually'. To me, the expected method is for line, neutral and protective conductor to enter together. IMHO what you have is functionally safe due to current levels, but may well be questioned. So have a close read of 521.5.1 and be ready to discuss.
Also, I think you might be questioned about IR > 2meg results. It is expected that this be the actual result you measured. A result near 2meg would be widely viewed as needing further investigation. Just saying.
A couple of others to confirm :
Max demand is 35A ?
Metal partitions are ticked in 8.14 ?
Adequacy of working space is blank in 9.4 ?
Hope that helps

 

[Pat H]

IR is normally recorded as the reading your test kit shows not that its above the required resistance.

 

[Midwest]

Lets not have the RCBO/RCD must be tested at the CU debate again.

As regards your 'main earth', have a butchers at this thread if you got a spare half hour;

Advice on regulation 521.5.1 - http://www.electriciansforums.co.uk/threads/advice-on-regulation-521-5-1.103579/

Can honestly say, I'm still confused and I read the thread at the time. Think the OP had it right at #105, as in your case, you using this job as your assessment. So ring your scheme and ask, and whatever they advise, do (IMO)

 

[westward10]

I hope you are going to submit nice neat paperwork and that is your trial run. I believe if your paperwork is untidy this gives an immediate poor impression.

 

[Pat H]

As a slight deviation has anyone re-generated these forms to be easier to use?
The actual layout isn't great and many entry locations are just too small to enter details in a legible way.

 

[Midwest]

I hope you are going to submit nice neat paperwork and that is your trial run. I believe if your paperwork is untidy this gives an immediate poor impression.

Think that's his 'working copy', well I hope so. I use to do that, until I lost a page of test results. Now use a desk diary, harder to lose

 

[Midwest]

As a slight deviation has anyone re-generated these forms to be easier to use?
The actual layout isn't great and many entry locations are just too small to enter details in a legible way.

Are you in a scheme?

 

[Nickj]

Thanks for all the input. I recorded ir as >2 as that's what the electrician I used to work for did. I note your points and will re-record the actual measurements tomorrow (they were all greatly in excess of 2 - none were under 600).

I'll call elecsa first thing tomorrow morning and ask their view. If they want it coming through the back then I'll need to scramble in order to get it done tomorrow as assessment is Tuesday!

Those results are indeed just a working copy. I'll use an electronic system after assessment but that system isn't in place yet.

Nightmare scenario!

 

[Nickj]

To pick up on maximum demand question.

I've put 35A. I came up wih that figure by adding all the breakers together and taking 40% of their total as maximum demand.

I see maximum demand done in so many different ways and I see different opinions on it.

Some say add all breakers and put that result regardless of whether it's greater than main fuse (an approach I'm not keen on?)

Some will use osg and put that figure.

Some use the 40% rule.

I have 32a ring (no diversity can be applied)

32a cooker circuit (I think I worked it out at 27a using diversity)

16a heating (no diversity allowed if immersion - unsure if there is one, then unsure how to calc if there isn't one)

6a lights - 8 lights in house, allow 100w per light so 800w, 3.5a.

Adding all that is greater than 60a.

What would you guys put?

 

[Octopus]

Thanks for all the input. I recorded ir as >2 as that's what the electrician I used to work for did. I note your points and will re-record the actual measurements tomorrow (they were all greatly in excess of 2 - none were under 600).

I'll call elecsa first thing tomorrow morning and ask their view. If they want it coming through the back then I'll need to scramble in order to get it done tomorrow as assessment is Tuesday!

Those results are indeed just a working copy. I'll use an electronic system after assessment but that system isn't in place yet.

Nightmare scenario!

IMHO thats a terrible way to do it and train you!

I can only assume that he did a global IR to achieve such numbers but in reality its a very unhelpful way of doing it when doing a CU change and introducing RCD's.

I always test IR before using a global measure (as a check) but then as the new CU takes "shape" I connect ALL the CPC's to the CU and test all the circuits individually L-N, L-E and N-E - that way you will see the "real" situation in the install.

It would seem that many people do the global IR as L&N to earth, which is simply lazy - the only way to check the installation IR properly you need to isolate as many of the appliances as possible......

As an example I did a global IR recently and got really terrible L&N to E readings - so went round unplugging as much as possible and then found it was 1 circuit in particular - further searching revealed a RCD FCU in the back of a cupboard - so this was swapped for a std unit and the readings improved dramatically

 

[Nickj]

I also checked each individeally murdoch (as did the electrician I worked for) I just put the >2 as that's what I believed should be entered.

I take notice of what's been said here and will remeasure and record them tomorrow.

Depending on what elecsa say I may need to remove the board to bring the main earth through the rear so taking ir again wont be much more effort.

I do hope they are happy for main earth to come through separately as to get all that work done the day before assessment is cutting way too close!

 

[Pat H]

The reason we state the measurement we get is that you may well get a result that's well above 2 mohm but lower than your kits max reading. That value is important as later measurements may show a degradation in IR.
So we record the actual meter result and that's implicit its greater than the minimum reg requires.

 

[Nickj]

Midwest, I have read that thread and I note what you said in one post

"Thought I would invest in the latest copy of Guidance note 1, Selection & Erection. Curious, I looked up 'Ferromagnetic enclosures, cable entering; (521.5.1) All the conductors of a circuit should generally follow the same route. Live cables of the same circuit may cause overheating if they enter a ferromagnetic enclosure through different openings (page 97, 7.4)"

I don't own gn1, I'm slowly buying and reading them all however gn1 is not one I've bought yet.

Reading that further confuses me. all conductors "should" not "must" therefore surely not required? I understand that l and n could cause issues but I don't see a problem with main earth coming through a separate hole.

I do hope elecsa can give me a definitive answer tomorrow at 8am. I'm worried about finishing on time of I have to reroute it all now.

Stress levels through the roof right now.

 

[Pat H]

Unless under fault conditions no current flows in the CPC so eddy currents related to that are immaterial. So as long as the L & N are together then fine.

 

[westward10]

Wouldn't lose sleep over earthing conductor through a separate entry, Pat is quite correct. Put your IR as >500 as >2 indicates a problem.

 

[NDG Elecs]

Don't stress too much Nick! The assessors are usually fine n easygoing. I would bring the point up with him about the main earth and have some other tech questions at the ready to; they like that kind of stuff!

Get a couple of packets of taste the difference cookies and you will be fine....

 

[Nickj]

Pat, I agree and I saw no reason not to run the earth through a separate hole until I read on this forum about 521.5.1.

Paragraph one is met as they are all in same enclosure.

It's paragraph 2 that causes issues,

"Where such conductors enter a ferrous enclosure, they shall be arranged such that the conductors are only collectively surrounded by ferromagnetic material"

Paragraph 3 is irrelevant I think in my case as the tails aren't SWA.

 

[Nickj]

In your experiences, would something like this cause a fail or deferral? I really can't afford for another assessment both financially and more importantly time wise.

 

[Octopus]

I also checked each individually murdoch (as did the electrician I worked for) I just put the >2 as that's what I believed should be entered.

I take notice of what's been said here and will remeasure and record them tomorrow.

Depending on what elecsa say I may need to remove the board to bring the main earth through the rear so taking ir again wont be much more effort.

I do hope they are happy for main earth to come through separately as to get all that work done the day before assessment is cutting way too close!

Recording the individual circuit readings is the best way to go ..... because if you called back later for, say tripping, you can compare readings to see if any have changed.....

There is NO requirement to bring the main earth through the same aperture as the tails..... so don't waste your time making the call. There are special glands that allow you to bring the tails and earth together but that's a "nice to have" .

 

[Pat H]

Yes but that relates to conductors conducting currents and the issues with eddy currents. The CPC isn't normally conducting so irrelevant.
I'd raise it with the assessor and say you've considered the regs and the reasons (you can even add you've taken advice) and you are happy with your solution.

I'm not in a scheme yet. hoping for my niceic assessment in the next month or so but in no rush.