Metallic containment

[Martin]

Gents

You'll be pleased to know this is not about Amd3 CUs, but I would value your opinions. I was called out to a failure at the weekend, a 16A Ceeform socket was dead. The location is a substantial wooden framed and clad building, in which is an incoming SWA that feeds into an isolator that feeds 25mm tails into a Hager CU. This only has two circuits that feed the lights and some socket outlets. I think this was done about 25 years ago. Subsequently two 16A Ceeform sockets were added to the exterior of the building. The installer, electrician is not the word here, fed them from individually housed RCDs', which were then supplied from the board. Now you, I, or just about any half decent spark would have added some extra mcbs and supplied the socket from them. Not this chap, the two 2.5 T&E were connected to the incoming tails in the Hager incomer, so no overcurrent protection, but not only that, the circuits CPCs were not connected. Words did and do fail me. Over time the phase connection has become slack and has seriously overheated, to the extent of melting the connection to the socket, hence the callout.

The building is going to be internally remodelled and I have been asked to quote for remedial works. All of the existing installation is scrap and I am in effect starting from scratch. The new additions to 422.4 imply, or can be read to imply, that in a building of combustible constructional materials conduit and trunking should be used. Do you agree with my reading of the regs. On a pragmatic level would you use conduit etc, strongly suggest that this is the correct way, and if neither what would you contemplate?

Thanks for ploughing through this, your thoughts are appreciated.

Martin

 

[Rob_the_rich]

iirc t&e passes bs en 60332-1-2, you might want to check with your supplier. So clipped direct should be fine.

If you do use conduit or trunking, it must comply with 422.4.204.

is it going to be domestic?

 

[Martin]

Hi Rob

No it's not domestic, but it is a listed building that has public access. I think that in normal operation clipped direct may well be satisfactory, although visually horrible. My concern is more containment under fault conditions which may go some way to minimise damage if overheating did occur. The National Trust house at Clandon had a horrendous fire that virtually destroyed it. I may be going OT here of course.

Regards

Martin

 

[binky]

I would have though normal PVc conduit or that D line trunking which comes in wood colours would be perfectly adequate. Millions of sheds, clipped direct  haven't burn't down.

 

[Rob_the_rich]

I wouldn't have clipped direct with public access either. Although fine in other areas. Seems like you need to get approval from others of the conduit or trunking system you intend to install, some are visually horrible too.

RCDs would help with the fire risk, and a metal DB

 

[Martin]

Thanks for the thoughts guys. Not certain which way we'll go on this yet.

Regards

Martin

 

[kerching]

This is one of the very few cases where you let the customer decide....give them the options, get their decision in writing

jobs a good'un

 

[Andy™]

 The National Trust house at Clandon had a horrendous fire that virtually destroyed it.

and the fire started in a LFB approved board...

 

[kerching]

and the fire started in a LFB approved board...

Some people , of should I say 'a more cynical nature', could find an element of humour in that....... :slap

 

[Tony S]

http://www.surreycc.gov.uk/__data/assets/pdf_file/0011/73982/Fire-investigation-report-Clandon-Park-.pdf

 

[apprentice87]

Oooerrr,,,

The cause of the fire was; 

"6.3.

The forensic examination of the distribution board shows evidence that there

has been resistive heating on the connection between the two lower left hand

side neutral bars that serve fuses numbered one to thirty six

.

6.4.

The affected connection was found to be considerably tighter than others

on the same component and in comparison with those on the right hand neutral

bar which serve fuses thirty seven to seventy two. This over tightening has

caused a deformation of the wiring and over time caused the connection to

loosen hence increasing the resistance of the wiring and subsequently

the generation of considerable heat. This heating process has degraded the

connection, including its insulation, continuously until full combustion (in the

form of open flame) has occurred"

Sooooo, everyone thinking that fires are caused by people failing to tighten terminations sufficiently could be wrong?? Tightening them what i would call "securely" might also be wrong....

john...

 

[ProDave]

.....The new additions to 422.4 imply, or can be read to imply, that in a building of combustible constructional materials conduit and trunking should be used. Do you agree with my reading of the regs. On a pragmatic level would you use conduit etc, strongly suggest that this is the correct way, and if neither what would you contemplate?

Thanks for ploughing through this, your thoughts are appreciated.

Martin

On that basis all timber framed houses should be wired in conduit. I have yet to see one that has been so wired.

 

[Sharpend]

http://www.surreycc.gov.uk/__data/assets/pdf_file/0011/73982/Fire-investigation-report-Clandon-Park-.pdf

Interestingly it put the blame on the manufacturer, suggesting that it had corn from the manufacturer like it, so can we now insist that the manufacturers correctly torque their part of the board please! 

Oh no, sorry I forgot, what they have down is put the onus on us to check their terminals are correct. 

Carry on lads, we're to blame. 

 

[Rob_the_rich]

I notice it says "connection", just one screw in the terminal I assume.

 

[Rob_the_rich]

I dont understand how they could possibly assertion that it was overtightened ? and then loose which caused it to overheat ?

Yes loose caused it to overheat but how do they know it was overtightened in the first place ?

Most wiring is deformed when the screw is tightened regardless of being tightened correctly or overtightened.

I have only seen poorly connected connections getting hot, never an overtightened one, bear in mind this was tightened by hand and not an impact driver......

possibly because the screw head was measurably nearer the block than the others, causing the cable not only to deform, but to squidge out the sides? So they measured the tightening but observed the looseness.

 

[apprentice87]

There is always the possibility they are not as smart as they think and are talking total Radishes..

You can tighten it all you like, you will not reduce the CSA of the conductor, merely deform it so MORE of the surface of the conductor is in contact with the terminal.. A good thing right??? The copper the conductor is made from will also work harden due to the "cold work" [being squashed] and this would reduce any further deformation as a result od expansion and contraction caused by repeated heating and cooling cycles?? Would it not???

john..

 

[Sharpend]

Anyone for an amendment???

 

[apprentice87]

Hi Canoe, Looks like we were thinking along the same lines at the same time!!!!

I agree asbestos it is!!

john..

 

[Essex1]

I don't know about anyone else but I was always taught over tightening is a bad thing. 

 

[ProDave]

They should have put in their report the findings that a metal CU does not contain a fire and they should now be looking at Asbestos CU's........

No they should have put in their report that we should have two screws on the incomer and neutral bar. Twice the contact area, half the chance of a screw being loose.

But since when has tackling the CAUSE been anyones concern? All they seem to want is to contain the fire, not prevent it, and that has proved to be ineffective.