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Topic Title: "Official" guidance on metal consumer units on TT
Topic Summary: Has any been published yet?...
Created On: 06 November 2017 05:52 PM
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 06 November 2017 05:52 PM
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paulskyrme

Posts: 1291
Joined: 12 February 2003

Hi peeps,

I am having a little debate with another about metal consumer units on TT installs.

There are a few areas of concern as I see them, all centred around the case of the consumer unit becoming live and inadequate fault current flowing to cause ADS.

The first scenario is if the incoming line conductor becomes detached from the incoming side of the main switch and contacts the enclosure.

The second is the single insulated flexible wiring on the line side within the enclosure shorting out on the body of the enclosure.

The third would be one of the main internal flexible Neutral links becoming detached from the N bar upstream of the RCD's, and contacting the body of the consumer unit.

The fourth would be chafing of the incoming line conductor against the body of the consumer unit and then this shorting to the housing.

These would cause the enclosure and thus all conductive metalwork in the installation to rise to mains potential with the fault current flowing being proportional to the Zs at the consumer unit, it would unlikely be an adequate fault current to cause ADS via the DNO fuse.

They are unlikely failure modes I admit, but they are there.

Is there any official guidance published as yet with regard to the use of glands, "tails clamps", 100mA S-type as main switch etc. please?

I have looked but, as yet found, not much!

I think that the 4th one can be resolved with suitable thought to how the tails enter the board, gland etc. plus taking the sheath right up to the incoming terminal provides additional mechanical protection to the wiring right up to the point of connection.

Fitting a "tails clamp" would tie both L & N tails together and hold them in place with regard to the body of the consumer unit, thus not allowing them to move out of position, which would address that potential failure mode.

The remaining issue is with the flexible links within the board itself, fitting a 100mA S-Type, or perhaps even 300 or 500 if available depending on the actual fault loop impedance at the consumer unit would address that issue.

Has anything actually been "officially" published as guidance though please?
There is a document from ESF that suggests glands and clamps, but that's it I could find, and it is not logo'd or anything as an ESF document.
Not necessarily by the IET, one of the schemes or something by Electrical Safety First would do with their logo on it.

I can't find anything concrete yet...
Still looking though.
If anyone knows of anything please let me know

Thanks,
 06 November 2017 05:59 PM
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dustydazzler

Posts: 1451
Joined: 19 January 2016

Manufacturers instruction manual
?
 06 November 2017 06:07 PM
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paulskyrme

Posts: 1291
Joined: 12 February 2003

Thanks dusty, been there but it's very "woolly"!
 06 November 2017 09:40 PM
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Alcomax

Posts: 248
Joined: 12 November 2009

Thanks dusty, been there but it's very "woolly"!


Of course it is "woolly" by design with a liberal amount of weasel words thrown in. It is distraction.

These are type A DBO's. However the "manufacturers instructions" do actually give guidance, though it is cryptic. After all, they cannot change basic electrical principles even with some pseudo "reinforced insulation creator" TM.

To quote from a typical "interface characteristics" provided by a well known brand.

" Types of system earthing for which the ASSEMBLY is designed : TNC-S, TN-S when installed in an electrical installation complying with BS7671"

So if you design and install to BS7671 it is pretty clear what you do, first and foremost respect ADS and use the ASSEMBLY for what it has been provided for.


edited to add : Sorry, tried my best not to reply to this post....will not post further...all been said and done.

Edited: 06 November 2017 at 11:05 PM by Alcomax
 07 November 2017 06:55 AM
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paulskyrme

Posts: 1291
Joined: 12 February 2003

Alcomax,
Thank you for posting.
I am quite happy with my interpretation of the requirements for safety with such things.

However, the other party is not and is adamant that this is just my interpretation and not that of the "industry", hence my looking for some pseudo official guidance.

Everything I throw at them with regard to this is thrown back as just my interpretation, or just my opinion.
That includes by the way, regulations from BS 7671, which I am being told that I am interpreting incorrectly.
Quite how Ohms law is wrong I don't know, but there we are.

Unfortunately the unit that has been fitted does not have such clear guidance as your quote in the sales literature, else I would use that.
The OEM sales literature only says recommends, thus, the other party is saying as it's only a recommendation, by the OEM, my interpretation of the regulations is wrong.

I'm currently in a no win situation, and the other party is intransigent because they don''t understand.

If I can locate a pseudo official document, then they have admitted that this may well change their mind, without it, I have no chance of them listening to reason and the requirements for ADS in the regs as they see they are right and I am wrong because at the moment, there is no fault present that can cause danger.
 07 November 2017 08:08 AM
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mapj1

Posts: 9577
Joined: 22 July 2004

it is a bit weak, but this article contains the golden word NICIEC

it has the usual errors claiming the source of magnetic heating as eddy current rather than what it actually is, i.e. magnetic hysteresis, and mentions the inadequate but spec compliant touch standard IPXXD.
But the section about fault currents on TT is technically correct and may help your case.

-------------------------
regards Mike


Edited: 07 November 2017 at 08:52 AM by mapj1
 08 November 2017 08:14 PM
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paulskyrme

Posts: 1291
Joined: 12 February 2003

Thanks Mike,

I'll see if I can get a printout that I can use as "evidence".
 08 November 2017 11:06 PM
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AJJewsbury

Posts: 16014
Joined: 13 August 2003

KFH says:
There will never be official guidance on metal consumer units on TT systems due to the high risk of issues if there is a problem due to the risks identified in the original post.
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