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Topic Title: Block and beam floor/ceiling
Topic Summary: Safe zones
Created On: 29 January 2013 08:03 PM
Status: Post and Reply
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 01 February 2013 03:05 PM
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Avatar for OMS.
OMS

Posts: 19759
Joined: 23 March 2004

Agreed Andy - flexi metal conduit would never have been an issue

The key problem as ever is the quality of manufacture of the cables in question though - there are some cables out there that met the old version of BS 8436 by means of what is effectively a tubular aluminium sheath - a bit like Alu MICC with an oversheath and an integral CPC. No dramas - they can carry kA fault levels and the breaker will be out before the sheath even thinks about getting warm.

Other types have a fag paper thin aluminium foil wrap, that in many cases barely overlaps - it'll pass a condutivity test from screen to CPC OK, and it'll just get through a nail penetration test at low fault currents (circa 170A) - as that current increases, the foil flashes back and that's it - if that nail ain't also touching the CPC then you might as well have used T&E.

Yet another example of the outfall derived from the unseemly haste to incorporate RCD protection by JPEL 64 - they didn't think it through the fisrt time, missed the opportunity to clarify at AMD 1 - and are now focusing on bloody car chargers - now that'll be fun given the ESQCR issues over caravans and the fact that many chargers will be appliances on a 13A plug top.

regards

OMS

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Failure is always an option
 01 February 2013 03:34 PM
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AJJewsbury

Posts: 11583
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Agreed Andy - flexi metal conduit would never have been an issue

Do you reckon it's an oversight the BS 7671 effectively prohibits its use for this purpose then?

(522.6.100(iii)/522.6.101(ii) demanding that the conduit satisfy the requirements of a c.p.c. while 543.2.3 prohibits using a flexible/pliable conduit as a c.p.c.)

- Andy.
 01 February 2013 04:13 PM
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Avatar for OMS.
OMS

Posts: 19759
Joined: 23 March 2004

I don't think it's an oversight Andy - last time I looked, BS EN 61386 doesn't have a test method for continuity of metallic flexible or pliable conduits. I'll have anther look later when I get back in touch with my Tech Indices portal.- Parts 22 and 23 I think cover it

So without a test method, you can understand the prohibition on it's use as a CPC.

I suspect it's in part all historical - I did one test a whole bunch of "kopex" and "adaptaflex" type conduits under quite high and quite low fault currents - circa 200A up to about 2kA just for the hell of it (well not quite but it wasn't rigerous testing in the sense of coming to a definite conclusion) and I'd have to say that regardless of how I strung them up and draped them about not one of them failed in any sense to survive those sort of currents (for up to 5 seconds anyway).

Personally speaking, I don't think it's a real issue but I guess JPEL 64 can only go on what is rigerous in terms of testing etc. It would be a real headache to convince several generations of electrical types that kopex was fine for a CPC - and I guess in your postulated application, we'd have a circuit CPC anyway inside it. So asuming proper glands at source end, at joints and at terminal points, then just how much length of conduit do we think may become electrically isolated from earth - not much I suspect, and zero if under mA DC testing for continuity anyway.

Still - what do I know

Regards

OMS

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Failure is always an option
 01 February 2013 04:34 PM
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AJJewsbury

Posts: 11583
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So without a test method, you can understand the prohibition on it's use as a CPC.

So is there (on paper at least) doubt in its ability to initiate ADS and so provide protection against electric shock in the case of insulation faults on the cores carried inside? - there must be a good reason why we need to earth it.

I guess I'm struggling to see the difference in required performance of an exposed-conductive-part and a c.p.c....

- Andy.
 01 February 2013 05:35 PM
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Avatar for OMS.
OMS

Posts: 19759
Joined: 23 March 2004

It's a good point Andy - in the classic scenario of the kopex to a motor, it's just possible that vibration would rub a hole through a core and make the kopex live - an internal CPC won't be much use.

And, as you say, it's that non conductive, then ADS isn't going to work - or if ADS does work, then effectively it's a CPC.

Wrong selection of cable for the external influence would be my guess at the answer to that scenario though

I can't access standards from this location, I'll have a look aver the wekend and see what comes up

A letter to IET maybe ? -

Regards

OMS

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Failure is always an option
 04 February 2013 01:30 PM
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Avatar for BigRed.
BigRed

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colin Hagget, can you grant PM, i've lost your number Re RSG
cheers
IET » Wiring and the regulations » Block and beam floor/ceiling

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