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Topic Title: Over cautious with shower cable?
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Created On: 08 January 2015 08:39 AM
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 09 January 2015 04:45 PM
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chrisbradley

Posts: 79
Joined: 13 September 2005

Hi all,

From a moderation point of view, please could I ask all forum users to keep things civil. The conditions of forum use require that we're all nice and respectful to each other, so if deep breaths could be taken by all concerned, that would be much appreciated.

Many thanks,
Chris - IET Digital Services
 09 January 2015 06:52 PM
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Phillron

Posts: 1411
Joined: 18 January 2007

Originally posted by: chrisbradley

Hi all,



From a moderation point of view, please could I ask all forum users to keep things civil. The conditions of forum use require that we're all nice and respectful to each other, so if deep breaths could be taken by all concerned, that would be much appreciated.

Many thanks,

Chris - IET Digital Services


I am pleased to read that moderation is swift when it is needed
 10 January 2015 06:32 AM
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davezawadi

Posts: 3954
Joined: 26 June 2002

Excellent Chris, thank you.
I hope I was sufficiently polite, if a bit specific!

Regards

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 12 January 2015 11:24 AM
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Zimmerman

Posts: 96
Joined: 31 July 2009

ahoneahtwo

Stick with the regs mate. Ignore those who tell you otherwise as they will not be there if you are ever in front of a court of law. When carrying out electrical work, detail everything you think needs looking at and, if you think necessary, take pictures as a back-up. When involved in inspection and testing, again, detail everything etc. it's what your paid for. If there are cables in cavities, that needs to be noted as they should not be there (Unsupported, in contact with insulation that may be detrimental to the PVC etc.) If the wiring is in insulation and you have reason to believe that they should be de-rated make a note of this and detail that in any paperwork. Follow the written guidance and you'll be fine. If you change from a 40 to a 50A C.B. then take a look at the Zs even if there is an RCD.

Be suspicious of individuals, or companies, looking to promote themselves and/or advertise their business on a public forum. If they stand for safety they should not advice you to ignore respected and proven guidance.
 12 January 2015 01:31 PM
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davezawadi

Posts: 3954
Joined: 26 June 2002

I trust that comment was not aimed at me Zimmer.

If it was
Perhaps you would care to quote the relevant reg numbers that my questions are checking are correctly implemented.

You obviously need to read the RCD section again very carefully.

I am getting a bit annoyed by this stream of comment which is getting close to libel, and is certainly not in the spirit of this forum.

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 12 January 2015 02:20 PM
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Zimmerman

Posts: 96
Joined: 31 July 2009

Dear Dave
I have no idea of what you are implying sir. I can however assure you that I am fully aware of the current wiring regulations and have no need of your help however well intended. I do however object to your overbearing attitude to some of the posters ( including myself) on this forum. This has been mentioned to the appropriate people as you were so keen to lead the way on. I thank you for your guidance and leadership in that particular matter.

Libel :-) With that one word a smile was on my face David. Thank you for that.


This is to everyone other than the knowledgeable David.

IMHO (and just my take on it and it may be flawed) Regarding the use of RCD's in a domestic or similar TN installation it is important to know the Zs of the circuit. RCDs in this case are for additional protection and, by definition, the word 'additional' implies 'added' to the protection already in place. In other words, enhanced open-circuit times at 300mS and 40mS. But RCDs fail closed circuit. It is required that all TN circuits open at either .4S or for distribution5S. So if the RCD fails at least under L/E faults the circuit should open in the required time and thus hopefully avoiding a touch voltage exceeding than 50V and ensuring that the conductor temperature does nor exceed the appropriate values.

As we know things change for a TT.

I may be wrong here but as far as I can see I'm happy with what I've written.
 12 January 2015 02:45 PM
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AJJewsbury

Posts: 16090
Joined: 13 August 2003

. It is required that all TN circuits open at either .4S or for distribution5S. So if the RCD fails at least under L/E faults the circuit should open in the required time and thus hopefully avoiding a touch voltage exceeding than 50V

While reserving an RCD for additional protection only is an approach I prefer, I don't believe it is a BS 7671 requirement. Regulation 411.4.4 (ii) seems quite clear that an RCD may be selected for ADS fault protection in TN systems if desired. You still need overcurrent protection of course - both for L-N faults and (usually) overload, but that's not subject to anything like the same time constraints as ADS.

BTW nothing in the ADS scheme limits touch voltages to 50V during the existence of a fault to earth. A quick look at the resistances involved would show that the voltage at the fault is likely to be around half Uo (e.g. 115V on a 230V system), or even higher where reduced c.s.a. c.p.c.s are used. Main bonding typically makes only a modest reduction in the touch voltage within the installation on TN systems as usually Ze is small compared with R2. BS 7671 only refers to a 50V limit for bonding for supplementary bonding, not main bonding. A close look at the formula will show that even then it only covers the case where the fault current is so low that the ADS protective device may not open within the required time. For larger currents, the 50V "limit" will likely be exceeded, although accompanied with a reduction in disconnection time.

- Andy.
 12 January 2015 02:46 PM
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Parsley

Posts: 1353
Joined: 04 November 2004

The RCD or MCB doesn't limit the value of the touch voltage, only the time it's there. The R2 limits the voltage.
If the RCD does fail in this example and there's no supplementary bonding present what would the Ut be? Would it be under 50V even if the Zs value was below the BS7671 values in table 41,,,?

Regards
 12 January 2015 02:56 PM
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Zimmerman

Posts: 96
Joined: 31 July 2009

That's what I get for typing off-the-cuff without engaging brain. I wish I hadn't have added the 50V now.. but too late, the deed is done.
 12 January 2015 06:36 PM
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davezawadi

Posts: 3954
Joined: 26 June 2002

RCDs may be used for ADS as Andy says. It is common and the only way to make a TT installation.

I make no further comment because the point is made.

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
IET » Wiring and the regulations » Over cautious with shower cable?

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