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Topic Title: Lead Sheath Earthing
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Created On: 20 November 2004 10:14 PM
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 20 November 2004 10:14 PM
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roseelec

Posts: 55
Joined: 18 November 2004

Does anyone know how to plumb a wire on to the lead sheath of a service cable where the supplier has not done this, and the Ze is good enough not to have to make it a TT system.

You cannot use a tenby clip as this compresses the cable and the lead creeps.

There is supposed to be a sprung clip available especially for lead sheath cables but I can't find these anywhere.

A copper strap is the solution I have been using up to now but have recently found that the lead can still creep under the clamping pressure, albeit after 2 years, but it still happened.

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 21 November 2004 09:49 AM
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deleted_monty123

Posts: 301
Joined: 15 October 2003

Years ago we used to use Moleskins, Tallow and Plumbers Solder for joints in cables. However, this seems like a sledgehammer to crack walnuts especially as B.S.7430:1998 Code of Practice for Earthing - 21.6.2 Earthing Conductors, also allows B.S.951 clamps to be used. I assume that you have fully assessed the usual risks associated with this method, i.e. mechanical strain, corrosion and the solder yielding when passing a sustained fault current, etc.?

Regards,

Dick


Edited: 21 November 2004 at 09:51 AM by deleted_monty123
 21 November 2004 02:06 PM
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roseelec

Posts: 55
Joined: 18 November 2004


But does B.S.7430:1998 Code of Practice for Earthing - 21.6.2 Earthing Conductors, allow B.S.951 clamps to be used on lead sheath. The B.S.951 standard does not allow for use on armoured without the use of an inner ring.

Quote

I assume that you have fully assessed the usual risks associated with this method, i.e. mechanical strain, corrosion and the solder yielding when passing a sustained fault current, etc.?


Pumbling has been the method used on lead sheath by supply authorities in some installations anyway, although the size of the conductor used is about 4mm sq and no longer conforms, but the conductor is then connected to a suppliers earth terminal, the comsumer side can then be connected to it. (although it does not make sense to connect 16mm to the terminal that is then connected to the sheath by 4mm - but it does allow an individual circuits fault current for up to 10mm sq T&E).

The Solder yielding when passing fault current is a very interesting point but the Ze would not have changed significantly since the original 4mm was plumbed onto the sheath so I assume this must be ok - unless the supply authorities ignored this point at the time.

What do you think.



Moleskins, Tallow and Plumbers Solder for joints in cables. How do you go about doing this
for plumbing a condutor to the sheath?
 22 November 2004 10:09 AM
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khales

Posts: 1271
Joined: 16 January 2002

See also this thread on the same topic:
TN-S earth Clamp

AIUI: BS951 and NICEIC both prohibit BS951 clamps from use on lead sheath (thread above). However, I am told that RECs are not bound to apply the IEE Wiring Regulations to their supply, and are therefore able to decide for themselves to use clamps, BS951 or other type, on their lead sheaths.

FWIW I have an REC-fitted BS951 clamp on the lead sheath of my domestic supply; but it is an old-style 2-part BS-951 clamp - I don't know if that is mechanically better for the lead sheath. Other's (thread above) have reported REC retro-fitting modern BS951 clamps to lead sheath. I also telephoned my REC engineering dept. and they said that (from their point of view) it was perfectly acceptable, indeed usual, to fit clamps to lead sheath. Remember: just because BS951 says the clamps are not to be fitted to lead sheath just means that doing so won't comply with BS951 (which is written for a different purpose). There is nothing to stop an Engineer (e.g. in an REC) deciding that a Tenby-type clamp, albeit designed for another purpose, is nevertheless a suitable type of clamp for lead sheath. So, if the REC fits a BS951 clamp it seems that's OK, it's just using a BS951 clamp for a different purpose, and is therefore outside BS951; but an electrician under NICEIC jurisdiction may not do the same.

Further info: Pirelli make clamps for lead sheath, rather like a Jubilee clip; but not in small sizes (see my link to Pirelli catalogue in thread above). The other alternative would be a cable-gland purpose designed for lead-sheathed cable; e.g.
Hawke Cable Gland; but these are not suitable for retro-fitting.

In conclusion: my understanding at present is that if the REC provides an earth connection by clamp to lead sheath, then one is entitled to use it in accordance with the supply & earth
characteristics they have given; but if they don't, then one can't necessarily fit one's own connection to their lead sheath. I'm also told that if an REC is asked to come and fit an earth terminal where none exists, they are nowadays likely to decline, and you then have to provide your own, independent, earth (TT-system). Such is progress!


-------------------------
khales
 22 November 2004 10:18 AM
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khales

Posts: 1271
Joined: 16 January 2002

Further to this: the Prysmian Cable Joints and Terminations Catalogue: on p.53 describes: MECHANICAL EARTH BOND KITS FOR LEAD SHEATHED CABLES; but for conductor sizes >=35mm². So, if the supply cable has conductors>=35mm², then problem solved @ £8.96/clamp!

-------------------------
khales

Edited: 07 February 2007 at 10:23 AM by khales
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