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OSG 7.3.2 cables in walls and partitions

In the above,item d states the cables can be protected by earthed steel conduit or,

Item e then states :mechanical protection sufficient to prevent penetration of nails,etc,

but no mention of earthing..

I have a situation with a short 200mm horizontal run outside the zones.

I could slide a lenght of earthed conduit over,but then the earth connection would be plastered

over and could not be inspected.

If I treat the conduit as mechanical protection,it would not need earthing?

Wondered what others would do.

                                                        Regards,Hz

  • Unless earthed, and suitable to act as a protective conductor, the mechanical protection needs to withstand penetration by nails and screws.


    These days, we usually advise this means "nail gun" and "tek screws" - so, you're really looking at very thick plate steel or similar to achieve that requirement.


    This advice is based on real cases of such fixings causing things like the metal frame of paroc and similar stud walls to become "live" - in one case, I understand a plumber was electrocuted as the puddle he was trying to fix the leak which caused it was livened up by the metal stud frame!

  • If I treat the conduit as mechanical protection,it would not need earthing?



    Correct - if the (sheathed) cable can't be pierced by a nail/screw then the (metal) mechanical protection isn't going to be made live - so doesn't need earthing.


    But as Graham points out, in these days of nail guns etc, using a purely mechanical means of protection is far from straight forward.


    Perhaps easier to use a cable type that has an in-built surrounding c.p.c. - e.g. BS 8436.


      - Andy.
  • Former Community Member
    0 Former Community Member
    Perhaps easier to use a cable type that has an in-built surrounding c.p.c. - e.g. BS 8436.


    Does that need qualifying as to the circuit-breaker type and rating Andy?


    Regards


    BOD
  • Former Community Member
    0 Former Community Member
    Try telling some electrical installers that the metal nail-on plate used by gas fitters isn't compliant!


    Regards


    BOD

  • gkenyon:

    This advice is based on real cases of such fixings causing things like the metal frame of paroc and similar stud walls to become "live" - in one case, I understand a plumber was electrocuted as the puddle he was trying to fix the leak which caused it was livened up by the metal stud frame!




    I heard that a cable was chafed and made the steelwork live, which wasn't a problem until it all got wet. So then there was a live puddle, which wasn't in itself a problem until the housewife completed the circuit by touching the leaking washing machine.


    But I think that this is a different situation. I reckon about 1/4" of steel plate would do the trick.

  • I remember a Napit talk at Harrogate where their man suggested a steel plate screwed over the main tails from the outside cabinet and up the wall to the cu,to avoid rcd protection of the tails.Hz.
  • Years back I was asked to lay some T & E on a trough on an old ceiling about 4" (100mm) thick. I insisted on a steel trough 4mm plate with screwd lid and earthed 6.0mm back to CU. The trough and lid wa a min of 1 1/2" (38mm) from joist tops and bottoms and  I was relatively happy with the safety of each (RCDd) circuit. Fingers crossed

  • Does that need qualifying as to the circuit-breaker type and rating Andy?



    Probably no more so than any reduced c.p.c. situation - the quoted k²S² for BS 8436 foil screen are usually quite favourable compared with the reduced c.p.c.s on T&E for example - and often SWA armours won't comply with table 54.7 so will need some calculation to show BS 7671 compliance.  The "rules of thumb" for compliance for different situations are different of course, but that all seems a lot of detail for a "you might want to consider something like this as an option" reply.


      - Andy.
  • For example, from Eland:

     

    Protective devices used for these cables shall be either Type B to BS EN 60898 or Type B RCBO to BS EN 61009-1. The protective devices shall have a maximum let through energy (l2t) of 42000A2s when used with 1.0mm2 or 1.5mm2 cable and 60000 A2s when used with 2.5mm2 or 4.0mm2 cable.



    Whereas the k²S² for a 1mm² c.p.c. (as in 1.0mm² and 1.5mm² T&E) would be just 13,225A²s and a a 1.5mm² c.p.c. (as in 2.5mm² and 4mm² T&E) would be just 29,756.25A²s.


      - Andy.

  • The conductors are surrounded by a bonded aluminium tape. I see that the cable also has a stranded C.P.C. Presumably it has been tested for occasions where a nail may just penetrate the aluminium covering part and touch the phase conductor as well. Or is the cable only compliant for burying out of zones if the C.P.C. is also touched by the invading nail?

    https://www.cef.co.uk/catalogue/products/3377923-2-5mm-2-core-shielded-lsoh-cable-white-100m-drum?&utm_source=google&utm_medium=cpc&utm_adgroup=&utm_campaign=&ppc_keyword=&gclid=EAIaIQobChMI4d7prvuR5wIVxLTtCh1IHgdkEAQYBCABEgIIy_D_BwE&gclsrc=aw.ds