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OSG 7.3.2 cables in walls and partitions

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OSG 7.3.2 cables in walls and partitions

Posted by hertzal123 on Jan 19, 2020 2:24 pm

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
 

Re: OSG 7.3.2 cables in walls and partitions

Posted by gkenyon on Jan 19, 2020 3:03 pm

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!
Graham Kenyon, Managing Director, G Kenyon Technology Ltd www.gkenyontech.com G Kenyon Technology Ltd Logo © G Kenyon Technology Ltd 2015-2019

Re: OSG 7.3.2 cables in walls and partitions

Posted by AJJewsbury on Jan 19, 2020 3:17 pm

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.

Re: OSG 7.3.2 cables in walls and partitions

Posted by perspicacious on Jan 19, 2020 8:52 pm

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

Re: OSG 7.3.2 cables in walls and partitions

Posted by perspicacious on Jan 19, 2020 8:54 pm

Try telling some electrical installers that the metal nail-on plate used by gas fitters isn't compliant!

Regards

BOD

Re: OSG 7.3.2 cables in walls and partitions

Posted by Chris Pearson on Jan 19, 2020 9:02 pm

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.

Re: OSG 7.3.2 cables in walls and partitions

Posted by hertzal123 on Jan 19, 2020 9:11 pm

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.

Re: OSG 7.3.2 cables in walls and partitionse

Posted by ebee on Jan 19, 2020 9:17 pm

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

Re: OSG 7.3.2 cables in walls and partitions

Posted by AJJewsbury on Jan 19, 2020 11:04 pm

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.

Re: OSG 7.3.2 cables in walls and partitions

Posted by AJJewsbury on Jan 20, 2020 10:20 am

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.
 

Re: OSG 7.3.2 cables in walls and partitions

Posted by Zoomup on Jan 20, 2020 10:33 am

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

 

Re: OSG 7.3.2 cables in walls and partitions

Posted by AJJewsbury on Jan 20, 2020 10:43 am

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?


I believe the BS 8436 test involves a 1.6mm dia nail connecting just the foil to a line conductor - e.g. https://www.fpcables.co.uk/6_NAIL_PEN.pdf

Presumably the situation is helped by the foil being in contact with the entire circumference of the nail and then (a bit like the soil surrounding an earth electrode) the effective c.s.a. of the foil gets significantly larger quite quickly as the current moves away from the nail.

   - Andy.
 

Re: OSG 7.3.2 cables in walls and partitions

Posted by Zoomup on Jan 20, 2020 4:27 pm

An enquiry about B.S. 8436 cable in Screwfix today resulted in blank looks by the staff as did an enquiry about 100 Amp. B.S. 88-3 cut out fuses.

Z.

Re: OSG 7.3.2 cables in walls and partitions

Posted by AJJewsbury on Jan 20, 2020 4:37 pm

An enquiry about B.S. 8436 cable in Screwfix today resulted in blank looks

Must have been new staff - they used to stock it a few years ago.

  - Andy.

Re: OSG 7.3.2 cables in walls and partitions

Posted by hertzal123 on Jan 20, 2020 5:32 pm

At that price they prob only sell a drum every 5 years.Hz😭

Re: OSG 7.3.2 cables in walls and partitions

Posted by Zoomup on Jan 20, 2020 6:07 pm


Flexishield logo
Developed for use in partitions and building voids

Flexishield cable

Flexishield has been specifically developed for applications where cables are required to be concealed within partitions and building voids, at a depth of less than 50mm.

Flexishield’s main feature is it’s ability to fail safe when used as part of an electrical circuit that is penetrated by a sharp metallic object. The cable has now been designed to fail safe at 200 amps under nail penetration conditions, as such it will operate an MCB of up to 40A Type B or up to 20A Type C this surpassing the requirements of BS8436.  A fully compliant cable, Flexishield is a cost effective solution to meet the 18th Edition of The Wiring Regulations BS 7671:2018 for concealed cables at a depth of less than 50mm, Regulations 522.6.201 to 522.6.204.

Flexishield is a protected fixed wiring cable, it uses a bonded aluminium tape which gives excellent mechanical resistance and acts as an effective screen to help reduce electrical interference. Flexishield has been independently certified to BS 8436 (300/500v) and IS 273 (600/1000v) by BASEC.

The Range

Flexishield as standard comes in the range of 1.5 to 4.0mm2 to BS 8436 300/500v and the 6.0mm2 to IS 273 600/1000v. All available with stranded conductors of 2, 3 and 4 cores and a CPC of equal cross sectional area. Standard sheath colour is white, core colours as per the harmonised wiring codes and supplied on 100m drums. Options by request are available with sheath and core colours, drum lengths and 600/1000v ratings on the 1.5 to 4.0mm2 sizes.

So do we just connect the C.P.C. to an earth terminal as usual? Or do we have to gland the cylindrical foil in any special way?

Z.

Re: OSG 7.3.2 cables in walls and partitions

Posted by Zoomup on Jan 20, 2020 6:13 pm

hertzal123:
At that price they prob only sell a drum every 5 years.Hz😭

I suppose that if it helps compliance and reduces labour time it is a good investment.

Z.

Re: OSG 7.3.2 cables in walls and partitions

Posted by AJJewsbury on Jan 20, 2020 6:20 pm

So do we just connect the C.P.C. to an earth terminal as usual? Or do we have to gland the cylindrical foil in any special way?

Just sleeve and terminate the c.p.c. - as you would for T&E. No glands required (although normal round glands can be useful where an improved IP rating is needed). The foil is in contact with the c.p.c. all along the cable, so connection to the foil is automatic.
  - Andy.

Re: OSG 7.3.2 cables in walls and partitions

Posted by OMS on Jan 20, 2020 7:05 pm

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

 

Should be good for a standard NATO round  - need a bit more for 7.62 x 51 AP 😉

OMS
 
The trap we've fallen into is to believe that a thousand incompetents properly organized can do the job of a few dozen outstanding people

Re: OSG 7.3.2 cables in walls and partitions

Posted by perspicacious on Jan 20, 2020 7:37 pm

I raised the protective device Andy because I had a distant memory of this cable foil only being required by the BS to pass 160 A, hence limiting it to B32 or C16 circuit-breakers.

From searching the old forum produces this:
 
24 April 2015 08:50 AM
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AJJewsbury

Posts: 17795
Joined: 13 August 2003
 
If the cable was being used for its screening purposes rather than its ADS foil screen nail penetration charateristics, could it be used with type C devices, as the nail test is irrelevant in this case?
It would be no worse than FP200 type cables that can be used with 'C's, and whose screen is inferior to the 8436 standard??
Makes sense to me.

BS8436 is limited to type B devices
That always felt like a bit of an oversimplification to me - if a B32 is OK, then why not a C16 for instance? - the maximum permitted energy let-through is the same. Or even a fuse if the conditions are right.
- Andy.


Who now doubts the necessity of retaining this archive of information?

Regards

BOD

Re: OSG 7.3.2 cables in walls and partitions

Posted by perspicacious on Jan 20, 2020 7:38 pm

Well apart from the copy and paste function selectively vertically displaying the post and date info!

Regards

BOD

Re: OSG 7.3.2 cables in walls and partitions

Posted by gkenyon on Jan 21, 2020 7:32 am

AJJewsbury:
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.
 

 

Definitely ... scarily, these are far below the maximum energy let-through stated in BS EN 60898 / BS EN 61009 for 6 kA breakers .... leading to a minimum csa of 2.5 mm2 ! (see Table 8.4 of the IET's Electrical Installation Design Guide).

Does this mean you can't use 6 kA fault level mcb's to protect most circuits in dwellings??

No ... manufacturers' quoted values need to be used, as these are much less ... see Table 8.5 in EIDG. But it does mean you shouldn't simply assume that any and every 6 kA fault-level breaker to EN 60898, EN 61009 or EN 60947 will be suitable for use with a particular cpc, particularly the 1.0 mm2 and 1.5 mm2 reduced cpc's in T&E.
Graham Kenyon, Managing Director, G Kenyon Technology Ltd www.gkenyontech.com G Kenyon Technology Ltd Logo © G Kenyon Technology Ltd 2015-2019

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