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Topic Title: Sizing SWA Flying Lead
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Created On: 08 July 2013 08:40 AM
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 08 July 2013 08:40 AM
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CPC

Posts: 32
Joined: 16 February 2011

Hi all.

Could any one confirm that I am sizing the flying lead correctly.

Have a 240mm2 XLPE/SWA which is glanded into a main switch panel but does nt have a flying lead from the gland to the earth bar.
I was thinking that because the steel amour has c.s.a of 289mm and has a copper equivalent of 289/8 = 36.13mm a 50mm could be used or should I be sizing using table 54.7 which S/2 = 240/2 = 120mm2.

Any help would be great.

Many thanks to all,
CPC.
 08 July 2013 08:46 AM
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perspicacious

Posts: 7199
Joined: 18 April 2006

If you've only got 36.13 mm2 coming in, is there any point in the last 1 m being any larger?

The question I'd ask is whether the SWA armouring is adequate.

Regards

BOD
 08 July 2013 09:20 AM
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CPC

Posts: 32
Joined: 16 February 2011

Hi Bod

According to my chart the amour of the cable is adequate to use as a cpc.

Regards CPC
 08 July 2013 10:20 AM
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zeeper

Posts: 1409
Joined: 11 July 2008

using 54.7 i've come up with 270 for the steel. so 289 is good.

The tail needs to be sized in accordance with 543.1.1
 08 July 2013 03:24 PM
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AJJewsbury

Posts: 11367
Joined: 13 August 2003

If you've only got 36.13 mm2 coming in

But you haven't got 36.13 mm2 coming in - it's 289 mm2 Steel - it's only equivalent to 36.13mm2 of copper in terms of resistance. The important thing if it's acting as a c.p.c. is it rate of temperature rise for a given fault current (which depends on the thermal capacity of the metal as well as its resistance) - i.e. the "k" values. If we were looking at 143 for Cu and 46 for steel, then the direct copper equivalent is about 93mm2.
- Andy.
 08 July 2013 03:42 PM
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Parsley

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I thought thermosetting xlpe 240mm2 4 core had a copper equivalent of 25.

Regards
 08 July 2013 04:49 PM
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OMS

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Joined: 23 March 2004

Forget copper equivalent conductance (unless it's also acting as bonding in a PME system and if it is, then I'd be worried at these sort of capacities) - all you need is the equivalent thermal capacity - ie K2/K1 - as Andy pointed out.

At that sort of size, I'd be astounded if there isn't enough metal between the gland plate and the earth bar by default any way. Bolting the bango to the gland plate is usually more than adequate without a fly lead. If you must have it then 50% of the selected (rather than calculated) thermally equivalent CPC size would do it

Regards

OMS

-------------------------
Failure is always an option
 08 July 2013 05:15 PM
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perspicacious

Posts: 7199
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"But you haven't got 36.13 mm2 coming in - it's 289 mm2 Steel - it's only equivalent to 36.13mm2 of copper in terms of resistance"

Andy, in my style I was trying to get the OP to reveal more about the install and to understand that as OMS pointed out, if PME, bonding where the SWA is 240 mm2, that suggests to me that the DNO is likely to have a copper equivalent cross-sectional area of the supply neutral conductor over 150 mm2 and then Table 54.8 calling for 50 mm2 will put the 240 mm2 SWA into a difficult situation and put the original question into the background whilst the issue of the SWA is resolved

As to gland plates, quite a few makes have just 4 or 6 self tapped screws holding the plate onto the main back frame of the DB and then the outgoing cables at the other plate rely on another 4 or 6 screws. If I install, I run a copper cable bolted to each plate to connect rather than rely on the self tappers

Regards

BOD
 09 July 2013 12:26 PM
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Parsley

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Joined: 04 November 2004

Below is GN8 recommendation on SWA terminations.

"It may be questionable whether the termination of steel or aluminium wire armouring with glands into metal gland plates, which themselves may only be bolted to the switchgear or controlgear frame, is adequate. Terminations made in this way will always leave some doubt about the effective current-carrying capacity, which needs to be of the order of several kiloamperes, through the metalwork joints, some of which may be electrically impaired by paint or other surface finishes. It is therefore always desirable, and often necessary, to employ 'gland tag washers' together with a copper protective conductor with cable lug to bridge the 'gap' between the armouring and the earthing terminal of the item of equipment. Figure 9.2 illustrates an example of a means of terminating an armoured cable. It shows the removable plate at the bottom of the enclosure, together with two retaining screws, through which all the earth fault current would pass were it not for the gland tag washer arrangement."


Regards
 09 July 2013 01:20 PM
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perspicacious

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Joined: 18 April 2006

My experience stems from 30 years ago seeing the consequences of an electrician using a screwdriver (non insulated blade) and hammer to punch out the knock out under a 30 A switch fuse that had been left wired ready for an additional service under a 400 A bus bar.

Apart from doing this when the bus bar was still live he didn't use a pair of "molegrips" on the blade to prevent "overshoot" nor did he put a block of wood to stop excess travel or the flying knock out. Needless to say, the blade overshot and out of the 50/50 chance of the blade touching the dead side of the "blade entry", it caught the live side and, according to site talk, the guy had to go home to change.....

I kept for a while the remains of the 30 A switchfuse whose 20 mm entry was now nearer 40 x 30 mm and the gland plate screws had circles of black marks where the current ( 4 kA) had made its way back to the MET. All the other cover fixing screws had similiar black marks and some of the male brass bushes also had splatters on them.

Nothing like seeing the results to modify your own behaviour and work practice

Regards

BOD
 09 July 2013 02:36 PM
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Parsley

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Nothing like seeing the results to modify your own behaviour and work practice
Regards
BOD


Yes it isn't much fun is it.

I stopped taking covers of old C50 DB's around 2006.

Regards
Statistics

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