Technical requirement for Non-Ferrous gland plates
j...@preston.demon.co.uk
<ksa...@emirates.net.ae> writes
> I am a member of a quality control team for inspection of
> electrical installations including Main switch boards, Sub main
> switchboards, final distribution boards, final sub-circuits, etc.
> During one of the inspection sessions, it was discovered that the
> supplier has utilised GI sheet for cable gland plates, instead of
> Non Ferrous gland plates for the cables (as requested by the BS
> standards) . However, when raising this issue to the contractor, he
> claimed that there is no necessity or technical requirement
> for such plates. I wonder if any one may guide me where I would be
> able to get the technical justification for providing a Non-Ferrous
> gland plates for Multi core cables glanding in Main &
> Sub-main Switchboards.
> Thanks in advance for all the help,
> Regards,
> Kamel
>
>[ A MIME image / gif part was included here. ]
>
How are you qualified to be a member of a QC team if you don't know the
relevant standards?
--
james edwards
Andrew Gabriel
"Kamel Sadek" <ksa...@emirates.net.ae> writes:
>This is a multi-part message in MIME format.
Please switch off the MIME and HTML postings
>I am a member of a quality control team for inspection of electrical =
>installations including Main switch boards, Sub main switchboards, final =
>distribution boards, final sub-circuits, etc.=20
>During one of the inspection sessions, it was discovered that the =
>supplier has utilised GI sheet for cable gland plates, instead of Non =
>Ferrous gland plates for the cables (as requested by the BS standards) . =
>However, when raising this issue to the contractor, he claimed that =
>there is no necessity or technical requirement for such plates. I wonder =
>if any one may guide me where I would be able to get the technical =
>justification for providing a Non-Ferrous gland plates for Multi core =
>cables glanding in Main & Sub-main Switchboards.
I don't know what "GI" is, but I'll assume it's ferrous (galvanised iron?).
You will end up with a problem if the current through each hole in the
sheet is not balanced. For example, if you have a 3 phase supply going
through the sheet, it will be OK if all the conductors pass through the
same hole, but not alright if any of them pass through different holes in
the sheet. Basically, you have something which looks rather similar to a
current transformer with no secondary winding. This will add impedance to
the supply cable. Also the sheet (core) will not be laminated, so it
could heat up through eddy currents induced in it.
This is the same problem you get with armoured cables where the armour
is magnetic - current in them must be balanced. If it isn't, non-magnetic
armour must be used, together with non-magnetic gland plates, etc.
I don't know what BS standards say, but I've recently bought a final
distribution board, and the gland plates it came with are definantly
magnetic.
--
Andrew Gabriel
Consultant Software Engineer
Virginia
Regulation 521-02-01, Paragraph TWO.
Regards,
Virginia
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In article <8jnuq8$38...@news.emirates.net.ae>,
Kamel Sadek <ksa...@emirates.net.ae> wrote:
> I am a member of a quality control team for inspection of electrical
> installations including Main switch boards, Sub main switchboards, final
> distribution boards, final sub-circuits, etc. During one of the inspection
> sessions, it was discovered that the supplier has utilised GI sheet for
> cable gland plates, instead of Non Ferrous gland plates for the cables (as
> requested by the BS standards) . However, when raising this issue to the
> contractor, he claimed that there is no necessity or technical requirement
> for such plates. I wonder if any one may guide me where I would be able to
> get the technical justification for providing a Non-Ferrous gland plates
> for Multi core cables glanding in Main & Sub-main Switchboards. Thanks in
> advance for all the help, Regards, Kamel
The basic reason is to minimise eddy current losses. If you pass a large
alternating current through a hole in a magnetic iron plate the field
produced by the current will induce currents in the plate. Iron is not a very
good conductor, and the plate can get hot.
It is difficult to give precise guidelines, but you should be asking
questions for currents in single core cables greater than 600A. For three
core three phase cables it is not normally a problem because the currents sum
to zero and the fields cancel out.
This should be covered by the type testing of the switchgear, but often cable
boxes are not properly included.
John Rye
--
EurIng J Rye CEng FIEE Electrical Engineering Consultant
18 Wentworth Close Hadleigh IPSWICH IP7 5SA England
Tel No 01473 827126 http://www.ukonline.co.uk/jrye/index.html
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