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Topic Title: Equipotential Bonding Query
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Created On: 14 June 2018 08:29 AM
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 14 June 2018 08:29 AM
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Spen008

Posts: 16
Joined: 09 July 2013

Hi,

I'm getting confused with equipotential bonding requirements if anyone can help me.

The scenario is that within a substation I have a wall mounted metal equipment enclosure (it's a busduct end feeder box). Now the CPC for the circuit consists of the cable armour and busduct metal encasing which passes the 7671 calculations.

My question is should I also consider bonding this enclosure to the substation earth ring? If the consensus is that yes you should bond the enclosure also, then should I size the bonding conductor to carry full fault current or just as table 54.8 as the full fault current will be via the dedicated CPC. (The supply characteristics of my installation is TNC-S).

Thanks
 14 June 2018 08:59 AM
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mapj1

Posts: 11223
Joined: 22 July 2004

Firstly
Can you touch this box and other metal items connected more directly to the substation earth simultaneously?
How far off the substation earth (resistively speaking) is the cpc of the box,and what is the ADS protecting the bus in the box ?
Could a fault from the bus live to box outer internally that did not quite operate any fuse or breakers, drop so much voltage in the cpc loop, that the outer surface of the box rose to a dangerous voltage compared to the substation earth?
if yes then additional bonding needed on safety grounds. Even if no, then it may still be desirable, or recommended by DNO rules.

-------------------------
regards Mike
 14 June 2018 09:28 AM
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Spen008

Posts: 16
Joined: 09 July 2013

Thanks for your response Mike.

Yes you can touch the box and substation steelwork simultaneously. The bus in the box is protection by an upstream ABB EMAX device with LSIG relay. A fault from the bus live to box should clear the fault as the Armouring of the cable feeding the box is the CPC.
I'd like to add the additional bonding for additional safety considering that the enclosure is sufficiently big, but do you size the additional bonding conductor to withstand the maximum short circuit fault level? My concern is that the equipotential bond could see some of the fault current and not be able to withstand it!?
 14 June 2018 09:28 AM
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Spen008

Posts: 16
Joined: 09 July 2013

Thanks for your response Mike.

Yes you can touch the box and substation steelwork simultaneously. The bus in the box is protection by an upstream ABB EMAX device with LSIG relay. A fault from the bus live to box should clear the fault as the Armouring of the cable feeding the box is the CPC.
I'd like to add the additional bonding for additional safety considering that the enclosure is sufficiently big, but do you size the additional bonding conductor to withstand the maximum short circuit fault level? My concern is that the equipotential bond could see some of the fault current and not be able to withstand it!?
 16 June 2018 02:59 PM
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AJJewsbury

Posts: 17169
Joined: 13 August 2003

but do you size the additional bonding conductor to withstand the maximum short circuit fault level? My concern is that the equipotential bond could see some of the fault current and not be able to withstand it!?

BS 7671's general principle seems to be that half the size of the c.p.c. should be adequate (both for main bonds (based on the earthing conductor size - reg 544.1.1) and supplementary bonds (544.2.2) - presumably on the assumption that the c.p.c. will carry at least half of the fault current (leaving no more than half for the bond) and perhaps that any reduction in loop impedance tends to decrease disconnection time resulting in reduced energy let-though (tends to be true for fuses, less so for circuit breakers). Your detailed knowledge of your situation might be needed to check whether BS 7671's approach is appropriate to your situation though. (Substations tend to follow DNO rules rather than BS 7671 and might have to contend with situations well outside BS 7671's scope, such as HV faults to earth, or possibly live working procedures)

- Andy.
 17 June 2018 07:56 PM
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ArthurHall

Posts: 754
Joined: 25 July 2008

Rise of potential touch voltages are a much bigger consideration in substations than ADS the general rule of thumb is bond everything with 70mm or 25x3 tape.
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