Author Topic: BS 9990  (Read 5937 times)

Offline jokar

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BS 9990
« on: August 12, 2015, 01:42:33 PM »
Clause 4.3 of this standard states that rising mains require earthing yet electricians tell me that BS 7671 refutes that.  Any comments?

Offline Owain

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Re: BS 9990
« Reply #1 on: August 12, 2015, 08:24:14 PM »
Clause 4.3 of this standard states that rising mains require earthing yet electricians tell me that BS 7671 refutes that.  Any comments?

As metallic services they should be main equipotentially bonded at the point of entry to the building back to the main earthing terminal (along with water main, gas main, etc). BS7671 applies.

My reading (of a freely available draft of the standard) is that the fire mains 'should be earthed in accordance with BS7671' which is not the same as saying 'should be earthed even if BS7671 says it shouldn't be'. So if BS7671 requires earthing of the fire main because it's considered an extraneous-conductive-part or because of some other criterion then it should be earthed. If, however, there is no requirement in BS7671 to earth the fire main, then BS9990 does not require willy-nilly addition of green/yellow wire.

In some circumstances BS7671 would explicitly prohibit earthing, such as earth-free locations (not sure how that would be workable with metallic fire mains)


Offline John Webb

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Re: BS 9990
« Reply #2 on: August 13, 2015, 09:51:53 PM »
Section 411 refers to the need to use Protective equipotential bonding on "other installation pipework and ducting" in addition to the water and gas installation pipes.

Section 418 includes a reference to 'Non-conducting location' but these are only allowed where the installation is under the supervision of skilled or instructed persons, which is not going to apply to most workplaces, I would have thought.

So I'd go along with Owain that the rising mains should be bonded to the main earthing terminal.
John Webb
Consultant on Fire Safety, Diocese of St Albans
(Views expressed are my own)