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Topic Title: Safety Services - Segregation of Circuits and Equipment
Topic Summary: Position of transfer switch
Created On: 04 October 2012 08:22 AM
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 04 October 2012 08:22 AM
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Avatar for timothyboler                                      .
timothyboler

Posts: 230
Joined: 25 July 2008

Hi everyone,

Been looking at the article in the recent Summer Wiring Matters on safety service supplies particularly page 14 with the layout diagrams. The reg. 560.7.1 states that that:

"Circuits of safety services shall be independent of other circuits - this means that any electrical fault, maintenance or modification work on one system must not affect the correct functions of the other. This may necessitate separation by fire-resistance materials of different routes or enclosures."

However the article suggests that this means that the mains/emergency transfer switch should be as close to the life safety load as possible and shows in figure 3 no less than 3 ATSs for each life safety plant location. In addition the emergency backup generator has its own segregated main switchboard and fire rated enclosure.

Does this mean that the common practice of providing a tie breaker switch on the (single) main switchboard with the generator and mains supply on either end does not comply? Clearly this is a much cheaper option as the wiring to the loads would not need to be duplicated. As long as the distribution and final circuit cables were fire rated and a Form 4 switchboard was used would this not provide all the necessary safeguards?

Lastly section 560.8.1 requires that all wiring systems to safety services (including emergency lighting systems) utilize fire rated cables. However BS 5266 Emergency Lightning section 8.2.2 allows PVC cable in rigid steel or PVC conduit. Is this a good idea; I would have thought the PVC inside would melt in a fire?

Cheers, Tim

-------------------------
Everyone loves a fireman - but hates the fire inspector.
 04 October 2012 09:54 AM
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Avatar for OMS.
OMS

Posts: 19602
Joined: 23 March 2004

Originally posted by: timothyboler

Hi everyone,

Been looking at the article in the recent Summer Wiring Matters on safety service supplies particularly page 14 with the layout diagrams. The reg. 560.7.1 states that that:

"Circuits of safety services shall be independent of other circuits - this means that any electrical fault, maintenance or modification work on one system must not affect the correct functions of the other. This may necessitate separation by fire-resistance materials of different routes or enclosures."

However the article suggests that this means that the mains/emergency transfer switch should be as close to the life safety load as possible and shows in figure 3 no less than 3 ATSs for each life safety plant location. In addition the emergency backup generator has its own segregated main switchboard and fire rated enclosure.

The 3 life safety ATS are 3 seperate life safey loads - it could be 3 seperate lifts in independant fire fightig shafts or say one lift, smoke pressurisation fans and AOV's

You'd expect the gen set to be in its own fire compartment for all sorts of reasons - the switchgear associated with it would also be segregated.


Does this mean that the common practice of providing a tie breaker switch on the (single) main switchboard with the generator and mains supply on either end does not comply?

Depends on the risk assessment - I've used seperate boards in seperate rooms linked via fire rated busbars, I done whayt you describe above on good quality form 4 Type 6 or 7 switchboards in remote locations where fire risk is low (ie external energy centres perhaps)

Clearly this is a much cheaper option as the wiring to the loads would not need to be duplicated.

How so - the ATS needs to be close up to the safety load and the two supplies to the ATS run over diverse routing

As long as the distribution and final circuit cables were fire rated and a Form 4 switchboard was used would this not provide all the necessary safeguards?

Not really - work out where you life safety ATS needs to be first - it's usually in the protected enclosure - look closely at Fig 3 for example - you have fire rated primary and secondary supply to the ATS - the load however isn't supplied in a fire rated cable - the assumptionbeing that if the fire ocurs in the protected enclosure then all bets are off anyway

Lastly section 560.8.1 requires that all wiring systems to safety services (including emergency lighting systems) utilize fire rated cables. However BS 5266 Emergency Lightning section 8.2.2 allows PVC cable in rigid steel or PVC conduit. Is this a good idea; I would have thought the PVC inside would melt in a fire?

Not in my copy it doesn't


Cheers, Tim


Regards

-------------------------
Failure is always an option
 04 October 2012 12:33 PM
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Avatar for timothyboler                                      .
timothyboler

Posts: 230
Joined: 25 July 2008

Thanks for the fast response OMS.

Depends on the risk assessment - I've used separate boards in seperate rooms linked via fire rated busbars, I done whayt you describe above on good quality form 4 Type 6 or 7 switchboards in remote locations where fire risk is low (ie external energy centres perhaps)


Ok, so the boards are still separated then i.e. it's not one continuous enclosure unless it's a low risk.

How so - the ATS needs to be close up to the safety load and the two supplies to the ATS run over diverse routing


What I was trying to illustrate was the main switchboard acting as the sole ATS, either fed from 2 independent transformers or a transformer and backup emergency generator with a tie breaker acting as the transfer switch. Clearly in this arrangement the main switchboard/ATS would not be close to the life safety loads and could be a significant distance away. I can't find any wording in the regs that precludes it though. We have a few installations like this feeding fire pumps. As far as I know only BS 5588-5 requires the ATS to be installed at the point of load (within the lift shaft in this case).

Not really - work out where you life safety ATS needs to be first - it's usually in the protected enclosure - look closely at Fig 3 for example - you have fire rated primary and secondary supply to the ATS - the load however isn't supplied in a fire rated cable - the assumption being that if the fire occurs in the protected enclosure then all bets are off anyway


Ok, makes sense.

Not in my copy it doesn't


I wasn't reading the wording properly. PVC in conduit is allowed with "additional fire protection"?

Thanks and regards, Tim


-------------------------
Everyone loves a fireman - but hates the fire inspector.
 04 October 2012 12:53 PM
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Avatar for OMS.
OMS

Posts: 19602
Joined: 23 March 2004

Originally posted by: timothyboler

Thanks for the fast response OMS.

Depends on the risk assessment - I've used separate boards in seperate rooms linked via fire rated busbars, I done whayt you describe above on good quality form 4 Type 6 or 7 switchboards in remote locations where fire risk is low (ie external energy centres perhaps)


Ok, so the boards are still separated then i.e. it's not one continuous enclosure unless it's a low risk.

Generally yes - you can buy a switchboard with an internal fire partition (look for slightly wider sections at the joins - the se can either stand alone or you then uild a wall that effectively puts each half of the board into seperate rooms - or you make the seperation wider and physically put the switchboards in two rooms with a fire rated tie busbar section


How so - the ATS needs to be close up to the safety load and the two supplies to the ATS run over diverse routing



What I was trying to illustrate was the main switchboard acting as the sole ATS, either fed from 2 independent transformers or a transformer and backup emergency generator with a tie breaker acting as the transfer switch. Clearly in this arrangement the main switchboard/ATS would not be close to the life safety loads and could be a significant distance away.

It won't be in BS 7671 - the location would be in relevant codes, BS 9999 or specific product standards etc such as BS EN 81 or BS 5588

I can't find any wording in the regs that precludes it though. We have a few installations like this feeding fire pumps. As far as I know only BS 5588-5 requires the ATS to be installed at the point of load (within the lift shaft in this case).

Generally yes, if the ATS for the fire pump isn't required to be in the same protected enclosure as the fire pump (or the pump doesn't have a seperate dieel engined drive) then the mains to generator transfer a the mai switchboard is effectively the ATS.


Not really - work out where you life safety ATS needs to be first - it's usually in the protected enclosure - look closely at Fig 3 for example - you have fire rated primary and secondary supply to the ATS - the load however isn't supplied in a fire rated cable - the assumption being that if the fire occurs in the protected enclosure then all bets are off anyway


Ok, makes sense.

Not in my copy it doesn't


I wasn't reading the wording properly. PVC in conduit is allowed with "additional fire protection"?

Thought so - buried flush in the concrete frame for example would give aditional fire protection - but I think the cable itself also has to be fire rated singles - which version are you reading 2005 or 2011

Thanks and regards, Tim



regards

OMS

-------------------------
Failure is always an option
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