Maybe have a look at 422.2.1 in 7671 with respect to the DB serving the firefighting shaft. Such a shaft is, of course, a protected escape route.

Further, my reading of Clause 28 of 9999 is that the lighting in the shaft is required to be maintained by the secondary supply and thus is a safety service requiring appropriate fire resistance.

With respect to your first question, I am not sure you will turn up a specific requirement. Rather, the guidance will suggest an objective. In terms of electrical services supplying life safety and fire protection equipment, 37.2.3.1 of 9999 would be a good starting point for a risk analysis.

Not my area at all, but it does occur to me that just because a fire is present within a compartment, it doesn't necessarily follow that the entre compartment is instantly affected. You might have a fire in one corner (where the cables run through) but not (for a while at least) elsewhere in the compartment (where there lights etc might be) - so there might be advantage in having fire rated cables feeding non-fire-proof lights - at least in that it might sometimes buy a little extra time.

   - Andy.

Good afternoon Gareth

A Safety service is defined in BS 7671 Part 2.

You will have to reference BS 7671 general rules and especially Chapter 56. Also BS 9999 and BS 8519 will also apply.

What does the Fire Strategy for the building say?

You say you have a common panel for for both primary and secondary supplies to the safety services? If so that is wrong, very wrong. The primary panel and the secondary panel have to be in to separate rooms with usually a 2 hour fire separation. The supply sources have to be independent of one another such as a DNO supply and a generator. The primary and secondary supplies have to take 2 separate diverse routes through the building again usually with a 2 hour fire separation. The primary and secondary cables have to be fire resistant usually for a 2 hour rating.The cables have to have metallic supports. The primary and secondary cables should only come together at the transfer switch, the transfer switch has to be close to the particular safety service.

This is only a summary and you need to follow the other requirements set out in the 3 references I have already mentioned and meet the fire strategy for the building.

Hope this helps?

JP

John Peckham said:

You say you have a common panel for for both primary and secondary supplies to the safety services? If so that is wrong, very wrong.

I read the OP as the DB serving normal lighting only in the firefighting shaft via two possible separate supply sources. Emergency lighting then being provided as appropriate via integral batteries. That being the case, I can’t see an issue. However, if you have a standard reference to indicate otherwise, JP, it would be helpful.

As you say, there should be more of a holistic approach with consultation sought with the responsible person who may feel it appropriate to seek the services of his fire engineer to assist in making a determination on acceptability. 
I don’t think I would make a call on a proposal for any work in a higher risk building without getting the nod from either BC or the FE, even if I was confident about code compliance! 

Extract from BS 9999 2017 so others can take a view.

37.2.3.3  Primary and secondary power supplies

To reduce the risk of the loss of electrical supply to fire protection systems that are required to

operate continuously during a fire [such as those listed in h) below], a secondary power supply

should be provided. This supply should be of sufficient capacity to maintain supplies to all life

safety and fire equipment installations. The secondary power system should be designed to operate

safely in fire conditions. The means for the provision of a secondary supply should include the

overall electrical distribution system within the building, and also the power needs for other

equipment requiring a secondary power supply.

NOTE 1 In some cases, where the power demand from a system is low (such as control systems for

natural vent actuators), a secondary supply can be achieved by the use of back-up batteries.

NOTE 2 For legal and technical reasons, power supply companies have reservations about offering a

power supply from a second substation to provide protection against the occurrence of a fault

(unconnected with the fire) on the high-voltage distribution system. Accordingly, a generator or an

independent power supply needs to be provided if protection against faults is required by the

occupier.

The management procedures for the building should prohibit the isolation of circuits supplying

power to the above mentioned equipment during a fire emergency.

Power supplies should meet the following specific recommendations.

a) A secondary power supply independent of the primary power supply to the building, e.g. an

automatically started generator or a supply from another substation, should be provided which,

independently of the primary supply, is of sufficient capacity to maintain in operation for at

least 3 h the following:

1) any powered smoke control systems (including systems using pressure differentials);

2) any fire and rescue service communication systems; and

3) any other fire protection or fire-fighting equipment, except automatic fire detection and fire

alarm systems and evacuation lifts.

b) The secondary power supply should be capable of providing the power supply for items a1), a2)

and a3) within 15 s of the failure of the primary electrical supply.

c) Where the secondary electrical supply is to be taken from a separate substation (whether utility

or private) to that supplying the primary electrical supply, the following criteria should be met.

1) The electrical supplies to the two independent substations should be taken from two separate

high-voltage supplies, and not originate from the same substation.

2) The failure of one substation should not lead to the failure of the other.

3) The two independent substations should be adequately separated. Where the substations are

located within the building they serve, the following criteria should be met:

1. i) each substation should be enclosed within a fire-resisting structure having a minimum of 2 h

fire resistance;

1. ii) the two substations should be located in two separate parts of the building.

4) Supply cables from the high-voltage substations should enter directly the

high-voltage/low-voltage switchrooms and not pass through the building.

5) The two sets of supply cables should be adequately separated from each

other to avoid a single fault affecting both supplies.

d) Cables supplying current to the life safety installations should be installed in accordance with

BS 7671 and the manufacturer’s instructions. The cables should have an inherently high resistance

to fire and be protected where necessary against mechanical damage. Cables, switchgear and other

equipment transmitting the secondary power supply should be separate from those of the primary

supply, or be physically protected so that a breakdown, or any cause of breakdown, on one supply

would not lead to a simultaneous failure of the other supply.

NOTE 3 Further guidance on the selection of cables is given in BS 8519.

e) The primary and secondary power supply cables should be terminated in a changeover device

located within the plant room(s) housing the life safety and fire protection equipment, or in the

case of a firefighters lift, within the fire-fighting shaft.

NOTE 4 This is not to be confused with the lift well.

f) The changeover device should automatically effect the transition from the primary to the

secondary power supply if the primary supply to the particular plant fails.

g) Any electrical substation or enclosures containing any distribution board, generator, powered

smoke control plant, pressurization plant, communication equipment, and any other equipment

associated with life safety and fire protection systems, should be separated from the building by

construction with a duration of fire resistance of not less than 2 h.

h) Secondary power supplies should be provided for the following:

1) sprinkler pumps;

2) wet riser pumps;

3) firefighters lifts;

4) fire-fighting shafts (associated equipment and normal lighting);

5) fire-fighting intercommunications installations;

6) pressurization fans (air supply and pressure relief);

7) depressurization fans (air supply and pressure relief);

8) smoke control system;

9) evacuation lifts.

"normal lighting" wouldn't be provided by self contained emergency luminaires. Usual practice would be to use a static inverter within the firefighting shaft, fed by dual diverse supplies from the primary & secondary source, then use fire rated cables from the inverter to the luminaires.

Just to confuse matter though, BS9991 doesn't require "normal lighting" thereby allowing the use of self contained luminaires to provide 5lux. It all depends on which standard the Fire Strategy/Fire Engineer is using. As others have said already, the multiple BS's covering fire safety in buildings can be a confusing muddle, but it is critical to establish which applies to your particular site.

regards, burn

Hi John

Thanks for the reply.

To clarify, we have two substations on site, feeding two separate landlords panels. Each landlord panel will provide secondary supplies for the life safety systems served by the other (see Figure 3 of BG 70 as an example). 

The Fire Strategy Report only contains the usual recommendations, e.g. service penetrations should not compromise the fire barrier. Wirings systems should not collapse prematurely in the event of a fire, cables or other electrical equipment shall not be installed in protected escape routes unless part of an essential fire safety or related safety system.......etc. 

The answer to your second question is no. Refer figure 1 of BS 8519:2020. The little arrangement in the top left has an ATS in a 2 hour fire resisting enclosure serving life safety equipment in an adjacent 2h fire resisting enclosure. The cables from the ATS which pass through from one enclosure to the other are shown as non-fire-resistant-cabling. 

Hi mrf

I've not noticed that before. And now I'm looking in more detail, it would seem that this is repeated in BG 70 figure 25. 

Make sure you have the current version of BG 70: 2021. See figure 2 and figure 32.