Heat resistant wiring to BS standard?

[Tom Sutton]

I received the following enquirey to day and would like to hear your opinions before I reply.

"My school has regular wiring throughout its fire alarm system. However, maintenance and certificating companies tell us we must rewire the whole alarm/detection system so that the wires are heat resistant. This would be at a cost of many thousands of pounds which we cannot afford.
 
Are they correct that every existing building must do this or are they just trying to make some money?
 
I contacted my local fire station but they make non committal answers, indicating that they do not know."

[Wiz]

TW, I believe they mean 'fire resistant' rather than 'heat resistant'.

The recommendation for fire resisting cable on the 'detection' element of fire detection and alarm systems appeared in the 2002 standard.

The recommendation for a fire resisting wiring system on the 'alarm' element of fire detection and alarm systems goes back much further, I believe to about 1970.

However, British Standards are not retrospective, so if the wiring system met the Standard at the time of installation, then there is no automatic need to meet a new recommendation when it is introduced.

The term 'wiring system' above is used because earlier standards accepted the use of non-fire resistant single insulated cables within metal conduit etc. to be acceptably fire resistant in many circumstances (not the case with current Standards).

I would also say that if the fire alarm system is over, say, 30 years old then it must be getting to the stage where replacement, with a view to embracing modern equipment and fault monitoring techniques etc., should be a considered a real benefit even if the old system hasn't actually failed and met the BS recommendations at the date of installation.

[Graeme]

The maintenance company can't tell you have to do anything only recommend and brings items that don't comply to your attention.

[Tom Sutton]

I accept your point Graeme its RP's decision. Wiz doesn’t this mean all wiring must be standard or enhanced fire resistant wiring if so why talk about critical paths why not say all wiring and explain which has to be enhanced or standard.

Sounder and supply circuits used MICC before 1970, I think the theory was if the detection circuits burned through the conductors would make contact and initiate the alarm so fire resistance wiring wasn’t necessary.

[Wiz]

TW, the requirement for fire resistant cable in both the 'detection' and 'alarm' circuits i.e 'critical path' has only been a recommendation since the 2002 standard.

There are many areas of wiring within a 'fire alarm system' not all of these have to be fire resistant (in respect of BS) and the inception of the term 'critical path' was provided to describe a new single area of wiring that was previously considered to be two different areas with differing recommendations.

If fire resistant cable was simply recommended for all fire alarm wiring certain circuits including where fire resistant cable isn't really necessary this would add extra costs. e.g. there is no need to wire electromagnetic door holders in fire resistant cable because these are 'fail-safe' and the failure of the circuit due to the cable burning through would just cause the fire doors to close anyway.

I'm sure the BS committees spend many hours agonising over the choice of words used in BS recommendations and trying to ensure that misinterpretation doesn't lead to 'over engineering' a system with the resultant unnecessary costs.

[GregC]

e.g. there is no need to wire electromagnetic door holders in fire resistant cable because these are 'fail-safe' and the failure of the circuit due to the cable burning through would just cause the fire doors to close anyway.

Then what would they be wired in, given that BS states

"All fire alarm cables should be of a single, common colour that is not used for cables of general electrical services in the building, to enable these cables to be distinguished from those of other circuits"

[Wiz]

GregC, in-line with the theme of your response, I would confirm that clause 26.2.j clearly states that All conductors should have a cross-sectional area of at least 1mm2.

[Tom Sutton]

Thanks Wiz

[Graeme]

GregC, in-line with the theme of your response, I would confirm that clause 26.2.j clearly states that All conductors should have a cross-sectional area of at least 1mm2.

Wiz

could be wrong but i think he may be asking "what type of cable?" i.e Flat grey twin and earth etc not to be used for door magnets as it's the same colour as general electrical cable.

[Wiz]

Graeme, I found GregC's reply confusing so I added a confusing reply as well!

Anyway, I think the interpretation is that things like door magnets are not actually part of the fire alarm system in respect of the recommendations of BS5839. These sort of things are seen as being ancillary to the the fire alarm system and are, in fact, subject to other other BS recommendations. These sort of things might be 'interfaced' with the fire alarm system but are not part of it.

[Tom Sutton]

There are many areas of wiring within a 'fire alarm system' not all of these have to be fire resistant (in respect of BS)

e.g. there is no need to wire electromagnetic door holders in fire resistant cable because these are 'fail-safe' and the failure of the circuit due to the cable burning through would just cause the fire doors to close anyway.

Wiz you said there are many areas not requiring fire resistant cable now you have discounted door magnets could you detail other areas that do not.

[GregC]

Could you also define what is and what isnt a critical signal path?

[Wiz]

There are many areas of wiring within a 'fire alarm system' not all of these have to be fire resistant (in respect of BS)

e.g. there is no need to wire electromagnetic door holders in fire resistant cable because these are 'fail-safe' and the failure of the circuit due to the cable burning through would just cause the fire doors to close anyway.

Wiz you said there are many areas not requiring fire resistant cable now you have discounted door magnets could you detail other areas that do not.

Well TW, maybe 'many' is a bit of an exaggeration! But certainly any cable to all the other different types of fail-safe door control devices could be included, plus links to fail-safe gas supply solenoids plus links to interface relays that are normally held energised etc.

I would also argue that the cable between a simple fire alarm repeat panel (one provided for convenience rather than necessity) and the main control panel wouldn't require fire resistant cable.

The BS5839 definition of critical signal path is;

all components and interconnections between every fire alarm initiation point (manual call point or automatic fire detector) and the input terminals on, or within, each fire alarm device.

i.e. from where the fire alarm signal is initiated through the wiring, the control panel, more wiring, to the sounder and/or beacon

[Tom Sutton]

Thanks again Wiz I have never been seriously involved in fire alarm systems and all this is good info.

[Wiz]

Thanks again Wiz I have never been seriously involved in fire alarm systems and all this is good info.

No probs, TW. Always a pleasure to be of some help.