Short cut to better fire alarm systems?

[Wiz]

I've wondered about this ever since it happened.

At the time it happened, there weren't any forums, like this, where answers could be sought.

A previous BS5839-1 update included the recommendation that a short-circuit of a monitored fire zone circuit indicated as a fault condition rather than the fire condition which was the norm previously.

Does anyone know the underlying reason for the change?

I've asked on www.firealarmengineers.com and nobody has an answer that makes real sense for making such a change.

[John Webb]

I have a feeling this amendment was to give warning of damage to cables or the like. That is, operating a call point used to put a short on the zone lines to give the alarm, but this was indistinguishable from mechanical damage to the cables which caused a short. So by putting the system into alarm with something like a few hundred ohms instead of the short the two different conditions can be distinguished. But this change came in after I stopped working on fire alarms so it is a bit of a guess.

[Wiz]

Thanks for your reply, John.

The point you make was raised in discussions on another forum, but is it so much better that a accidental short-circuit on a cable raises a fault than a fire condition? Were there many unwanted alarms caused by this?

The earlier standard also allowed non-fire resistant cable to be used on fire zone wiring and the shorting of the conductors as the pvc melted in a fire would give a fire condition. It was almost as if the zonal wiring was one long fire detector!  It might also be argued that the later recommendation for using the more expensive fire resistant cable was partly required because the earlier change recommending that a short now gave a fault condition instead of a fire condition.

Some engineers have found that accidental overloading of a zone with too much equipment is now giving only a fault condition whereas it used to give a fire condition. One example explained on another forum is where a particular manufacturer used to make a detector where the 'operated' indicator was part of the mounting base, and then they designed a new model where the detector itself incorporated the indicator, but this new detector could still mount on the old-style base (although it wasn't sold to be used in this way). This meant you could now had two 'operated' indicators, one on the detector and one on the base, but if the detector then actually operated, the combined current taken by both indicators was enough to cause the panel to given a fault condition (short-circuit) rather than a fire condition! I know this is due to a specific set of mistakes being made, but it did, evidentally, happen somewhere. In the previous set-up it the problem would have given a fire condition.

I would have thought it is better that a rare, but possible, fault condition gave a fire warning rather than a real fire only giving a fault warning!

I'm sure there must be a good and valid reason for the change, but I still think we haven't heard it yet.


p.s this subject is only relating to non-addressable/conventional fire alarm systems

[John Webb]

I think the change may have been made because a short-circuit due to damaged cable is something that needs a different sort of inspection to that looking for an operated BGU. It might also be to do with the increasing use of detectors? Obviously a short-circuit on the older type of system might be a fire or might be cable damage - if the latter then all your BGUs and detectors are out of action and you need to do something about it fairly quickly. But as I said above, this change occurred after I stopped working on fire alarm systems so I can only describe it as "informed conjecture"!
Would the current BSI Secretary of the committee responsible for BS5836 know?

[Wiz]

I understand what you are saying, John.

But in the scenario that you descibe, the current method of monitoring/fault indication would have exactly the same effect during a short-circuit of the cabling i.e. none of the detectors and call-points on that zone would 'work'. But whereas previously the panel would have indicated fire, it would just indicate fault. I think the older way seems more 'fail-safe'.

There has got to be a better reason for the change than we have so far thought of.

Hopefully, as you say, someone who actually 'knows' the reason for the change will enlighten us.

[Bill J]

Wiz,

The older way was more fail safe, however the number of false alarms, false evacuations and callouts of the fire service, really demanded that a solution be found. Buiders work and water ingress would lead to buildings being regularly evacuated, which in turn lead to ignoring the FA system completely!

This coupled with the fact that more and more techie things could be done witha FA system such as sounders on the same cables as detectors, detectors on differant zones on the same legs of cable, made non fire proof cabling an issue.

In a perfect world I agree, short to fire would suffice, but folks need to improve what works, despite the belief of if it aint broken......

Bill

[Wiz]

Thanks for your input, Bill.

I can see that the fairly common problem of water ingress into parts of the fire alarm zone circuits could cause a problem of unwanted alarms. Short-to-fire would definitely be affected more then the 'modern' sensing method, but unwanted alarms could still be given by water ingress in the 'modern' sensing method. I can also see that 'builders work' is more likely to cause a 'full short' than a 'resistive short of the right value'

I never realised that the water ingress and 'builders work' problem was so big that it impacted so much on unwanted alarms. However, I accept your knowledge in this area and so I believe the reasons you have highlighted is probably correct. Especially as no-one has come up with more likely alternatives.

The problem I have is, that whilst unwanted alarms may have been reduced because of this change, I believe the potential for an actual fire condition appearing as only a fault condition has increased. I believe we have lost a useful 'fail-safe' situation.

However, since this change was made quite a number of years ago and my concerns have not been raised by others during that time, I accept that the problem isn't nearly as big as I seem to consider it!

[colin todd]

Wizzypops, there was a fire that I will not mention by name in which it was held by some that the fire alarm system had been intentionally isolated (though that is not the official version) because there had been numerous false alarms as a result of building works in the premises. There was someone on the committee at the time who felt very strongly that in that day and age there was no need for false alarms to arise from a s/c and that systems should be able to distinguish. Some conventional systems at the time did that anyway.

Some of us were not convinced that false alarms from cable damage were prevalent regardless of whether or not it was relevant to the fire I mentioned above. But there was a school of thought that regardless of the actual number of cases why expose people to the potential problem when it could be solved easily.

In truth, speaking personally I felt about 60:40 against the change but I had no very strong feelings so was prepared to go along with the change and not object.

[Wiz]

Thanks for your reply Mr C.T.

I was aware that some panels were already available at the time of the change that had the 'zone short-circuit=fault' monitoring. I wager that these panels had a surge in sales before the other manufacturers had the time and resources to re-design their own panels!

It is good to hear that there wasn't some stunning improvement in the overall integrity of systems in making the change, because try as I might, I couldn't think what it could be.

I'm still not sure that the loss of the almost 'fail-safe' warning of a circuit being overloaded has been a good thing. I'd rather a system gave a fire signal instead of a fault condition in these circumstances but I suppose it doesn't matter that much.

Out of interest, do you think, that if the change mentioned above hadn't been made, that the use of non-fire resistant cables would still have been acceptable (or considered even maybe as a benefit) on zone circuits of non-addressable systems, other than those also containing alarm warning devices?

[colin todd]

Wiz, the question is a bit hypothetical, but there is a case for non fr if it fails to alarm, but the changewas somewhat  and reflected custom and practice in any case.