Using fire resistant cable for detector wiring in fire alarm installations might seem like a sensible decision. In most cases, however, it may not be necessary says Don Scott.

Most fire professionals would agree that, within reason, adding more fire detectors to an installation over and above the bare minimum needed to satisfy all requirements is likely to increase the effectiveness of the installation. After all, adding more detectors means that the likelihood of a detector being close to the source of a fire, and therefore responding rapidly, is increased.

So why is it that fire alarm installations in the UK are almost invariably designed to minimise the number of detectors used, whereas this is most certainly not true in the remainder of Europe?

The answer isn't hard to find – it's all down to cost. That's not to suggest that our European colleagues are necessarily more willing to spend money on their installations than we are, it's just that the cost of fitting a detector in the UK is many times higher than that of fitting the same detector in Europe.

The reason is the cabling. In Europe, detectors are almost invariably wired with standard Belden-type cable. This means that the installation cost per detector is somewhere in the region of £30. In the UK, standard practice is to use fire-resistant cable for detectors, which pushes the installation cost up to around £200 per detector. Small wonder UK systems are designed to keep the number of detectors to a minimum!

Critical signal paths

This difference in practice between the UK and Europe can be directly traced to BS 5839, which requires that "all critical signal paths" within a fire protection installation should be wired in fire resistant cable. This is defined as including the wiring to the detectors. But does the use of fire resistant cabling for the detectors give any useful benefits? Before answering this question, let's make it absolutely clear that we are talking only about detector wiring. There is no suggestion that moving away from fire resistant cable for other parts of the installation – such as alarm circuits – could in any way be justified.

Returning to detector wiring, however, let's start by considering the primary function of a detector. Its job is to raise the alarm when the fire starts. Once it has done this, its work is finished. What possible argument is there, therefore, for wiring the detectors with cable that will continue to connect them to the alarm panel long after the fire has started and the alarm has been raised?

So it's easy to see that the argument for using fire resistant cable is, to say the least, of dubious value. However, there is one interesting point that is sometimes mentioned in favour of using it – if the detectors are still functioning when the fire brigade arrives at a fire, the order in which the detectors operate will provide useful information about the direction of spread of the fire. This will help firefighters to ensure they enter the building behind the fire rather than in front of it.

With older fire alarm installations, this argument had some validity as, in these systems, the detector wiring usually only had short-circuit isolators fitted between zones. With this arrangement, if a cable burned through, signals from all of the detectors in a zone would be lost.

Modern detectors, however, such as those in the Siemens Sinteso range, have built-in short-circuit isolators. This means that when a cable burns through, the only signal lost is from the affected detector and those immediately adjacent to it on either side can still function. Therefore, even if the cable integrity is compromised, the system will continue to provide the fire brigade with the information it needs to minimise risk when entering the building.

Standard practice
Taking these arguments together, it's easy to see that there is little reason for the continued use of fire resistant cables for wiring detectors except, perhaps, ‘that's what it says in the standard'. Indeed, that is what it says, but it's worth noting that BS 5839 isn't entirely inflexible; it does allow deviation from the prescribed requirements "for good reasons". Surely a good reason is the installation of more detectors to make the system more efficient, while containing costs by using Belden-type cable for detector wiring? An "agreed variation" to cover the wiring of detectors is certainly possible, although it may be a little troublesome to put in place as it must be agreed by all interested parties including, for example, the building owner and the insurers. In this respect it is worth making it absolutely clear that an installation in line with BS 5839 – which includes one or more agreed variations – is still fully compliant with the standard.

The proof of the pudding is, of course, in the eating. As we've already seen, European practice is to use non fire resistant cable for detectors, and generally to install more detectors. How does this affect the fire fatality statistics? According to the World Fire Statistics for 2002, the number of fatalities from fire for the UK was 1.18 per 100,000 people. In Germany, the corresponding figure was 0.98, in Italy it was 0.82 and in Switzerland it was just 0.62.

Clearly, the use of fire-resistant cabling for detectors has no discernable beneficial effect, so how much longer will the UK plough a lonely furrow following this outmoded practice that is more likely to compromise than enhance fire safety?

Don Scott is fire engineering consultant at Siemens Building Technologies, Fire Systems.