We all need peace of mind when it comes to re performance

In today's increasingly sophisticated world, the buildings we live in, work in and play in are using ever more complex technologies to keep us safe. With re alarms using multi-sensor technologies distributed throughout the building, it becomes possible to identify potential problems at the earliest possible moment and take appropriate staged action.

Such systems obviously require secure and continuous communication between the sensors, sounders and the control panel and this places new demands upon cabling systems.

The revision to BS 5839 goes some way to addressing these issues and increases demands when designing, commissioning and maintaining re detection and alarm systems. It specically recognises addressable systems and places requirements upon cabling systems to maintain communication under re conditions. It also requires that false alarms be minimised.

BS 5839-1 (2002)

This standard now species two cable types for critical signal paths: standard and enhanced.

Enhanced cables should be used in any building in which the designer, specier or regulatory authority believes they are appropriate. They must be used in unsprinklered buildings higher than 30m, unsprinklered buildings with four or more evacuation zones and unsprinklered buildings where cables from one zone pass through another zone.

Enhanced cable must be able to maintain insulation and continuity during ame tests at temperatures of 950°C + or - 20°C for 2 hours whilst subjected to water spray and mechanical shock.

Standard cable must be able to maintain insulation and continuity during ame tests at temperatures of 850°C + or - 20°C for 30 minutes whilst subjected to water spray and mechanical shock.

Whilst both of these tests place a higher level of fire stress upon cables than the current BS 6387 CWZ, there is continuing debate about the water test for enhanced cables. Other organisations have taken a different approach.

For example, in the US, cables for circuits designed to operate during a re are tested to UL 2196. The cable must maintain continuity and insulation when subjected to 1010°C followed by the effects of a powerful water spray from a re hose. Notonly is the test at a higher temperature but also direct mechanical stress is applied to cable by the water deluge rather than just light tapping to the cable support. Only the most robust cables like Mineral Insulated (MI) to BS EN 60702-1 can achieve these levels unless additional protection is applied.

Similarly, London Underground Limited (LUL) developed a test for its essential circuitry after the King's Cross tragedy. Cables must complete a three-hour ame test combined with impact on the area of cable subjected to the most Intense heat. The same piece of cable must then be energised and subjected to 15 minutes of water spray, bent at 180 degrees into a U-shape around a mandrel and given more impact before nal immersion in water for a further hour. After this, It must sill maintain continuity and insulation.

Data compatibility

Whilst BS 5839 requires that cables used in addressable systems must maintain data compatibility during re situations, it gives no guidelines concerning how to demonstrate compliance.

BS 5839 - Part 1 Fire Protection in Public Buildings

by Geoff Williams

Computer data is routinely sent down data cables and the requirements that those cables must meet - from transmission line theory - are well understood. For the rst time these considerations need to be added to those for re performance, where cables are to be used in addressable alarm systems.

Data compatibility is demonstrated by a constancy of characteristic impedance across the required frequency range (data speed) together with a low level of signal loss along the length. Each of these easily measurable parameters are functions of the core resistance and inductance, inter-core conductance and capacitance. To make a re survival data cable, it is simply necessary to ensure that the re does not change these physical parameters.

Soft-skinned cables use polymer based insulation systems and although each manufacturer uses different formulations and combinations of other materials, all of these are changed when exposed to re, water and impact. The characteristic impedance of the burnt and un-burnt sections of soft-skinned cables must therefore be different.

At the transition, some signal strength wilt be lost and some of the signal will be sent backwards the wrong way down the cable. As the signal becomes weaker and weaker, the vital message becomes degraded to a point where communication becomes corrupted. For the want of the right cable, the sophisticated high technology protection system designed, installed, and maintained to ensure our safety has become useless. Quite simply; Wrong message = wrong actions taken = risk to life.

Whilst the electronic design of the senders and receivers must be compatible with the nominal characteristic impedance of cable to be used, its actual value is less important than ensuring that it remains constant. An appropriate, realistic and achievable limit is that the cable's characteristic impedance should not change by more than + or - 10% when exposed to the appropriate re conditions. Any cable meeting this limit can be considered to be a re survival data compatible product.

Mineral Insulated is such a cable as it remains unaltered during a re. Data communications are maintained thus ensuring that the sophisticated high technology protection system continues to operate and the correct action is taken.

Why compromise?

There is clearly scope for a more stringent approach incorporating some of the concepts from other existing tests to increase the level of re stress and to add explicit tests for data compatibility.

Indeed, it is in all interests that the standard covering this eld should go further and be more specic, whether you're a manufacturer, specier, distributor or contractor.

The new amendment to BS5839 -1 (2002) represents a signicant step forward to increasing safety in this extremely complex and important area. Rather than being overly prescriptive, it forces all those involved in re safety - the designer, the specier and regulatory authority - to undertake and review formal risk assessments in choosing the appropriate technological solutions.

This innovative approach makes the standard future-proof as new technologies are automatically incorporated - but it places more demands and continued responsibility upon those involved. It is no longer possible to hide behind third party certication. So why compromise on cables?

We all need peace of mind when it comes to re performance

In today's increasingly sophisticated world, the buildings we live in, work in and play in are using ever more complex technologies to keep us safe. With re alarms using multi-sensor technologies distributed throughout the building, it becomes possible to identify potential problems at the earliest possible moment and take appropriate staged action.

Such systems obviously require secure and continuous communication between the sensors, sounders and the control panel and this places new demands upon cabling systems.

The revision to BS 5839 goes some way to addressing these issues and increases demands when designing, commissioning and maintaining re detection and alarm systems. It specically recognises addressable systems and places requirements upon cabling systems to maintain communication under re conditions. It also requires that false alarms be minimised.