Pyro MI cables success with BS7346-6

Tyco Thermal Controls Pyro MI cables successfully witness tested to the stringent fire test methodology of BS7346-6.

Tyco Thermal Controls has led the way by example in having its mineral-insulated Pyro MI cables undergo Lloyds Register witness testing under the new, more stringent, fire test methodology of BS 7346-6:2005.

This cable successfully achieved the highest performance requirement of 120 minutes survival time for fire fighting systems.

The Standard applies to cables above a notional size of 20mmOD, but also recognises smaller cables that can be accommodated in the test rig.

As many MI cable sizes typically used for life safety and fire fighting circuits are significantly smaller and lighter than other cables of an equivalent rating in the Standard, Tyco Thermal Controls also took the opportunity to subject Pyro MI cables down to12mmOD to the same rigorous tests and were able to demonstrate that these too will meet the highest fire performance requirements.

BS 7346-6:2005 is the recently introduced Standard for fire-rated cable systems, defining the performance requirements of various types of fire-rated cables (up to 600/1000V) in maintaining circuit integrity for life safety, fire-fighting and property protection systems under fire conditions.

With the introduction of BS7346 in respect of fire-rated components, and specifically in Part 6 - Specification for cable systems, the British Standards Institute addressed the long standing concerns with respect to fire test methodology.

These had been earlier recognised in the commentary to BS 5839-1:2002, for fire alarm cables that identified a need for revision of the test methods of BS 6387 (for which the test rig is limited to smaller cables up to approx 20mmOD) to more accurately replicate the physics of a real fire scenario.

BS 7346-6:2005 introduces new test methods to assess cable performance under conditions of direct application of fire, direct mechanical impact and fire-fighting water shock - all conditions associated with a real fire incident.

Where previously each element of a test was conducted on a separate cable sample, this Standard requires the complete test to be applied to a single cable sample and, for the first time, it is subjected to direct mechanical shock and water jet impacts.

The test methodology has been devised, for a single cable sample, to validate that the fire-rated cable will maintain circuit integrity for the required survival time whilst simultaneously:.

* withstanding a constant temperature attack at 842DegC.

* withstanding repeated direct impact shock of approx 10N at 10 minute intervals.

* withstanding direct water jet impact equivalent to a 2 bar water hose at the point of mechanical impact during the last 5 minutes of the required survival time.

The BS 7346-6 Standard describes two classes of system, Life Safety and Fire Fighting which, when applied, determine the appropriate performance requirement according to the function of each circuit.

Life Safety requires control systems to remain functional for alarm and to support evacuation.

Suggested survival time requirements assessed as being for instance 30 minutes for single stage evacuation or 60 minutes for multi-stage evacuation.

Fire fighting systems require circuit integrity for 120 minutes to serve active systems for protecting and assisting fire-fighters in carrying out their role and would typically include pressure differential systems for fire-fighting, smoke and heat exhaust ventilation, sprinklers and fire fighting lifts.

Contained in the Standard are a table and an annex which provide a matrix of suggested survival times to meet different fire performance objectives.

Obviously, if for practical reasons a single cable type is to be employed for all applications then it would need to have the maximum fire survival performance to maintain full system integrity.

Most active fire safety systems in buildings rely on a dependable electrical power supply for detection, alarms, evacuation and communication thereby enabling safe evacuation and assisting the fire fighting response.

In the event of fire it is critical for electrical circuit integrity, for power, signal and control, to sustain operation systems incorporating equipment such as, sprinkler pumps, shutters and smoke curtains, smoke ventilation systems, fire fighting lifts, smoke control dampers and pressurisation fans.

The electric cables which maintain power, control and signals to such equipment are fundamental to the task.

By introducing a more accurate replication of fire dynamics into cable test methodology and classifying objectives, BS 7346-6 offers fire safety engineers a meaningful guide to the capability of different fire performance cables in respect of their application.

Within the new Standard, building services engineers and fire engineers, responsible for specifying fire-rated cable systems into ever more complex buildings, are provided with clear guidance for the selection of appropriate fire-rated cables to support Life Safety and Fire Fighting applications.

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