When is fail-safe really fail-safe?

Fail-safe systems exist in many industrial sectors. However, there are no suitable standards in shaft smoke extraction. If there is a power failure, the ventilation flap has to open. But there are also other error sources…

Safety-relevant installations adhere to a design principle in mechanical and plant engineering. They are designed to be ‘fail-safe’. The system is automatically placed in a safe condition in the event of a defective component or human failure. The precondition for applying this principle is the existence of a safe system condition.

This safe system condition also exists in installations for conducting smoke out of lift shafts. This is then achieved if the flap that is closed in normal operation on the shaft head has opened.

Fail-safe in shaft smoke extraction

In the absence of such suitable standards or generally acknowledged technical rules for systems for shaft smoke extraction, manufacturers like to make use of related standards for smoke and heat extraction technology. For example, inspecting and distributing the closing flaps according to EN12101-2 as natural smoke and heat extraction devices is customary.

There is no standard applicable to the actuation of these flaps; inspection can only occur alternatively according to 12101-10. The following definition can be found there under 3.1.12: Fail Safe System: "Smoke and heat extraction system that automatically enters the safe condition upon a power failure, the smoke extraction position. "

What types of fault must a shaft smoke extraction system protect against in a "fail-safe" manner? There is no question that in the event of a power failure, the safe condition of the system (open flap) must be established – as described in EN 12101-10. However, a shaft smoke extraction system does not automatically qualify as "fail-safe" if only this characteristic is met.

For example, other kinds of faults can occur that have to be taken into consideration:
- missing components (e.g. punctiform smoke detectors that have been removed from their mounts)
- power interruption (e.g. separation / cutting of a line)
- short-circuits in a line (e.g. damage to a line by penetration of foreign bodies like nails)

While the installer has no influence on the risk of a power failure, the probability of damage to lines and components depends on who has access to the installation location (shaft).

The danger of such damage increases if other building trades, such as the lift maintenance company or untrained or poorly trained subcontractors, work in the lift shaft, apart from the smoke extraction system installers. Therefore, special attention must be paid to fail-safe design of the system regarding shorts to another wire or line interruptions if the installer is not alone in working in the shaft.

Active or passive systems

There are different approaches to protecting a system for smoke extraction against such failure sources. A spring-loaded drive is often used, for example, that opens automatically in the event of a power failure.

Active line monitoring checks every wire for interruption and short circuits and opens the connected flap in the event of malfunctions. Passive systems rely on relay technology and interrupt the supply voltage when the smoke detector or manual button is actuated.

Without additional measures, penetration by a foreign body can produce the kind of short to the lines that actuation of a smoke/fire alarm by one of the underlying elements remains undetected and without consequences. Consequently, additional measures should be taken to prevent this case too.

Thus, when designing a system for smoke extraction from lift shafts, the "fail-safe" attribute is only justified if faults and shorts in lines are actively detected or the safe condition is brought about by other suitable measures.

Martin Müller
The author is a graduate electrical engineer and works for Bluekit as Head of Business Development.

More information: bluekit.de