Thursday, 20 October 2016

Carl Turbitt, ABB's UK HVAC Drives Team Leader writes: I just wanted to take this opportunity to remind the building design community of a very important, yet often overlooked, feature of a variable-speed drive (VSD): the fireman’s override or, as it is sometimes referred to, “fire mode”.

It is an important tool in the design of building safety systems and performs three distinct functions: fireman’s override, fume extraction and stairwell pressurisation.

So what is “fire mode”? It is a switch or input that allows the fire service to take control of those VSDs controlling fans and turn them into smoke extraction units to maintain escape routes. It maintains the VSD’s operational availability whilst it is being used as part of the emergency fire control operations.

Fireman’s override

The fireman’s override allows the VSD to be programmed to a pre-set or controlled speed for assistance in emergency situations or emergency evacuation route protection.

The mode is usually triggered with a special key at the fireman’s control station. Upon receipt of a contact closure or a signal from the building’s fire alarm system, the VSD enters “fire mode” and overrides all other inputs whether they be analogue/digital, serial communication and all keypad commands. It ignores reset faults and warnings to ensure a “run at all costs” operation and forces the motor to run at the adjustable, pre-set speed or PID controlled speed.

Fume extraction

In the event of a fire, today’s buildings feature dedicated smoke management and extraction systems that localise and extract smoke such that the rest of the building remains smoke free and can be evacuated safely.

The VSDs operate the HVAC motors in reverse to remove smoke from the building, assisting with visibility and safety during firefighting.

Stairwell pressurisation

The emergence of stairwell pressurisation to ensure escape routes are accessible is easier to achieve with a VSD in PID control than with an uncontrolled direct-on-line (DOL) motor. This is because DOL runs the motors at full speed without any control, whereas PID control maintains the pressure in a stairwell at constant value, typically 50 Pa, thereby keeping the stairwell positively pressurised to keep smoke and fire out.

Furthermore, if there is a sudden pressure change caused by doors opening or windows blowing out, the PID controller detects such changes and alters the motor speed accordingly to keep the pressure correct – DOL cannot do this as it is ON or OFF.

Careful use of speed control and the ability to reverse motors, or even run them at higher than synchronous speeds, makes VSDs ideal for inclusion in escape route management. Being able to run at all speeds and in all directions means correct pressures can be controlled, in an ever changing building.

Also remember that motors running too fast (DOL speeds) can generate pressures that are too great, and perhaps cause overpressure which can make occupants in the stairwell uncomfortable, hence needing a controlled set-point. Too great a pressure could blow out windows or open doors, defeating the point of an escape route.

To learn more about fireman’s override, download our eBook: Top Tips No.21
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