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BSEE FIRE SAFETY N


There are many, apparently, innocuous features built within today’s variable‐speed drives that may go unnoticed. ABB’s Andy Preston discusses the benefits of a fireman’s override.


ot all variable- speed drives (VSDs) have a fireman’s override or, as it is


sometimes referred


to, “fire mode” – simply because not all applications require such a feature.


A fireman’s override 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. As such, this feature is most commonly found in VSDs designed for commercial or industrial buildings and tunnels.


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.


The VSDs then operate the HVAC motors in reverse to remove smoke from the building, assisting with visibility and safety during firefighting. Fire mode cannot be triggered by any other signal or manual option. Once the VSD enters fire mode it cannot exit this mode until the fire condition signal is reset.


Updating today’s specifications


Today, most consulting engineers’ specifications feature a bypass along with the VSD. However, these specifications assume that bypasses are always needed. But the reliability of today’s VSDs are such that they rarely fail and even include TUV safety certification. A fireman’s override helps consultants to design functioning escape routes and escape strategies within a building. It allows emergency services to easily trigger these essential functions to aid in the evacuation of personnel or whilst fighting fires.


Also, 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, thereby keeping the stairwell positively pressurised to keep smoke and fire out.


Furthermore, if there is a sudden pressure change cause by doors opening or windows blowing out or walls burning away, 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.


Variable-speed, therefore, allows more specific escape routes to be designed by employing areas of positive pressure that hold stairwells open. Because the speed can be variable under PID control, changing building conditions can be accommodated, such as doors opening and closing.


Advertising: 01622 699116 Editorial: 01354 461430


TAKING CONTROL OF SMOKE EXTRACTION Why the fireman’s override in VSDs is a critical decision


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.


Choosing the right motor


For smoke ventilation, smoke rated motors must be used when the VSD’s fireman’s override is activated. Therefore, it is essential that motors suitable for use with VSDs are selected at the outset. Modern VSDs, with their ability to control speed, can enable a more elegant and adaptive smoke control strategy than traditional DOL approaches.


Adhering to the regulation


EN12101-3 allows the use of smoke rated motors, together with safety margins. In addition, the regulation demands that VSDs have suitable output filters, as the motor insulation, being used in high temperatures, may require further protection. VSDs are proven to be more reliable than contactors and the new regulation allows them to be part of the safety systems in machinery, hence it is


recommended to use the latest technology to bring the best safety system with the simplest design, and higher safety integrity.


Further advice


VSDs are typically designed with a keypad that can accommodate programming and controls. When the smoke control system is activated either automatically or manually at the firefighter’s control panel, this keypad needs to be overridden and all control/ programming functions disabled. In addition, the keypad may have an “off” feature. When in fireman’s override mode, this “off” function must be disabled to ensure no accidental stopping can occur. Since the override function is safety related, changing any of the functions should require a password before changes can be made.


Summary


Without the fireman’s override built into the VSD, the function would have to be handled by the building management system (BMS). This makes programming and installation more complex.


Also, since the VSD has to locally control the motor so that it runs to destruction (ignoring all faults), this function could not be performed by the BMS on its own, as the VSD needs a special mode to ignore these issues internally.


Simpler DOL systems would have to be employed which would be less elegant at control, and modern high rise buildings would be harder to protect.


Email: energy@gb.abb.co.uk – and request the Fireman’s override eBook


www.abb.co.uk/hvacdrives


uThe UK trade organisation, Gambica, has produced a brief guide for using VSDs in “fire mode” – http://www.gambica.org.uk/resourceLibrary/gambica‐; guide‐‐‐fire‐mode‐in‐variable‐speed‐drives‐pdf.html


uToday’s variable‐speed drives have a host of features built in to make systems more efficient and buildings safer, including fireman’s override.


26 BUILDING SERVICES & ENVIRONMENTAL ENGINEER JUNE 2017 VISIT OUR WEBSITE: www.bsee.co.uk


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