Variable speed drives and fire
Engineers and managers involved with buildings are well aware of the benefits of variable speed drives, but many have concerns about specifying them in applications where they need to continue operating in the event of a fire. A lot has, however, been done to address those concerns, as Georgina Perkins of Siemens Building Technologies explains.
There are many building-related applications where the use of variable speed drives (VSDs) is highly desirable for reasons of convenience and energy efficiency. In some of these applications, such as stairwell pressurisation and air extraction, it is essential that the drives should continue to operate for as long as possible in the event of a fire, in order to give the building occupants the best chance to exit safely.
But what is the best way of ensuring that VSDs continue to operate in fire? Simply using a standard drive and hoping that it will provide a satisfactory performance isn’t an option. Thermal and other protection built into the drive to safeguard it in normal operation will almost certainly shut it down very quickly under fire conditions, which is clearly unacceptable.
What’s needed is a VSD which behaves like any other under normal conditions, but has a special operating mode – often called essential services mode – to cope with fire. To date, the standard that most specifically addresses essential services mode and the behaviour of VSDs in fires is the Australian and New Zealand Standard AS/NZS 1668.1:1998.
This standard covers: “The use of ventilation and air conditioning in buildings – fire and smoke control in multi-compartment buildings”, and makes it very clear that: “The fundamental purpose of this document is life safety.”
It would be easy to think that, with this standard to hand, VSD manufacturers would have to do no more than follow its recommendations in order to implement an acceptable essential services operating mode for their products. Unfortunately, it’s not that simple. The standard does not explicitly tell VSD manufacturers what to do, nor does it provide instructions for any tests to be carried out.
Further, some of the statements in the standard seem to fly in the face of good engineering practice. For example, after stating that VSDs should be selected to provide a high degree of reliability under fire conditions, Section 4.12.1 of the standard goes on to say: “Overload safety devices of variable frequency inverters in equipment shall be automatically overridden when operating in the essential services mode.”
Unfortunately, this statement has been reflected in some tender specifications in the phrase ‘run to destruction’, a phrase which, incidentally, is never used in the standard. A small number of VSD manufacturers and consultants have gone further by interpreting this to mean that all protection in the VSD should be bypassed when it is operating in essential services mode.
The idea is that removing all protection – including over-current, over-voltage, short-circuit and ground-fault protection – will somehow maximise the opportunity for the VSD, in the words of the standard: “To continue operating in spite of any predicament occurring either at the motor, the cabling or surrounding environment to the VSD.” Unfortunately, this idea is based on wishful thinking rather than sound engineering facts.
It is true, however, that in essential services mode, the role of protection in a VSD changes considerably. Under normal conditions, the protection is designed to safeguard the drive, the cabling and the motor. In essential services mode, the only requirement is to safeguard life, by ensuring that the VSD continues to operate for as long as possible.
Siemens believes that this requirement cannot be met by a VSD which has had all protection removed. Consider, for example, what would happen if such a VSD were subjected to short term overvoltage on the mains supply, something which is quite likely in a burning building. With no protection, the electronic devices in the VSD would almost certainly be destroyed, causing complete and irreversible drive failure.
Similar examples could easily be quoted for other forms of protection, leading to the inevitable conclusion that removing all forms of protection in essential services mode is much more likely to shorten the period for which the VSD operates rather than extend it.
But what’s the alternative? Extensive research has led Siemens to the conclusion that basic protection, including over-voltage and over-current, must be retained, but should be automatically reset after a trip, and that an unlimited number of automatic reset attempts should be permitted. In short, the drive should be protected, but it should keep trying to run.
It is not, however, necessary to retain all forms of protection. Over-temperature protection, for example, is designed primarily to prolong the service life of the VSD, which is hardly a concern in the event of a fire. Bypassing this protection is, therefore, beneficial.
Deciding on a suitable level of protection is, of course, not the whole story when it comes to selecting VSDs for fire-related applications, and the Australian standard contains other important requirements for the VSD:
• Its required function will be initiated and continue during the specified fire conditions. This is best achieved by means of a hard-wired signal from the fire-control panel to the VSD which will switch it into essential services mode.
• Its local failure will not render the smoke control system inoperable. This is only really possible if the VSD is complemented by a standby system, either in the form of a backup VSD or a conventional motor starter.
• It cannot be inadvertently deactivated, programmed or otherwise left in a non-functional condition by operating or maintenance personnel, thus rendering it incapable of operating correctly in essential service mode. This requirement can be met by ensuring that, once essential service mode has been entered, the only way the VSD can be stopped is by a signal from fire control panel, or by turning off the power.
• Equipment based on electronic circuitry will be installed so that it functions normally, or is tested and verified, at least four times a year. Most of the VSDs will be in regular use as normal drives, so this requirement will be satisfied automatically. For those that are not, routine testing is easily carried out as part of a regular maintenance schedule.
VSDs are commonplace in modern buildings, and they are unrivalled in the opportunities they offer for effective control and for saving energy. Reservations about their performance in the event of a fire have, however, been a barrier to their widespread adoption in some applications.
Recent work carried out by Siemens has provided an effective answer to these reservations and, while no supplier can ever claim that its products will not fail in a fire, the result is VSDs which offer a sensible and realistic approach to essential service mode operation. With this in mind, consultants, engineers and facilities managers can now take full advantage, in almost every application, of the many benefits which VSDs undoubtedly provide.