Martin Blount, Technical Manager at Craig & Derricott, takes a look at the requirements for automatic transfer switches for life safety, fire-fighting and other critical applications, in accordance with the British Standards.
An automatic transfer switch (ATS) is an intelligent power switching device governed by dedicated control logic. The principal purpose of an ATS is to ensure the continuous delivery of electrical power from one of two power sources to a connected load circuit. As such, ATSs are commonly used for life-safety systems, data centres and other equipment where a loss of supply could cause damage to equipment or present a danger to health and safety. Since the introduction of BS EN 60947, any items used for the purpose of power transfer must comply with BS EN 60947-6-1 (Low-voltage switchgear and controlgear – Multiple function equipment. Transfer switching equipment).
One standard does not cover everything
It is common for designers and specifiers to overlook other relevant standards that may affect the function, build or operation of an ATS. For example, forms of separation are rarely considered in the design of an ATS, even though BS EN 61439-2 very much applies and access to the equipment can be necessary during commissioning and maintenance. There’s a small, hidden comment hidden in an annex of BS 9999 (Annex V.4.5), that states for evacuation lifts and fire-fighting lift installations, “A failure of the primary power supply should be simulated once a month. If a generator provides the standby power supply, it should energize the lift(s) for at least 1h.”. It’s easy to overlook associated standards when specifying ATSs but reputable manufacturers should be aware of the applications of their product and can offer advice and training on specifying the correct equipment for the application.
The introduction of BS 8519
One standard that is often referred to when specifying ATSs is BS 8519 (Selection and installation of fire-resistant power and control cable systems for life safety, fire-fighting and other critical applications – Code of practice). Replacing BS 7346-6, BS 8519:2010 dedicated a whole four paragraphs to ATSs, offering little guidance other than the ATS should conform to BS EN 60947-6 and “conditional to the occupation of the building, a bypass arrangement should be incorporated to enable the changeover device to be maintained without loss of service from the critical plant.”. Effectively, incorporating a single-line bypass feature to an ATS was all that was needed to fulfil the requirements of the standard. At least it was for a decade… BS 8519:2020 greatly expands on clause 9, entitled “Automatic changeover devices”, which was to be expected. At a time when life safety and fire-fighting systems were under intense scrutiny, deeper consideration of all the associated systems was welcome. The section now covers a whole page and has necessitated the biggest change in ATS design in years, however, not all of the changes were expected, or even accepted as progress.
Contactors
The standards insist that “the ATS device should meet the PC classification conforming to BS EN 60947-6-1”. This on its own does not rule out the use of contactors as BS EN 60947-6-1 accepts contactors that have a manufacturer assigned conditional short-circuit current rating and which meet certain temperature rise criteria. Therefore, contactors can meet the CC and PC classification, however, the comments, “The automatic transfer switch (ATS) should be a single component with integrated controller from the same manufacturer” and “should be based on switch technology” does rule out the use of contactors with separate controls for life safety circuits.
Motors and their use in critical systems
The mention that the ATSs should be specified with a utilisation category of AC33A or AC33B was no surprise, as many life-safety and fire-fighting loads that the standard refers to contain motors, i.e., fire-fighting lifts, smoke extract fans, sprinkler pumps.
Maintenance bypasses
Although BS 8519:2020 goes into slightly more detail on the bypass, it passes the responsibility of deciding whether it is indeed required and the type of bypass needed to the designer or consultant. “The designer should carry out a design risk assessment for the specific application to identify the impact of the loss of service, for example, during planned maintenance. It should identify whether there are alternative management activities that could be put in place that would overcome the need for the bypass.” This suggests that building schemes could comply with the standard without the use of a bypass in the critical ATSs on the condition that a risk assessment has been carried out to prove that their inclusion wouldn’t negatively affect the safety of the system. All ATS manufacturers should be aware of BS 8519:2020. If they see a specification referring to this standard and there are no details on the type of bypass required on the ATSs, then questions should be asked. The standard puts the responsibility on the designer to specify this so no reputable manufacturer will risk the liability of quoting the incorrect product.
Conclusion
The impact of BS 8519:2020 was considerable and instantaneous. Seemingly overnight, ATS specifications were demanding compliance to the standard, with or without the necessary information on bypass types or application information, leaving manufacturers chasing information from customers and chasing material from suppliers that are battling with unprecedented demands. External factors such as Brexit and COVID-19 have only made supply chain issues worse. There’s no doubt that the standard is a half-step in the right direction, but when specifying critical ATSs now, it’s important to fully understand it, along with any other associated standards that may affect the application. Craig & Derricott offers an extensive range of low voltage products, including automatic transfer switches, EV chargers, electrical switchgear and control stations, rail rolling stock components and rail LED lighting, all supported by a bespoke design service. We provide a wide of Automatic Transfer Switches that meet the requirements of both BS 8519: 2010 & BS 8519:2020, as well as a series of CPD approved training presentations covering different topics within the Low Voltage Switchgear industry.
To get more details about Craig & Derricott’s range of Automatic Transfer Switches click here
