Relay eliminates power failures

It's no secret that building evacuations disrupt productive workflow. And building evacuations may be caused when power fails to restore automatically through automatic and reliable transfers. Engineers can minimize power failures through transfer switches by following the model set by ARX Engineering, Alberta, Canada.


It's no secret that building evacuations disrupt productive workflow. And building evacuations may be caused when power fails to restore automatically through automatic and reliable transfers. Engineers can minimize power failures through transfer switches by following the model set by ARX Engineering, Alberta, Canada.

Within a one-year period, three major facilities in Calgary, Alberta, experienced a failure of the utility source when power failed to restore automatically—resulting in building evacuations.

The increased frequency of power-transfer scheme failures is attributable to the age of the equipment in these large commercial buildings and the use of archaic technologies. These designs use devices such as electromechanical relays and pneumatic timers, which have a limited service lifecycle, in comparison to the more advanced micro-processor-based devices.

“Rather than continue to repair these problematic systems, ARX was determined to design a new modern protection and control scheme,” said Roland Davidson, R.E.T., operations manager at ARX Engineering. “The goal was a solution that would not only improve reliability but also enhance the performance of the system.”

With the backing of Brookfield Property Management and Oxford Properties, ARX Engineering designed a retrofit transfer scheme that improves reliability, meets the current utility requirements, provides enhanced protection features, and offers feedback to the building operators.

After consultation with Schweitzer Engineering Laboratories, Pullman, Wash., the design was constructed with a powerful and flexible microprocessor-controlled protective relay, the SEL-351 Directional Overcurrent and Reclosing Relay. The SEL-351 Relay is widely used at utilities and substations around the world because of its extensive monitoring, protection, communications, and automation features.

“At one facility, we installed two SEL-351 Relays on each of the main breakers plus four SEL-551C Overcurrent/Reclosing Relays on the feeder breakers,” Davidson said. “The scheme is a 25 kV preferred source with a hot alternate. Many of the major office towers in downtown Calgary are serviced by a 25 kV network with the preferred feed in one building being the alternate in an adjacent building. Upon the failure of one of the 25 kV feeds, the affected buildings will transfer automatically to their alternate source. The local utility (ENMAX) will then restore the buildings back to their original state after the problem is corrected under a defined procedure.”

Transfer scheme design parameters include a selectable preferred source, automated and synchronized transfer to a second source in case of power sags or other disturbances, and remote alarm notification to the building engineering staff should there be a loss of the standby (secondary) source. Overcurrent protection and lockout features are standard functions of the SEL-351 Relay.

“Event reports, communications, load profiling, ease of installation, flexible programmable logic, and enhanced protection features have considerably improved the reliability of this system for the property owners,” Davidson said.

With the successful installation of this system at five facilities within the Calgary marketplace, the building owners have seen the benefits of the installation in that they have not experienced any malfunctions and have renewed confidence in the event of a problem with the alternate supply. An unforeseen benefit is that they now are aware when the utility transfers them onto the alternate source without notification, and they are aware if there is a problem with the utility alternate source.

Information provided by ARX Engineering, Calgary, Alberta.


Three facilities in Calgary, Alberta, experienced power failures due to poor transfer switches. Rather than repair problematic systems, ARX Engineering solved the problem by designing a retrofit transfer scheme constructed with the microprocessor-controlled SEL-351 Relay from Schweitzer Engineering Laboratories.

The relay features a breaker wear monitor; station batter monitor; remote and local control switches; synchrophasor measurements; and overcurrent protection that uses a secure mix of phase, negative-sequence, and ground overcurrent. For more information on the relay, go to

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