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Standby Power and Transfer Switching

Posted By Leslie Stoch, Wednesday, March 01, 2000
Updated: Monday, February 11, 2013

In this article, we will cover some of Canadian Electrical Code requirements for standby power and transfer switching. The National Building Code specifies the minimum requirements for emergency standby power supplies for different building sizes and classifications, for high-rise residential, commercial, industrial and commercial buildings depending upon size height and occupancy. It specifies the minimum electrical backup requirements for critical emergency facilities including fire alarm systems, fire pumps, elevators, lighting, exit signs, ventilation systems and emergency voice communications.

While the National Building Code explains what must be provided, the Canadian Electrical Code provides the rules for installing emergency backup power supply systems in buildings where they are required by the NBC. For the most part, these requirements are found in the CEC, Section 46, Emergency Systems, Unit Equipment and Exit Signs. Other sections of the CEC also contain rules for different aspects of standby power supplies and transfer switching.

When required by the NBC, Rule 46-202 specifies that an emergency power supply may consist of either:

  • A bank of storage batteries capable of maintaining at least 91 percent of their full voltage for the minimum time specified in the building code or at least ½ hour; or
  • An automatically started generator, having sufficient capacity to supply the required emergency loads.

Storage batteries must be rechargeable and equipped with a battery charging system. Due to their time and storage capacity limitations, battery systems are normally restricted to emergency lighting and exit sign applications. Larger buildings, required by the NBC to provide more than emergency lighting and exit signs (elevators, fire pumps, ventilation, etc.), are normally supplied by a fuel-driven emergency generator (diesel, gasoline, natural gas). The generator must have a sufficient supply of fuel to permit the generator to operate for the required period of time.

Rule 46-204 stipulates that the emergency power supply must be switched automatically to ensure that the standby supply comes on promptly on failure of the regular power supply. This rule also requires that transfer switching equipment be installed in a location inaccessible to unqualified persons. This is to ensure that such equipment is not inadvertently disabled, resulting in failure of backup power for emergency facilities at the time when needed.

As indicated above, an emergency power supply must not become accidentally disconnected. For this reason, Rule 46-206 also specifies that the building emergency panel supplying critical loads such as the fire alarm system, elevators, emergency voice communications, etc., must not have a readily accessible main switch, fuses or circuit-breaker. Main overcurrent protection for the emergency panel should only be located in a locked room containing the emergency generator or batteries. Once again, this requirement also helps to guarantee continuity of the standby power supply when it is needed.

To ensure that operating personnel become aware when the emergency power supply is out of service, Rule 46-208 requires:

  • Audible and visible alarms that warn of problems such as failure of the battery charging system, or emergency generator problems such as overheating, low fuel or high bearing temperatures.
  • An audible alarm may be silenced, but a red trouble light must remain on to remind personnel that a problem still continues.
  • When the problem has been corrected, either an audible alarm must remind personnel to reset the trouble signal, or it resets automatically.

Section 32, Fire Alarm Systems and Fire Pumps in the Canadian Electrical Code also contains some special requirements for electrical backup and transfer switching for fire alarm systems and fire pumps.

To minimize the possibility of being accidentally disconnected from its backup supply, Rule 32-108 requires that a fire alarm system must be connected as close as possible to the terminals of the transfer switch where the fire alarm receives its emergency supply when other equipment is also supplied. Usually this means connection to an emergency electrical panel supplied from the regular and standby power sources.

To ensure that fire pumps are capable of operation during a fire, Rule 32-204 permits a separate electrical service for the fire pump(s). In an exception to other rules, this service may be located remotely from the main service. This helps ensure that any faults in the main electrical installation do not hinder operation of the fire pump(s). A remote location may also be necessary for placement of the fire pump service in relation to the emergency power supply and transfer switching.

To minimize the possibility of an electrical failure during disruption of the main electrical supply, Rule 32-206 requires that each fire pump have a dedicated automatic transfer switch, approved for fire pump service. In other words, each fire pump must have its own switch. The switch must be located either in a barriered compartment of the fire pump controller or in a separate enclosure adjacent to the controller and labelled to identify it.

Other sections of the Canadian Electrical Code contain requirements for backup power supplies and transfer switching. For example, Rule 14-612 in Section 14, Protection and Control requires that transfer switching between the regular and emergency standby power supplies must prevent the inadvertent interconnection of the normal and standby sources. This means that the transfer switch must disconnect the regular source before connecting the standby source during a power failure. An accidental interconnection between normal and standby supplies could create an electrical hazard to personnel working to correct the problems in the regular supply. Rule 6-106 in Section 6, Services and Service Equipment also stipulates that where a service is supplied by more than one system, the switching must be arranged to prevent systems from interconnection.

Uninterruptible power supplies (UPS) are used to provide computers and other sensitive or critical electronic equipment with a clean source of power and protection against voltage surges, frequency fluctuations, with temporary protection against power failures. Normally, the UPS has solid state transfer switching between the incoming and the uninterruptible power supplies to provide rapid switching when the inverter fails. Rules 14-700 to 14-704 of the CEC prohibit the use of this solid state switch as a disconnecting means. A separate circuit-breaker or disconnect switch must be used to isolate these systems from each other when personnel must work on the regular supply with the UPS in operation. This is intended to avoid electrical shock hazards due to leakage through the solid state devices or failure of the solid state switch.

As with previous articles, you should consult your local electrical inspection authority for a more exact interpretation of any of the above.


Read more by Leslie Stoch

Tags:  Canadian Code  March-April 2000 

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