When required by the National Building Code of Canada, the Canadian Electrical Code, Section 46, Emergency Systems, Unit Equipment and Exit Signs provides installation and maintenance requirements for standby power for essential services when the regular power supply fails. Other sections of the code contain some requirements as well, including Section 32, Fire Alarm Systems and Fire Pumps. Miscellaneous requirements also show up in other sections such as Section 14, Protection and Control, and Section 10, Grounding and Bonding.
Rule 46-202 specifies that an emergency power supply may consist of either:
a) rechargeable storage batteries of sufficient size to carry assigned loads and maintaining at least 91 percent of full voltage for minimum ½ hour or the time period specified in the National Building Code; or
b) an automatically started generator.
The supply to essential safety services such as elevators, fire pumps, lighting and ventilation must be transferred to the emergency supply automatically when the regular electrical supply fails.
Rule 46-206 specifies:
(1) No device capable of interrupting the circuit, other than the overcurrent device for current supply of the emergency system, shall be placed ahead of the branch circuit overcurrent devices.
(2) The branch circuit over- current devices shall be accessible only to authorized persons.
What does the code intend by Rule 46-206? Through this rule, the code wants us to ensure that the emergency system will always be capable of responding when emergency power is needed. That’s why it specifies that there must be no main disconnect or main circuit-breaker ahead of the emergency panel, except when located in the generator or battery room. The emergency supply circuit should be as uncomplicated as possible to minimize chances of failure. The main disconnect must be accessible only to authorized people to minimize the possibility of someone inadvertently disabling the emergency system. The emergency panel must also be locked, and accessible only to authorized people.
Emergency systems must be well maintained to avoid the risk of failure at a critical time. For this reason, Rule 46-208 specifies that: “audible and visual trouble-signal devices” be installed to alert people when the emergency power supply has a problem and when it is operating. When it sounds, the audible signal may be shut off, but a red warning light must remain on to remind people of the current circumstances. When the emergency supply is restored to normal, an alarm sounds as a reminder to reset the silencing switch, or it may reset itself automatically.
When an emergency power supplies fire pumps, Rule 32-206 specifies that each fire pump must have its own transfer switch. A fire pump transfer switch may be located inside the fire pump controller or in a separate enclosure near the controller. The transfer switch must be approved for fire pump service and marked with permanent identification.
Emergency generators can also present special problems, especially when the building’s power supply includes switchboard ground-fault protection. You will probably recall that Rule 10-204 only allows the electrical system to be grounded:
a) once at the main electrical service;
b) once upstream from the main service, at the transformer; and
c) with no grounding connections downstream from the main service, except when one building supplies another building.
Some 3-phase, 4-wire generators have an interconnection between the generator case and the neutral point in its windings. This may cause two problems:
a) With such an interconnection, it is practically impossible to avoid grounding the system neutral through the generator case, past the main service in violation of Rule 10-204.
b) A downstream ground connection may prevent the main switchboard’s ground-fault protection from sensing a ground fault and preventing it from tripping when it should be protecting the system from ground-fault damage.
There are two practical solutions, by either selecting a generator with no interconnection between its case and its windings, or connecting the generator to the main supply through a 4-pole transfer switch that disconnects the system neutral from the generator when it is not in operation.
Another problem may arise when an uninterruptible power supply (UPS) is used to provide clean, uninterruptible power to sensitive electronic equipment. The UPS normally uses an electronic transfer switch that closes quickly to supply its load directly from the regular supply when the UPS is out of service. The electronic transfer switch remains open when the main power supply fails. However, Rule 14-700 prohibits reliance on solid state devices as isolating or disconnection means. Therefore, a circuit-breaker is normally connected in series with the electronic transfer switch to isolate the UPS output from the main supply when it is out of service, and people need to work on the main supply.
As with previous articles, you should check with the electrical inspection authority in each province or territory as applicable for an exact interpretation of any of the above.