Life safety systems are defined under Rule 46-002 of the CE Code and required by Articles 3.2.7.4 through 3.2.7.10 of the National Building Code (NBC) to be supported by an emergency power supply, i.e. either a generator or batteries, to ensure continuity of operations. These loads are important for public safety in general and crucial for evacuations during fires and other emergency situations in particular. Examples include exit signs, egress route lighting, fire alarms, elevators and smoke-venting fans.

Under Rule 46-108, the wiring for an emergency power supply and any electrical equipment that is not defined as a life safety system must be kept entirely independent of all other wiring and equipment (except in transfer switches and emergency lights supplied from normal and emergency sources). Separation is required beginning at the first point of distribution for either power source.

An essential electrical system is defined under Rule 24-002 as having “the capability of restoring and sustaining a supply of electrical energy to specified loads in the event of a loss of the normal supply.” In healthcare facilities, an interruption of the normal supply can jeopardize the care of patients, as well as the safety of staff and the public.

Essential electrical systems are divided into three categories: vital, delayed vital and conditional branch. CAN/CSA-Z32 provides guidance as to what loads should be supplied by each.

The vital branch is the portion of the system in which the circuits require power restoration within 10 seconds. Healthcare examples include the intensive care unit (ICU) and emergency lights. The delayed vital branch is the portion where circuits require power restoration within two minutes, e.g. blood bank refrigerators.

The conditional branch is the portion where essential loads such as medical records storage may be transferred to the emergency source at a time dictated by the prevailing need. Loads on the conditional branch typically sustain the continued operation of the facility and some additional services, but are not as crucial to patients.

A need to clarify

As both Sections 24 and 46 are amendatory and NBC requirements are also applicable in healthcare facilities, there has been a need to clarify how Rule 46-108 can be applied within the scope of Section 24 where there are essential electrical systems. A new version of Rule 24-302 in the 2018 edition of the CE Code provides two options as alternatives to meeting the requirements of Rule 46-108.

The first permits the wiring of vital, delayed vital and conditional branches to be combined with that of life safety systems, provided all overcurrent devices of the essential electrical system are selectively co-ordinated. This ensures a fault in a conditional load does not cause power loss to a vital load, aligning with Clause 8.7.2 of CSA C282, Emergency Electrical Power Supply for Buildings.

The second permits the wiring of vital and delayed vital (but not conditional) branches to be combined with that of life safety systems without selective co-ordination of all overcurrent devices.

It is important to note it could be impractical to satisfy the first option of providing selective co-ordination down to the branch circuit level. Commentary Clause B.18 of CSA C282 clarifies that while most systems use moulded-case circuit breakers for protection, the ability to achieve selective tripping under short-circuit conditions is extremely limited. The use of air or power circuit breakers with a true short time rating and no instantaneous trips allows a much greater level of co-ordination.

The commentary further clarifies it is generally not possible to ensure selective tripping under short-circuit conditions between downstream panel breakers and those that feed the panels, given the moulded-case breakers. Selective tripping is achieved only when the magnitude of the fault current in the branch circuit is so low, it does not release the instantaneous trip of the breaker that feeds the panel. The cost of air circuit breakers for these applications is prohibitive.

One approach to satisfy the requirements of the new Rule 24-302 is to combine both options—that is, where possible at higher-level distribution, provide selective co-ordination, then start separating the branches when it becomes cost-prohibitive and, with regard to volt/current level, impractical.

The ultimate purposes of these rules are to maintain power to loads for public safety and patient care and to minimize the risk of power interruption to crucial loads due to faults in less crucial loads.

Nansy Hanna, is director for engineering at Ontario’s Electrical Safety Authority (ESA), chair of the Canadian Advisory Council on Electrical Safety (CACES) and a member of the ULC Advisory Council, CSA Technical Committee on Industrial, Consumer and Commercial Products and CSA CE Code-Part I, Sections 24, 32, and 46. She can be reached at nansy.hanna@electricalsafety.on.ca.