Usually we can assume that the rules of the Canadian Electrical Code are based on some basic principles, which don’t vary a whole lot — to minimize the possibilities of electrical fire and shocks. But are the rules ever in direct conflict with each other or their principles?
Thankfully, we don’t need to consider this question too often. Rules are generally written to be consistent with other rules and some firmly held electrical safety principles. However, Rule 14-l00(d) in the 1998 Canadian Electrical Code seems to have strayed some distance from the fold. Here is the new 1998 code rule:
“Each ungrounded conductor shall be protected by an overcurrent device at the pointwhere it receives its supplyof current and at each pointwhere the size of the conductor isdecreased,except that such protection shall be permitted to be omitted: (d) Where the conductor:
(i) Forms part of the only circuit sup- plied from a power or distribution transformer rated over 750 kV with primary protection in accordance with Rules 26-252(1), (2), and (3) and that supplies only that circuit; and
(ii) Terminates in a single overcurrent device with a rating not exceeding the ampacity of the conductor(s) in the circuit; and
(iii) Is protected from mechanical damage
The basic electrical safety principle provided in the code is pretty clear — we have to provide overcurrent protection for conductors at points of supply or where conductor size is reduced. Rule 14-100 gives us a few exceptions under some very controlled conditions. The reasoning — to ensure that unprotected conductors in buildings don’t create any undue electrical fire or shock hazards. This new sub-rule of Rule 14-100 seems to imply that these principles are now changed when the transformer primary voltage exceeds 750 volts.
What Rule 14-100(d) says — we can now run the secondary conductors from a power or distribution transformer as far as we like without overcurrent protection if the transformer primary voltage exceeds 750 volts. The conductor run must still be protected from damage and terminate in a single circuit-breaker or set of fuses which protect conductors at their current ratings.
It seems odd that sources of power should receive different treatment when they are larger (power and distribution transformers with primary voltages above 750 volts). Thisseems even less reasonable when we consider that Table 50 (for primary voltages above 750 volts) allows primary overcurrent protection up to 600%. This of course allows the same multiple of transformer secondary current at the transformer secondary. Add to that the fact that larger power and distribution transformers can deliver higher phase-to-phase and phase-to-ground fault currents, which are not easily detected or interrupted by the transformer primary side protection.
Other 1998 CEC rules appear to support the general principle that conductors must always be protected by properly sized overcurrent and/or ground fault protection, with some closely controlled exceptions. Some examples:
1) Rule 14-102 was originally written to provide ground fault protection, to prevent damage due to low level arcing ground faults in large services and feeders. Obviously, the effectiveness of ground fault protection is greatly reduced when conductors are allowed to travel a long distance through a building before reaching the point where they are protected.
2) Rule 6-206(e) specifies that that the main service box must be “as close as practicable to the point where the consumer’s service conductors enter the building.” Once again, this rule is designed to limit the length of wiring in a building and thereby limit the possibilities for unprotected arcing faults.
3) For the same reasons, Rule 14-100(b) only allows us to reduce wire sizes when conductor runs are restricted to maximum of 3 m and when installed in a totally enclosed raceway, armoured cable or metal-sheathed cable.
4) The same for Rule 14-100(c), which allows us to reduce wire size down to 1/3, but only up to 7.5m with mechanical protection.
5) Rule 14-100(f), a variation of Rule 14-100(c) allows us to reduce primary conductors to a transformer down to 1/3 as long as both the primary and secondary conductors are protected against damage and their total lengths limited to maximum 7.5 m.
If we can assume that all of these rules are designed to minimize the occurrences of arcing faults for unprotected conductors in buildings, why is Rule 14-100(d) so out of tune with the rest? I would be pleased to find out your answers to this puzzling question.
As in previous articles, you should consult the inspection authority in the province or territory as applicable for a more precise interpretation of any of the above.