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Main protective and control devices for emergency generators — are we consistent on this issue?

Posted By Ark Tsisserev, Saturday, January 01, 2011
Updated: Wednesday, September 05, 2012

Let’s say, you are a supplier of an emergency generator or a designer of an emergency distribution system, and your task is to select an emergency generator and main disconnecting means and overcurrent devices that will manually or automatically disconnectthe electrical system supplied from the emergency generator.

The question is, Must the generator be provided with a singleprotective and control device?

This innocent question appears to create lots of confusion for the suppliers of emergency generators, electrical designers and electrical safety regulators; and the approach to this issue in the industry is far from being consistent.

Perhaps, the best place to find the answers is the Canadian Electrical Code — the standard for the electrical installation criteria.

Our first stop would be Section 28, as this section covers motors and generators. Rules 28-900 – 28-908 govern protection and control of generators, and it would beseemed logical to seek the answer within these rules. We may consider ourselves lucky, as the very first rule in this subsection (Rule 28-900)deals with disconnecting means required for generators.

This rule in part states that, "Generators shall be equipped with an indicating switch or circuit breaker by means of which the generator and all protective devices and control apparatus are able to be disconnected entirely from the circuits supplied by the generator…”

So, the issue appears to be solved, and the answer to our question above is deemed to be "Yes.”

But let’s take a deep breath and review the requirement mandated by Rule 28-900.

First of all, thisrequirement relates to generators in general, and it does not necessarily deal with an emergency generatorthat is governed by Section 46 of the CE Code.

Secondly, the requirement of Rule 28-900 appears to be inconsistent with the scope of the CE Code, as it does not deal with the installation criteria but seems to coverrequirements of "approved” product installed under rules of the Code.This latter observation means that Rule 28-900 does not appear to belong to the installation Code, as installation Code covers only installation of "approved” equipment. This also means that if a generator is, indeed, required to be equipped with "an indicating switch or circuit breaker,” such requirement should be in the CSA safety standard C22.2 No.100 that covers design and constructionof motors and generators.

S/C responsible for Section 28 has acknowledged this inconsistency, and the proposal to modify this requirement is currently being deliberated by this S/C.

Therefore, the answer to our question posed at the outset of this article has not been provided as yet.

Now is the time to make the second stop — Section 46. This section covers installation criteria forsources of the emergency power supplymandated by the National Building Code of Canada for life safety systems. Life safety systems are defined in Section 46, and this article is not intended to dwell on this matter. For the purpose of our discussion, the important fact is that Rule 46-202(3)(c) states that where a generator is utilized as a source of required emergency power supply for life safety systems, then such generator mustconform to the CSA standard CAN/CSA C282.

Before we’ll review this standard, let’s look at another provision of Section 46 in respect to the overcurrent device for an emergency power supply.

This next step will lead us to Rule 46-206(1). This rule states that "the overcurrent device for an emergency power supply shall be coordinated with the overcurrent devices of feeders and branch circuits supplying life safety systems and other electrical equipment connected to the emergency power supply in order to provide selective operation of the branch circuit overcurrent device when a fault occurs in that branch circuit.” If we’ll review this statement carefully, and if we’ll check out figure 8 in Appendix B Notes on Section 46 (page 430 of the CE Code), we’ll acknowledge that this rule does not specifically mandate the installation of the overcurrent device for an emergency power supply (as this O/C device may be mandated by the CSA standard C282). This rule simply states that the O/C protective device for an emergency generator must be coordinated with downstream O/C devices installed in accordance with the CE Code. And such coordination is required only between the emergency generator O/C device and O/C devices installedin feeders and branch circuitsthat supply life safety systems connected to the emergency generator. Figure 8in Appendix B Notes on Section 46 also appears to explain that the single O/C device for an emergency generator (which must be coordinated with the O/C devices of downstream feeders) is outside the scope of the Code, as it is a part of an emergency generator in accordance with provisions of the CSA standard C282.

But before we’ll move to this standard, let’s take a look at the CEC generic requirements in respect to protective and control devicesat any point of supply.

Section 14 of the CE Code is the place for these requirements. Rule 14-010 of the Code provides general criteria for such protective and control devices. This rule mandates that unless specifically indicated in the Code, all "electrical apparatus and ungrounded conductors” must be providedwith:

(a)automatic devices that open the electrical circuit when the current in the circuit "reaches a value that will producea dangerous temperature in the apparatus or conductor” or "in the event ofa groundfault”; and

(b) manual devices that "will safely disconnect all ungrounded conductors of the circuit at the point of supply simultaneously.”

Another rule of Section 14 could be appropriate for reviewof the O/C protection requirements of conductors. This rule is 14-100. It states in part that "each ungrounded conductor shall be protected by an overcurrent device at the point where it receives its supply of current…”

Thus, from the CE Code perspective, it is abundantlyclear that a single protective and controldevice must be provided at the point of supply.

Now is the perfect time to evaluate C282 provisions on this subject.

Subsection 8.7 of C282 is dedicated to "overcurrent devices” under scope of this standard.

Clause 8.7.1 of this Subsection states in part that "the overcurrent devices in the emergency distribution system shall be coordinated to maximize the selective tripping of branch circuit breakers when a short-circuit occurs.” This clause does not appear to specifically mandate a single disconnecting means/overcurrent protective device for an emergency generator, but simply requires coordination between the O/C devices in the emergency distribution system. This requirement is consistent with Rule 46-206(1) of the CEC, as was discussed earlier.

However, Clause 8.7.2 of this subsection states the following: "A lockable automatic device shall be used to disconnect the generator from the loads in the event of a fault current. The minimum rating of the automatic device(s) shall be equal to the full current of the LTP rating.”

This is interesting…..and a bit confusing. Does the wording of Clause 8.7.2 imply or direct a standard user that there can be only one main disconnect (i.e., breaker) device on an emergency generator?

Let’s review this latter requirement of C282 in light of the earlier observations. Based on the provisions of the CEC quoted above, it is clear that a single set of control devices must be provided with an emergency generator. Another piece of relevant information could be found in the ULC standard S524. Article 3.2.4 of this standard covers requirements for an emergency power supply derived from an emergency generator. Clause of this article mandates that "the disconnecting means for a generator emergency power supply” must be fitted with a locking device. This requirement of the ULC S524 indicates that each generator conforming to C282 and intended to provide an emergency power supply source to a fire alarm system must be equipped with the disconnecting means.

Therefore, the logical conclusion based on this discussion is that an emergency generator must be provided with a single protective and control device.

Now is the time to support this conclusion by recommendation to amend Clauses 8.7.1 and 8.7.2 of C282.

Such recommendation has been sent to the technical committee responsible for the development of C282.

Meanwhile, as usual, the regulatory bodies with jurisdictional power must be consulted in order to clarify this subject.

Read more by Ark Tsisserev

Tags:  Canadian Perspective  January-February 2011 

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