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#102080 - 12/15/04 02:56 PM Transformer secondary protection  
qualtech  Offline
Junior Member
Joined: Dec 2004
Posts: 2
Pittsburgh, PA, USA
I am having trouble finding a reference for how many circuit breakers or fuses are allowed to serve as transformer secondary protection in the absence of a secondary main device. I only have access to the 2002 Ontario code, but I would expect to find a footnote on Table 50 or some other reference to detail what secondary protection may consist of. In contrast, the US National Electric Code's transformer protection table (450.3) has a note as follows: "Where secondary overcurrent protection is required, the secondary overcurrent device shall be permitted to consist of not more than six circuit breakers or six sets of fuses grouped in one location. Where multiple overcurrent devices are utilized, the total of all the device ratings shall not exceed the allowed value of a single overcurrent device. If both breakers and fuses are utilized as the overcurrent device, the total of the device ratings shall not exceed that allowed for fuses."

We have a customer who has switchgear with nine 1200A feeder breakers and no secondary main. The nine breakers are fed from a 3000kVA, 600V transformer. The trip settings add up to nearly 8000A, or 275% of the rated secondary current. This would definitely be a violation per the US NEC. Does anybody have any information that would clear this up for me. Is there a reference in another code book?


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#102081 - 12/15/04 07:05 PM Re: Transformer secondary protection  
Tony Moscioni  Offline
Moderator
Joined: May 2001
Posts: 144
CANADA
Rule 26-252 Overcurrent Protection for Power and Distribution Transformers Rated Over 750 V

(1) Except as permitted in Subrules (2), (3), and (4), each transformer shall be protected by an individual overcurrent device on the primary side, which shall be rated at not more than 150% of the rated primary current of the transformer in the case of fuses, and which shall be set at not more than 300% of the rated primary current of the transformer in the case of circuit breakers.

(2) Where 150% of the rated primary current of the transformer does not correspond to a standard rating of a fuse, the next higher standard rating shall be permitted.

(3) An individual overcurrent device shall not be required where the feeder or branch circuit overcurrent device provides the protection specified in this Rule.

(4) A transformer having an overcurrent device on the secondary side rated or set at not more than the values in Table 50 or a transformer equipped with co-ordinated thermal overload protection by the manufacturer shall not be required to have an individual overcurrent device on the primary side, provided the primary feeder overcurrent device is rated or set at not more than the values in Table 50.


Rationale for Rule 26-252.

This Rule describes the type and rating of overcurrent (short-circuit) protection for high-voltage power and distribution transformers.


Intent for Rule 26-252

Overcurrent devices located on the secondary side of the transformer will not protect the transformer against a short-circuit condition. Therefore, protection is required on the primary side. Normally, we ask for an individual overcurrent device on the primary side, and the rating of that device will vary, depending upon whether a fuse or a circuit breaker is used. The difference in this rating is based upon the difference in the operating characteristics between fuses and circuit breakers. Subrules (3) and (4) list those special circumstances where an individual overcurrent device on the primary side of the transformer is not required.


Maximum Setting or Rating of Overcurrent Device
as a Percentage of Rated Current of Transformer

Primary Side Over 750 V

Transformer Circuit Breaker Fuse Rating
Rated Setting in % Setting in %
Impedance


Not more than 7.5% 600 300

More than 7.5%
and not
more than 10% 400 200


Secondary Side Over 750 V

Transformer Circuit Breaker Fuse Rating
Rated Setting in % Setting in %
Impedance


Not more than 7.5% 300 150

More than 7.5%
and not
more than 10% 250 125


Secondary Side 750 V or less

Transformer Circuit Breaker or Fuse Rating
Rated Setting in %
Impedance


Not more than 7.5% 250

More than 7.5%
and not
more than 10% 250


Tony Moscioni
Electrical Inspector
Electrical Safety Authority


#102082 - 12/15/04 08:07 PM Re: Transformer secondary protection  
qualtech  Offline
Junior Member
Joined: Dec 2004
Posts: 2
Pittsburgh, PA, USA
Thanks for your reply Tony. I understand that primary protection is necessary for short circuit protection of the transformer and I understand all of the code references that you provided. What I am missing is; where secondary protection is required and is provided by multiple devices (e.g., nine feeder breakers with no secondary main), how do you determine if the secondary protection meets the intent of Table 50 and Rule 26-252? Do you add the trip ratings of the nine feeders as though they were a single device? The code only mentions secondary protection in a singular sense, such as

Rule 26-252(4): A transformer having an overcurrent device on the secondary side rated or set at not more than the values in Table 50...

Rule 26-254(4): A transformer having an overcurrent device on the secondary side rated or set at not more than 125% of the rated secondary current of the transformer shall not be required to have...

Note that regardless of whether the customer is currently complying with the code requirements, we will recommend that the customer install a secondary main circuit breaker, as this greatly reduces the potential blast energy in the event of an arcing fault on the switchgear's main bus. The secondary breaker can be adjusted to clear the arcing fault much faster than if clearing is dependent on a primary current limiting fuse. Of course, the secondary main breaker does not help if an arcing fault develops between the transformer secondary winding and the line side of the main breaker. For this type of fault, it must be cleared by the primary fuse.


#102083 - 12/16/04 09:23 AM Re: Transformer secondary protection  
Tony Moscioni  Offline
Moderator
Joined: May 2001
Posts: 144
CANADA
Qualtech,

I better understand your question now.

A single "main" disconnecting means is required on the seconday side.

Let me try to explain . . .

Here in Ontario, we have added an additional requirement to Section 14.


Rule 14-418 states:

1) A single disconnecting means shall be provided either integral with or adjacent to the distribution equipment:

a) within each unit of a multi-unit building,other than a dwellimng unit;

b) within each area common tpo m ore than one building; such as an underground parking area; or

c) within each building when fed from another building

2) Subrule (1) does not apply to circuits from part of fire alarm, fire protection and emergency systems.

We have no similar rule as Article 450.3 in the NEC.


Tony Moscioni
Electrical Inspector
Electrical Safety Authority



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