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
