Technical information

CONTROL CIRCUITS

The protection to the primary should not be more than 1.88 x 2.5 = 4.7;

4 A fuses are used.

The protection on the secondary theoretically should not be more than 18.8

x 1.25 = 23,5 A. However, note [a] allows the use of the standard size

immediately above and 25 A and fuses can be used.

If,say,the secondary powers three different circuits, each with their own

protection (e.g. 4 A, 10 A and 11 A), the fuse can be omitted pr

o viding that the

sum of the protection is not more than 25 A. If it is (e.g. 4 A, 10 A and 15 A) a

dedicated component graded at 25 A should be used again.

The conductor to the primary should have a capacity of no less than 4 A:

using UL508A table 28.1 over a AWG 16 (10 A) is chosen through over sizing.

The conductor to the secondary should have a capacity of at least 25 A. From

table 28.1 of UL508A a AWG 10 (30 A) is chosen: this creates an absolute

advantage compared with AWG 8 that would have been necessary for

protecting just the primary.

2) CEC: it is possible to protect “only on the primary” or “on the primary and

secondary”.The protection “just at the primary” is only admitted for dry

transformers up to 750 V with a single secondary, regardless of the type of

connection.

•Only primary: the protection should be graded to a value of not more than

125% of the rated current to the primary. If the rating does not correspond

with a standard value (see the publication prior to section “BCP”) the one

immediately above the one calculated is permitted.

The conductor that powers the primary should have a capacity of no less

than the rating of the protection while the conductor to the secondary

should have a capacity of no less than 125% of the nominal current to the

secondary.

•Pr imary and secondary: the protection to the primary should be graded at

a value of no more than 300% the rated current to the primary provided

the secondary has protection graded at a value of no more than 125% of

the rated current to the secondary.

Control Circuit