Short Circuit Settings

[Kanu_01]

I have transformer of 75kVA, 480/208V and z=3.6%. I am calculating the breaker settings Ir, Isd and Ii.
My breaker is of 250A.

Based on FLA, I am setting Ir and Ii by seeing fault current. I am confused about setting Isd. How should I set my values of Isd?

Can you please give me an example of how to proceed with settings of relays and releases in any network?

 

[waross]

Here that is a matter for the code.
1. Rating of the breaker;
Not more than 125% to 300% of the rated current of the transformer. It depends on other protection.

2. Setting of the breaker;
Selection of overcurrent devices with too low a rating for the primary of a dry-type transformer can result in
unintended operation when the transformer is being energized (such as might occur after a power outage). To
avoid such operation, the overcurrent device should be able to carry
(a) 12 times the transformer rated primary full load current for 0.1 s; and
(b) 25 times the transformer rated primary full load current for 0.01 s.
NOTE THE DIFFERENCE BETWEEN THE RATING OF THE BREAKER AND THE SETTING OF THE TRIPS.