Transformer protection and CB discrimination

code requires that the primary of your transformer be protected by fuse at 150% or 300%with cb.so by using a cb as your protection of primary transformer winding ,you should be able to protect your submains without jeopardizing your main feed.I have used this aproach on high demand load situations and not have had any problems,so far.

Thanks for your reply professor,

The HV side is being designed by an external consultant, but I will check the CB rating o that side.

So you are saying that the breaker on the TF primary will protect the submains to the MSB? It will also protect the transformer from overloading... How do I ensure it won't trip under an LV fault?

So I then don't need any protection on the LV side of the TF. I am free to design the main switch/circuit breaker to protect the MSB, and to grade with the downstream breakers

Have I interpreted you correctly?

you will stil have to protect your submains depending on current demand for each submain,but with your main at 300% the submains should trip first,because the submains should be rated at 80% according to code.let me know how this works out for you.

Professor, thanks for your ongoing help,

The TF is protected on the HV side by a 35A CB (3.3kV) which is close enough to 100% load and very far from 300% load. I think I will ask the HV consultant for a rationale for this CB size.

Hey, when you say 'code' which code are you referring to? which country are you in?

Breaker is better at transformer secondary. Cable from transformer to breaker is protected by protective device at primary

So you recommend a breaker at the primary -and- the secondary?

Or do you mean the secondary breaker to be located downstream of the LV main, in the MSB?