xfmr primary/secondary device coordination

[lielec11]

Please review your electrical setup. There are no fuses involved. A CB will either trip or not, no melting time, only mechanical lag. Just remember that CB's actuation time will depend on how big a difference the fault is to the CB's trip setting (inversely proportional) plus a little mechanical lag.

What you have are 4(four) circuit breaker and the solid line curves are the corresponding damage curves of your lines and the transformer. CB TCCs will give you the earliest trip time and the longest trip time possible after the CB senses the fault.

When coordinating, you either separate the trip curves by time-grading (vertical separation) or amplitude separation (horizontal separation). That is why, it could have been better if you can tweak the fault value settings and add time delays to the CB curve to lift the tripping characteristics. If not, just keep the faith that your horizontal separation works as designed, IMO.

I understand there are no fuses I was just discussing TCC curves in general. If you didn't read that portion you missed my entire argument which has to do with unlatching time vs. fault clearing time. You refer to it as mechanical lag. My concern is that the two curves are in a position where the downstream breaker will begin to unlatch but may not clear the fault before the upstream breaker then beings to unlatch. Or as you say, there isn't enough mechanical lag between the downstream and upstream breaker.