TCC coordination, fuse only up to 300sec.

[binwork91]

Hi, see attached TCC. I have question about the coordination. As you can see two TCC show two fuses JFL and JKS. And both fuse only up to 300 sec, however, the breaker up to 1000 sec. Can you tell if any TCC have selective coordination? i mean after 300 sec, I don't know how the fuse curve look like.

 

[jim dungar]

You cannot extrapolate fuse or breaker performance, unless you have manufacturer's trip curve formula.
In my experience, once you reach the end of a curve, or the time axis crossing, you are only guessing at what happens.

 

[David Castor]

You cannot accurately predict what will happen. The manufacturer stops the curve at 300 sec because fuse performance is unpredictable at lower values of current. Also, nothing magical about 1000 seconds. It just where TCCs traditionally end. That said, 300 seconds is 5 minutes at a pretty significant overload. It's pretty uncommon that we worry a lot about coordination for faults in that region - usually something happens before that.
The old electromechanical overcurrent relay curves didn't start until 1.5 x the pickup setting due to lack of repeatability at lower current. Schweitzer still does this for their digital relays, which is a little annoying.