Fuse Coordination with SR750 1

[SELEC]

I am doing a coordination study for a ficility by using ETAP and SKM.

What is confusing me is, the same type of fuse was showing on SKM and ETAP TCC differently. on SKM,there is almost no overlap, because the top left portion of 900E was cut off, which was showing on ETAP. the top left portion is for overload is long time delay part of overcurrent protection.
Is it an acceptable practice to overlap on the top left portion or we should customize the fuse curve to get a better coordination? The fuse has been purchased, so we cann't change it to another type? Who has a better solution for this problem. Thank you in advance.

 

[davidbeach]

First off, I would never use any device out of a program library without comparing it to the manufacturer's published data. What does your fuse curve look like on the manufacturer's TCC? All of the programs have to make some compromises in representing the device characteristics; are they the same compromises you would make? Any time I do a coordination study, all of the device library entries I use are ones I created/modified/verified my self.

Second, a fuse is what it is unless it is something like the S&C Fault-Fiter with electronic controls.

Overlap with what?

A 900E would be strange animal. I'm not aware of any E rated fuse above 800E, nor of any single barrel above 400E, so if it existed, a 900E would be a triple barrel fuse, and I've not seen any of those either. Not to say it can't happens, but it would be a new one to me. Who's fuse, and what product family?

Frankly, a fuse with a 900 rating at medium voltage sounds like a bad design. That's way too much load to trust to a fuse; it really ought to be a circuit breaker with a relay. Besides, a fuse can only respond to phase current, not ground fault current. If it is really a 900E, you could have an 1800A phase-to-ground fault that the fuse would never clear; be afraid, be very afraid, and stay very far away. Even if it is an ungrounded system, or an impedance grounded system, a second ground fault could easily draw enough current to do vast damage without ever getting to the minimum melt on a fuse that large.

 

[SELEC]

It is Ferraz Shawmut fuse 3barrels. 900E fuse overlapped with upstream overload protection portion of feeder curve.

I studied this system again. 900E fuse is used to protect VFD bus, which feeds VFD controlled motors( 2 motors, one starting one running). I am assuming VFD should have over load protection itself, it is unlikely that VFD controlled Motors are over loaded. It is high resistance grounded system.

 

[alehman]

I agree with the above comments. Seems like a poor design.

You mention an SR750, but not where it is in the system. That is a digital feeder relay with a variety of time-current curves. Have you looked all of the TCC options in this relay? (They're not all in SKM, as I remember.) Also, it allows you to create your own TCC's if you can't find a pre-programmed one you like.

It is likely, as you mention, that the motor controllers will provide primary overload protection, with the fuses serving as short circuit protection.

 

[SELEC]

As I mentioned before, The motor is VFD controlled(running and starting)and VFD should have overload protection itself, the fuse is used for short circuit protection.