Med Voltage Fuse Co-ordination Problem

[rusk]

Hi, All.

I have a question about medium voltage fuse (14400V) behaviour. I have an breaker upstream of my fuse that has an instantaneous setting of 2240A.

I need a fuse sized at 40E for my transformer to co-ordinate properly with the secondary main bkr.

I have two choices. #1 I can choose a 40E fuse that is fast acting such that at 1400A it will completely clear at 0.01 sec. #2 I can choose a 40E fuse that is slower and won't start melting until 3500A.

My maximum fault current at the location of the fuse is 14000A. What I am questioning is the ability of the faster fuse to clear a 14000A fault before the inst of the upstream bkr. Will the fuse blow like I anticipate leaving my upstream bkr in service? Or will my bkr trip and without my fuse operating completely leaving me with a possibly damaged but not opened fuse.

Alternatively, if I choose the slower fuse and a fault of 14000A occurs will the upstream interupt before any damage occurs to the fuse.

Which choice (or other alternative fuses) is more sound than the other and why?

Thx, All.

 

[Plinko]

I would think if your IT (instantaneous trip) of your upline device is faster than the minimum melting time (at any given fault magnitude...min. fault to the max available fault current(14000A)) then your fuse should be safe. But without seeing the instantaneous TCC for each device it's hard to predict what might happen to the fuse.
I probably didn't help too much with this post, but there may be more to this than meets the eye. I'd be happy to discuss this issue in more detail.

Plinko
plunkettbrian@hotmail.com

 

[jghrist]

<<two choices. #1 I can choose a 40E fuse that is fast acting such that at 1400A it will completely clear at 0.01 sec. #2 I can choose a 40E fuse that is slower and won't start melting until 3500A.>>

I assume you mean 40A, not 40E. The &quot;E&quot; designation refers to a particular speed fuse.

If you are talking about non-current limiting fuses, you cannot assure clearing before tripping the breaker on instantaneous. Even at a high fault current, the fuse will allow at least one-half cycle of current through which may trip the breaker.

You also cannot keep the fuse from blowing before the breaker trips. The breaker may take 3 cycles to trip. Even a very slow fuse like a Type KS will clear in less than 3 cycles at 14000A.

If you want to coordinate the transformer fuse with the breaker, you have four choices:

1. Set the instaneous pickup higher than the available fault at the transformer.

2. Add some time delay in the relay, making it less than instantaneous. This can be done with programming (if it is a microprocessor relay), a separate time delay relay, or by using a definite time relay or one with a not very inverse time-current characteristic.

3. Use a current limiting fuse sized to keep the let-through current below the instantaneous pickup.

4. Use reclosing with the instaneous disabled after the first trip. The breaker will trip and reclose. If the fuse hasn't cleared already, it will before the second breaker operation which will be on a delayed curve.