Current Limiting Fuse recommendation for Arc Flash reduction
Current Limiting Fuse recommendation for Arc Flash reduction
(OP)
Hi All,
Before asking this question, I have read a number posts on ArcFlash. I think I have a solution but I guess your words might prove a little more reassuring.
Case#1 : I have a 13.8KV/600V 750 KVA transformer, It is a Power Distribution centre with a S&C 15 KV SM-4, 50 E disconnect switch in the primary and a CDP panel with 5 feeder breakers on the secondary side. These three items are enclosed in one enclosure.
Traditionally for a standard transformer:
Primary Inrush Current: 376 A (12 X I(FL) for 6 cycles)
Secondary Inrush Current: 10377 A
## My question is that if I install a secondary breaker
Will the breaker see this amount of secondary inrush current for .1 sec?
Case # 2 In the situation above, if I don't use the secondary breaker, If I have to replace the SM-4 fuse, Could you suggest any current limiting fuse, which does not blow on the transformer inrush.
I would appreciate a word.
Thanks
Before asking this question, I have read a number posts on ArcFlash. I think I have a solution but I guess your words might prove a little more reassuring.
Case#1 : I have a 13.8KV/600V 750 KVA transformer, It is a Power Distribution centre with a S&C 15 KV SM-4, 50 E disconnect switch in the primary and a CDP panel with 5 feeder breakers on the secondary side. These three items are enclosed in one enclosure.
Traditionally for a standard transformer:
Primary Inrush Current: 376 A (12 X I(FL) for 6 cycles)
Secondary Inrush Current: 10377 A
## My question is that if I install a secondary breaker
Will the breaker see this amount of secondary inrush current for .1 sec?
Case # 2 In the situation above, if I don't use the secondary breaker, If I have to replace the SM-4 fuse, Could you suggest any current limiting fuse, which does not blow on the transformer inrush.
I would appreciate a word.
Thanks






RE: Current Limiting Fuse recommendation for Arc Flash reduction
If you energize the transformer from the high side, the low side breaker sees no inrush.
You can certainly fit a current limiting fuse on the primary that will not blow on inrush. It's done all the time.
BUT, if you are thinking putting a current-limiting fuse on the primary side is going to reduce arc-flash on the secondary side, you are mistaken.
The primary fuse will do very little to reduce arc-flash on the low side. In fact, the SM-4 may do a slightly better job than the current limiting fuse.
RE: Current Limiting Fuse recommendation for Arc Flash reduction
dpc, After I posted this post, I have been simulating ETAP and your statement looks like a blessing to me.
You mentioned
"If you energize the transformer from the high side, the low side breaker sees no inrush"
HOW COME ?
because I had selected a Cutler Hammer ND style with 1200 A Frame, 1000 A rating plug with LSIG trip.
I had selected a 5X trip rating for the instantenous settings. My arc flash level is Cat 0. But if I reflect the inrush on secondary, sec breaker will trip based on 12X I(FL) on secondary.
Thanks for your quick response dpc. I would appreciate if you could kindly elaborate.
Thanks
RE: Current Limiting Fuse recommendation for Arc Flash reduction
RE: Current Limiting Fuse recommendation for Arc Flash reduction
My apologies for missing one more point, You mentioned a current limiting is used all the time.
Actually, the first current limiting fuse I tried was
GE 960-EJ01 50E, It helped reducing the energy on sec. bus but I wanted to make it does not trip on transformer in rush, which is 376 A for a 750 KVA transformer.
12 times for 6 cycle. I plotted 376 A on the current limiting fuse and it goes off before .1 sec.
Would you agree with me that it is the way to justify the application of a fuse.
Thanks
RE: Current Limiting Fuse recommendation for Arc Flash reduction
My perception is that Transformer primary Inrush gets reflected on the secondary side as well.
Is this not correct ?
V2 I1
- = -
V1 I2
Thanks
RE: Current Limiting Fuse recommendation for Arc Flash reduction
RE: Current Limiting Fuse recommendation for Arc Flash reduction
I think this is a case of placing too much emphasis on a computer program and not thinking about the basics of electrical engineering. Computer software definitely makes the mechanics of analysis easier BUT this must be tempered with a feel for does the answer make sense. Remember the basics.
RE: Current Limiting Fuse recommendation for Arc Flash reduction
Basics are the key. Understand, Simmi, that a transfomer is an application of electromagnetism theory. To understand the why is there an inrush current, you must understand the magnetic relation between the windings and the magnetic core of a transformer.
I agree with wbd. Computer software like ETAP (I use Cyme for my part) make things easier and faster. But I have to be able to always explain the results or be able to noticed a modelisation mistake. So, it was necessary that I have deep understanding of the models be it for cable, motor, generator, filter, etc.
So, go back to basics.
Danny Garant, ing.jr
Groupe Stavibel Inc.
www.stavibel.qc.ca
RE: Current Limiting Fuse recommendation for Arc Flash reduction
RE: Current Limiting Fuse recommendation for Arc Flash reduction
1- Utility
2- Motors and generators
If there was inrush current at the secondary (I know there is not), contribution to Arc Flash would be minimal due to a very short duration.