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Current Limiting Fuse recommendation for Arc Flash reduction
2

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




 

RE: Current Limiting Fuse recommendation for Arc Flash reduction

What are you trying to accomplish?  

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

(OP)


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

2
Where would the secondary inrush current come from/go to?  What is the circuit?

RE: Current Limiting Fuse recommendation for Arc Flash reduction

(OP)
dpc,

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

(OP)
davidbeach,

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

Think about it a bit.  Hint 1, current needs a circuit, a complete loop in which to flow.  Hint 2, consider a more complete model of the transformer that includes the magnetizing (excitation) branch.  Hint 3, your voltage/current relationship is true for load currents, but is irrelevant for the topic at hand.

RE: Current Limiting Fuse recommendation for Arc Flash reduction

A star to you David.

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

I give a star to david too.

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

Regarding the primary fuse - it must be large enough to withstand the transformer inrush.  That is a basic requirement.  If the fuse is properly sized, it will provide no current-limiting for  a secondary fault   It may help reduce arc-flash levels on the secondary side by clearing eventually, but it is not current-limiting.   

RE: Current Limiting Fuse recommendation for Arc Flash reduction

For Arc Flash studies, fault contributions mostly come from:
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.   

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