Current Limiting Fuse recommendation for Arc Flash reduction 2

[Simmi123]

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

 

[dpc]

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.

 

[Simmi123]

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

 

[davidbeach]

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

 

[Simmi123]

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

 

[Simmi123]

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

 

[davidbeach]

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.

 

[wbd]

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.

 

[Morquea]

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.

http://www.stavibel.qc.ca

 

[dpc]

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.

 

[unclebob]

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.