Fuse Rating for Short Circuit 1

[rammie47]

Hi,

I need to hook up 138kV, 1ph ssvt (inductive type) to a bus via fuse link disconnect. The maximum SLG fault current at that bus can be 20kA (symmetrical) from my study. But, the maximum fuse rating available in the market is only 12.5kA (symmetrical). Can I still use this fuse to protect the SSVT. The insulator and the remaining structure can withstand 20kA for sometime.

I learn that typically the 138kv ssvt units are hard bus connected without any fuse or disconnect switch. Considering that, I thought what I propose is definitely a better solution if not better.

Will a fast acting fuse serves better than normal speed fuse? After all this fuse is meant to blow anything more than 5 amp current

 

[alehman]

No. Probably better to do without than have an under-rated fuse.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[rammie47]

Alehman, Can you explain why? I may be missing some basics here

 

[Marmite]

If the fuse is over rated there is no guarantee that it will interrupt the fault current at all and in attempting to do so it may fail disruptively or catastrophically.
Regards
Marmite

 

[rammie47]

Marmite - In this case the fuse is actually underrated for short circuit fault. Its never meant for overcurrent.

Available short circuit at the bus - 20kA
Fuse capability - 12.5kA

 

[rbulsara]

What Alan said. Marmite probably meant over "dutied" (or underrated).

The fuse SCC rating has to be equal or greater than the available max. fault current, which is 20kA in this case.

Rafiq Bulsara

http://www.srengineersct.com

 

[rammie47]

Thanks Rafiq. I agree that engineering design and perhaps code also does not allow for underrated fuse.

I thought the fuse will anyways blows off much less than 12.5kA...

 

[electricnewbie]

Rafiq,

So you are saying the fuse should blow just fine, except it may not be able to interrupt the current during a fault (since it is rated well below the availble fault current)?

 

[rbulsara]

derekv:
Where did I say that?

Rafiq Bulsara

http://www.srengineersct.com

 

[rammie47]

But in any case I am still trying to understand the consequences of installing underrated fuse?

Sure, I will not recommend it

 

[rbulsara]

consequences of installing underrated fuse

What Marmite said...., politely.

Rafiq Bulsara

http://www.srengineersct.com

 

[jghrist]

Even if the fuse were adequately rated, the bus protection would trip anyway for a VT fault. The only advantage of the fuse would be that you would know where the fault was. Chances are, with or without a fuse, the VT would be smoke anyway and obviously the source of the fault because only a VT failure would blow the fuse.

 

[rammie47]

Depends on how sensitive the 138kV differential is set to trip. The SSVT is only 100kVA @ 138kV = 0.73Amps. That's too small a current for differential to pick - up.

 

[Marmite]

Rammie, what makes you think that the fault current would be limited to 0.73A, and hence not trip the busbar protection? I don't follow the logic.
Regards
Marmite

 

[Electic]

Things that could happen from an over-dutied fuse, include that it will launch the ends of the fuse cartridge at great speed, or it might initiate an arc that then jumps to ground or other phase. These are catastrophic failures that can burn buildings down and/or kill people.

 

[rammie47]

Marmite - I agree that the fault current will be more than 0.73Amps. I take back my statement, I gotta get back to my basics. Thanks for the correction!