HRC Fuse

[RAVWARD]

Hi,

How do we decide, if need a HRC fuse for a particular application. I spoke to one of the ferraz shawmut application engineer and he advised me that all of their fuses are HRC. Is there any other type also?

Thanks

 

[RAVWARD]

Actually, what I am trying to do here is:
Selection of a fuse for 1000 KVA, 15.5 KV . The FLA is 37 Amp. Cutler Hammer 40 C was suggested but no reasoning given. I am sure, you guys could advise, what is the criteria for selection of fuse for transformer protection, I beleive overload of transformer is also considered.

Please advise me, if 40 Amp selection is fine.

Thanks

 

[dpc]

I assume you are talking about a fuse on the primary of the transformer. If the FLA is 37 A, the 40 fuse seems too small due to the transformer inrush current.

 

[RAVWARD]

Dpc agreed,

That means my so called "Intermediate engineer" is not right. What options do I have.
I make it 45 or you could advise me a transformer fusing protection philospohy. I read that it has to do with the transfomer over load chr. also .This bothers me that how to get in to the Over Load thing. Please give me few tips, where to look or how to make a decision. I think that will be helpful.

Thanks

 

[davidbeach]

S&C has a publication available that goes in to great gory detail as to how one goes about sizing fuses for transformers.

 

[DanDel]

The bet way is to plot the transformer damage curve and inrush point along with the fuse curves and see what works.

 

[dpc]

Sizing will depend on the fuse type. But I would think the minimum would be a 50E. Check with your fuse supplier of choice and get a recommendation.

 

[RAVWARD]

Thanks DPC.
Here is the SLF

http://img141.imageshack.us/my.php?image=fuseuq6.jpg

The fuse I selected is 50 amp from Ferraz Shawmut. All of their fuses are current limiting(General Purpose= Current Limiting + Over Load).
The transformer damage curve should be on the right hand side of the fuse curve. The problem is that for primary fault current it is fine, but when I want to go for a single line to ground fault on secondary. The damage curve shifs to the right of fuse curve. Could you please advise me, how can I satisfy both conditions.The fauly that occurs before the secondary protection and the secondary terminals of transformer.
Thanks

 

[oldfieldguy]

The primary fuses will protect for faults on the line between the fuse and the transformer, inside the transformer itself and for phase to phase faults on the secondary. Depending on the available L-G fault current available, they either won't protect on a secondary line to ground fault before the secondary protection, or they will be much slower to open due to the lower current for the L-G fault as opposed to the L-L. This is something you have to live with in the real world if you're using fuses for protection.

old field guy

 

[rbulsara]

For quick check, ferraz-shawmut has free dowloadable program "Select-a-fuse". There is MV transfomer fuse selector in them.

IEEE Buff Book (std 242) is proper reading, so is S & C 's publication for transformer fuse selection.

For tighter protection E rated fuse are selected closer to FLA rating of the transformer, but not more than 125% of the FLA. (remember E rating is not same as regular ampere rating). Also people make distinciton between normal rating and fan cooled ratings.

 

[dpc]

Agree with oldfieldguy - fuses cannot generally fully protect transformers for through faults.

 

[rbulsara]

See thread238-106115

 

[RAVWARD]

Thanks Everybody.

I recall reading this thread, I guess I should have remembered that. Could you advise me, I always come across
E type fuse. Why cannot we use K type or T Type fuse. I beleive K type is a fast acting fuse. Could you advise me in what situation I would use a k type fuse.

Thanks

 

[rbulsara]

Type of fuses has to to with application and how they are mounted and where they are mounted. E-rated fuses (and R rated for motor protection) are most common in a switchgear (air interrupters) as they do not "explode" when operating. Some of type of fuses are mounted on pole-tops which can afford to "explode" so to speak, not necessarily causing shrapnels but lot of smoke and sound.

Also type of fuse holders makes the difference.

Refer to applicaiton notes of the manufactures.

 

[dpc]

K and T are fuse links used outdoors in special tubes for distribution line protection. They have low interrupting ratings. And cannot be used indoors. They will provide even worse transformer protection than a full range power fuse.

The best transformer protection for through faults (using a fuse) is generally an expulsion-type power fuse such as S&C SM type. Current-limiting fuses have very steep curves that generally don't coordinate well with anything else.

 

[davidbeach]

A current limiting fuse is also a poor choice for transformers as trying to stay above the inrush and below the damage curve is more difficult with CL fuses than it is with something like the S&C SM series.