Medium Voltage Power fuses
Medium Voltage Power fuses
(OP)
Hi all!
I'm having a discussion with a consultant about the caliber to use for the primary side of a 5000 kva transformer.
I'm using the nominal current to size my fuse. Primary is 25 kV so the nominal current is about 116 A. I'm going with a 150E SM-5 S&C fuse.
The consultant wants to use a 100E to fully coordinate with the upstream relays. By using another type of fuse, he claims he can use a 80E !
I'm trying to convince him with this paper (see attachment), but he's using the fuse TCC to prove to me that a 100E is good enough up to 200A!
The 5000 A, 600 V switchgear fed by the transformer doesn't have a main breaker. I'm suspecting that he wants to lower the incident energy at the 600V side by using a smaller caliber. Also, I think he obsessed with coordination...
I'm running out of ideas to convince him he's not right.
Do you have any comments?
BOB
I'm having a discussion with a consultant about the caliber to use for the primary side of a 5000 kva transformer.
I'm using the nominal current to size my fuse. Primary is 25 kV so the nominal current is about 116 A. I'm going with a 150E SM-5 S&C fuse.
The consultant wants to use a 100E to fully coordinate with the upstream relays. By using another type of fuse, he claims he can use a 80E !
I'm trying to convince him with this paper (see attachment), but he's using the fuse TCC to prove to me that a 100E is good enough up to 200A!
The 5000 A, 600 V switchgear fed by the transformer doesn't have a main breaker. I'm suspecting that he wants to lower the incident energy at the 600V side by using a smaller caliber. Also, I think he obsessed with coordination...
I'm running out of ideas to convince him he's not right.
Do you have any comments?
BOB






RE: Medium Voltage Power fuses
A 150E expulsion fuse is not going to think about melting until about 300 amps.
RE: Medium Voltage Power fuses
RE: Medium Voltage Power fuses
100E will probably limit your transformer loading capacity.
Here's a link to another S&C guide I use.
It's a large file and you may have to extrapolate for a 5000kVA.
http://www.sandc.com/edocs_pdfs/EDOC_006526.pdf
RE: Medium Voltage Power fuses
So, I don't see the theory behind selecting a fuse that is has a rating less than the transformer rated current. I do suppose the fuse could work if the enviroment was cooler than the specified ambient temperature used to specify the rated current.
I'll have to read the S&C guides if I have some time. Maybe they say something different.
RE: Medium Voltage Power fuses
This size of transformer at that voltage could easily be a coordination concern when trying to blow a fuse to limit transformer damage on a through fault, but also allow maximum possible loading up to and possibly excedding nameplate value.
Sounds to me like you guys are having a philosophy argument vs. fuse melt characteristics.
RE: Medium Voltage Power fuses
Also, the arc-flash incident energy at the low side bus (since no main breaker) will be a function of the primary fuse time-current curve. One size smaller fuse can make a significant difference in the arc-flash level.
RE: Medium Voltage Power fuses
I actually read the question again and it appears you have no other protection besides these fuses. In that case, I'd have to say that's a rather poor installation. As dpc posted, E rated fuses by themselves are lousy at providing full range protection. I'd also doubt any practical sized upstream fuse could get the 600V side to a safe workable arc flash rating.