Fuse sizing for load or coordination?

[Plinko]

I'm looking for the most common philosophy on fuse sizing for distribution taps (12.5kV system). In general,are most utilities sizing fused taps according to load or for maximum coordination with upline devices. Example: A single phase overhead tap off the main feeder 10 spans from the sub. I'm hanging one 25kVA Xfmr fused with a 6T (minimal chance for expansion)...a 10T would be sufficient for that tap. However, a 65T will give me max coordination with the substation breaker. Is it more common to use the 10T or 65T for this tap? I know this example opens the door to many other variables but I'm just looking for a general philosophy. I'm looking to implement a standard for fuse sizing at my utility and am trying to get the pros and cons of both philosophies....Thanks for your response!

 

[stevenal]

Does "(minimal chance for expansion)" mean you expect no load growth on the tap? If so, I would fuse it on the low side. The lower fuse will clear faster, limiting equipment damage and the voltage sags experienced by nearby loads. I tend to not go lower than 25 to avoid nuisance fuse blowing due to lightning. If load growth is expected, I'll try to fuse accomodate it. In general, our philosophy is not to try to save fuses with instantaneous trips and reclosing at the substation. A fuse saving philosophy would call for a higher fuse.

 

[wbd]

Actually, you could even fuse the txf with a 3T fuse since this has a min melt of 6A in 300 seconds. This would protect the txf but would not allow the use of the overload capability of the txf. (Using a txf overload curve where the txf can be OL for 60 seconds at 4.75 times the kva)

Using the 6T would be what my utility is using right now for your situation. I am presently studying using a Dual Slofast fuse which would follow the txf overload curve while still providing protection for the txf and ckt.

For the upstream (protected) fuse, I would probably go with the 10T. This should still co-ord with any upstream fuses or reclosers.

For general info, you should see about getting a copy of the "Electrical Distribution System Protection" book by Cooper Power Systems and also A.B.Chance company has some good literature.

 

[dpc]

I suspect the 65T would be much more common than the 10T, based on my experience. Utilities fuse for short circuit protection and don't worry as much about overloads. Service continuity is a major concern, so they don't want fuses blowing unless really necessary.

Fuse manufacturers have "preferred" and "non-preferred" ratings. If you stick with the preferred ratings, using smaller sizes as you go downstream, coordination is generally assured. As mentioned by others, you need to factor in the reclosing philosophy and reclosing sequence.