Medium Voltage E-Rated Fuse Sizing for Transformer Protection

[tlona]

Hello. I am trying to size the primary fuse for 3.75mva-33kv/480v transformer. The NEC allows primary only protection at 250% for supervised installations which is my situation. My FLA is 65amps. When looking into fuse vendor data (ie., S&C, cutler hammer, little fuse, etc...) many list a tables calling out the fuse size based on transformer KVA and primary KV. For my application the E-rated fuse typically listed is an 80E or 100E. The E-Rated fuses, be definition, trips between 200% & 240% of the E-rating in 5minutes but the curves never show at what time the fuse blows at the E-rating. This means for the 80E the fuse will blow in 5minutes between 160 & 192 amps which is out side of the required NEC 250% requirement. For a 100E blows at 200 & 240amps. I would think specifying the E rated fuse the rated should be close to the FLA? Can anyone provide any insight as to what I am misinterpreting. Thank-you very much for your time and knowledge.

 

[davidbeach]

Go with the fuse manufacturer’s recommendation for voltage and kva. Hard to go wrong there. The number is the number, no need to dig deeper. If you come up with 162A, any E rating below that meets the requirement.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[RRaghunath]

E-rated fuses are full range fuses and hence should operate even for currents just above the fuse rated current as well.
Time taken at low interrupting currents could be as high as 1 hour and can be read from the TCC curves supplied by the manufacturers.
In your case with 65A rated current of transformer, you can choose a fuse of 160A rating and still be within NEC stipulation. This will ensure the fuse withstands transformer inrush currents better and give long life.

 

[tlona]

Thank-you both for the feedback. Yes I will be going by manufactures recommendation. What I still do not understand is that I could specify a 100E fuse for my 65A FLA transformer. The 162A current (250% NEC requirement) never hits the fuse curve of a 100E fuse which fuse blows in 5 minutes somewhere between 200 & 240 amps, well above that allowed by NEC?.

 

[LionelHutz]

The 5 or 10 minutes is just one point on the fuse curve. The fuse doesn't go from never opening at 199A to opening in 5 minutes at 200A.

E fuses are classed as general purpose current limiting fuses. This is Merson definition - "A general purpose current-limiting power fuse is one that is capable of interrupting all currents from its rated interrupting rating down to the current that causes melting of the fusible element in one hour." Their curves only show to 1000 seconds, so the longer time part of the curves are unknown. The part that is ambiguous in this is that they don't specify the current that causes melting of the fusible element in one hour.

 

[RRaghunath]

NEC allows fuse rating to be 250% of transformer full load current if circuit breaker (with overload protection) is provided on secondary side. This way, I believe the fuse is not expected to protect for overloads on secondary side and is only for meant for short circuit faults (which anyway yield much higher currents than 250% of rated current).

 

[tlona]

Thank-you all for the feed back as it is greatly appreciated. The 1 hour does provide clarity as you have mentioned these fuse curves never go out that far and appear to be a straight line after the 5 minute mark. The clarify our situation over 600V and supervised with a fuse rated at 250% max. No secondary protection is required per NEC table 450.3(A). Thank-you all

 

[RRaghunath]

tiona,
"over 600V and supervised with a fuse rated at 250% max. No secondary protection is required per NEC table 450.3(A)" - You are right. Secondary protection is required if fuse rating is 300% (and not for 250%). Thanks, I stand corrected.