E Rated Fuse

[mbk2k3]

Why are E rated fuses rated 400E but when observing the TCC, the curve starts melting at like 900A?
If the purpose of E rated fuses is to clear inrush and meant only for short circuit protection, why is it called a 400A fuse?

This applies to almost all medium voltage fuses that I've seen. Is this because fuses are not meant for overload protection? And are exclusively for short circuit protection only?

 

[jraef]

Short Circuit Only? I wouldn't take it quite that far, but the part about not providing overload protection, defined as low level over current, then that is true. Applying MV fuses is not the same as applying LV fuses, you can't equivocate.

"Fuses with an E-rating larger than 100E must open within 600 seconds (10 minutes) when subjected to an rms current of 220-240% of the fuse's continuous current rating."


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[JJHorak]

Overload protection is best done by design of the attached load, not by tripping fuses and relays. If you rely on fuses and relays to keep you from overloading the system, you have an unreliable system and a notable design problem.
Fuse amp rating is the current they can carry without damage. The faster they trip, the more sensitive they can be to damage from transient overcurrent, and E fuses can be very fast, so the load current stays well under the trip TCC. If you want overload protection at medium voltage, you need relays. Then they will tell you to set the relay so high it is not good overload protection either.

J. Horak, P-R Engineering, Colorado

 

[mbk2k3]

Great response JJ Horak.

Thanks.

It still doesn't completely satisfy my curiosity about the amperage ratings.
So if a 400E fuse curve starts meltiing/clearing somewhere in the 800A region, can the relay safely handle currents only upto 400A? What about 500A? 600A?

 

[Parchie]

mbk2k3 (Electrical)
So if a 400E fuse curve starts meltiing/clearing somewhere in the 800A region, can the relay safely handle currents only upto 400A? What about 500A? 600A?

Fuse melting is a function of time and current. You just can say "melting/clearing in the 800A area".
If you wanted to precisely choose a fuse for your specific application, get a TCC curve from the fuse manufacturer and compare with your expected load amperes. If your time-current plot falls below and to the left of the fuse TCC, you're choice is good.

 

[mbk2k3]

@ Parchie

But thats what I'm saying. When I look up the TCC of a 400E rated fuse, which is advertised as good for 400A, the melting current portion of the curve starts at ~800A.

So yes, I personally think that the fuse is good for 500A, why is it advertised/marketed as a 400A fuse....

 

[LionelHutz]

The curves I have seen simply doesn't go lower because they stop at a time which is too short to allow the lower currents to be shown.

Mersen says their E-rated fuses are general purpose. Their text says something along the lines of the fuse can interrupt all currents between the current that causes melting in 1-hour up to it's interrupt rating. So, the curves would have to go to 3600 seconds as a minimum to show the lowest level.

Still, it's not a great idea to use E-rated fuses without a protection relay.

 

[wroggent]

In addition to what jraef said, the rated current of a fuse is that which results in the maximum allowable temperature in C37.40 at a specified ambient temperature. As lionelhutz mentioned, C37.41 specifies that general purpose fuses should be tested for currents that result in tripping in the 0.01 to 3600 second range. Full-range fuses are tested in the 0.01 to 10000 second range and are intended to interrupt currents ranging from the rated current up to the interrupting rating. Note that the ~800A that your curve starts at is less than 220% of 400A. The general purpose fuses should not be relied on for interrupting currents in that 400A to 800A range not covered by the TCC.