fuses blowing

[barrydmi]

7.5 kva dry type transformer, 480 pri. 120/240 sec., priamry fuses 15 amp type J blow when supply power is turned on. Tried 15 amp type RK5 fuses instead and it works fine. Primary voltage is good, secondary voltage is good, transformer was energized with no load. Why would a type J fuse keep blowing? Thanks for the help.

 

[bphgravity]

Re: fuses blowing

The designator of "J" and "RK5" are classes and do not automatically infer type. Is it possible that the class J fuse you are using is of the quick acting type which is responding to the inrush? Most class RK5's are time delays.

 

[physis]

Re: fuses blowing

It would make sense to me. I wouldn't expect a fast acting fuse to work.

 

[barrydmi]

Re: fuses blowing

Thanks for the input. The class J fuses were duel element time dealy.

 

[kiloamp7]

Re: fuses blowing

Usually you are better off using 20A fuses in pri. of this xfmr, time lag preferred.

Believe that all RK5 are time lag.

Xfmr inrush will get you sometimes, whether there is a sec. load or not.

 

[tom baker]

Re: fuses blowing

There you go. The forum is like having a old timer in your back pocket.

 

[bowman]

Re: fuses blowing

have seen problems w/ inrush current especially when transformers were reverse fed
if i remember correctly i think its 25x's the norm

 

[barrydmi]

Re: fuses blowing

Again, thank you guys for the input. To me it boils down to using the FRS-R-20 which is the one we finally installed is the write fuse, and the LPJ-20SP which is the one that was blowing is not. Even though the Bussman catalog decribes the LPJ-SP as sutible for inrush current,and sutible for motor protection. I learned something. Thanks. Barry.