blown fuses

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spongebob

Member
I have a situation where the primary fuses to a single phase transformer are blowing periodically at power up. I have a single phase transformer, 480 volt pri. to 240 volt sec. at 5kva rating.
With no load connected to the secondary, the 2- 10 amp, bussman LP-CC time delay fuses will blow instantly. I have ruled out the wiring, since I replaced it all(even though I detected no problem with it).
That leaves me with only the transformer in the circuit. Since the secondary is also fused, the code allows for an increase in primary fuse size. I installed 15 amp fuses and they blew as well. I now have 20 amp fuses in and so far, no problem.
What am I seeing with this transformer when power is applied? This situation is happening on 2 brand new installations.
 

ron

Senior Member
Re: blown fuses

The 15A time delay fuses should hold on inrush for a 480V primary on a 5kVA xfmr.
Did you get a similar result with load on the secondary? Now that it holds with the 20A fuses, what is the load measurement on startup and steady state?
 

wanderer20001us

Senior Member
Re: blown fuses

Is this in a system with significant non-linear loads or high harmonic content? It may be a harmonic inrush problem.
 

kthbrwn

Member
Re: blown fuses

When using "cc" fuses for motor protection, they automatically need to be sized larger, as opposed to say a LPJ style fuse. For a transformer application though, I think you would have better luck, with a ATQR style in the cc class, and then sizing at 125%.
 

ron

Senior Member
Re: blown fuses

The LP-CC is for time delay type applications. Reviewing the time current coordination curves reveals that the LP-CC should handle the inrush of a transformer as well or better as most other fuses that may be selected. For example, the 15A fuse will melt approx 125A @ .1 seconds. Another good fuse selection would be FNQ-R as mentioned, since it is also a time delay type and its characteristic would melt at approx 100A @ .1 seconds.
 

spongebob

Member
Re: blown fuses

Thanks for all the good info.
I don't have a current reading at start up,(can't shut them down) however the running current draw is only 1 amp on the primary side.
I did forget to mention the problem was intermittent, and the transformers are brand new.
I don't know how I missed the FNQ-R fuses for this application, however I am going to install them at a lower rating and watch the results.
 
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