100A Time Delay current limiting class RK5 200,000A rms keeps blowing on B phase.

[Big Jerm]

I have went out to this fasility for the second time with the same blown fuse, the fuse is before the meter and is the first fuse on the system (three phase 480V ) the line side comes from the 400A disconect or power side of the mall in the electrical room which also feeds a number of other stores all 100A .
The load side feeds two 100 amp fused disconects in the store and the two 100A panels ,there are 277/480v circuits in each panel (ac etc) then each panel feeds a step down trans and feeds 40A breaker which feeds a panel of 120/208V circuits.
My question is why is the problem by-passing the other fuses in the store and how can I track this down without hours of opening ever electrical device for loose connections and faulty wires?

 

[Cow]

My question is why is the problem by-passing the other fuses in the store and how can I track this down without hours of opening ever electrical device for loose connections and faulty wires?

I see two options that'll let you do this without opening a bunch of panels:

1. Wave a magic wand.
2. Have a helper do it.

 

[augie47]

There could be a variety of reasons.
I would suggest approaching the problem from three directions. The fuse is probably opening from (a) heat, (b) overload, or (c) fault.
Is there evidence of some type of "fault" when the fuse blows ? Is there evidence of a problem before or after replacing the fuse ?
Have you monitored the load ? How heavily is the fuse loaded ?
Have you performed an IR check for overheating ?
I would say that in a vast majority of the time I have observed this situation, there is something causing a heat problem... loose clips, poor connection at the terminals or within the switch itself
Is there any discoloration ?

 

[Jraef]

Or...
The fuse is doing its job and you have not been willing to spend the time to troubleshoot the entire system.

If it isn't something immediately obvious as augie47 mentioned, get (or rent) a recording meter and start at the fuse itself. If you see high current, move down stream of the first split off to see if a single branch circuit is the culprit. If it is, keep moving down stream until you isolate the source. If there is no single branch circuit causing it (as I would suspect because no down stream device is clearing), then it's a cumulative issue and you have a load balancing problem, i.e. a lot of the single phase loads are tapped off of A-B or B-C, with not enough off of A-C to keep them in balance.

 

[Big Jerm]

Thank you

Thank you

I checked for obvious problems and did not find any, I called and told the customer I would require more time (money) to narrow down the problem and he is going to get back to me after talking to his boss.
Thanks for all the advice I will let you know what it was when I get to further troubleshoot it .

 

[brian john]

1. Perform a fall of potential across the the disconnect line to load for each phase, if the VD is higher on B phase narrow down.
2. Measure the load.
3. Determine if the load is fairly consistent or could possibly at some point overload the fuse.

I bet on a high resistance connection or a faulty appliance, that has a bad circuit breaker.