Overloads on a 3 phase motor

[CROW]

I have a hydraulic press which is fed from a 200a switch fused to 125a with 100a fuses in the panel, AB size 4 MS with W74 OL's (82a) when the pump starts it pulls 180a and blows 2 of the 125a switch fuses. Why those fuses and not the OL's or the 100a fuses in the panel? Obviously there is a problem with the motor or hydraulic circuit, but...

 

[iwire]

Overloads are designed to work slowly.

What is the HP and voltage of the motor?

Are all the fuses the same or are the 100s 'slow blows'?

 

[CROW]

That might help. 480v 60hp rated for 72.5 fla.

 

[Controls]

When did this problem start? Did anybody replace the fuses and did not install the original type of fuses that was there?

Just like Iwire mentioned. O.L are slow acting. If your motor is going bad (windings), if you a have a short circuit, your fuses will blow before O.Ls come in play. If you were able to run without blowing the fuses at start up, O.Ls would kick out in order to save the motor.( if there was an over current situation with the motor itself), not the circuit feeding the motor.

 

[iwire]

That might help. 480v 60hp rated for 72.5 fla.

Per table 430.250 we need to figure that motor at 77 amps.

That means approximately ...

One time fuse as large as 225 amps

Time delay fuse as large as 125 amps

Standard breaker as large as 175 amps

It seems to me that the existing short circuit and ground fault protection is too small.

Now if it had been working fine for years and suddenly it does not there certainly may be other issues but the existing set up is under sized.