Fuse blew feeding group of motors

[powersoff]

I have (1) Class J 600A fuse that blew feeding a group of motors. The load consist of 20 fan motors plus a small heater. All of the motors have individual fusing and IEC style overload protection(several OL's tripped but none of the fuses protecting the individual motors did). When the fuse blew the load was roughly 240amps (12 to 14 motors running). I have checked wires to ground with ohmeter , resistance between all motor leads (4ohms), Operation of all the contactors with the 480volt power off, and terminations at Main Fuse Disconnect. The nameplate for 19 of the motors 27Amps and 1 is 14Amps. Would anyone have a suggestion to what else I should check?

 

[itsmoked]

Study the actual fuse block. Very often either the wire terminations or the fuse grips are loose. This instantly translates into a voltage drop which means power dissipation.

It doesn't take much power dissipation in a place like that, where it shouldn't be happening, to raise the fuse's ambient temperature. The fuse's blowing point IS a function of the fuse's ambient temperature. Your fuse can be derated by that and then blow unexpectedly.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[edison123]

May be a defective fuse or a loose fuse connection since you say several OL's tripped.

 

[waross]

I agree with Keith. Look for discoloration of the fuse blades and fuse clips. Check the wire connections for tightness.
If one fuse blew the system would be single phased and tripped overloads are just doing their job.

Bill
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"Why not the best?"
Jimmy Carter