fuses, heater overloads on motor circuit

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jmbs

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I've done a lot of installs but not a lot of troubleshooting.20 amp brkr to disconnect with 5amp fuses. From disconnect to motor starter with 2.3 amp thermal overloads. 480 volts.2 hp motor on conveyer. All three 5 amp fuses blew today. Replaced fuses runs like new. See if I'm headed in the right direction?- If it happens again I plan to check resistance on the windings and if that checks out meg the whole circuit. For three fuses to bblow that means a dead short right? For future reference, when would overloads kick out before fuses? Thank you for the help.
 
fuses, heater overloads on motor circuit

Depending on the class of fuse, a locked rotor condition could blow fuses before the overloads trip.
However, I can’t ever recall all 3 fuses blowing on a motor load.
 
Fuses protect against Short Circuits, the thermal OL protects against long-time Over Current.

Hard to decipher your data in the post because of poor punctuation. Is that 2HP or .2HP? The FLA value doesn't compute either way: 2HP should be closer to 3.4A FLC, 0.2HP would be something closer to 0.24A, you said the heaters are set to 2.3A? I would think you would be getting nuisance tripping of the OL relay, although maybe someone wanted protection at a much much lower level for some reason.

But if it IS a 2HP motor, the fuses can be as high as 6A if they are time delay, and if they are NOT time delay they can be as high as 10A. If you have non-time delay fuses that are only 5A, I wouldn't be surprised at them all blowing from inrush current.

Post more details if you want better answers.
 
Jraef said:
Non-time delay fuses sized too low for the application, one blows on inrush, the other 2 blow later because of single phasing the motor.
Click to expand...

I would only expect one of the last 2 to blow with proper fuses.
But I agree, with undersized, fast-acting fuses, both could.
 
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