Why Didn't the breaker trip?

[hockeyoligist2]

I had a call that an aerator (480v) wasn't working, the operator said it tripped the day before and he reset. And it didn't come back on. When i got there I turned it off and the starter dropped out????!!!!, after being on all night. I ohm'd out the wires at the starter and they checked ok. Checked the amps, all legs were reading 15 amps (normal for it without a load) so I assumed the prop came off. pulled it and two legs had worn into about midways of the power cable. So, it had been on for at least 16 hours and never tripped with only one leg feeding the motor!!!!!!!! Put a new cable on and the motor is fine. How can this happen? it Should have tripped again, or burned the motor up, it shouldn't have read a complete circuit , or showed same amps on every leg. I'm kinda baffled with this.

 

[69boss302]

Seen it lots of times. The wires aren't always shorting or grounding. Could be just enough moisture and as soon as it arcs drys it out. Could be the cable hits against the conduit or whatever cuts into the cable, arcs and blows itself clear for a little while.

Whe had a nitrogen pump that went for close to 2 years. Breaker trip, check it out, everything fine, same as you found. It would run for about 2 months and trip again. Again nothing run for 2 months and trip. They replaced the motor once and 2 months later the breaker triped. Never so much as a fraction of current imbalance or sign of a ground. Somebody finally pulled a conduit cover and could see some torn insulation (but no copper) put the megger on while somebody pulled on the wire and watch it jump back and forth to zero and bounce up to 1000 meg. That's only one time, been others.

 

[ron]

Generally, for a motor, the upstream breaker is for short circuit protection only. Overload protection would be provided by heaters/overloads. Check to be sure they are set properly.