Septic tank pump motor ohming out to ground on neutral lead?

[sw_ross]

I was called out for a service call to a facility that has a septic tank with a 3-float control system with two pump motors. The above ground control box has the leads for 3-floats and 2 pump motors that come from the tank. The disconnect switch for each motor is a 1-pole 20 amp breaker. The call ticket said that one of the breakers keeps tripping. The motors are 120v.

When I got there the disconnect switch/breaker in the control box for motor 2 was tripped. I also noticed a separate test switch on the control panel for motor 2 that was in the "manual-on" position, while the test switch for motor 1 was in the "auto" position. I reset the disconnect switch/breaker to the on position. Since the test switch for motor 2 was in the "on" position the contactor for motor 2 pulled in and there was 120volts L-G at the terminals that feed motor 2. I couldn't hear the motor running, and put a clamp meter on the motor leads, which showed 0 amps. The disconnect switch/breaker held at this point, and didn't trip at all while I was doing any of the following testing.

When I turned the switch for motor 1 to the "on" position I could hear it running and read 6.5 amps with my clamp meter on the motor leads. After turning everything off I reversed the motor leads and got motor 1 to operate manually off either set of terminals, but motor 2 wouldn't run off either set of terminals. I tried ohming out the motor leads for motor 2; no continuity L-N, or L-G, but I got continuity N-G. I didn't know what to think about this so I did the same test on motor 1, which didn't show continuity L-N, L-G, or N-G.

At this point I'm making the assumption that since the motor 2 switch was in the "manual-on" position and that it was the "lag" motor (motor 1 is the "lead" motor) that it burned up since it was running constantly. I don't know how they are positioned in the tank in relation to each other. Does this sound plausible? Is there any other test I should have done? I did run enough water in the building to get the "lead" motor to start running and eventually get the alarm to go off, without getting motor 2 to start up.

Any feedback is appreciated.
Thanks,
Sky

 

[__dan]

If they ran #2 manually on and forgot it that way, they would have burned out the pump seals, but maybe not the motor.

You should be able to read the motor winding continuity L to N, and it sounds like #2 has burned a winding open and shorted N to G.

Could be connections too, but depending on how those pumps are applied, or misapplied, you could burn them out every three months until the right equipment goes in. Plan on pulling and replacing the pump, but check the other factors. If it is the right type pump for the job. There's a big difference between effluent lift pumps and sewage grinders. Your amp draw says you have an effluent lift pump, and if you're clogging it with solids, you'll be changing it again pretty quick..

 

[ceb58]

Sounds more like someone kept resetting the breaker and trying the pump. Breaker would trip, try again thinking they would get different results until they have burned the windings into and they are now shorting to ground. You have taken pump one and placed it on controller two and every thing works. That eliminates the controls. Pump 2 is shot pull it, replace it, get paid for it and be done with it.

 

[copper chopper]

there also could be bad seals or a crack in motor 2s housing letting in septic and causing the continuity.