hockeyoligist2
Senior Member
- Location
- close to greenville sc
Two identical 460v sewer grit pumps. They both have had the wire short to ground and/or burn one wire in two inside the concrete enclosed conduit several times since they went online a year ago. The original contractor has replaced it under warranty and it is good for a while, then we get a call that it is tripping.
Every time I or one of the other electricians get there we find that it is shorted or lost a leg. Hook up temp wires, everything is good. Last week I got the dreaded call, checked no power on one wire. Hook up our temps again, everything good. Next day the other one lost a leg. Temped it.
Got the call again today, temps still connected, I checked for shorts, power, and motor, everything good. Turned it on and it tripped immediately. Went downstairs to check the motor again and it was running without power and was cold! Pulled the disconnect and it was still turning. I could hear water coming through the pipes. It throttled down and stopped.
I traced out the pipes and found a backwash valve that clears the lines after it shuts down to prevent the lines and pump from stopping up with grit. It is controlled separately by the plants computer system. The computer system gets a signal from another valve telling it that it is closed, then it purges the pipes. We don't have very good control drawings, so we never knew it was there.
The pumps cycle every 4 1/2 minutes, the PC was telling it to purge for 3 minutes. The valve takes about 1 1/2 minutes to close, so the pump is still spinning backwards when it tries to restart! I changed the PC timer to 2 minutes which is more than needed to purge.
I'm guessing that the wire is the weakest link and it eventually breaks down. Sound reasonable?
Every time I or one of the other electricians get there we find that it is shorted or lost a leg. Hook up temp wires, everything is good. Last week I got the dreaded call, checked no power on one wire. Hook up our temps again, everything good. Next day the other one lost a leg. Temped it.
Got the call again today, temps still connected, I checked for shorts, power, and motor, everything good. Turned it on and it tripped immediately. Went downstairs to check the motor again and it was running without power and was cold! Pulled the disconnect and it was still turning. I could hear water coming through the pipes. It throttled down and stopped.
I traced out the pipes and found a backwash valve that clears the lines after it shuts down to prevent the lines and pump from stopping up with grit. It is controlled separately by the plants computer system. The computer system gets a signal from another valve telling it that it is closed, then it purges the pipes. We don't have very good control drawings, so we never knew it was there.
The pumps cycle every 4 1/2 minutes, the PC was telling it to purge for 3 minutes. The valve takes about 1 1/2 minutes to close, so the pump is still spinning backwards when it tries to restart! I changed the PC timer to 2 minutes which is more than needed to purge.
I'm guessing that the wire is the weakest link and it eventually breaks down. Sound reasonable?