Submerged Well Pump Puzzle

[indshane]

A municipal customer recently had a problem when a flange broke on a fresh water well. The water filled the block wall building until the water reached a height of 7 feet. At this point the door blew open allowing the water to escape. When the water had reached the VFD, the VFD blew up which shut down the well. Unfortunately, the broken flange was downstream of the check valve. Water from the system continued to fill the building. This submerged all the controls in the building but did not trip the circuit breakers outside. The 3 phase 480 VAC CB was not tripped nor was the 1 phase 240 VAC CB that feeds a NEMA 1 subpanel. The customer said the lights were on when he got to the well site at 3am. Why did the CB's not trip on a phase to phase short or phase to ground assuming the grounding is properly done. Thanks for the input.

 

[petersonra]

Re: Submerged Well Pump Puzzle

Originally posted by indshane:
A municipal customer recently had a problem when a flange broke on a fresh water well. The water filled the block wall building until the water reached a height of 7 feet. At this point the door blew open allowing the water to escape. When the water had reached the VFD, the VFD blew up which shut down the well. Unfortunately, the broken flange was downstream of the check valve. Water from the system continued to fill the building. This submerged all the controls in the building but did not trip the circuit breakers outside. The 3 phase 480 VAC CB was not tripped nor was the 1 phase 240 VAC CB that feeds a NEMA 1 subpanel. The customer said the lights were on when he got to the well site at 3am. Why did the CB's not trip on a phase to phase short or phase to ground assuming the grounding is properly done. Thanks for the input.

probably because water is not an especially good conductor of electricity so the current flow was not enough to trip either CB.

here is something you can do to test this. fill up a 5 gallon gucket with tap water. hook up test leads to your ohmmeter. drop one lead into the bottom of the bucket and set the other into the top of the water. read the meter.

 

[charlie b]

Re: Submerged Well Pump Puzzle

I once saw an Instructor, in an effort to prove a point about leakage currents, unplug the overhead projector from the extension cord, immerse the female end of the extension cord into a glass of water (without first unplugging the male end from the wall outlet), take a drink from the glass

Nor would I ever count on water as an insulator, to prevent shock, or as a conductive path to trip the breaker and terminate a ground fault.

 

[electricman2]

Re: Submerged Well Pump Puzzle

Fresh water is a relatively poor conductor. In fact strange as it seems absolutely pure water is an insulator.