Operator getting shocked from wash down hose, Help?

[hockeyoligist2]

The operator of a gravity belt thickener (sewer plant) is getting shocked when he touches the metal tip while hosing down the floor. He said that the only time he gets shocked is when the hose is laying across a VFD controlled pump, 30 hp. He has to put it across it to get to certain areas.

I have checked the grounds and everything seems OK. I have checked the motor to other grounding points and the only thing I get is .89v DC to the wet floor. Nothing to anything else and no AC voltage.

The ground wire is connected to the cabinet housing the VFD, so I added an additional ground wire to the VFD frame in hopes that it would correct this.

Could the motor be creating a static charge in the hose? The hose has nylon in it which could create static also , I would think.

 

[mike_kilroy]

did you check for AC voltage with the motor running? if the motor lost its case ground, or never had one, it will charge to 300/600vac (230vs460v supply) thru capacitive coupling to the motor windings inside [from the high freq PWM pulses]. You should be able to see this with a decent meter, but of course only when the motor is running. Adding another ground wire at the VFD cabinet probably won't help you; try running a ground wire from the motor case itself to a good ground.

 

[hillbilly1]

It may have nothing to do with the pump, since it only happens in that one area, Instead of checking for voltage from the tip to the motor, check for voltage from the motor ( Running/not running) to the surrounding areas that get washed down. The voltage may be comming from another source in that area through the concrete back to the semi grounded water hose.