Genie Lifts And Parallel Currents

Hawkeye_Pierce

So I have an interesting story, just for the sake of telling a story I guess.

I work in theatre as a lighting designer. Today when I went up on a genie lift, as the motor was pushing the genie up I grabbed onto the light I needed to focus. . .Zaaap!!!! Got bit!

I later measured 60volts between the genie's chassis and the steelwork in the ceiling that the lights hang off of above the stage. This voltage was only present while the motor is running in the genie. I did this using a receptacle in the auditorium that we usually use.

Now if I run the genie off one of the 200Amp 3ph PD boxes that the dimmer system uses, no 60 volts, no sparks. This PD box is one of three that serves 200 some odd lights in the air attached to the buildings steelwork. (200+ lights with grounded metal chassis' attached to metal lighting grid)


So whats going on? There are no shorts I can find anywhere on the genie lift itself. Is this happening because there is a difference in voltage between two grounding systems in the building. . . even though that uhhh shouldnt be?! The receptacle used is fed from the other side of the building.

Extension cord to genie is grounded, and receptacles ground tests positive with one of those plugin testers, not that that is definitive.

JohnJ0906

Was the lift plugged in while this happened?

Exactly how are these lights controlled?

__________________
John from Baltimore
"One Day at a Time"
All replies based on the 2008 NEC

Hawkeye_Pierce

Yes Genie was plugged in like I said.

Lights are run to scr dimmer system with temporary cable aka 12/3 SOOW, but I dont think that has much to do with the situation other than when lights which are attached to the steelwork share their neutral / ground with the genie, the current goes away.

firemike

Ground/Neutral reversed anywhere? At the plug? In the extension cord?

Can you measure voltages between the different systems, such as hot from one to neutral and ground of the other system to see if there is a difference. Are all the electrical systems bonded back to one point (ie: the main service equipment?) If not, possibly creating a differential potential somewhere.


Sounds like the problem is with the plug/system you usually use, as when you tap into the lighting supply you get no voltage. Possibly a weak/loose/corroded neutral connection somewhere, giving one source a better neutral than the other.

__________________
-Mike-

JohnJ0906

I wonder if the dimmer panel is properly grounded/bonded. Anyway you look at it, you need to find the cause of this.

__________________
John from Baltimore
"One Day at a Time"
All replies based on the 2008 NEC

te12c02w

I hope you find the problem. We have encountered a similar situation involving church chandeliers and a genie lift. I was told that the people cleaning the chandelier got a shock when they touched it. I went to the church and used the same lift, which was a rental and pretty old, plugged into the same receptacle and never did get any shocks or voltage readings across the lift frame and metal parts of the chandeliers. Better cord?, lift problems?, gremlins? I never did find out although those cleaning people still occasionally ask me about it. Good luck. Jim R

Hawkeye_Pierce

I think I will do a deeper inspection into the where the two systems derive and see if I spot anything shady. None of the outlets are reversed polarity.

I sure hope the the dimmer systems panel is properly grounded! Thats a looota juice to not be bonded correctly.

I have a feeling firemike is spot on the money. Good idea measuring for a difference between hot on one and neutral on the other. There is no way they all derive from the same system.


All I can say is judging by the rigging in the place, the disabled smoke alarms above the stage, and the condition their lighting equipment was kept in before us new people came in, A ****ty neutral connection somewhere would not surprise me.

JohnJ0906

Keep us updated Hawkeye. I like to find out how these things resolve.

__________________
John from Baltimore
"One Day at a Time"
All replies based on the 2008 NEC

Hawkeye_Pierce

Will do guys!

On a side note, I sure wish there was some way to get certified as an theatrical electrician. I feel like Im the only person in theatre who even knows Article 520 exists

firemike

From what you have described, apparently all you need is a screwdriver, pair of dikes, and some tape to be a theatre electrician, so consider yourself certified!

__________________
-Mike-

Hawkeye_Pierce

Dont be mean! I have to ballance phases, do tie ins, and build quad boxes too!


Oh wait, **** all that takes is a screwdriver, pair of dikes and some 33+

GenieBlue

This is a very old post but I will add a bit to it for the record.

Many lifts come with non-marking tires. The lack of carbon conduction through these tires can cause a capacitive build up to occur and a discharge can take place when the operator touches something that will conduct this charge.
These units come equipped with static straps that drag along the floor to keep the lift from building up the static charge. Many times the straps have been ripped off the machine and no longer provide the discharge path.