Chain Motor ground on Truss

[peterleif]

Hey all i just had a weird incident at my job a few days ago; we were setting up some truss and motors for a projection setup. All the motors were hooked up to the beams and to motor control, as we were testing the motors to make sure they were all working one of the chains hit the truss and started sparking. It looked as if the oil on the chain was popping off like in a grease fire, this was on 2 of the motors attached to the same piece of truss, one of the motors ceased to work right after this but the other was fine, so I'm not sure if that would have any correlation.

Just wanted to see if anyone has ever ran into anything like this before and if you know what was causing this?

-Thank you.

P.S. We were using a 30 amp 5 five wire plug, don't know if that makes a difference.

 

[Les]

Just an absolute shot in the dark but it sounds like the now-dead chain motor shorted to ground.

 

[LavaASU]

Uh, I'm pretty sure *in factory supplied condition* that the chain would be grounded via the motors ground. Did the sparks thing happen between just one motors chain and the truss, or was the show repeated with both motors chains, and did they spark at the same time if so or separately? Was there anything else on the truss and was it receiving power if so?

Some possibilities:
*the motor was not grounded (be a fault in the motor or elsewhere in the system) and a separate electrical fault made the motor chassis hot... this would also require the truss to be grounded... and the motor shorted to ground through the chain
*something else on the truss had a fault making the truss hot and it shorted to ground through the motor (you mentioned video truss-- certain projectors have whats called high leakage current requiring them to have a solid ground to prevent the chassis ending up live... so that would be a single failure situation that could explain this)

 

[WooferHound]

Related story about chain hoist connectors and electrocution
http://www.controlbooth.com/threads/chain-hoist-electrocution-hazard.20975/

 

[porkchop]

I think LavaASU mentioned two pretty likely situations. Given that it happened simultaneously to 2 chain motors I would guess it's the second situation where something on the truss what grounding through the chain and the motors ground. An important detail you haven't mentioned is what stopped the sparking? Did a breaker trip, was the main disconnect opened by a stagehand, did it stop suddenly without real explanation, etc...?

 

[peterleif]

well we cut the breakers for power to all motors and changed out the cable and motor and the problem stopped. We checked the cables afterwards but didnt see any problems.