Overhead crane/monorail shocking operator

[hockeyoligist2]

Almost New install, It has 2 motors (480v 3P), one for lifting the other for moving it side to side. I haven't been able to talk to any of the people that got shocked. They were all contractors and they reported it to the plant supervisor before leaving.

I have checked all of the wires for bare spots. Ohm-ed them all to ground. I checked for voltage between the control support cable and a box on the wall and I got 24v AC. I added a ground wire to the control transformer and it dropped to 11v. The only thing I can come up with is the rail is painted and not allowing a good ground connection. Any ideas?

 

[mxslick]

Other than the ground wire you added, was the control transformer secondary tied to the frame of the crane?

Did you add the ground to the primary, secondary, or frame of the control transformer?

I would first remove the ground you added then if possible energize each motor and the control transformer separately, one at a time, until you find out which is causing the voltage to appear.

It is also possible that the rail sliding contacts' insulators are contaminated and tracking to ground.

 

[zog]

I have checked all of the wires for bare spots. Ohm-ed them all to ground. ?

Does that mean you meggered them?

 

[hockeyoligist2]

Does that mean you meggered them?

No, I didn't megger them, the megger was at the shop, I will try that tomorrow. I just used my fluke to check all of the wires back to ground. I am also going to get another electrician to go with me tomorrow so that I can check volts to ground on the cable while the motors are operating.

Quote "It is also possible that the rail sliding contacts' insulators are contaminated and tracking to ground."

They were covered with spider webs, I cleaned those off and cleaned the contacts. There are 3 sliders, one for each phase, it depends on the travel rail/building for it's ground. Most of the other ones that i have worked on had 4 rails to include the ground.

I did a kinda dumb thing, I know I will get scolded... I grounded myself and brushed the back of my hand across the support cable and I felt nothing.

Could it be static? Every time it is used it has to go down about 60 foot. So I'm thinking that maybe the cable coming off of the reel is causing static? It would help if I could talk to the guys that got shocked and determine exactly what they were doing at the time of the shock.

 

[K8MHZ]

I did a kinda dumb thing, I know I will get scolded... I grounded myself and brushed the back of my hand across the support cable and I felt nothing.

On my very first job as an electrician's helper (1976) the electrician I working with was on a ladder and checking 480 buss by licking the back of his hand and touching it to each buss. I stood there with a nice Triplett in my hand and told him I had the meter right here. He looked at me and said, "You trust your life with that meter? What if it suddenly stops working and you put a wrench to a hot 480 bus you thought was dead while standing on a ladder 20 feet off the ground? This way I know it's safe for me, and more importantly safe for you."

So.....even now, after I have checked with the meter, before I put a wrench to a big piece of copper I check it with the back of my hand.

Back on topic, is there any way you can make contact with the contractors that got the shock to determine the current path? If you knew what they were touching when it happened that would do much to help figure things out.

 

[hockeyoligist2]

I talked to the contractors this morning. They said that they were only touching the support cable for the control. They were standing on a metal grate one time and standing on a pump the other time.

I megged both motors and the wires and everything checked out fine. I talked with a guy from the crane company that we use. He said we need to install another contact rail for the ground because we aren't getting a good ground back to the building. Sounds like a good idea to me.

 

[hockeyoligist2]

Finally found my solution, Section 610.61 of the NEC

?All exposed non?current-carrying metal parts of cranes, monorail hoists, hoists, and accessories, including pendant controls, shall be metallically joined together into a continuous electrical conductor so that the entire crane or hoist will be grounded in accordance with Article 250. Moving parts, other than removable accessories, or attachments that have metal-to-metal bearing surfaces, shall be considered to be electrically connected to each other through bearing surfaces for grounding purposes. The trolley frame and bridge frame shall not be considered as electrically grounded through the bridge and trolley wheels and its respective tracks. A separate bonding conductor shall be provided.?