480v crane grounding sorry long story

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dm9289

Industrial Maintenance Electrician
Location
Pennsylvania
Occupation
Industrial process repair/ maintenance Electrician
In my plant we had a 1955 crane retrofit with a new wireless control system due to many wiring and contactor problems. The contractors only re-utilized the motors and house ungrounded 3 delta 480v connection rails (new high temp wire and festoon system). The hoist and trolley are reversing contactor type, the bridge is a drive to give us different speeds.

We almost always use gloves while using the crane but a day after the contractors left we were lowering their junk and an employee took his glove of due to having some problem getting used to the control buttons (wireless) while holding the hoist wire rope standing on a metal frame and got shocked.

We called the contractor back and found during certain different bridge positions the voltage to the wire rope would vary from 0 volt most of time to up to 120v. During our last use of this frequently unused crane ( used mainly during annual 1 week maintenance project) it did toast a contactor blew some fuses and we noticed wiring in need of replacement. The contractors believe the shock it is due to a bad motor which seems likely.

Finally to the questions

Our old school electric buyer wants to replace the motor as suggested but also wants to replace the drive due fear that regenerative voltage is causing the problem. Does this make sense to replace the drive with contactors?


The contractor in addition to replacing the motor suggested adding a 4th rail for a gnd to eliminate the shock hazzard in the future. The old school buyer does not want to add the rail due to cost and feels the cranes metal wheels on our building should be adequate. Does anyone agree with this being an adequate gnd system?

Short of a 4th rail does anyone have any ideas or experience with grounding this sort of crane short of adding a 4th rail?
 
dm9289 said:
The contractors believe the shock it is due to a bad motor which seems likely.
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I'm not a motor or crane expert by any means, but they believe? That implies that they haven't even checked it, say, with a megger. First step, megger the motor. Second, inspect and megger the rest of the wiring.

dm9289 said:
Our old school electric buyer wants to replace the motor as suggested but also wants to replace the drive due fear that regenerative voltage is causing the problem. Does this make sense to replace the drive with contactors?
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Doesn't to me. If the motor has a bad winding, why not have a motor shop rebuild/rewind it? Whether there is a regen drive or not, something is leaking to where it shouldn't. That's the problem to find and fix.

dm9289 said:
Short of a 4th rail does anyone have any ideas or experience with grounding this sort of crane short of adding a 4th rail?
Click to expand...

Usually the running rails of a crane can provide an EGC, provided that they're properly bonded. Are they? You mentioned that it's fed by an ungrounded delta. Is there a ground detector? Can you change the feed to corner-grounded?

250.116 Nonelectrical Equipment. The metal parts of the
following nonelectrical equipment described in this section
shall be connected to the equipment grounding conductor:
(1) Frames and tracks of electrically operated cranes and
hoists
(2) Frames of nonelectrically driven elevator cars to which
electrical conductors are attached
(3) Hand-operated metal shifting ropes or cables of electric
elevators
FPN: Where extensive metal in or on buildings may become
energized and is subject to personal contact, adequate
bonding and grounding will provide additional safety.

and 610.61
"Moving parts, other than removable accessories, or attachments
that have metal-to-metal bearing surfaces, shall
be considered to be electrically bonded 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."
 
Not the regen

Did they by any chance need a neutral for a control circuit and bootleg it from the ground/metal components/steel rail? I've seen that one done more than once. You did say fed from an ungrounded source though didn't you?
 
Was fed from ungrounded source and it appears they did not bootleg neutral it.

They will rebuild or buy the motor leads in case burnt going into motor.

I also believe that the drive is no issue it is leakage , thanks for your confirmation.

The building frame is at gnd potential, so I assume it could be converted to a corner grounded but I have no experience with this. The entire building uses this 480v source via various overhead feeds.

Am I reading the references correctly it seems like they want a 4th railin both cases
 
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