Floating Voltage on Hoist Hook

[ldh04]

Has anyone experienced a small(< 30V) voltage on a hoist hook when the crane (480 VAC) is grounded through the wheel-to-rail connection as allowed by NEC 610.61. Is it a problem? How large can the voltage get.

 

[Skogsgurra]

First of all: Are the rails grounded? If not, you are not adhering to NEC (or any other code). Check to see if you get the voltage from rails to ground first. If you have, ground both rails.

Then, there are at least two "gaps" in the path between crane frame and rails. The gaps are the bearing gaps and the wheel to rail gaps. If you have a second carriage, you will have yet one gap to consider.

The bearing gaps (grease film between balls and raceways) usually break down at somewhere between ten and fifteen volts and if you have four (my guess) wheel, there should always be at least one or two bearings that break down below that voltage. So, you can have no more than fifteen volts across the bearing. There is one exception; if the temperature is low, the voltage can be greater since the break-down voltage increases when temperature goes down.

The second gap is the one between wheel and rail. If the rails are clean and the wheels are clean, you should not expect much voltage there. Less than one volt, I would say. But, if the rails are painted, you can have a lot more.

So, find out where your voltage drop develops: Rail to ground, across the bearings or between wheels and rails and do whatever is necessary to reduce the voltage.

 

[busbar]

 
If the crane is served from a solidly-grounded wye source, then it may be possible to see ~280V on the hook.

Two conditions must happen to energize the hook—a ground fault has occurred somewhere on the 480V system {usually from insulation failure) and the crane assembly is poorly grounded or lacking adequate bonding to other exposed surfaces to limit potential difference.

To further characterize the problem, besides measuring the undesired voltage, consider also measuring current when shorting the energized hook to various {grounded} conductive surfaces that people may contact. If the measurements are widely fluctuating, that may correspond to equipment served by a magnetic starter.

1) Disconnect and perform 1kV insulation-resistance tests on various electrical components to locate insulation failure. 2) Using a Kelvin-connected ohmmeter with a current on the order of 10 amperes, assure that the movable parts or the crane are not floating or inadequately bonded together. Both problems must be found and repaired to prevent likely increasing hazard.