Interesting. I'm no electrician, but wouldn't the drum already be grounded through the motor?
The drum should be grounded because there are 500 VDC connections made on the drum from the slip ring to the cable the drum is reeling.
I don't think I should be relying on the grease covered interfaces between the grears or bearings to provide a safety ground.
Where have you seen slip rings used as ground connections?
EGC through slip rings and collector/brushes are used in the packaging equipment at our plant, see above example of manufacturer's products. Per NFPA 79, Electrical Standards for Industrial Machinery, Chapter 8 Grounding, read 22.214.171.124 Equipment Grounding. and 8.2.3 Continuity of the Equipment Grounding (Protective Bonding) Circuit. We have to insure that our bonding is intact all the time. I don't know how else the OEM of these machines would get the EGC and metal parts of the rotating machinery bonded otherwise.
ZC, I can't get that site to open, would you check it.
However the fact that _bonding_ through the bearings will _probably_ work doesn't make it a good idea. Current flow through bearings is a great way to destroy them. First in a ball bearing the areas of contact between the races and balls are very small, so you can have intense local heating. Additionally, you should have at least some grease layer, which can cause micro-arcing and pitting of the balls or races.