Grounding Rotating Parts

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human7

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Could someone direct me where to find information on grounding rotating part in the NEC? Our customer would like us to ground a large winch drum through a slip ring..not sure if this is Kosher or what the rules are.
 

human7

Member
Interesting. I'm no electrician, but wouldn't the drum already be grounded through the motor?

No, the motor drives a gears on a pedestal. The drum has a bullgear on one edge that meshes with the gears on the pedestal. Power passes through a slip ring and onto the drum.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
why does the drum need to be grounded? other than a customer's wish?

as a practical matter, these things are often effectively bonded together, even if they do rotate.

one would think that a listed slip ring that can be used for powering something could also be used for grounding it. and in fact this is pretty common. i can't see why it would be prohibited to use just the grounding connection.
 
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human7

Member
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?
 

SG-1

Senior Member
My 8 KW generator uses one slip ring for the grounded connection.

Which side of the DC is grounded ? Positive or Negative ?

One end of the wire is going to be connected to the drum & the other end connected to what ?
 

ZCBee

Member
Location
Reno, NV
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?


http://www.insul-.com/productline.cfm?Ids=473,469,471,470,468,472,475,467


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 8.2.1.2 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.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)

http://www.insul-.com/productline.cfm?Ids=473,469,471,470,468,472,475,467


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 8.2.1.2 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.
 

SG-1

Senior Member
ZC, I can't get that site to open, would you check it.

Augie, I got it to open, but there were a thousand ways to go forward & I gave up. I do not doubt it is done.

The thing I am wondering is why they do not just ground the positive or the negative, if the purpose is to clear the fault? No new slip ring is required.

If they are going to connect the egc to a grounding electrode system then all that will happen during a ground fault is more DC energized equipment, like everything connected to the grounding electrode system. Still, nobody will know there is a fault.

At 500 volts or if rectifier derived 250.162(A) they are not required to ground. What is the wench pulling & what happens when it stops ?? A ground detection system may be a more logical solution. You finish the pull and shut down in a orderly fashion.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
If the DC supply is grounded, and all the metal is properly bonded, then a fault between the cable and the drum will cause fault current to flow, and OCPD will likely open. Ball bearing generally make pretty good electrical contact at more than a few volts. At the contact points the grease layer is very thin if present at all.

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.

Now, if as SG-1 mentions, your DC supply is not grounded, then a ground fault won't cause much current flow, and won't trip OCPD. SG-1 is also making an important distinction: you probably don't really want to _ground_ this drum, but instead you want to _bond_ this drum back to the source of electrical supply. Since most supplies are in fact grounded, this bonding will also result in grounding the drum. The key is that if you were to somehow ground this drum without also bonding it back to the source, you would actually create significant safeyt issues.

-Jon
 

Mgraw

Senior Member
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.


-Jon

You are 100% correct. The pitting of the ball bearings and race can cause major damage to the equipment. Just a small arch can eventually destroy bearings and races. When I worked for a crane manufacturer we always used slip rings.
 
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