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Old 07-12-2012, 07:07 AM   #1
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In industry, Neutral - ground connection


We had purchased CNC mc along with servo stabilizer. Our electrical source from Electricity board is 3 phase with neutral only.
We are make separate earthing by using earth bit and connected to the CNC via copper plate.
But CC service engineer came and checked the voltages between the ground and neutral, mulimeter shows 9.3VAC. And he complained earth fault.
Kindly suggest me, how to recover this issue? Can we connect the neutral to the ground copper rod together? acceptable or not?
[CNC mc doesn't needs neutral connection except servo stabilizer]
Kinldy suggest me assap.

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Old 07-12-2012, 07:10 AM   #2
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In industry, Neutral - ground connection


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Originally Posted by sriram View Post
We had purchased CNC mc along with servo stabilizer. Our electrical source from Electricity board is 3 phase with neutral only.
We are make separate earthing by using earth bit and connected to the CNC via copper plate.
But CC service engineer came and checked the voltages between the ground and neutral, mulimeter shows 9.3VAC. And he complained earth fault.
Kindly suggest me, how to recover this issue? Can we connect the neutral to the ground copper rod together? acceptable or not?
[CNC mc doesn't needs neutral connection except servo stabilizer]
Kinldy suggest me assap.
You realize this is a DIY site, right?

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Old 07-12-2012, 07:13 AM   #3
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In industry, Neutral - ground connection


Call an electrician. If you/your company has the $$$ for the machine, you have the $$$ to hire an electrician.
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Old 07-12-2012, 08:01 AM   #4
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In industry, Neutral - ground connection


You may not connect neutral to ground at branch circuit receptacles or inside appliances.

Like in a household electrical system you ground the neutral (bond the neutral to the grounding electrode conductor) only at the first main disconnect which can sometimes be upstream of what you consider to be the main breaker panel.

In a branch circuit it is possible for neutral to ground to measure some voltage. This should be negligible, generally no more than a volt or two, but will tend to be larger if more amperes are being drawn. Significant ground to neutral voltage under "normal" conditions may mean a loose connection somewhere, either in the neutral path back to the panel or the ground path (equipment grounding conductor) back to the panel.

If you drive a rod or bury a copper plate for use as a ground, that must be interconnected (aka bonded) with existing ground rods for the building using #6 gauge copper wire as a GEC (can end at and be clamped to an existing such #6 copper wire and need not be splice free).

In most parts of the U.S. only a licensed electrician may do work in buildings other than single family houses.
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Old 07-12-2012, 09:26 AM   #5
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In industry, Neutral - ground connection


You.ve got a potential difference between your main ground and your new ground. Hire someone to wire it correctly.
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Old 07-12-2012, 03:51 PM   #6
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In industry, Neutral - ground connection


You guys totally screwed this up, and should get an electrician to do the installation. Neutral and ground MUST BE BONDED AT THE SERVICE, and your machine MUST BE CONNECTED TO THE SYSTEM'S GROUNDING BUS. It sounds like you installed a separate grounding electrode for this machine only, and connected the chassis to only that electrode and not to the system's grounding bus. This is a dangerous condition. You have no protection from ground faults, because it is unbonded.
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Old 07-12-2012, 03:58 PM   #7
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In industry, Neutral - ground connection


Quote:
Originally Posted by AllanJ View Post

In most parts of the U.S. only a licensed electrician may do work in buildings other than single family houses.
I think it's pretty clear that this guy is not in the US or Canada.
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Old 07-12-2012, 05:19 PM   #8
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In industry, Neutral - ground connection


Quote:
Originally Posted by AllanJ View Post

If you drive a rod or bury a copper plate for use as a ground, that must be interconnected (aka bonded) with existing ground rods for the building using #6 gauge copper wire as a GEC (can end at and be clamped to an existing such #6 copper wire and need not be splice free).
Your statement is not correct, Check out 250.54


These ground rods serve no useful purpose what so ever, and quite possibly do more harm than good. Just stating the facts folks!

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Old 07-12-2012, 06:01 PM   #9
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In industry, Neutral - ground connection


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Originally Posted by stickboy1375 View Post
Your statement is not correct, Check out 250.54

These ground rods serve no useful purpose what so ever, and quite possibly do more harm than good. Just stating the facts folks!
Yeah, they're pretty worthless. But his statement was closer to correct than incorrect: 250.54 does not require that auxiliary electrodes be bonded the same way as main electrodes, but it does require that they be bonded to the equipment grounding conductor, which must comply with all the normal requirements for equipment grounding and bonding. So the auxiliary electrode must be bonded to the system neutral and other grounding electrodes through the branch circuit equipment grounding conductor. It just doesn't have to be a #6 wire.
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Old 07-12-2012, 06:05 PM   #10
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In industry, Neutral - ground connection


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Originally Posted by mpoulton View Post
Yeah, they're pretty worthless. But his statement was closer to correct than incorrect: 250.54 does not require that auxiliary electrodes be bonded the same way as main electrodes, but it does require that they be bonded to the equipment grounding conductor, which must comply with all the normal requirements for equipment grounding and bonding. So the auxiliary electrode must be bonded to the system neutral and other grounding electrodes through the branch circuit equipment grounding conductor. It just doesn't have to be a #6 wire.
Very true, I wasn't trying to make him feel sheepish, just wanted to clarify the exact intent of the NEC.
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Old 07-12-2012, 06:11 PM   #11
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In industry, Neutral - ground connection


Quote:
Originally Posted by stickboy1375 View Post
Your statement is not correct, Check out 250.54


These ground rods serve no useful purpose what so ever, and quite possibly do more harm than good. Just stating the facts folks!
It says that more than one ground rod can be connected to the equipment grounding system following rule 250.118 but doesn't have to meet all the normal requirements of a ground rod. This new ground rod can't be your ground fault protection. In an industrial environment you have a building ground and a system ground. So at each panel you'll have your ground that you pulled in with your service conductors and you'll also have building ground which is basically just a bare 3/0 copper running all over the place tied into cases of electrical equipment by means of a lug mounted on the outside. Panels, machinery, cable tray all get grounded. I think what the rule means is that it doesn't have to be tied into the building ground system but there still needs to be a ground run in the pipe feeding the unit for ground fault protection. The ground fault protection is likely enough to level out the ground potential being as it's ultimately tied into your service ground. OP said there's no ground with the feed wires.

edit: took me too long to do all the reading and i got beat to the punch

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