How to measure DC resistance between the common point of connection and earth ground shall not exceed 3 ohms.

[Neo202001]

I am trying to meet the following requirement, what is some options of doing this measurement:

Requirement I am trying to meet: "Electrical systems shall be connected having a grounded-neutral conductor in accordance with the requirements of NFPA 70. A common point of connection to this ground shall be required in all special purpose areas. The DC resistance between the common point of the connection and earth ground shall not exceed 3 ohms."

 

[zbang]

I love vague specs. Does "earth ground" mean the dirt (and where?) or does it mean the top of any grounding electrode? It sounds like they want ground bars in specific areas and that they have a low resistance to the grounding electrode system (GES).

Would you give us some context for that requirement?

 

[Neo202001]

The requirement calls for an annual check for calibration work benches. Another requirement calls for all work benches to bonded together and not have the ground connection connected in serial.

 

[roger]

The forum rules do not allow us to assist DIYers so I am closing this thread in accordance with those rules.

This site is designed for:

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* This NEC� Forum is for those in the electrical and related industries. Questions of a "How-To" nature by persons not involved in the electrical industry will be removed without notice.

Roger

 

[roger]

After a conversation with the OP it was noted that there was some confusion with the registration to the forums, he is an EE looking for assistance, the thread is being reopened

Roger

 

[hbiss]

I am trying to meet the following requirement, what is some options of doing this measurement:

Electrical systems shall be connected having a grounded-neutral conductor in accordance with the requirements of NFPA 70. A common point of connection to this ground shall be required in all special purpose areas. The DC resistance between the common point of the connection and earth ground the grounded neutral conductor shall not exceed 3 ohms."

Try this and an ohm meter.

-Hal

 

[Neo202001]

that’s the plan, but at what two points should I take the measurement at?

 

[hbiss]

Are these ESD safe work benches? Since you say all benches must be bonded together but not in series, I assume a star configuration from a common ground point somewhere between the benches connected to the building EGC. That can be as simple as a plug plugged into a receptacle with only the ground pin wired back to the common ground point or as elaborate as a separate wire run back to the service panel ground bus.

They are saying that the DC resistance of the conductor from the common ground point to the point where it is bonded to the building EGC can't exceed 3 ohms. That wire is what you would measure.

This is how I interpret your question, I could be way off.

-Hal

 

[Neo202001]

Hal,
1 bench is a esd, the others are electronic work benches where electronic equipment maybe repaired and risk of exposure to energized sources.

I am totally clueless and been researching but most things I have come across has been methods of measuring resistance of conducting rods in the earth.
The only common point of ground connection for all the benches is within the service panel.

 

[zbang]

I would ask the fine person who wrote the spec what they actually had in mind.

 

[hbiss]

1 bench is a esd, the others are electronic work benches where electronic equipment maybe repaired and risk of exposure to energized sources.

Usually one would never do electrical or electronic work on a metal bench or conductive surface, especially grounded. Learned that when I was 12 and servicing AC/DC radios and TVs.

ESD benches use a mat or surface made out of a high resistance material or a high value resistor in series with the ground lead to minimize electrical shock while still bleeding off any static charge. In any case, bench tops are wood.

So I have no idea what this is other than to certify ESD bench surfaces.

-Hal

 

[Neo202001]

I found this figure in a reference document, it calls for resistance bridge do you think a digital multi-meter work just as well?

 

[zbang]

If all this is checking is basic continuity from a "grounding bar" on a workbench or in a lab to the GES, then practically anything will work (even a battery & buzzer). If this is an ESD ground, then resistance almost doesn't matter; as Hal said, ESD systems generally have a high resistance built it. If this is not an ESD protection system but rather a EMI reduction system, then a low impedance is important but anything under a few ohms DC is likely to be fine.

I still think you need to ask the spec writer what they were/are thinking.

IMNSHO, most lab "grounding" specs are mostly made up of guesses and "well *I* think"s, with very little actual physics. (Like, why 3 ohms? Why not 5? or 25?)

 

[Neo202001]

I feel someone pulled those numbers from a magic hat. So meet with the facility engineers and the game plan is to measure dc resistance from the earth conductor rod initial bond point, and every delta bond point measurement at each electrical service panel up to the ground bond point of the work benches. Sum all the dc resistance deltas and see if it meets the 3 ohms or less requirement.

 

[hbiss]

I feel like somebody pulled this whole thing out of someplace else, but knock yourself out.

-Hal