Ground Rod resistence

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jmd445

Senior Member
In lieu of using a ground rod resistance tester Such as an AEMC 3711 or the fall of potential methods, how do you test for ground rod resistance in a residential setting?

Jim
 

bphgravity

Senior Member
Location
Florida
Re: Ground Rod resistence

Clamp-on ground resistance tester.

tes5600.jpg
 

triphase

Senior Member
Re: Ground Rod resistence

Jim I did not think one could without using the instrument. Just Rely on soil conditions and methods of grounding.....Others will have the answer....sorry
 

jmd445

Senior Member
Re: Ground Rod resistence

These instruments are relatively speaking new to the industry and rather costly. That is where my question is derived, If I were to use my trusty well used Fluke, how would I test.

Jim
 

triphase

Senior Member
Re: Ground Rod resistence

Bryan, I just checked that web site... Jim $295.00 which ain't that costly considering My hydraulic knock out was $1300.00 :)
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: Ground Rod resistence

Why test? Just drive 2 rods and be finished. Also with the use of concrete encased electrodes, rods are no longer required and there would be no testing required.
Don
 

bphgravity

Senior Member
Location
Florida
Re: Ground Rod resistence

Don's method is the best yet. Is there some AHJ or other entity actually requiring you to test the ground resistance?
 

zog

Senior Member
Location
Charlotte, NC
Re: Ground Rod resistence

ANSI/IEEE 142 requires the resistance between the main graounding electrode and "ground" to be less than 5 Ohms for large commercial or industrial systems and 1 ohm or less for generating or transmission stations.

We get called to do testing all the time, it is required on many new installations that meet the above criteria, we use the fall of potential method, the clamp on method can only be used for certian situations and you have to know what you are doing. Megger, has a good article on the differences between fall of potential and clamp on testing if anyone cares.
 

bphgravity

Senior Member
Location
Florida
Re: Ground Rod resistence

Originally posted by zog:
...ANSI/IEEE 142 requires the resistance between the main graounding electrode and "ground" to be less than 5 Ohms for large commercial or industrial systems...

Why?
 

zog

Senior Member
Location
Charlotte, NC
Re: Ground Rod resistence

To ensure prober (rapid) operation of the ground fault protection relays. Too high of a resistance will cause current to be too low and prevent or delay the relay from operating.

The IEEE standard requires the system to be tested every 3 years.

AMEC has a lot of good information on thier website on the importance of a good ground connection for industrial systems.
 

bphgravity

Senior Member
Location
Florida
Re: Ground Rod resistence

Originally posted by zog:
To ensure prober (rapid) operation of the ground fault protection relays. Too high of a resistance will cause current to be too low and prevent or delay the relay from operating...

What are the systems you are talking about that rely on earth as an effective fault path?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Re: Ground Rod resistence

NETA (National Electrical Testing Association) has similar specifications for acceptance testing.


Grounding Systems
1. Visual and Mechanical Inspection
a. Verify ground system is in compliance with drawings, specifications, and NFPA 70 National Electrical Code.
b. Inspect physical and mechanical condition.
c. Inspect anchorage.
2. Electrical Tests
a. Perform fall-of-potential or alternative test in accordance with ANSI/IEEE 81 on the main grounding electrode or system.
b. Perform point-to-point tests to determine the resistance between the main grounding system and all major electrical equipment frames, system neutral, and/or derived neutral points.
3. Test Values
a. The resistance between the main grounding electrode and ground should be no greater than five ohms for large commercial or industrial systems and 1.0 ohm or less for generating or transmission station grounds unless otherwise specified by the owner. (Reference ANSI/IEEE 142)
b. Investigate point-to-point resistance values which exceed 0.5 ohm.
 

zog

Senior Member
Location
Charlotte, NC
Re: Ground Rod resistence

I dont know how to make it any clearer than I have already explained, I am not talking about a GFCI, or a dwelling unit, I am talking about GFP in industrial power systems. Can anyone else reword what I have said to make it make more sense?
 

bphgravity

Senior Member
Location
Florida
Re: Ground Rod resistence

I can understand this requirement for transmission and distribution systems. But I can't for commercial and industrial facilities.

I'm looking for data supporting the need, not the theory.

GFPE relies on connection to Earth?

[ February 27, 2006, 01:01 PM: Message edited by: bphgravity ]
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: Ground Rod resistence

I am talking about GFP in industrial power systems.
The connection to earth should not be needed to make GFP work in a code compliant system. The only time that the earth should be playing a part in opening protective relays is with utility distribution systems.
Don
 

tom baker

First Chief Moderator
Staff member
Re: Ground Rod resistence

Zog:
There is a new IEEE paper out on "How to Protect you house and its contents from lighting".
Mike Holt sent it out as a newsletter. This IEEE standard states a low resistance earth ground is required for proper operation of a TVSS, which is false.
There can be a advanatage to having a low resistance earth ground for lightning protection, but not for TVSS operation or GFP operation.
My point is even the IEEE has bad information.
 

jwelectric

Senior Member
Location
North Carolina
Re: Ground Rod resistence

Originally posted by jmd445:
In lieu of using a ground rod resistance tester Such as an AEMC 3711 or the fall of potential methods, how do you test for ground rod resistance in a residential setting?

Jim
I don?t but should the need ever arise to where I needed to test the resistance I would use Ohm?s Law.

I would connect a live circuit to the ground rod and take an amp reading and divide the amperage into the voltage and there is the resistance.
:)
 
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