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#71992 - 11/13/06 07:04 PM When should a ground rod be replaced?
Rich Thomas Offline
Member

Registered: 03/09/05
Posts: 48
Loc: Seattle, Washington, USA
How long are ground rods good for?
How can a failing ground rod be determined?

Any thought?

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#71993 - 11/14/06 05:57 AM Re: When should a ground rod be replaced?
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
Rich,
As I said in the Chat room.
There is a tester that you can hire to test the resistance of the rod to Earth, in relation to 2 other rods placed at least 12 foot from the in-situ rod.
What this tester does is gives a comparison of earth resistance of the two other electrodes and tells you how good the in-situ one is.
If you think you have a failing ground rod, it's probably best that you replace it, as a precaution.
BTW,Rich, what brings you to this conclusion?.
A poor connection can do the same thing as a bad rod.
Galvanic corrosion, get your metals right.

[This message has been edited by Trumpy (edited 11-14-2006).]
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin

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#71994 - 11/14/06 07:56 AM Re: When should a ground rod be replaced?
renosteinke Offline
Cat Servant
Member

Registered: 01/22/05
Posts: 5305
Loc: Blue Collar Country
This was a very good discussion in the chat last night ... yet another good reason to come and hang out some time, all!

It's also a good forum topic. We all just see the ground rod there, and assume it's working.

Has anyone ever solved a problem by replacing a faulty ground rod? How did you identify the problem?

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#71995 - 11/14/06 08:40 AM Re: When should a ground rod be replaced?
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
I have had some bad clamps but never identified a bad rod, unless I could just pull it out.
_________________________
Greg Fretwell

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#71996 - 11/14/06 03:04 PM Re: When should a ground rod be replaced?
RODALCO Offline
Member

Registered: 12/08/05
Posts: 863
Loc: Titirangi, Akld, New Zealand
use an Earth Bank Tester

We use the AVO Megger Digital earth tester
DET 5/4R
for utility work in the Vector Network on distribution transformers and new customers 1Ø, 2Ø and 3Ø.

The 2 test leads are on 100 meter reels and are placed 50 and 80 meters away from the earth rod under test in a straight line.
_________________________
The product of rotation, excitation and flux produces electricty.

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#71997 - 11/14/06 07:49 PM Re: When should a ground rod be replaced?
Almost Fried Offline
Member

Registered: 09/04/06
Posts: 104
Loc: Madison County, Ark. USA
One of the excellent ideas gleaned from this board is to sell up service calls. I have for years, recommended that customers have me replace their rods if they are galvanized-those units are half eaten away in the soils around here in 10-15 years. I always have used copper clad rods, and now that the code requires 2 rods, this could be an excellent source of additional work. Andif they have a computer, remind them that surge arrestors are only as effective as the grounding path to dissapate the surge.

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#71998 - 11/15/06 01:30 AM Re: When should a ground rod be replaced?
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
When should a ground rod be replaced?

Who knows, IMO almost never unless something unusual has happened to it.

We test electrodes with a three point fall of potential test like Mike (Trumpy) mentioned.

The cost of buying, renting, or hiring someone with one of these testers greatly exceeds the cost of installing a new rod.

So if you are not comfortable with a ground rod than I say forget testing it just add a new one to the grounding electrode system.

If you where to test it what value would you be comfortable with..5 ohms...25...100 ohms?

There is a ton of myth about what ground rods can do for premise wiring systems operating at less than 600 volts and what they can do for systems that operate at a relatively low 120 volts to ground.

Rods and other electrodes can do a lot for high voltage distribution systems.

If any one is interested I recommend taking a look at this thread at Holt's. IMO there is a lot of interesting information.

That said never accept what you read on the Internet as fact without doing more research or accept that our current thoughts on grounding electrodes are entirely correct.

What does a grounding rod do for us at a typical home service?

 Quote:
now that the code requires 2 rods


That is an overstatement, the code does not require two rods, the code does not actually require any rods.

If the only electrodes available are rods one rod is fine if you can prove the magic 25 ohms.

If you have other electrodes you may not need rods at all.

Bob

[This message has been edited by iwire (edited 11-15-2006).]
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#71999 - 11/15/06 02:07 AM Re: When should a ground rod be replaced?
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
By the way we need to keep in mind who does what.

RODALCO and Trumpy both spend (or have spent) a lot of time working directly with high voltage distribution.

In my opinion that experience is far different from those of us that spend our time under 600 volts.

The effectiveness and necessity for good electrodes in high voltage distribution is greater than in our essentially 'low' voltage.

Say a homes service did not have a effective ground fault path and relied on just a grounding electrode.

Lets say that electrode had a 20 ohm resistance.

If we have a ground fault in that system you would only find 6 ams of current flow which will never trip a breaker in a home

120 volts / 20 ohms = 6 amps

On the other hand if the utility had the same ground fault on a 13,800 volt (7800 to ground) line things are greatly changed,

7800 volts / 20 ohms = 390 amps which is many, many times the value of the utility cut out (fuse) and will dump the circuit in short order.

Bob
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#72000 - 11/15/06 03:35 AM Re: When should a ground rod be replaced?
Fred Offline
Member

Registered: 04/30/01
Posts: 461
Loc: Straughn, IN 47387
I replace a ground rod when the length is suspect or the diameter is too small. For instance: When upgrading a service for a residence and the existing ground rod appears to have been cut or otherwise damaged on the visible top end, I drive a new one. Likewise, if the existing rod is less than 1/2" in diameter, made of bare or galvanized steel or pipe and has some age on it I will drive a new one. Basically if I have any doubt about an existing rod I will spend the $8.00 and add another one. Can't hurt a thing.

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#72001 - 11/15/06 05:15 PM Re: When should a ground rod be replaced?
BOBELECTRIC Offline
Junior Member

Registered: 07/22/06
Posts: 7
Loc: Eighty Four,Pa.U.S.A.
New service complete includes 2 new ground rods that I installed to code.

Bob o. 84,Pa.15330
_________________________
Bob O.84,Pa.15330

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