5 ohms or less

MBLES

Senior Member
we are currently constructing a cell tower site. we are installing the ground ring and need to take ohms reading for close out package my requirements require me to get less than 5 ohms when testing, The master ground bar ties directly to building steel and the rest of the equipment ties to Master ground bar. I get 2-3 ohms at Building steel connection. i get 12-14 ohms at Master ground barconnection from Building steel, but i get 2-3 ohms from to master ground bar to each piece of equipment. do you guys have any experience with this type work? why would i get 12-14 ohms at master ground bar ground coming from building steel but get lower ohms reading at equipment and building steel connections. why wouldnt i get less than 5 ohms at master ground bar. what do you guys think im missing??
 

Sahib

Senior Member
Location
India
i get 12-14 ohms at Master ground barconnection from Building steel, what do you guys think im missing??
Ground resistances of very close places usually do not differ much. There may be contact resistance at Master ground bar connection from building steel. Disconnect and remake the connection after cleaning the contact surfaces.
 

just the cowboy

Inactive, Email Never Verified
Location
newburgh,ny
Mid-point

Mid-point

From what you describe you are only reading half the circuit. If the start is building then ring then equip, you would read from building to equip and get a total the two readings. As said redo the building to ring connection
 

big john

Senior Member
Location
Portland, ME
5Ω is a very common spec for the resistance of a grounding-electrode system when measured to remote-earth.

When checking the quality of bonding the spec is often 0.5Ω.

Be careful about what the specs actually call for and what test you are performing: The electrode system will need to be certified with a fall-of-potential apparatus.
 

GoldDigger

Moderator
Staff member
Be careful about what the specs actually call for and what test you are performing: The electrode system will need to be certified with a fall-of-potential apparatus.
Which can be almost impossible with an extensive ground electrode system surrounded by other structures.
The distance to the closest fall of potential electrode should be at least several times the scale distance of the GES.
 

Ingenieur

Senior Member
Location
Earth
Good primer on the subject http://www.weschler.com/_upload/sitepdfs/techref/gettingdowntoearth.pdf

as mentioned the far probe should be 5-7 times the largest dimension of the ges
the mid 62% of that distance
there is a table in the manual

A common issue is too close
the farther the lower the R
as d increases area A grows as the square of d
and since R = p d/A if A>d R gets smaller

a guy was measuring a mine bed (2 beds actually, station and ngr)
by law R<4
he was getting 7-8
added casings, etc, no change
I told him use the manual guidelines
got down to 2

keep a copy on my phone
 

big john

Senior Member
Location
Portland, ME
Is it the product or the concept?:D
It has the same application problems as a traditional fall-of-potential test: The readings will be negatively affected by anything in the zone-of-influence of the electrode.

The trick is that while there are steps you can take to verify your FOP test, there's nothing you cab do to double-check those clanps. Also, they are not a recognized test method per IEEE.
 

GoldDigger

Moderator
Staff member
The tester (also available from Fluke and other makers) can do a reasonable job of measuring the earth resistance of a single electrode as long as you understand its limitations.
But you cannot combine the single electrode measurements to get a complete GES value if the electrodes are within each other's sphere of influence.
 

cuba_pete

Senior Member
Location
Washington State
This is geared towards linemen.

My emphasis:

"Note: For multi grounded systems only. In a multi-grounded system, the larger the number of grounding poles, the more accurate the measured value."

If the grounding rods are installed in another's (of the same GES) sphere of influence then they aren't really installed "correctly" anyway. Simple single-point/one-step test equipment wouldn't work as well.

;) Of course, the NEC leaves this up to the electrician:

250.53(A)(3) Informational Note: The paralleling efficiency of rods is increased by spacing them twice the length of the longest rod.
 

big john

Senior Member
Location
Portland, ME
This is geared towards linemen....
Yep, that seems to be the one place where that tool would shine: Doing ground checks on rural pole lines. Lots of tests to perform, and you also know there's very unlikely to be any electrode interference.

Unfortunately, they are marketed as a general purpose electrode tester, when in all likelihood they will be inaccurate on 99% of structure electrodes.
 

GoldDigger

Moderator
Staff member
Yep, that seems to be the one place where that tool would shine: Doing ground checks on rural pole lines. Lots of tests to perform, and you also know there's very unlikely to be any electrode interference.

Unfortunately, they are marketed as a general purpose electrode tester, when in all likelihood they will be inaccurate on 99% of structure electrodes.
For situations where they are inaccurate (such as overlapping electrode SOIs) is a standard basic Fall Of Potential test going to be much better?
 

big john

Senior Member
Location
Portland, ME
For situations where they are inaccurate (such as overlapping electrode SOIs) is a standard basic Fall Of Potential test going to be much better?
Definitely: The results of a clamp on test are based on blind faith: Unless you compare them to a FOP test you may not even realize you have interference. Whereas the FOP test is self-proving based on the plot of the results.

And if the FOP fails, there are at least some options that may yield acceptable results: Change direction, increase distance, try Tagg-slope calculations. None of that exists for a clamp meter.
 

Sahib

Senior Member
Location
India
The clamp on resistance tester can not be used for the checking of ground resistance in OP's case due to presence of ground ring. But it may be used for checking the ground continuity resistance, where the issue is IMHO.
 

GoldDigger

Moderator
Staff member
Definitely: The results of a clamp on test are based on blind faith: Unless you compare them to a FOP test you may not even realize you have interference. Whereas the FOP test is self-proving based on the plot of the results....
Which assumes that the user of an integrated three point FOP tester actually runs a plot using different middle electrode positions instead of setting that probe at the "correct" distance and just reading the meter which pre-calculates the earth impedance based on an ideal voltage versus position plot.
It has the capability to be self-proving, but what I referred to as the basic test (perhaps a misnomer) does not do this.
 
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