[Towertalk] RF & Elec Earth Measuring

[kb9cry@attbi.com]

One cannot just simply use their VOM to measure 
resistance of an earthen grounding system.  Before I 
start let me say that the following statements are what 
I've learned as a Chemical Plant Engineer (22 yrs.) and 
as directed by company Electrical Engineers. To properly 
measure a grounding system resistance, we use special 
instruments made by Biddle Instruments.  We use a Megger 
Direct-Reading Earth tester.  The resistance to earth of 
any system can be theoretically calculated by the 
following formula:  R=p x L/A  where p is resistivity of 
earth in ohm-cm, L is length of conducting path and A is 
cross sectionaly area of the path.  Two types of tests 
are recommended, the Direct, two terminal test or the 
Fall of Potential or three terminal test.  The Megger 
essentially applies a voltage between the two (or two of 
the three terminals in a three terminal test) and 
measures the current flow.  The voltage between the two 
(or the first and the third terminal in a three terminal 
test) is measured. Using V=IR, the meter determines the 
resistance of earth or of the grounding system.  Biddle 
Instruments offers a manual on the above which has all 
the technical information and is available from them at 
510 Township Line Road, Blue Bell, PA  19422.  Manual is 
titled, "Getting down to Earth, Manual on Earth 
Resistance Testing for the Practical Man".  The NEC 
states that resistance should be not less than 25 ohms 
but we use a 5 ohm max here at the chemical plant.  We 
handle a lot of flammable materials and grounding is a 
big deal.  Your local industrial (usually not the 
commercial/residential guys) electrical contractor 
should have these instruments.  Hope this all helps.  Be 
safe.  Phil  KB9CRY
> How can you measure the resistance (?) of a cable running to earth? If I
> wanted to check if a cable already providing an electrical earth connection
> was working to optimum how could I do it without breaking the bank for test
> equip? Is there a difference in any results obtained for RF & Electrical
> earths?
> 
> --
> George Shaw
> 
> eMail:                          george.shaw@btinternet.com
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