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
> eMail: george.shaw@ukf.net
> Home: +44 (0) 28 9062 8924
> Mobile: +44 (0) 77 4036 1163
> ICQ#: 1741675
>
>
>
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