Megger Limited PWS / Nov 06
DET4TC/R Launch
19
Let’s say the reading came up as 4.5 ohms. Then because we know that the ICLAMP
requires at least 1/20
th
of the test current, we now know that resistance ‘A’ must be at
least 90 ohms.
Reading 4.5 X 20 = 90 ohms
Enough information to know whether the earth path A, be it an electrode, connection
to a mesh or whatever, needs attention.
The problem with Spheres of Influence
However there is another factor to take into account, and that is the sphere of
influence around the earth electrode/s and the buildings earth paths, whether it’s
through water/gas pipes or metal framework.
Consider the following diagram;
Here, the spheres of influence are outside each other, resulting in the equivalent
circuit shown underneath. ART should, subject to the 20:1 rule, work fine.
Note: If you are unclear as to the meaning of “Sphere of Influence”, more information
can be found in the Megger publication “Getting down to Earth”, part number 21500-
072.
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oint
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