EARTH TESTING TECHNIQUES |
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Practical test measuring setup |
In a practical earth test temporary earth stakes are used,
one stake usually noted as H or C2 is used as a path for test
current which is injected in to the earth
system under consideration.
The second stake which is usually noted as S or P2 measures
the potential developed by the injection of the test current.
A nominal distance of 20m between the stakes is recommended.
This will vary depending on the nature of system under tests. |
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3pole- and 4pole-measurements |
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The standard earth test, usually
called the Fall of Potential Method uses either 3 or 4 poles
or connections. The four pole method compensates for any voltage
drop there may be in the wire used to connect to the earth E
under test. The fourth wire is particularly important if the
connection to the E is a long distance from the instrument. |
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Selective measurement method |
Selective earth testing is based on the standard Fall of
Potential Method but has the advantage that the earth system
does not need to be disconnected.
The E point is connected to the nstrument; a current transformer
is used to measure the test current through selective parts
of the earthing system.
If there are parallel paths in the earthing system the individual
earth resistances of these parallel earths can be measured. |
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Stakeless measurement method |
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In this method the temporary stakes
are replaced by two current clamps. The test current from the
E and the H/C2 terminals of the instrument is fed through a
current clamp. This clamp is placed aroung the conductor connecting
the earth under test. A second measuring clamp is placed around
the same
conductor.
The resulting measurement is that of the earth under consideration
with a series element which is the combined parallel resistance
of all other connected
elements. As the combined resistance of the parallel paths is
small compared to the resistance of the earth being measured.
Clamps should be at least 10 cm apart. |
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