5．4-wires Precise Earth Resistance Measurement
In the testing of earth resistance, should firstly confirm
the earth voltage of earthing wire, the voltage between
C
(H)
and E or P(s) and ES must under 20V, the meter
showing NOISE symbols when the earth voltage exceed
5V, and the measured value of earth resistance may
produce error. At this time, firstly interrupt power supply
to earthing equipment, make sure the earth voltage
decrease, and then test earth resistance again.
4-wires measurement: The 4-wires method can eliminate influence of
contact resistance (usually result from dirty or rusty) between measured
earthing body, auxiliary earthing rods, test clips, meter’s input interface.
The 4-wires method can also eliminate influence of line resistance. So it
is better than 3-wires measurement.
As shown below: Start from the measured body, interval 5m-20m,
respectively insert P
(S)
, C
(H)
auxiliary earthing rods deep into the earth in
a straight line, and then connect testing wires (black, green, yellow, red)
from E, ES, P(S), C(H) interfaces corresponding to measured earth
electrode E, auxiliary voltage electrode P(S), auxiliary current electrode
C
(H).
The distance from measured earthing body to current
electrode C(H), should at least five times longer than the
embedded depth of measured earthing body, or the
embedded electrode depth (d).
When measuring the total earth resistance of complex
earthing system, d is the length of the biggest diagonal
line of this earthing system.
Testing wires can’t be winded together, or may affect the
measuring accuracy.
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