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composite resistance of the ground branch circuit, it includes the contact
resistance, lead resistance and ground body resistance of the branch circuit to
the public ground wire. Value measured by the traditional voltage current method
in tripping is only the ground body resistance.
Clearly, the former measurement value is greater than the last one, and the
difference reflects the amount of contact resistance between branch circuit and
public ground wire.
It is required to specify that ground resistance stipulated in the national standard
conclude the ground lead resistance. Noun terms in the DL/T621-1997 AC
Electric Equipment Ground states that: the sum of ground resistance of polarity
or natural polarity and ground wire resistance is called the ground resistance of
ground equipment.
The stipulation is equally stated clearly as in term of lightning prevention, the wire
resistance and ground body ground resistance is equivalent.
2 Selection of the Measurement Point
In some ground systems, as shown in the following picture, you should select a
correct measurement point of measure or get dif ferent measurement results.
In measuring at point A, the branch circuit measured have not formed into a loop
and the clamp meter displays OL and replace the measurement point.
In measuring at point B, the branch circuit being measured is a loop formed by
metal conduct and clamp meter display L 0.01 or the resistance value of
metal loop is just few hundredths , quite small, and should replace the
measurement point.
In measuring at point C, the branch circuit being measured is the ground
resistance value of it.
UT276A/278A OPERATING MANUALUT276A/278A OPERATING MANUAL
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