AEMC instruments 6422, Руководство пользователя

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22 Ground Resistance Testers Models 6422 & 6424
2.3 GROUNDING RESISTANCE (3P)
This function performs a 3-pole (3P) test to measure grounding resistance
when the electrical installation to be tested is de-energized (for example a new
installation). It uses two auxiliary rods, with the third rod being the grounding
electrode to be tested (hence the name 3P or 3-pole). To perform a 3P test, the
instrument:
1. Generates a 128 Hz square wave between the H and E terminals, with an
amplitude of 10 V peak. If the measurement is unstable, the test frequency
will automatically switch from (128 to 256) Hz to improve the measurement.
2. Measures the resulting current I
HE
along with the voltage between the S and
E terminals U
SE
.
3. Calculates the value of R
E
= U
SE
/ I
HE
.
The 3P test can also be performed on an existing electrical installation, but the
power must be OFF. Whether testing a new or existing installation, the grounding
electrode system under test must be isolated from other grounding connections
during the measurement.
2.3.1 3-Point Test Theory of Operation
3-Point measurement is used to measure resistance to ground of auxiliary
ground electrodes and grids. The potential difference between rods E and
S is measured by a voltmeter, and the current flow between rods E and H is
measured by an ammeter.
By Ohm’s Law E = RI or R = E/I, we may obtain the ground electrode resistance
R.
If E = 20 V and I = 1 A, then:
It is not necessary to calculate all the measurements when using a ground
tester. The ground tester will measure directly by generating its own current and
displaying the resistance of the ground electrode.