AEMC 6422 User Manual page 20

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 128Hz square wave between the H and E terminals, with
an amplitude of 10V peak. If the measurement is unstable, the test
frequency will automatically switch from 128Hz to 256Hz to improve the
measurement.
2. Measures the resulting current I
and E terminals U
3. Calculates the value of R
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 = 20V and I = 1A, 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.
20
HE
.
SE
= U / I
E SE
E 20
R= = = 20 ohms
I 1
along with the voltage between the S
.
HE
Ground Tester Models 6422/6424