MI 3295 Step Contact Voltage Measuring System
Measurements
25
For a longer exposure the touch voltages must stay below 50 V.
7.1.2 General on specific earth resistance
The specific earth resistance (soil resistivity) is measured to determine the characteristic of
the soil. The measurement is carried out in order to assure more accurate calculation of
earthing systems e.g. for high-voltage distribution columns, large industrial plants, lightning
systems etc. The results are used to properly dimension earthing systems (size, depth,
number and position of earthing rods). Specific Earth Resistance value is expressed in
m
(or
ft).
7.1.3 Measurement
During the measurement a test current is injected into the earth through an auxiliary probe.
The resistance of the auxiliary probe should be as low as possible in order to inject a high
test current. The resistance can be decreased by using more probes in parallel or using an
auxiliary earthing system as the auxiliary probe. A higher injected current improves the
immunity against spurious earth currents.
Step voltage
The measurement is performed between two ground points at a distance of 1 m as shown
on
. The 25 kg measuring probes simulates the feet. Alternatively step voltage
plates can be used. The voltage between the probes is measured by a voltmeter with an
internal resistance of 1
kΩ that simulates the body resistance.
Figure 7.2: Step voltage measurement
Contact/Touch voltage
The measurement is performed between an earthed accessible metal part and ground as
shown on
. The voltage between the probes is measured by a voltmeter with an
internal resistance of 1
kΩ that simulates the body resistance.
