HT instruments HT7051 - Manual (Page 17)

HT2055
EN - 16
7.2 STEP/CONTACT VOLTAGE MEASUREMENT
The measurements of step/contact voltage are performed by a voltmeter with an internal
resistance of 1k
 simulating the body resistance and with two weight probes of 25 kg
each simulating feet. Alternatively it is possible to use two metal plates instead of
weight probes on the ground where the operator stands.
CAUTION
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 earth system as
auxiliary probe. A higher injected current improves the immunity against
spurious earth currents.
As the test current is usually only a small fraction of the highest fault current the measured
voltages must be up scaled according to the following equation:
Gen
Fault
Measured C S
I
I
U U

,
U
S,C
………..calculated step or contact voltage in case of fault current
U
Measured
….measured voltage during the test
I
Fault
………..maximal earth current in case of a fault
I
Gen
………..injected test current
Installation of station unit
1 Place the station unit close to the earth system under test (distance max. 10m).
Connect the unit at the input terminal C1/H by using the supplied cable
2 Fit the auxiliary metal probe (1m length) on the ground at a distance of at least 5 times
the maximum diagonal of the earth system under test (see Fig. 17). Connect the input
terminal C2/E to the probe by using the 50m long cable (if enough) or use another
customized cable for longer distances. The nature of the measurement method permits
the use of different cables without any resistance compensation. Use also several
probes connected in parallel among them if it is necessary to reduce the contact
resistance on that point.