MI 3295 Step Contact Voltage Measuring System
Measurements
24
7 Measurements
7.1 Theory of measurements
7.1.1 General on earthing
An earthing electrode / grid depleted into ground has a certain resistance, depending on its
size, surface (oxides on the metal surface) and the soil resistivity around the electrode.
The earthing resistance is not concentrated in one point but is distributed around the
electrode. Correct earthing of exposed conductive parts assures that the voltage on them
stays below dangerous level in case of a fault.
If a fault happens a fault current will flow through the earthing electrode. A typical voltage
distribution occurs around the electrode (the “voltage funnel”). The largest part of the
voltage drop is concentrated around the earth electrode.
and Contact/Touch voltages occur as a result of fault currents flowing through the earthing
electrode / grid in the ground.
Fault currents close to power distribution objects (substations, distribution towers, plants)
can be very high, up to 200 kA. This can result in dangerous Step and Contact/Touch
voltages. If there are underground metal connections (intended or unknown) the voltage
funnel can get atypical forms and high voltages can occur far from the point of failure.
Therefore the voltage distribution in case of a fault around these objects must be carefully
analyzed.
Figure 7.1: Dangerous voltages on a faulty earthing system
Standard IEC 61140 defines following maximum allowed time vs Contact/Touch voltage
relations:
Maximum time of exposure
Voltage
>5 s to ∞
U
C
50 V
AC
or
120 V
DC
0.4 s
U
C
115 V
AC
or
180 V
DC
< 0.2 s
U
C
200 V
AC
< 0.04 s
U
C
250 V
AC
Table 1:
Maximum time durations vs fault voltage
