Megger DET2/2, User Manual

Page: 24 / 116

24
Determining ‘Touch’ Potential
‘Touch’ potential is the potential difference a person
would experience across his body if he were, for
example, standing on the ground outside the earthed
perimeter fence of a substation and touching the fence
at the time a fault occurred.
Firmly connect the instrument as follows:-
1) Terminal 'C1' to the substation earth.
2) Terminal ' C2' to the Current spike inserted in the
ground some distance away.
3) Terminal 'P1' to the structure being tested e.g. the
perimeter fence.
4) Terminal 'P2' to the Potential spike inserted in the
ground 1 metre away from the perimeter fence
adjacent to the point where a person might stand.
5) Press the Test push, and take a reading in
the normal way. This is the effective resistance
between the point of test on the fence and the
Potential spike as seen by the test current.
The maximum value of the current that would flow in
the earth when a fault to earth occurred at the
substation must be known. The maximum fault current
has to be calculated from the parameters associated
with the substation ratings involved. From Ohms Law
(V = I x R), the Touch potential can be calculated.
Determining 'Touch' potential.
Measuring Techniques - Testing Earth Electrodes