Earth Testing Safety Precautions - Megger DET2/2 Guia Del Usuario

Earth Testing Safety Precautions

Electrode Isolation or Duplication
It is preferable that the earth electrode to be tested is
first isolated from the circuit it is protecting, so that only
the earth is measured and not the complete system.
When this is done, the circuits and equipment must be
de-energised. If however this is not possible, the earth
electrode should be duplicated, so that when it is
disconnected for test purposes, the other one provides
the necessary circuit protection.
'Live' earth safety precautions
The DET2/2 allows earth testing to be done at a
relatively safe voltage using a maximum of a 50 V RMS
square wave at a frequency of nominally 128 Hz. In use
it is normally connected only to electrodes which are at
earth potential.
A 'Live' earth is one that carries current from the mains
supply, or could do so under fault conditions.
When working around power stations or sub stations
there is a danger that large potential gradients will
occur across the ground in the event of a phase to earth
fault. A wire which is connected to ground many metres
away will then no longer be at the same potential as
local ground, and in some cases could rise to above
1 kV. The following safety precautions are essential.
1. All persons involved must be trained and
competent in isolation and safety procedures for
the system to be worked on. They must be
clearly instructed not to touch the earth
electrode; test spikes; test leads, or their
terminations if any 'Live' earths may be
encountered. It is recommended that persons
involved wear appropriate rubber gloves, rubber
soled shoes, and stand on rubber mats.
2. The 'P2' and 'C2' terminals should be connected
through a double pole isolation switch, the rating
of which will cope with the maximum fault
voltage and current. The isolation switch must
be open whilst any personal contact is made
with the remote test spikes, or the connecting
leads, e.g. when changing their position.
Fault
current
Voltage rise of
earthing system
under fault
conditions
Earth
resistance
A method of disconnection where fault conditions
may occur.
D.P. Isolation switch
Remote
spikes
Fuses
True earth
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