ELM USER MANUAL ISSUE 4
-2-
2.1 Earth Leakage Systems
Desirable though it may be, it is impractical to
provide
automatic
protection
against
electrocution as a result of direct contact with a
live conductor, particularly where the electrical
reticulation is exposed to a humid or damp
atmosphere.
Table 1 indicates the current values affecting
human beings.
Current mA
Symptom
1 or less
Causes no sensation - not felt
1 to 8
Sensation of shock, not painful,
individual can let go at will, as
muscular control is not lost
8 to 15
Painful shock, individual can
let go at will, as muscular
control is not lost
15 to 20
Painful shock, muscular
control of adjacent muscles
lost, cannot let go
20 to 50
Painful, severe muscular
contractions, breathing difficult
50 to 100
(Possible)
100 to 200
(Certain)
Ventricular fibrillation (a heart
condition that may result in
death)
200 and over
Severe burns, severe muscular
contractions that are so severe
that chest muscles clamp the
heart and stop it for the
duration of the shock. (This
prevents ventricular
fibrillation)
Table 1
It will be seen from the foregoing table that the
passage of a current of as low as 15mA
through the human body can cause loss of
muscular control to the extent of preventing
the recipient from disengaging from the live
conductor. Whereas a current in excess of
50mA is sufficient to produce a critical heart
condition from which there is little or no
chance of recovery.
It follows that as an effective safeguard against
electrocution resulting from direct contact with
a live conductor, it would be necessary to
introduce earth leakage protection designed to
operate with a fault current below 15mA,
which in the majority of cases, would be
impractical.
The automatic protection of circuits is not
intended to take the place of sound installation
practice and the regular maintenance and
testing of electrical apparatus.
Care must be taken in the selection and
installation of all electrical equipment with due
regard to its required duty and the conditions
under which it may be called upon to operate.
Where automatic earth leakage protection has
been installed it is essential that its operation
be tested often and to facilitate this a means for
testing is incorporated in all approved earth
leakage relays.
2.2 Methods of Earth Leakage Protection
Earth Leakage Protection Relays for use in
mining applications have to be designed with
reference to AS2081 for use on fault-limited
systems.
There are two methods of protection used.
They are the Core Balance and the Series
Neutral earth leakage protection systems. The
Core Balance relay performs the primary
protection in an installation protecting the
outlet supplying power to a machine. In this
application the time delay is set at
instantaneous. The neutral earth leakage relay
is the back up relay of the installation.
2.3 Core Balance Protection
With this method the three phases are passed
symmetrical through the toroid. If there is no
earth fault present, the vector sum of the
currents in a three-phase supply is zero. If
current from any phase flows to earth the
system becomes unbalanced. The toroid
produces an output, which trips the relay.
A test current is injected through the window
of the toroid to test the operation of the relay.
See typical circuit, Page 7.
2.4 Series Neutral Protection
With this method the neutral is passed through
the toroid. An earth fault on any of the phase
conductors causes an earth current which
returns, through the toroid, to the star point of
the transformer.
A test circuit can connect a test resistor
between a phase and earth or inject a current
through the toroid as previously described. The
test resistor to earth method is recommended
