ELD V2 EARTH LEAKAGE RELAY
Issue: R8, Date: 10/08/2011, CRN: 8530
ELDB006 Rev8 ELD V2 (2002) Earth Leakage Protection Relay User Manual
Page 8
It can be seen from the previous table (Table 4.2) 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.
4.3
Methods of Earth Leakage Protection
Earth Leakage Protection Relays for use in mining applications have to be designed and tested to AS/NZS 2081.3-
2002 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 backup relay of the installation and can have a time delay up to a maximum of 500mS.
4.3.1
Core Balance Protection
With this method the three phases are passed symmetrically 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 toroid flux
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 16 - Drawings.
4.3.2
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 with this type of protection as this test also
proves the neutral to earth connection.
See typical circuit, Page 16 - Drawings.
