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 7
4
The Need for Earth Leakage Protection
4.1
Overview of Protection Problems
Unless properly controlled the occurrence of an earth fault can be hazardous because it may cause:
Frame to earth voltages dangerous to personnel.
Electric arcing, which may initiate an explosion or fire when arcing occurs in an underground mining
operation.
The main purpose of earth fault protection is to safeguard personnel and electrical apparatus. However it is found
that relays designed to operate on fault limited systems are not suitable for direct personal protection, i.e. users of
portable drills, grinders etc, which require trip levels of 20-30mA, with instantaneous operation. (Refer AS/NZS 3190).
The most common apparatus faults in mining applications are cable faults. Cables are most susceptible to damage
and are the major source of dangerous electrical incidents. This applies particularly to the flexible trailing cables
supplying power to mobile mining machines.
Cable construction is such as to provide every phase conductor with an individual conductor screen so that crushing
would cause a low single phase to earth fault current. The protective device, such as an Ampcontrol ELD earth
leakage relay, would then isolate the cable and contain the sparking within the cable before a heavy short circuit
current due to a phase to phase fault occurs.
Earth fault protection has been applied with considerable success in limiting faults and providing quick disconnection
of electrical apparatus from the supply in the event of earth fault situations.
A definite time operating characteristic is provided with adjustable trip sensitivity and time delay.
Time delay between protective units is introduced to allow the unit close to the fault to isolate the faulty circuit without
causing the healthy part of the system to be de-energised.
4.2
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 4.2 (taken from IEC Standards) 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; so severe that chest muscles clamp the
heart and stop it for the duration of the shock. (This prevents ventricular fibrillation)
