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Introduction to Earth Leakage Relays.

Earth leakage relays perform two main functions on mining installations: to protect machinery from damage and to make the in­stallation safer for personnel. In the typical distribution circuit shown in Fig 1, an earth leakage relay provides protection against leak­age to earth and earth faults. The principle of this method is that all the current flowing to the load must return from the load. If current leaks to the frame of the motor and returns via the earth bond back to the star point of the supply transformer then this difference is detected by the earth leakage relay which triggers the breaker, thus disconnecting the load.

It becomes apparent that the earthing must be solid and reliable for electrical installa­tions to be safe. For this reason, machinery is earthed via the supply cable, using either a copper conductor of the cable or the armour. The ground on which the machinery stands is normally not considered a reliable earth. Me­chanical damage to the earth bond, corrosion of the armour or bad workmanship may result in a high resistance point in the earth return. If an earth fault occurs, voltage will be developed across the high resistance region. This voltage poses a potential hazard to any personnel who may be exposed to it. Depending on the resis­tance value, the current flowing and the reaction time of the earth leakage relay, this could be fatal. It is obvious that continuous monitoring of the earth bond is imperative if safety standards are to be maintained.

SWITCHING SYSTEMS' range of EARTH CONTINUITY RELAYS provides a cost effective solution to the problem.

Product description

SWITCHING SYSTEMS' Earth Continuity Monitors are grouped into two categories:­

PILOT WIRE CONTROL RELAYS, pre­fixed GBP, MP & MSR. These relays offer simple and cost effective solutions. They have remote stop/start capability. The maximum resistance level is permanently set at 22Ω for GBP/MP relays, and at 100Ω for MSR Relays. They are used prima­rily underground with short cables and medium power loads.

Three generations of relays are available:

•  The first, MSR8-NI, is not intrinsically safe.

•  The second, MSR9-NI & GBP4-NI, is intrinsically safe, and is suitable for environments where limited electrical noise is present.

•  The third, MP130-NI, provides for noise immunity of up 12 V in full operation mode & from 12 to 50 V in restricted mode. These relays can be used as direct replacements for older models and they are the preferred products for all new applications.


This relay is micro-processor controlled and can withstand high induced AC voltages and injected DC voltages. The loop resistance is constantly indicated on a 7 segment LCD display. The trip level can be set using the rotary switches on the front panel. Fault condi­tions such as open circuit, short circuit and high-resistance are displayed. A separate in­terface module provides a 4 to 20mA output for remote monitoring. The use of this module requires factory modification of the earth conti­nuity relay. These relays are mainly used for high voltage reticulation trailing cables on dra­glines and drills.

ECR Range>Earth Leakage Relays