Earth Leakage Relay

Introduction

Majority of us have might have experienced electrical shock while using electrical equipment at some or the other instances in our lives. Though momentary, it is quite dangerous. This happens because electrical current passes to the Earth / earthing potential through human body; instead of completing its normal path in the electrical circuit. This is an example of earth leakage.

Earth leakage occurs due to reasons like normal wear and tear of equipment or moisture around terminals which can result in partial breakdown of insulation between supply and earth. Earth leakage currents are dangerous as it can lead to heat generation and insulation failure of cables. This can result in a major catastrophe thus leading to significant loss of property and human lives.

Difference between Earth Fault and Earth Leakage

According to IEC 60947-2, Earth fault current is the current flowing to earth due to insulation fault and Earth leakage current is the current flowing from the live parts of the installation to Earth in the absence of an insulation fault.

Conventional Short Circuit Protection Device (SCPD) are not designed to detect earth leakage currents. Earth Leakage Circuit breaker (ELCB or RCCB) has integral current breaking device. It detects as well as protects the system by opening the protected circuit when the residual current exceeds the set value. ELR is a relay that sends a signal to the shunt coil of a circuit breaker (MCB/MCCB or ACB) whenever the leakage current exceeds the set level.

Effect of earth leakage on human body

Earth Leakage current beyond 30mA can be lethal leading to death. 30mA sensitivity is required for protection in domestic installations where a person may come in direct contact with electric equipment in locations such as labs, schools, workshops etc. 100mA and 300mA protection is required wherever there is a possibility of indirect contact or persistent current flowing due to insulation failure in the cables.

Did You Know: L&T's ELR with Type class 'A' true RMS measurement (as per IEC 60947-2) provides the user benefits that go an extra mile. The user can set the threshold level ranging from 30mA to 30A. In case of earth leakage, the LED indicators will glow depending upon the percentage of set threshold value. For eg: If the set level is 30mA & the leakage current is around 23mA then 75% LED indicator will glow which provides a visual alert to the user. This empowers the user to take corrective actions before any accident.

Earth Leakage Relay with CBCT

The Earth Leakage relay with Core Balanced Current Transformer (CBCT) provides protection from earth leakage with advanced intimation of impending occurrence of the event. The user can proactively take action to avoid occurrence of any mishaps.

Earth Leakage relay is a microcontroller based device meant to measure low level of leakage current and isolate the faulty circuit from the system. Leakage current is sensed through core balanced current transformer.

Definite Time Trip occurs when Earth Leakage Current exceeds the trip time which is adjustable by means of a potentiometer. Core Balanced Current Transformer (CBCT) uses the technology of residual magnetic flux. All conductors to be protected shall pass through the core balance current transformer. The vector sum of all the currents should be equal to zero.

Īr + Īy + Īb= 0 for 3 phase 3 wire system.

Īr + Īy + Īb + Īn =0 for 3 phase 4 wire system

The CT wires should be placed adequately away from high current carrying conductors or source of strong magnetic field to avoid noise pickup.

The CT wires should be placed adequately away from high current carrying conductors or source of strong magnetic field to avoid noise pickup.

In case leakage current is present, vector sum will not be zero but some current will get detected.

L&T Electrical & Automation has comprehensive range of Earth Leakage relays with following Advantages: Wide auxiliary supply range: 110 - 240 V AC, 220 - 415 V AC; Selectable range 30mA – 30A in a single module; Configurable Earth leakage Trip time: 0 (instantaneous) to 10s (delayed); Manual /Remote reset feature; Base / DIN Rail mounting; Usage as a component with ACB, VCB or MCCB's with shunt or UV release; Test feature; 2 module width for usage in DBs; Indication of 30%, 45%, 60%, 75% of set Earth Leakage level with LED indication; Tamper proof cover on the front

Applications

Let us see major application areas for ELR.

Motor Control Panels and Switchboards

The relay combined with the CBCT capable of monitoring the supply conductors can be separately mounted within the confines of the switchboard. ELR's features such as test / reset, alarm output and continuous digital indication for the residual current value are more suitable to industrial applications. ELR is used along with shunt release of MCCB in motor applications. Leakage of current from the motor body to earth causes the CBCT to sense the earth leakage current and hence provides protection to human personnel working in the near vicinity.

Earth Mines

In Earth mines any leakage current above the allowed level is lethal. Workers operating in mines with various instruments face a severe danger of fires. The ELR must be used in conjunction with circuit breaker of appropriate rating.

Typical usage areas for ELR are:

Steel Plants, Generators and Transformers, Cement plants, Oil Refineries, Buildings, Mobile Operating equipment, Control Panels, Switchboards

Summary

An earth leakage relay is a device used to sense earth leakages and open a circuit breaker when the leakage current exceeds the preset value. It can protect human beings from electrocution and prevent fire hazards. Earth leakage relays are used in combination with Core balance current transformers (CBCT) or Zero phase current transformer (ZCT).

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