Earth Leakage Circuit BreakersByV.SaravananAssistant Professor EEETCE, Madurai

Definitions

Residual Current: is the vector sum of the currents of all the phases and associated neutral passing through the core balance transformer of an RCCB.Equipotential Zone: the zone within which all conductive parts are maintained at substantially the same potential by bonding to Earth.

Function of ELCBSwitch which disconnects the circuit by electro-mechanical means if any current flow through earthFor exampleAn insulation defect makes a fault current flow through the tool operator

Principle of ELCBThe fault current or leakage current is detected by a comparative measurement of the forward and reverse current in the power consumerIf the reverse current is not equivalent to the forward current, the earth-leakage circuit breaker is activatedThis happens so quickly that the operator is adequately protected

Function of ELCB

A Safe operation conditions, insulation in order: no leakage current is flowing through operator I1 = I2.B Insulation damage: some of the current is flowing through the operatorI2 is reduced by IF and is smaller than I1 & I1 I2. Earth-leakage circuit breaker disconnects.1 measurement of I12 measurement of I2

3 circuit breaker (disconnects if there is a variation between I1 and I2)4 power consuming device (e. g. a motor)I1 current to consumerI2 current returning from consumerIF leakage currentMP conductor0 zero conductor (earth)

RCCBIs an Residual Current Device (RCD) which will cause disconnection of electrical supplyShould a residual current passing through the device exceed a specified levelResidual Current Circuit Breaker with Overload protection (RCBO) is an RCD which will cause disconnection of electrical supply Due to residual current exceeding specified limits together with integral overloadOver current and short circuit protection associated with a miniature circuit breaker.

When is it advisable to install an RCD?For protection againstRisk of fire due to live to earth fault where fault current is insufficient to cause over-current protection device to operate. Risk of shock from indirect contact with equipment suffering a live to earth faultShock in potentially hazardous environmentAs supplementary protection against shock from directly touching 'Live' parts

Principles of Operation of RCCB

Operating FundamentalsWhen a Load is connected to the circuit supplied through an RCD current flows from the SUPPLY through the RCDWhereby both phase and neutral form the primaries of a CORE BALANCE TRANSFORMER arrangementThe SECONDARY of which is used as a SENSING COIL to detect any out of balance between the current flowing through the live and neutral conductors in the circuit

If a fault occurs on the load side of the RCD whereby a fault current (Ifn) flows between Live and Earth. The Load still demands a current return through the neutral of the RCD of I amps whilst the current flow through the Live becomes I+ Ifn From this imbalance a corresponding current will be induced electro-magnetically in the sensing coil which if of sufficient magnitude and duration will cause the actuator to function and trip the RCD thereby disconnecting the supply

However it should be noted that other disturbances that may cause imbalance between phase and neutral can emanate from upstream and/or downstream sources to give rise to the effect of unwanted tripping as identified, in 'Trouble-shooting'.

Test CircuitA test circuit is also incorporated whereby connection is made from load phase to supply neutral via a TEST COIL and RESISTOR and activated by a TEST BUTTON.This test circuit is designed to pass a current well in excess of the related tripping current of the RCD in question

What trip current rating should be selected?30mA - to give a high degree of protection against electric shock in a situation where supplementary protection against shock from accidental direct contact is required when it must be able to trip within 40 milliseconds when a fault current of 150mA is detectedThis will also satisfy the IEE/BS condition for supplementary protection of sockets feeding portable equipment outside the equipotential zone

100mA - to give a degree of protection against electric shock due to indirect contact situationGenerally this rating should be used to protect groups of circuits and provide overall protection against fire riskIf lower rated RCD devices are employed down stream then a time delayed 100mA RCD should be employed to ensure discrimination between same.

300mA - gives overall protection against risk of fire from electrical faults in wiring etc, only where sufficient current (typically less than 500mA) may cause incandescence of metal parts in suitable circumstances and in consideration that installed over current devices would require far in excess of 300mA to operate.If lower rated RCD devices are employed down stream then a time delayed 300mA RCD should be employed to ensure discrimination between same.NOTE: 10mA; 30mA and 100mA also inherently protect against this risk

Commercial RCCB

Thank You