86 Chapter 6 • Protection 6.2 Core balance earth leakage relay
Core balance earth leakage relays are basically current transformers which operate tripping devices when they detect an earth fault current which exceeds a predetermined value.
▲ 6.1.6 Te neutral conductor shall not be connected direct to earth or to the earth continuity conductor on the consumer’s side of the installation. If it does get connected to the consumer’s side of the installation the earth leakage protection device will not operate which will obviously result in an extremely dangerous installation.
Earth leakage relays with wound primaries are being phased out. Most core balance earth leakage devices have conductors which simply pass through the core, fig. 6.1. Tese conductors can be considered as primary windings with half a turn.
Operation
When the load circuit is faultless, the current through the live and neutral conductors (primaries) in the core, are equal in magnitude but opposite in direction. Te magnetic flux set up by the two conductors (primaries) neutralise each other, i.e. the phasor sum of the two currents is zero, and no emf is induced in the secondary winding.
If, however, an earth fault current develops, a path parallel to the neutral point of the transformer is formed through the earth. Tis will result in an imbalance (the one conductor carries more current than the other) and the phasor sum will no longer be zero.
As a result, the magnetic fluxes set up by the two primaries, no longer neutralise each other, and the resultant flux induces an emf in the secondary winding (transformer action).
When the earth fault current exceeds the predetermined value of 30 mA, the emf induced in the secondary winding, will trigger the tripping circuit in the printed circuit board (PCB). Te tripping coil (shunt trip) is energised and the circuit is disconnected from the supply.
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