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Current operated RCDs work on the principle illustrated in the diagram above.
The line and neutral conductors of the electricity supply to the installation or apparatus to be protected, pass through a toroidal transformer core.
When no earth leakage currents are present the currents IL and IN are equal and opposite and no magnetic flux is induced in the transformer. If an earth leakage current IE occurs, then IL and IN are no longer equal.
The difference in current induces a magnetic flux which is sensed by a coil wound on the toroid which operates the tripping relay of the RCD.
Principle of EMU
This is used to ensure the suitability and integrity of the Earth connection between an electrical supply point and an appliance e.g. electric motor, welding transformer.
There must be a substantial connection between the appliance metal work and the earth connection at the supply point. The EMU continuously monitors the impedance of this earth path and disconnects the supply should it rise above the predetermined level.
Note: The cable from the appliance must have an additional pilot core for the return path of the monitoring current. |
