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Residual-current device - Wikipedia, the free encyclopedia

Firstly if occurring an insulation fault ,then the current resulting from a fault is named "fault current or short-circuit current " .. on the other hand , leakage current is the current which ,in absence of a fault , returns to the source via the earth or protection conductor … while the residual current represents the RMS value of the vector sum of the currents flowing through all live conductors in a circuit at a point of the electrical installation , this current is mostly associated with a breaking device …

You know! There is no perfect electrical insulation. Any insulation will have - what is called - an insulation resistance, may be in Mega-Ohms. When a voltage is present across this insulation, as it happens in any elctrical installation, some amount of current will definitely flow through this insulation. (I = V/R). This current is called the LEAKAGE CURRENT. Normal magnitude is milliamperes.

In a perfectly balanced three phase electrical system, there shall not be any "residue" because at any given instant, the residual current is the vector sum of all the phase currents at that instant. As each phase current at any instant is cancelled by the vector sum of the other two phase currents at that instant, there will not be any "balance or residue" current. Once, this balance is affected, then there will be some residue and this residue is called "Residual Current". Magnitude is milliamperes or amperes.

Fault current is the current that would flow in case of a fault like short circuit or earth fault, in the electric system. Normally its magnitude will be very high, in kilo amperes.