The combination of adder and multiplexer special toroidal current transformers and normal current transformers is indicated in the following principle diagram. All X/5 current transformers used for this application must have the same winding ratios and powers (at least 10VA), and their accuracy class must be at least 0.5. Connections should be exactly as follows.
When there is no earth leakage, the vectorial sum of the measurement taken from the current transformers will also be equal to zero. Result of this, no current will flow from the measurement coil with terminal 5 and 6 input (our special toroidal leakage current transformer). Thus, no voltage will be induced from outputs 1 and 2 connected to the RCR series residual current relay which will not put the relay in the error state. However, when an earth leakage occurs, the vectorial sum will be different from zero, and a voltage will be induced from the 1 and 2 terminal outputs, which will switch the RCR and ELR series residual current relays to the fault position.