WHAT IS RESIDUAL CURRENT DEVICE
RCD’s are electrical devices which are used for the protection against electric shocks and the fire caused by earth faults, this faults will be cleared by cutting off the flow of electricity or it will actuate an alarm when it senses a leakage of current from a circuit
Residual current devices are made up of at least two components they are sensors and relays, sensors are used to supply electrical signal which is useful when the sum of current flowing in the live conductors is different from zero. and then the relay it will compare the electrical signal supplied by the sensors with the set-point value and sends with a deliberate delay, the opening order to the associated breaking device
CURRENT SENSORS IN RCD OPERATION
- Toroidal transformer- it is most commonly used to measure leakage current
- Current transformers – it is used in HV and MV and in Lv
TOROIDAL TRANSFORMER
By using the toroidal transformer we can check all the live conductors and is excited by the residual magnetic field which will be corresponding to the vector sum of the currents flowing from phase and neutral.
These type of sensors are used to detect residual currents from a few milliamperes to several dozen amperes
CURRENT TRANSFORMER
These transformers are used to measure the residual current of a three-phase electrical circuit
In this the load is connected to supply through the RCD the line and the neutral conductors are connected to toroidal transformer through the primary windings the secondary winding is used to connect to the relay and is also used as a sensing device if the line and the neutral is balanced it produces equal and opposite magnetic field If the line and the neutral is not balanced it will produce an out of balance flux and this will induce a current in the secondary winding and that is used to and it will create the tripping mechanism
