The residual current device (rcd) is used to detect earth fault currents and to interrupt supply if an earth current flows. The main application is to prevent electrocution but RCDs can also be used to protect equipment, especially against fire. The earth fault currents that operate an rcd can range from 5mA up to many amperes. For typical domestic applications the typical trip current would be 30 mA. The RCD can be opened and closed manually to switch normal load currents, and it opens automatically when an earth fault current flows which is about 50 per cent or more of the rated tripping current.
The main features of an rcd are shown in Fig. 1. The key component is a toroidal transformer, upon which the load current (live) and return current (neutral) conductors are wound in opposite directions. The toroid also carries a detecting winding. If no earth fault current is flowing, then the load and return currents are equal. In this case the mmfs generated by the load and return current windings are equal; there is no resultant flux in the toroid and the detecting winding does not generate any current.
When a fault current flows there is a difference between the load and return currents which generates a resultant flux in the toroid and induces a current in the detecting winding.
The current generated in the detecting winding operates a relay which opens the main contacts of the RCD. The detecting winding has to produce from a very small output, sufficient power to operate the tripping mechanism. Two alternative methods are used. In the first method, the output signal from the detecting coil is electronically amplified and the second method uses a polarized relay operating on a sensitive mechanical trip mechanism. The operation of a polarized trip relay is based on the magnetic output of a small coil nullifying the field from a permanent magnet, causing the release of an armature. The basic operation is illustrated in Fig. 2.
The operation of an rcd has here been described for single-phase operation, but it may also be applied in a three-phase application where typically it might be used in a light industrial system for protection against fire. There are two arrangements of a three-phase rcd. Either the three phases are wound around a current transformer, or the three phases and the neutral are wound onto a balancing transformer.
The RCD has only limited breaking capacity and it is not a replacement for overcurrent protection devices such as the MCB. The residual current breaker with overcurrent (RCBO) is now available; this is an rcd with an overcurrent tripping mechanism and enhanced contacts to cope with interruption of fault conditions.
RCDs are designed and tested according to the requirements of IEC 1008 and IEC 1009.