RCD = RESIDUAL CURRENT DEVICES
RCD’s work on the principle of a load being in balance – that is, the current on the Phase conductor is equal to that flowing out of the Neutral conductor. The Phase and Neutral cables in an RCD pass through a magnetic ring which detects any imbalance. If equal, the RCD remains operational, if imbalanced it causes the mechanism to trip.
Modular RCDs come in 2-pole and 4-pole versions. 2-pole versions are usually mounted in consumer units to protect a number of circuits at once. However, both 2-pole and 4-pole versions can be used separately as protection devices in enclosures.
There are two usual ways in which an imbalance of the load can occur:
Indirect Contact: defined in the IEE Wiring Regulations as ‘contact of persons and livestock with exposed conductive part made live by a fault which may result in an electric shock e.g. the casing of an appliance or motor.
Direct Contact: defined in the IEE Wiring Regulations as ‘contact of persons and livestock with live parts which may result in an electric shock.’ e.g. exposed damaged cable. In both cases RCDs should not be the only method of protection used - see BS7671 wiring regulations for further information. To provide all-round protection RCDs must be used in conjunction with suitable MCBs.
RCDs are available in the following sensitivities:
30mA the most popular sensitivity in the UK. In a shock, current flowing through body at 240V could be 80 to 240mA, depending on the resistance of the body in question. To ensure no harmful effects the RCD operates within 300mS at 30mA and 40mS at 150mA.
100Ma may provide protection against electrocution. However, there is a likelihood that the earth fault current may be below the sensitivity of the RCD - increasingly likely if additional resistances to that of the human body are in the current path.300mA - provides protection against risk of fire only, not against electrocution in shock situations. A typical application is lighting circuits where risk of electric shock is small. Note, a current of <500mA flowing in a high resistance path is enough to cause metallic parts to potentially start a fire.