What is the RCD (Residual Current Device / RCCB(Residual Current Circuit Breaker)

What is the

RCD (Residual Current Device / RCCB(Residual Current Circuit Breaker)

CHARACTERISTICS

·         Phase (line) and Neutral both wires connected through RCD.

·         It trips the circuit when there is earth fault current.

·         The amount of current flows through the phase (line) should return through neutral .

·         It detects by RCD. any mismatch between two currents flowing through phase and neutral detect by -RCD and trip the circuit within 30Miliseconed.

·         If a house has an earth system connected to an earth rod and not the main incoming cable, then it must have all circuits protected by an RCD (because u mite not be able to get enough fault current to trip a MCB)

·         RCDs are an extremely effective form of shock protection

The most widely used are 30 mA (milliamp) and 100 mA devices. A current flow of 30 mA (or 0.03 amps) is sufficiently small that it makes it very difficult to receive a dangerous shock. Even 100 mA is a relatively small figure when compared to the current that may flow in an earth fault without such protection (hundred of amps)

A 300/500 mA RCCB may be used where only fire protection is required. eg., on lighting circuits, where the risk of electric shock is small.

Limitation of RCCB

·         Standard electromechanical RCCBs are designed to operate on normal supplywaveforms and cannot be guaranteed to operate where none standard waveforms are generated by loads. The most common is the half wave rectified waveform sometimes called pulsating dc generated by speed control devices, semi conductors, computers and even dimmers.

·         Specially modified RCCBs are available which will operate on normal ac and pulsating dc.

·         RCDs don’t offer protection against current overloads: RCDs detect an imbalance in the live and neutral currents. A current overload, however large, cannot be detected. It is a frequent cause of problems with novices to replace an MCB in a fuse box with an RCD. This may be done in an attempt to increase shock protection. If a live-neutral fault occurs (a short circuit, or an overload), the RCD won’t trip, and may be damaged. In practice, the main MCB for the premises will probably trip, or the service fuse, so the situation is unlikely to lead to catastrophe; but it may be inconvenient.

·         It is now possible to get an MCB and and RCD in a single unit, called an RCBO (see below). Replacing an MCB with an RCBO of the same rating is generally safe.

·         Nuisance tripping of RCCB: Sudden changes in electrical load can cause a small, brief current flow to earth, especially in old appliances. RCDs are very sensitive and operate very quickly; they may well trip when the motor of an old freezer switches off. Some equipment is notoriously `leaky’, that is, generate a small, constant current flow to earth. Some types of computer equipment, and large television sets, are widely reported to cause problems.

·         RCD will not protect against a socket outlet being wired with its live and neutral terminals the wrong way round.

·         RCD will not protect against the overheating that results when conductors are not properly screwed into their terminals.

RCD will not protect against live-neutral shocks, because the current in the live and neutral is balanced. So if you touch live and neutral conductors at the same time (e.g., both terminals of a light fitting), you may still get a nasty shock.