Electrical Installation - RCDs, Fuses & MCBs

The electricity supply into your home comes through the ESB Networks main fuse and meter. Both of these items are sealed by ESB Networks and must never be interfered with. Newer installations have an isolating switch between the ESB Networks meter and the customer's distribution board; older installations have direct connections between the meter and the distribution board. The customer is responsible for the electrical installation on his or her side of the ESB Networks meter.

Repair work on the fixed electrical installation or to electrical appliances should only be carried out by a competent and qualified person and should not be attempted by the layperson.

RCDs, Fuses and MCBs (minature circuit breakers)

MCB Fuse

The customer's distribution board (or fuse board) contains the MCBs or fuses which protect individual circuits that carry electricity to your sockets, cooker, lights etc. These circuits have different size wires (large for a cooker, small for the lights) and, for this reason, have different strengths of MCB's or fuses protecting them. The strength is measured in Amps and will be clearly marked on the MCB or fuse.

An RCD is a Residual Circuit Device (also sometimes known as a Trip Switch or Earth Leakage Trip Switch). An RCD works by detecting a leakage of current from the circuit. Once a fault is detected, a circuit is disconnected. An RCD is found in the fuseboard and it should be tested frequently.

An MCBs or fuse is a safety device which cuts off the flow of electricity to a circuit if a fault in the circuit or in an appliance causes an overload. An MCB can be reset when the fault is identified and repaired; a blown fuse must be replaced with one of similar strength.

RCD's, Fuses & MCB's Video
This short video explains the purpose of a Residual Circuit Device (RCD) also sometimes known as a Trip Switch or an Earth Leakage Trip Switch. The video shows how an RCD works, what it looks like and where it is found.


Most modern installations will have MCB's fitted instead of fuses. They are more convenient to use. When the fault/overload which caused the MCB to switch off is located and remedied the MCB is reset to the upward ON position.


It is most important when a fuse fails that it is replaced by one of the same strength and NEVER by a stronger one.

A fuse has a small coloured disc in the base and the colour identifies the strength. The colours are:

Disc Colour Strength Circuit Type
Green 6 Amp Lights
Red 10 Amp Lights
Brown 16 Amp Socket outlets - radial
Blue 20 Amp Water heater, storage heater
Black 35 Amp Socket outlets - ring, Cooker, Electric Shower