RCDs and MCBs

A Residual Current Device (RCD) is a device like a switch, which monitors the current flowing between live and neutral. If it detects a difference of between 20 and 30 milliamps (20/30 thousandths of an amp) it determines that this current is going to earth somewhere (possibly through you!) and it will disconnect the circuit or circuits being monitored. It cannot be stressed enough that this device can save lives either directly through electrocution, or indirectly from fire.

A Miniature Circuit Breaker (MCB) is the modern equivalent of a rewirable fuse, only much more efficient, switching off an overloaded circuit usually within 100 milliseconds. If a fault occurs causing this to happen, once the fault is rectified the MCB is simply switched back on. No more fiddly fuse wire!

From 1st July 2008, BS7671 (17th edition) came into force and now all circuits that have cables in walls that are less than 50mm deep and are not mechanically protected, must be RCD protected. As it is contrary to BS7671 to have an RCD protecting the whole installation, this means that all new consumer units will need at least two RCDs. All circuits in bath/ shower rooms must also be RCD protected, regardless of depth of cables in walls.

Installations that have any circuits that are not RCD protected (i.e. smoke detectors/ security alarms) and are supplied via overhead cables (and therefore need an earth rod) must also have a 100mA time delayed RCD as the main switch on the consumer unit.

For more information contact Brian Coleman on:
Mobile: 07956 510307
Tel/fax: 01322 613185
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