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Diagrams and notes are the copyright of Mike Joseph. Please contact me should you wish to use them.

MCB - Miniature Circuit Breaker

The miniature circuit breaker (mcb) and the re-wireable fuse protect the cable from being overloaded and thereby causing a fire - they protect property only. Typical values are 5 or 6 amps (lighting), 15 or 16 for an immersion heater, 30 or 32 amps for ring mains and upto about 40 amps for cooker and shower circuits.

The advantage of the mcb is that it is more reliable and can be easily reset - it is no fun fixing fuse wire by torchlight!

RCD - Residual Current Device

The residual current device (RCD) is designed to trip VERY quickly at a very low amperage. The reason for this is that the human nervous system works at about 50 milliamps - ie 1/20th of an amp will KILL. No messing - DEAD. The reason for this is that, effectively, it induces a heart attack.

There is a further device called a residual current breaker with overload (RCBO) which combines the functions of an RCD with an MCB, ie it provides shock and overload protection. Again, it has a test button which should be pressed at 3 monthly intervals.

Breakers

The RCD will trip at 30 milliamps (about 1/30 of an amp) and in less than 300 milliseconds (1/3 of a second). In this way it prevents an electric shock.

Basic rules:

  1. If one or more RCDs are fitted to your fuse board (aka Consumer Unit) then you MUST press the test button every three months to test that it still works and 'exercise' the internal mechanical parts. If it fails to trip - call an electrician IMMEDIATELY!

  2. If using ANY power tools out of doors and if your fuse board is not fitted with an RCD (or it is rated at more than 30mA) then ALWAYS use a plug-in RCD (available from Homebase, B&Q, etc). This is because there is a huge chance that if you cut the cable then you could get a fatal belt as the body/grass/damp soil forms an excellent return to earth for the power - you become the cable!

Diagrams and notes are the copyright of Mike Joseph. Please contact me should you wish to use them.

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