What is a Fuse, MCB, RCD, RCBO, AFDD, SPD and what do they do?
Fuse
-this is your basic circuit protection, can be contained in many items such as a plug top or even an old style “fuse board”. It has to be rated at the correct size for the circuit or item it protects and as such the circuit has to be rated to take that size fuse.
In the event of too much load being drawn through the fuse either by over loading or a short circuit, the fuse will blow.
MCB
-Miniature Circuit breaker, this acts like a fuse, protecting the circuit from an overload or short circuit. Unlike a fuse it can be reset once the problem has been rectified.
Fuses and MCBs do not detect earth leakage, they will not detect the current going through a human body down to earth, the body will be seen as just any other electrical appliance.
RCD
- Residual Current Device, this safety device has been around for many years and although only been a requirement on all circuits in recent years, it detects earth leakage, not having one is like driving a car without seatbelts, why would you risk it ?
This device makes sure that the current leaving it and going round the circuit is the same as the current returning, if they are not balanced it will trip, disconnecting the circuit from the electric supply. This is done in a matter of milli seconds.
In other words, if an electric current is diverted somewhere it shouldn’t by a fault, or a human body, it will trip in time to stop a lethal shock developing. It can literaly be a life saver.
There are many types of RCD now days and the correct one for the job it is required to do must be chosen. In todays modern world, new types of RCD which will detect DC leakage current should be used where DC loads are present (LED lighting is a prime example of a DC load, also motors such as a washing machine)
A common misconception is that a RCD will trip because I am using too much power.
fact-a RCD is unlikely to trip due to too much load being drawn, it does not work like a fuse and hence it cannot be used on its own, although rated like a fuse, this is purely to select one man enough for the job. A 40amp RCD will not trip when exposed to more than 40 amps.
RCBO
-this is a combined RCD & MCB and as such will detect both overload and earth leakage. Like an RCD and MCB it can have many different specifications and so the correct type will need to be chosen. It is like having airbags and seatbelts in your car.
AFDD
-Arc Fault Detection Device, this device can detect dangerous arc faults which can occur in faulty appliances and wiring. Many house fires are caused by electrical arc faults, loose connections and damaged wiring insulation are typical examples of faults that will cause an electrical arc.
SPD
-Surge Protection Device, this device protects the electrical installation from high voltage spikes such as that caused by a lightning strike or fault in the incoming supply. Similar to surge protection devices that have been around for some time on some extension leads but on a larger scale.