Electrical circuit protection explained
Please note that all electrical wiring and installation details given on diydata.com is for information purposes only. From 1st January 2005, the Building Regulations Part P requires, in England and Wales, that only certified persons can carryout electrical installation work, or the work must be certified upon completion - see this page for more details.
Always isolate any electrical circuit before working on the circuit.
Fuses, MCBs, RCDs, and RCBOs are all devices used to protect users and equipment from fault conditions in an electrical circuit by isolating the electrical supply. With fuses and MCBs only the live feed is isolated; with RCDs and RCBOs both the live and neutral feeds are isolated.
A fuse is a very basic protection device which is destroyed (i.e. it 'blows') and breaks the circuit should the current exceed the rating of the fuse. Once the fuse has blown, it needs to be replaced.
In older equipment, the fuse may just be a length of appropriate fuse wire fixed between two terminals (normally screw terminals). These are becoming rarer as electrical installations are updated - the presence of such fuses usually indicates that it is about time that the installation is updated.
Modern fuses are generally incorporated within sealed ceramic cylindrical body (or cartridge) and the whole cartridge needs to be replaced.
Cartridge fuses are used in older type consumer units, fused sockets, fused plugs etc.
Miniature Circuit Breaker (MCB)
An MCB is a modern alternative to fuses used in Consumer Units (Fuse Boxes). They are just like switches which switch off when an overload is detected in the circuit. The advantage of MCBs over fuses is that if they trip, they can be reset - they also offer a more precise tripping value.
Residual Current Device (RCD)
Modern alternatives (better) to Earth Leakage Circuit Breakers and fuses in the Consumer Unit. RCDs are tripped if they detect a small current imbalance between the Live and Neutral wires above the trip value - this is typically 30mA.
RCDs can be wired to protect a single or a number of circuits - the advantage of protecting individual circuits is that if one circuit trips, it will not shut down the whole house, just the protected circuit.
RCDs are available in at least 4 basic configurations:
- As hard wired in units, where both the inputs and outputs are wired into the unit - ideal for a workshop etc where all the sockets within can be protected. Each individual circuit taken from the RCD is protected by a MCB of an appropriate value.
- As protected outlets - normally a protected socket can be fitted as a direct replacement for a standard, no protected outlet socket.
- As a plug-in unit which can convert any socket into to a protected circuit - this gives good flexibility as, for example, a lawn mower or a hedge trimmer can be plugged in at different times. However, as the individual appliance could still be plugged into an unprotected socket, you need to remember to fit the plug-in unit when it is required.
- As a plug for wiring on to the lead of an individual appliance, this does make it less flexible than the plug-in unit above but it does ensure that the piece of equipment is always protected. One very usefully use to to fit it to the end of an extension cable, then whatever you plug into the extension lead is protected.
Residual Current Breaker with Overload protection (RCBO)
A RCBOs combines the functions of a MCB and a RCD in one unit. They are used to protect a particular circuit, instead of having a single RCD for the whole building. Generally these are used more often in commercial building than domestic ones.