MCBs or Miniature Circuit Breakers
- MCB stands for
Miniature Circuit Breaker
- MCBs are used for protection against electrical overload.
- To explain that in simple terms, think of a wiring circuit in your house which serves several rooms and can usually cope with the appliances used. But if too many appliances are switched on at once, that circuit could become overloaded.
- Overloading can result in excessive heat being created in the wiring cable, which can in turn lead to cables melting and cause fires.
- It is not uncommon to find overloading happening around Christmas time when excessive use of extension leads with trailing sockets for plugging in decorations. Likewise, the temptation to plug a whole bank of Christmas decorations and heaters into an electric timer can also be dangerous.
MCBs - Miniature Circuit Breakers
Electrical Overload on Extension Lead
MCBs in a Consumer Unit
- Fires are often the result of such overloads, unless some protective device has been installed to detect when an overload is occurring, and then deliberately breaks the circuit to stop the currently flowing and thus removing the danger.
- Miniature Circuit Breakers are designed for just this purpose, but the use of the correct value MCB is very important.
- For example a lighting circuit serving numerous lighting points in your home is usually well within the current carrying capacity of a 1mm2 or 1.5mm2 diameter cable, with an MCB designed to
break (or trip) at 6 amps. But if you attempt to connect a socket outlet to that circuit and plug a toaster into it, the cable may melt and may cause a fire, so the MCB will detect the excess load (the
overload) and break the circuit before any damage is caused.
- For this reason you may have noticed that whilst lighting circuits are wired in 1mm2 or 1.5mm2 diameter cable, circuits serving socket outlets are usually wired using 2.5mm2 cable and are protected by MCBs designed to trip at 32 amps. Circuits designed to supply more powerful equipment such as cookers and electric showers are likely to be wired in 6mm2 or 10mm2 and protected by an MCB designed to trip at 40 amps.
- Please note that there are many reasons why variations to MCB values and cable thickness can be found, such as the installation method, heat and length. All I intend to provide with this overview is to help you understand what MCBs are, and not to specify what should be installed in your home.
The electrician who installed the circuit should have determined the correct size (value) of MCB. This is not a job recommended for an unqualified person to undertake.
- In many respects MCB Types B, C and D are similar.
- They are all designed to trip (or break the circuit) if the fault current exceeds their
nominal rating, but some MCBs are designed to allow for for certain pieces of equipment causing surge currents on start up, such as fluorescent lighting, motors, etc.
- Other devices typically found in a domestic consumer unit include RCDs and RCBOs.