MCB

MCB means 'miniature circuit breaker.' MCBs replace fuses in most modern CUs.

(Front view of some MCBs installed in a consumer unit)

Ratings

Lighting circuits

• usually use 6A MCBs.
• Retrofit MCBs are generally 5A.
• 10A lighting circuits are also sometimes used, primarily in larger buildings.

There was a restriction in the 16th edition regs (now removed in the 17th) that precludes the use of any light fittings that make use of miniature BC or SES lamp holders on circuits protected at 10A.

Socket circuits

• Ring circuits usually use 32A MCBs,
• retrofitted MCBs are generally 30A.
• Less common radial socket circuits typically use a 16A or 20A MCB

Other loads

• Other values are used for other loads, and occasionally for socket & lighting circuits as well.

RCBOs

An RCBO is a combination of MCB and RCD in one device. Use of these enables much greater RCD discrimination, and greatly reduces the chance of and consequences of RCD nuisance trips. RCBO cost is significantly higher than MCBs + RCD.

Types B,C,D

Modern MCBs contain 2 trip elements, one thermal, one magnetic. The thermal element responds to sustained overload currents, and the magnetic element responds rapidly to substantial fault currents. Note that a MCB is unlikely to respond at all to a sustained low level overload currents.

New MCBs come in 3 types: type B, type C & type D.

• Type B is used for the great majority of domestic MCBs, and has the quickest magnetic tripping of the 3.
• Type C is a little slower acting and a little less prone to some types of nuisance trip
• Type D is used where more tolerance of short term overcurrent is wanted, and is thus the least prone to nuisance trips. Use of type D MCBs requires careful calculation to ensure that adequate protection is provided for the circuit cables in the event of a fault.

The minimum current needed to trip an MCB in 0.1 seconds is:

• Type B: 5x rated current
• Type C: 10x rated current
• Type D: 20x rated current

For new installs, circuits feeding fixed equipment (including lighting circuits) require a maximum 5 second disconnect time, and socket circuits a maximum 0.4 sec disconnect time. These times are straightforward to meet using type B MCBs. In some cases type Cs will also comply, but often not.

Some installations with mains filament lighting on type B MCBs are prone to nuisance trips. Whether use of type C or D MCBs can reduce nuisance tripping has been the subject of discussion on uk.d-i-y. Sometimes it cures the problem, sometimes it makes little difference.

Type A MCBs don't exist (use of the "A" suffix was deliberately avoided to save confusion with the current rating of the device in Amps).

Comparison with Fuses

Rewirable fuses, cartridge fuses and MCBs all do the same job, and are all fully compliant with 16th edition regs, and may be installed new. There are minor differences in performance between them. It is often assumed that MCBs are safer because they're newer, however in reality there are various minor pros and cons on both sides.

• MCBs trip more quickly on medium overcurrent
• Fuses trip more quickly on heavy fault currents (which start fires faster)
• MCBs are more likely to nuisance trip on bulb failures. This can cause safety problems as well as inconvenience, especially with stair lighting. (Stair falls kill and injure orders of magnitude more people each year than electric shock)
• Use of MCBs enables a slight increase in circuit current rating (from 5A to 6A, 30A to 32A)
• MCBs are easily reset
• MCBs may be abused because of this, by being repeatedly reset when a real fault occurs. It should be noted that in such an event, MCB or fuse action is intended to much reduce the risk of fire, but it does not prevent it, and a repeatedly tripping mcb is not a safe situation.
• Rewirable fuses can be abused by rewiring with the wrong fuse wire
• Rewireable fuses also require that during design of a circuit the current carrying capacity of it needs to be de-rated to 7/10ths of its nominal value, to allow for their slower response.
• A card of fusewire kept on the CU much reduces the chance of this, but does not eliminate it
• Cartridge fuses are not vulnerable to this, however spares need to be stocked to avoid problems.
• Fuses have no mechanism of failure to act, so they can not fail to act (as long as they're not grossly mis-specified to begin with).
• MCBs are active mechanical devices, and do fail to act occasionally (creating a dangerous situation)
• Note that neither fuses nor MCBs are designed to act on all fault currents, and failure to trip does not necessarily indicate a fault with the MCB or fuse.
• High current fuses (eg for showers) can get hot in use.
• All domestic CU fuses are rated to the necessary 6kA breaking capacity
• Some MCBs, particularly retrofits, are only rated to 3kA, and in some installations this is insufficient for safe reliable breaking.
• MCBs are prone to tripping before the mains plug fuse when an appliance is faulty, leaving the appliance with an intact fuse and still able to cause serious injury.
• CU fuses have much better discrimination in the above situation.

Breaking capacity (3kA, 6kA etc)

Retrofit 3kA MCBs

Any fuse or MCB will carry a rating known as its "maximum breaking capacity". This is the largest fault current that it can safely interrupt in the event of a circuit fault. Should the fault current exceed the device's breaking capacity, then its possible it will still open the circuit, but sustain damage in the process. Alternatively it may fail to interrupt the current flow altogether (hopefully an upstream device would open the circuit in these circumstances). Damaged MCBs may seize - either the electrical contacts welding together, or the mechanism failing such that it will be impossible to switch the device back on again.

In domestic situations its fairy rare for this to be a problem, however supplies (typically in towns or very close to a substation) which are very "stiff" (i.e. have very low impedance), and very short final circuits with only a few meters of cable between CU and point of use, may encounter prospective fault currents that exceed the breaking capacity of some devices.

• Most domestic MCB can break 6kA
• Some ranges will also do 10 or 16kA (often those designed for commercial installation and three phase CUs)
• Some plug in replacement MCBs (such as those pictured) can only break up to 3kA.

Circuit designers may need to provide additional "back up" breaking capacity to ensure adequate protection when designing for systems with these very low fault impedances.

See Also

• Fuse
• Wiki Contents
• Wiki Subject Categories