MCB

MCBs replace fuses in most modern CUs.

MCB means 'miniature circuit breaker.'

Ratings

• Lighting circuits usually use 6A MCBs. Retrofit MCBs are generally 5A.
• Ring circuits usually use 32A MCBs, with retrofits generally 30A.
• Other values are used for other loads, and in some cases for the above 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.

Types B,C,D

Modern MCBs contain 2 trip elements, one thermal, one magnetic. The thermal element responds to prolonged low overloads, and the magnetic element responds rapidly to heavy overloads.

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.

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

Some installations with mains filament lighting on type B MCBs are prone to nuisance trips. Whether use of type C or D MCBs can improve nuisance tripping has been the subject of discussion on uk.d-i-y.

Type A MCBs don't exist.

Comparison with Fuses

Rewirable fuses, cartridge fuses and MCBs all do the same job, and are all fully compliant with 16th edition regs, so 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 overcurent
• Fuses trip more quickly on heavy fault current
• 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 many each year)
• 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, and 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 wire
• 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.
• Fuses have no mechanism of failure to act, as long as they're correctly specified to begin with.
• MCBs are active mechanical devices, and do fail to act occasionally
• 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.
• Domestic CU fuses are rated to the necessary full 6kA breaking capacity
• Some MCBs, particularly retrofits, are only rated to 3kA, and in some installations this is insufficient for safe reliable breaking.
• MCBs sometimes trip before the mains plug fuse when an appliance is faulty, leaving the appliance with an intact fuse and still able to cause serious injury.

See Also

• Fuse
• Wiki Contents
• Wiki Subject Categories