MCB
MCB means 'miniature circuit breaker.' MCBs replace fuses in most modern CUs.
Ratings
Lighting circuits
- usually use 6A MCBs.
- Retrofit MCBs are generally 5A.
- 10A lighting circuits are also sometimes used, primarily in larger buildings. Note however that use of protection at 10A precludes the use of any light fittings that make use of miniature BC or SES lamp holders.
Socket circuits
- Ring circuits usually use 32A MCBs,
- retrofitted MCBs are generally 30A.
- Less common House Wiring for Beginners#Radial 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 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
For new installs, lighting circuits require a 5 second disconnect time, and socket circuits a 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 improve nuisance tripping has been the subject of discussion on uk.d-i-y.
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.