A Complete Guide to MCBs

How Does a Miniature Circuit Breaker Work?

Mini circuit breakers are triggered by overcurrent - electrical current that exceeds a designated safe current and makes use of a relatively robust mechanical mechanism designed to minimise failures and false alarms.

Excess current causes the bimetallic strip within the MCB to heat, bend, and trip. This releases a switch that moves the electrical contact points apart to confine the arc (electrical discharge). The arc is divided and cooled by an insulated metal strip called the arc chute. The contacts close again once the fault has been fixed and the MCBs are reset.

An MCB is designed to protect against both overloading and short-circuiting. These are detected differently using separate processes. Overload protection is provided by the bimetallic strip using thermal operation, whereas short-circuit protection is provided by the tripping coil via electro-magnetic operation.

If the discharge is especially high, the MCB will trip (activate) very quickly – within one-tenth of a second. When the overcurrent is closer to the safety limits, the component will be slower to respond.

Different MCB Types Explained

Several different MCB types are available – types A, B, C, D, K, and Z. However, the three key versions are type B, type C, and type D. Each is designed to be responsive to the likely strength of electrical surges in different settings. These variations are typically known as their ‘trip curve’ but can also be known as their tripping characteristics or overcurrent characteristics.

Let’s take a look at the differences between each primary type:

Other MCB Types

A few more specialised MCB models are available. These include:

• Type K MCBs – these will trip when the current reaches eight to twelve times the recommended maximum. They are a good choice for motors

• Z-type MCBs – these are highly sensitive MCBs, tripping when current exceeds the rated load by only two to three times. They are used with more delicate devices prone to short circuits, such as semiconductors

How to Choose the Right MCB

The type of MCB that you should buy depends on the specifics of your device or installation. When shopping for an MCB, compare the following factors:

• The tripping characteristics

• The breaking capacity – the maximum current a circuit breaker can interrupt without destruction or releasing an arc. This needs to match the likely strength of any surges in the vicinity of the installation. Breaking capacities are measured in kiloamperes (kA), each of which is 1,000 amperes (amps), the standard unit of electrical current

• The number of poles or trippable switches within the MCB housing. Pole options include single, double, triple and neutral, and four-pole. Triple or three-pole models are common; these can simultaneously break the current on all three circuits if a fault occurs on any one

MCB Choices by Ampere Rating

The following lists some of the most frequently-encountered load capacities for commercial MCBs:

• 2 amps

• 6a

• 10 amps

• 16a

• 20 amps

• 25a

• 32 amps

MCB Choices by Breaking Capacity

The following are the most frequently-encountered breaking capacities of commercial MCBs:

Chart Comparing Typical MCB Applications

Image	Shop Now	Type	Rating	Number of Poles	Applications
	Buy here	Type C	16A	1	Computers
	Buy here	Type B	32A	3	Domestic appliances
	Buy here	Type B	16A	3	General-purpose
	Buy here	Type D	40A	3	Industrial
	Buy here	Type C	6A	1	Motor control

MCBs by Brand

Key MCB brands are compared in the following chart:

Image	Brand & Product Type	Main Benefits	Typical Applications
	ABB MCBs	Complete range of MCBs to cover most applications Space and time-saving design	Residential, commercial and industrial applications
	Eaton MCBs	Professional range of MCBs Safe and reliable	Commercial and industrial
	Legrand MCBs	Leading-edge technical features High-performance ranges	Residential, commercial and industrial applications
	Schneider MCBs	Excellent circuit protection Rigorously tested	Wide range of applications; industrial and commercial
	Siemens MCBs	High-performance range Innovative mounting options	Wide range of applications
	Europa MCBs	Perfect for consumer unit fitting Fully tested and conform to all relevant standards	Perfect for residential applications

FAQs

How Do You Test MCBs?

To test an MCB after installation, you will need a suitable portable voltage meter from a reputable manufacturer. Electricians also recommend manually inspecting the switching mechanism of MCBs before installation; typically, a greater degree of pressure is needed to open and close more reliable models.

Can You Mix MCBs?

Specifications matter more than brand when choosing MCBs, so in theory, you can use any component that is compatible with a particular device. However, mixing brands in the same installation is not recommended as it makes testing less reliable and can invalidate warranties and installation guarantees.

Why are MCBs Preferred to Fuses?

MCBs serve a similar function to electrical fuses, which are designed to melt and thereby break a circuit if the current flowing through exceeds a designated limit. However, fuses can be less reliable than MCBs - the latter work better in lower voltage settings and do not require replacement after use.

What is the Difference Between MCBs and MCCBs?

Moulded case circuit breakers (MCCBs) have a very similar function to MCBs but they also have a higher capacity. MCBs are all sub-100-amp devices and are designed for low voltage circuits, so their trip curves cannot be adjusted. By contrast, MCCBs feature adjustable trip characteristics, which means that they can be used with higher voltages – as much as 2,500 in some instances.

What is the Difference Between MCBs and ELCBs?

ELCBs (earth-leaking circuit breakers) use earthing as their primary method of controlling electrical current and preventing electrocution. They work by detecting any stray voltage on the enclosure of a device and then breaking the circuit if it exceeds a designated level. They perform a similar function to RCDs, but the latter detect stray voltage directly and so are now more favoured by electricians.

What is the Difference Between MCBs and RCDs?

Residual current devices (RCDs) are a different form of electrical safety equipment. While MCBs have a general function, RCDs are specifically built to protect against the frequently fatal risk of electrocution from touching exposed wires or incorrectly earthed cables. They operate directly within electrical circuits to detect faults and cut off potentially hazardous currents.

RCDs are also available in various different types – types A, B, C, D, K, and Z. In the United States and Canada, they are known as ground fault circuit interrupters (GFCIs).