Technical Article

MCB Basics

[picture of Xpole FAZ MCBs] Since circuit breakers play a big part in ensuring the safety of electrical installations, it's important to make sure that the right breaker is used for each application. By explaining the basics of MCBs, Gavin Stoppel of Moeller Electric provides some useful guidance.

Let's start with the basics: What is a circuit breaker? In essence, a circuit breaker is a device that automatically opens an electrical circuit when a fault occurs, thereby protecting the attached equipment and wiring. In many cases they are a more flexible alternative to fuses, as they can be easily reset and do not need replacement if an electrical overload or fault does occur. The types of fault in question are usually overloads and short circuits, but breakers are also available which protect against earth faults.

As leading supplier, Moeller's extensive range demonstrates that circuit breakers can come in a huge variety of capacities, shapes and sizes. They range from huge vacuum and oil-filled devices used on high-voltage distribution networks, to the miniature circuit breakers (MCBs) found in consumer units and distribution boards. This article will concentrate on MCBs, as these are the devices that electrical contractors will come across most frequently.

Even when we limit our discussion to MCBs, there are several different types to consider. The most basic being a simple thermal breaker that incorporates a bi-metallic strip, through which passes the current of the circuit to be protected. This heats the strip, which bends by an amount commensurate with the current. Under overload conditions, the strip bends further than usual, and activates the breaker trip mechanism.

Thermal breakers such as Moeller's FAZ range are simple and inexpensive, but they provide only limited protection against short circuits. They should only be used where overload protection is the most important requirement and where short circuit currents will not exceed 1,000A.

More versatile and more frequently used are thermal-magnetic MCBs. These have a similar thermal trip mechanism to that just described, which provides effective protection against small and moderate overloads. In addition, however, they have an electromagnetic mechanism that provides virtually instantaneous tripping for large overloads and short circuits.

Thermal-magnetic MCBs can be designed to provide high breaking capacities, and to have various types of trip characteristic. They are ideal for applications where comprehensive protection is required at a moderate price, and where high short circuit levels may be encountered.

Temperature Compensation

It is worth mentioning that temperature-compensated MCBs are available which reduce their tripping current automatically as the ambient temperature rises. This allows for the reduction in current-capacity of electrical wiring at higher temperatures, and ensures that effective protection is provided over a wide range of ambient conditions.

Choosing an MCB

When choosing an MCB for a particular application, four major factors need to be considered. These are the standard to which the MCB conforms, its current rating, the type of trip characteristic and its breaking capacity. Let's look at each of these in turn.

In the UK, the applicable standard for most MCBs is BS EN 60898. This covers low-voltage breakers for use in domestic and similar applications. If, however, an industrial installation is under consideration, it may be necessary to refer to BS EN 60947-2. Breakers for use in the UK should always conform to the appropriate standard, as this will guarantee that they correctly carry out their protection functions.

The current rating of an MCB is the maximum current that it will carry continuously without tripping. MCBs should always be chosen so that their current rating matches, as closely as possible, the maximum load current of the circuit they are protecting.

Trip Characteristics

On the face of it, an MCB that trips as quickly as possible under fault conditions sounds ideal. In fact, this is what is required for short-circuit faults but, for overload protection, the situation is more complicated. Many items of equipment, such as fluorescent lights, transformers and motors, draw a high peak current for a short period when they are switched on.

An MCB, which reacts instantaneously, would trip every time such a peak occurred, which would make it unusable. Fortunately, the thermal element in MCBs does not react instantaneously, as the bi-metal strip takes time to heat up. It is, therefore, hardly affected by short-term current peaks. By changing the design of the bimetal elements, MCB manufacturers can determine what size of peak current a particular MCB will ignore, and for what length of time. This relationship between current and tripping time is usually shown as a curve, known as the MCB's trip characteristic.

To avoid the need for users to work with the curves, BS EN 60898 defines several types of standard characteristic, the most important of which are Types B, C and D, all of which are included in Moeller FAZ range. In most cases, it's easy to choose one of these types to match the application in hand, and only in really specialised applications will users need to work with the full characteristic curves.

Type B MCBs react quickly to overloads, and are set to trip when the current passing through them is between 3 and 4.5 times the normal full load current. They are suitable for protecting incandescent lighting and socket-outlet circuits in domestic and commercial environments, where there is little risk of surges that could cause the MCB to trip.

Type C MCBs react more slowly, and are recommended for applications involving inductive loads with high inrush currents, such as fluorescent lighting installations. Type C MCBs are set to trip at between 5 and 10 times the normal full load current.

Type D MCBs are slower still, and are set to trip at between 10 and 20 times the normal full load current. They are recommended only for circuits with very high inrush currents, such as those feeding transformers and welding machines.

Breaking Capacity

The final factor to be considered when choosing an MCB is its breaking capacity. This must always be greater than the prospective short-circuit current (PSCC) at the point where the MCB is installed.

It should not be difficult to make sure that this is the case, as part of the testing required on electrical installations involves measuring the source impedance of the supply, from which the PSCC can easily be calculated. Many modern installation testers carry out this calculation automatically.

Most manufacturers now offer ranges of MCBs with breaking capacities of 10kA, Moeller's FAZ range is 15kA and this has been extended to 25kA for certain products. This higher breaking capacity can, in some applications, reduce costs substantially by allowing MCBs to be used where previously more costly moulded case breakers would have been required.

Earth-Fault Protection

At the start of this article, breakers that provide earth-fault protection were mentioned. Typically these are known at RCDs (residual current devices) or RCCBs (residual current circuit breakers), and their primary function is to guard against the effects of electric shock.

There isn't room to discuss them in detail in this article, but it is worth noting that MCBs that incorporate earth fault protection are now available. These are often called RCBOs (residual current breaker with overload), and are useful in many applications, not least in domestic installations.

Summary

Modern MCBs are reliable, inexpensive devices which, when correctly chosen, provide a high level of protection both for wiring and equipment. Selecting the right MCB is not a difficult process and the information contained in this article should make it easier still.

This page last updated: 31 August 2005