Technical Article

Breaker Briefing

[picture of NZM units] Fitting the right circuit breakers is an essential step in ensuring the safety and reliability of every electrical installation. But, with a huge variety of types on offer, how should contractors set about choosing between them? Colin McAhren, Product Manager at Moeller Electric provides the answers.

Circuit breakers come in all shapes and sizes, but between the extremes are the devices with which most contractors (and wholesalers) have daily contact: miniature circuit breakers (MCBs), moulded-case circuit breakers (MCCBs) and residual current devices (RCDs). It's these everyday products we'll discuss in this article.

[picture of MCB units] The most basic type of MCB is a simple thermal-only breaker designed around a bi-metallic strip. Thermal-only breakers are simple and inexpensive. They provide good protection against overloads but, as the bimetallic strip is slow to respond to large currents, they give poor protection against short circuits. They should therefore, only be used where overload protection is the main requirement and where short-circuit currents will not exceed 1,000A.

For most applications, thermal-magnetic MCBs are a better choice. These have a thermal trip mechanism which is similar to that used in thermal-only MCBs, and which provides effective protection against small and moderate overloads. In addition, however, they have an electromagnetic trip mechanism which provides virtually instantaneous tripping in the event of a large overload or a short circuit.

Thermal-magnetic MCBs can be designed to have very high breaking capacities and to provide a variety of trip characteristics. Widely used, they offer comprehensive protection and appropriate types can be used where high short-circuit currents may be encountered.

A useful refinement which may be found in some types of MCB is temperature compensation, an arrangement whereby the tripping current is automatically reduced as the ambient temperature rises. This compensates for the reduction in current-carrying capacity of the wiring at higher temperatures, allowing effective protection to be provided over a wider range of ambient conditions.

As we have seen, in the majority of cases, thermal-magnetic MCBs are the best option. With this decision made, the next step is to decide which thermal-magnetic MCB to use. There are four main factors which influence this decision: the standards to which the MCB conforms, its current rating, the type of trip characteristic it offers, and its breaking capacity. Let's look at these in turn, starting with standards.

In the UK, the standard applicable to most MCBs is BS EN 60898, which covers low-voltage breakers for use in domestic and similar applications. For equipment which forms part of an industrial installation, however, it may be necessary to refer to BS EN 60947-2.

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

The choice of trip characteristic is a little more complicated. It would be easy to think that it would be ideal for an MCB to trip as quickly as possible if the current increased above its normal value. This is, in fact, exactly what's needed for short circuits faults, where the quicker the current is interrupted, the better. For overloads, however, the situation isn't quite so clear-cut.

Many items draw high peak currents for a short period when they are switched on. An MCB with instantaneous overload response would trip every time it saw such a peak, which would clearly make it unusable.

Fortunately, as was mentioned earlier, the bimetallic strip in an MCB doesn't react instantly and is, therefore, very little affected by short-term current peaks. By carefully adjusting the design of the bimetallic strip, MCB manufacturers can determine what size of peak current an MCB will ignore, and for how long.

The relationship between the current and tripping time for the MCB is the trip characteristic, and is often presented as a graph. In most cases, however, contractors will not need to refer directly to these graphs, as BS EN 60989 defines several types of standard characteristic.

Selecting an MCB with the right characteristics means that it will provide the best possible protection whilst minimising the risk of 'nuisance tripping'. The best-known MCB characteristics are Types B, C and D which cover the majority of traditional applications. These have however, recently been joined by Type Z, K and S characteristics which offer improved protection in certain applications.

Type B react quickly to overloads, and are built to trip when the current passing through them is between 3 and 4.5 times the normal full load current.

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

Type D are slower still, and are set to trip at between 10 and 20 times normal full load current. They are recommended only for circuits with very high inrush currents. However, MCBs with the new S or K characteristics may provide better protection in some applications of this type.

Type K are designed to trip at between 8 and 12 times normal full load current, placing them between the traditional Type C and Type D breakers. In most cases, they allow improved cable protection to be provided in circuits which include motors, capacitors and transformers, where it would previously have been necessary to use Type D devices.

Type S have a characteristic which is optimised for protecting control circuit transformers. They are designed to trip at between 13 and 17 times normal operating current. This means that they are somewhat similar to Type D MCBs, but have a more closely defined tripping range which allows them to provide good transformer protection with minimal risk of nuisance tripping.

Type Z are designed to trip instantly at between 2 and 3 times their normal operating current. This makes them suitable for protecting electronic components and devices which can be quickly destroyed even by small current surges. They are also suitable for protecting high-impedance cables.

The final factor which needs to be considered when selecting MCBs is the breaking capacity. This must always be greater than the prospective short circuit current (PSCC) at the point where the MCB is to be installed, or there is a risk that the MCB will be unable to clear faults safely.

Typically, modern MCBs have a breaking capacity of between 6kA to 10kA, but it is worth mentioning that recent developments by Moeller Electric have allowed this to be extended to 25kA for certain products. This can, in some applications, reduce costs by allowing MCBs to be used where more costly MCCBs would previously have been needed.

At this point, it's worth looking at the difference between an MCB and an MCCB. Essentially, it's one of ratings. As we've seen, MCBs are available with rated currents up to 100A or so, and with breaking capacities up to 25kA. For comparison, Moeller Electric's NZM range of MCCBs includes models with ratings up to 1,600A and with breaking capacities up to 150kA.

In the past, MCCBs used the same type of thermal and magnetic tripping mechanisms as MCBs, but microprocessor-based electronic trips are becoming much more common. This opens the way for enhanced functionality, such as communications facilities which allow the breakers to send information about current and trip status to a fieldbus network or a building information system.

Unlike MCBs, MCCBs also usually have trip characteristics which can be adjusted by the user. They don't therefore need to be ordered with a specific built-in characteristic.

To round off, let's take a quick look at RCDs. These are a special class of miniature circuit breaker; they respond not to overcurrents, but instead detect current flowing to earth. This can indicate that someone has touched a live part, or that the insulation of the equipment which the RCD is protecting is faulty. Their main purpose is to protect against electric shock.

RCDs, which are sometimes called RCCBs (residual current circuit breakers) or earth leakage trips always operate instantaneously, and are rated by switching capacity and sensitivity. The switching capacity is the maximum current which the RCD can safely carry and switch and must be greater than the normal full load current of the circuit. The sensitivity of the RCD is the level of earth leakage current needed to make it trip, with typical values being in the range 10mA to 100mA.

Many manufacturers now provide devices which combine the functions of an MCB with those of an RCD. Known as RCBOs (residual current breaker with overload), these can provide space and cost savings. When choosing RCBOs, the MCB and RCD functions should be considered separately, applying the same criteria as if two separate devices were being used.

Circuit breakers provide dependable, convenient and cost-effective protection for electrical installations of all types. If they are to give their best performance, however, they must be correctly chosen to suit the application. In most cases, this is not a difficult task, and hopefully the information in this article will have provided some useful guidance.

This page last updated: 24 September 2007