Make or break

Fitting the right circuit-breakers is an essential step in ensuring the safety and reliability of every electrical installation. But there’s a huge variety available on the market, and it’s not always easy to make the right decision.

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). We’ll be taking a closer look at these everyday products.

The most basic type of MCB is a simple thermal-only breaker designed around a bimetallic 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 only be used where overload protection is the main requirement and where short-circuit currents will not exceed 1000 A.

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.

So, you’ve opted opt for thermal-magnetic MCBs. The next step is to decide which thermal-magnetic MCB to use. Four main factors influence this decision: the standards to which the MCB conforms, its current rating, the type of trip characteristic it offers and its breaking capacity.

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 that forms part of an industrial installation you may need 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 the maximum-load current of the circuit as closely as possible.

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

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

Fortunately, as mentioned earlier, the bimetallic strip in an MCB doesn’t react instantly and is thus affected very little by short-term current peaks. By adjusting the design of the bimetallic strip, MCB makers 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 while minimising the risk of nuisance tripping. The best-known MCB characteristics are Types B, C and D, which cover most traditional applications. These have recently been joined by Types Z, K and S, which offer improved protection in certain applications.

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 6 kA and 10 kA, but it is worth mentioning that recent tests by Moeller Electric have extended this to 25 kA for certain products. This can, in some applications, reduce cost 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 rests on ratings. As we’ve seen, some MCBs have rated currents up to 100 A or so, with breaking capacities up to 25 kA. By comparison, Moeller Electric’s NZM range of MCCBs includes models with ratings up to 1600 A, with breaking capacities up to 150 kA.

Unlike MCBs, MCCBs usually have trip characteristics that can be adjusted by the user. So they don’t 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 do not respond 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 or earth-leakage trips, always operate instantaneously and are rated by switching capacity and sensitivity. The switching capacity is the maximum current that 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 in the range 10 mA to 100 mA.

Many manufacturers now provide devices that combine the functions of an MCB with those of an RCD. Known as residual current-breakers with overload (RCBOs), 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 offer their best performance, however, they must be specifically chosen to suit the application at hand. In most cases, this is not a difficult task. But don’t allow yourself to be short-circuited by the wealth of options available.