Picking The Right Protection with Eaton
Choosing the right devices for final circuit protection can be a confusing task: modern MCBs, RCCBs and RCBOs are available in a bewildering array of types and sizes. To help resolve this confusion, David Pitt of Eaton’s Electrical Sector provides some useful guidance.
In modern electrical installations, three types of device are commonly used to provide final circuit protection: miniature circuit breakers (MCBs) that provide over-current and short-circuit protection, residual current circuit breakers (RCCBs) that provide earth leakage protection, and residual current breakers with overcurrent (RCBOs) that combine the functions of an MCB and an RCCB in a single device.
Since the introduction of the 17th Edition of the IET Wiring Regulations, RCBOs are being used more widely, but when looking at the characteristics of protection devices it’s easier to start by considering MCBs and RCCBs separately.
MCBs are commonly available in versions with Type B, C or D tripping characteristics, the main difference being the ability of the MCB to handle current surges without tripping. Type B devices trip at fault currents between 3 and 5 times the rated current (3 In to 5 In) and are most suitable for domestic installations and for light commercial applications where switching surges are small.
Type C devices trip between 5 In and 10 In and are often chosen for domestic, commercial and industrial applications where electric motors or significant numbers of fluorescent lights are in use. Type D devices, which trip between 10 In and 20 In, are most often used in industrial applications where equipment that produces high inrush currents – such as an arc welding machine or x-ray machine – is in use.
When selecting MCBs, it’s essential to remember that their primary function is to protect cables downstream of the device. Type B and Type C devices can normally be selected to provide tripping times that will give adequate cable protection, but this can be more difficult with Type D devices, which may need a lower loop impedance (Zs) to achieve the tripping time prescribed in the Wiring Regulations.
Now let’s look at RCCBs, which are also available in various different types that are designated by letters, just like MCBs.
This is a potential source of confusion so it’s worth remembering that a Type B MCB, for example, is not related to a Type B RCCB!
The types of RCCB most likely to be encountered are Type AC, Type A, Type B, Type B+ and Type S. Type AC RCCBs are sensitive to ac currents and are suitable for most domestic and commercial applications. Type A RCCBs provide additional protection to Type AC in that they are sensitive to ac currents and pulsating dc currents. Type B RCCBs are sensitive to ac, pulsating ac and steady dc leakage currents. This type should always be used in photovoltaic (PV) solar energy installations where the inverter type could allow leakage of dc current.
Type B+ devices are similar to Type B, but respond to ac leakage currents over a wider frequency range, which is useful in some specialised applications. It’s worth noting that Type B and Type B+ devices can be used wherever a Type AC or Type A device is specified, as they provide the same functionality as these types and more.
But what of Type S RCCBs? These are selective devices intended for use where circuits include more than one RCCB. For example, an installation could include an upstream RCCB to provide protection for several downstream circuits, some of which also have their own RCCB or RCBO. If an earth fault occurs on one of the downstream circuits with an RCCB or RCBO, this device should trip rather than the upstream RCCB. Using a Type S RCCB for the upstream device will provide the necessary selectivity.
Now let’s turn to RCBOs. In principle, as these are functionally an MCB and an RCCB in the same device, it would be possible to produce versions with a huge range of combinations of MCB and RCCB characteristics. In practice, manufacturers limit themselves to the most commonly used combinations, such as a Type B or C MCB characteristic with a Type A or AC RCCB characteristic. Where other combinations are needed, it often means that an RCBO can’t be used and that the necessary protection must be provided by using a separate MCB and RCCB. Some manufacturers, however, offer field-fittable residual current units that can be used to convert an MCB into an RCBO, and this makes possible a wider range of combinations.
Two interesting new product developments are worth mentioning at this point. The first is that in addition to standard RCBOs, which are bigger than an ordinary MCB, leading manufacturers are now offering compact RCBOs that save space in consumer units or distribution boards and are easier to wire. These compact devices are usually available with the same combinations of characteristics as standard RCBOs.
The second development is the recent introduction of RCCBs utilising digital electronic technology, which offer a very wide range of characteristics and are particularly suitable for use in “difficult” applications, such as the protection of circuits supplying large numbers of fluorescent lamps with electronic ballasts.
This article has looked only at operating characteristics for protection devices and is intended to provide guidance on choosing the best type of device for use in specific applications. When choosing individual devices, however, there are many other factors that must be taken into account including the current rating for MCBs and RCBOs, the sensitivity for RCCBs and RCBOs, and the short circuit capacity for all devices.
There’s no room to cover these factors here, but comprehensive guidance can be found in the IET Wiring Regulations to which reference should, of course, be made when designing any type of electrical installation. Additionally, leading suppliers of protection devices, such as Eaton, are always happy to provide expert advice and guidance.