In modern electrical installations, three types of device are commonly used to provide final circuit protection: MCB that provide over-current and short-circuit protection, RCCB that provide earth leakage protection, and RCBO that combine the functions of an MCB and an RCCB in a single device.
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. so How to picking the right circuit protection?
■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.
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!
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, 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.
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.
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.
Additionally, professional suppliers of protection devices, such as Meba Electric Co.,Ltd, are always happy to provide expert advice and guidance.