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Tips for Choosing the Best MCB or RCBO

Life was a lot simpler for those working in the area of LV power distribution and protection a few decades ago. Choice was limited largely to what fuse wire size to select. Thanks to advances in technology, installers today have a lot more to consider.

One of the most important decisions involves choosing the right MCB or RCBO for the job, based on the levels of protection against electrical faults required.

Protective devices include:

  • Miniature Circuit Breakers (MCBs)

  • Residual Current Circuit Breakers (with overcurrent protection) (RCBOs)

  • Residual Current Circuit Breakers (without overcurrent protection) (RCCBs)

  • Arc Fault Detection Devices (AFDDs)

Before deciding which protection device to use in commercial, residential or industrial buildings, it’s important to understand the differences between devices. This knowledge can help avoid a range of issues including unnecessary tripping, disconnection timing problems for earth fault protection and complications with upstream protection devices.

The central purpose of MCB and RCBO devices is protection of the downstream cable, so the device needs to be selected in keeping with the wiring rules.

Classifying MCB Devices

B, C and D devices are distinguished by their capacity to manage surges without tripping. Surges are generally inrush currents, resulting from reactive loads like lighting or loads containing battery charging equipment or motors. B, C & D types can be classified as follows:

  • Type B. These devices are most suited to domestic appliances, but they may be appropriate for light commercial application if surges are minimal or non-existent.

  • Type C. The devices of choice for industrial and commercial applications where a certain level of electrical inrush is normal.

  • Type D. These devices are usually restricted to industrial applications where high inrush currents are expected, including substantial battery charging systems, transformers, motors, x-ray machines and some forms of lighting.

The basis of the classification of devices is the fault current rating that triggers operation (usually under 100ms) as protection from short-circuits. It is essential that the circuit-breaker is not tripped unnecessarily by equipment which has high inrush currents, but it must trip when required to prevent a genuine short-circuit which could cause damage to circuit cables.

Tripping features:

  • Type B – trip when fault currents are 3-5 times the rated current (In) This is 30-50A for a 10A device.

  • Type C – trip when fault currents are 5-10 times the rated current (In). This is 50-100A for a 10A device

  • Type D – trip when fault currents are 10-20 time the rated current (In). This is 100-200A for a 10A device.

Cable ratings refer to continuous service under specific installation conditions, but obviously cables can carry higher currents for short periods of time without acquiring permanent damage.

In addition to protecting cables from short circuits and overloads, MCBs can be used to protect against earth faults and the effects of electric shock on fixed and portable equipment. Whether an MCB can provide an appropriate disconnection time is determined by the earthing arrangement and the circuit’s total earth loop impedance (Zs) value.

Avoiding Unnecessary Tripping

Natural inrush currents are just one of the causes of tripping for Type B devices in retail and domestic environments. Another is the failure of lamps and components which can take place due to high arcing currents.

If unnecessary tripping persists, particularly in commercial applications, substituting a Type C device for a Type B device can resolve the problem. Another option is to make use of a higher rated Type B MCB, such as a 10A instead of a 6A. Whichever option is chosen, the device must always be installed in keeping with wiring rules.

Substituting a Type D device for a Type C device should only happen after carefully considering the installation conditions, particularly operating times.

Other Factors to Consider

When selecting circuit-breakers, it is imperative to buy from trusted manufacturers. There have been cases of imported products claiming to have a low 6kA capacity which have failed critically when tested.

It is significant that combined overcurrent and residual current circuit breakers (RCBOs) have become a common feature in commercial buildings and are increasingly being installed in residential buildings. Because RCBOs combine both RCD and MCB technology, the disconnection times to guard against electric shock can now be achieved irrespective of curve type.  

Why AFDDs?

AFDD devices protect against parallel and series arc faults that cannot be detected by the other technologies discussed, offering another level of protection. AFDDs have been recognised in the USA for years and are now mandatory in some countries, including Germany. They are recommended by International and European wiring rules.

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