Technology continues to evolve in the field of LV power distribution and protection. 30 years ago the specifier/installer was typically faced with the decision on what size fuse wire to use. Now they must consider which MCB or RCBO to install, with even more choice than ever on what levels of protection against the effects of electrical fault to afford.

Due for release in the summer of 2018, BS 7671: Requirements for Electrical Installations (IET Wiring regulations) 18th edition will reference 4 potential types of protective device:

In this particular article, we will look at the selection of MCBs and RCBOs and points to consider in this process.

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The decision to use Type B, C or D MCBs or RCBOs for final circuit protection in residential, commercial or industrial buildings can be based on a few simple rules.

However, an understanding of the differences between these types of device can help the specifier or installer overcome problems of unwanted tripping, disconnections times for earth fault protection or issues associated with discrimination with upstream protective devices.

The primary purpose of circuit protection devices such as MCBs is to protect the cable downstream of the device. The first requirement, therefore, is to select a device in accordance with the latest revision of BS 7671:Requirements for Electrical Installations.

The essential distinction between Type B, C or D devices is based on their ability to handle surge currents without tripping. These are, typically, inrush currents associated with reactive loads such as lighting, or loads containing motors or battery charging equipment.

The classification of Types B, C or D is based on the fault current rating at which instantaneous operation occurs (typically less than 100ms) to protect against short-circuits. It is important that equipment having high inrush currents should not cause the circuit-breaker to trip unnecessarily, and yet the device should trip in the event of a short-circuit current that could damage the circuit cables.

Normal cable ratings relate to continuous service under specified installation conditions. Cables will, of course, carry higher currents for a short time without suffering permanent damage. 

As well as protecting cables against the effects of overloads and short circuits, MCBs can also be legitimately used to provide protection against earth faults and protect against the effects of electric shock, on both fixed and portable equipment. However, the earthing arrangement and total earth loop impedance (Zs) value of the circuit will dictate whether an MCB is able to provide a suitable disconnection time.

As well as natural inrush currents, sometimes failure of lamps/components can trip Type B circuit-breakers in domestic and retail environments. This is caused by high arcing currents occurring at the time of failure.

A Type C device may be substituted for a Type B device where unwanted tripping persists, especially in commercial applications. Alternatively it may be possible to use a higher rated Type B MCB, say 10A rather than 6A. Whichever solution is adopted, the installation must be in accordance with BS 7671.

A change from Type C to Type D devices should only be taken after careful consideration of the installation conditions, in particular the operating times required by BS7671.

The importance of selecting circuit-breakers from reputable manufacturers cannot be over-emphasised. Some imported products, claiming to have a lower (typically residential) 6kA short-circuit capacity, have failed dramatically under test.

Combined overcurrent and residual current circuit breakers (RCBOs) are now commonplace in commercial buildings and growing in popularity in residential. RCBOs combine MCB and RCD technology, meaning disconnection times to protect against the effects of electric shock can typically be achieved regardless of curve type.

And finally, new for BS 7671:Requirements for Electrical Installations 18th Edition are AFDDs (Arc Fault Detection Devices). International and European wiring rules recommend the use of AFDDs for protection against arc faults, with some countries making their use mandatory. Recognised in the USA for many years and mandatory in German installation standards: AFDD devices have the potential to take UK installation safety to an even higher level, providing protection against series and parallel arcing faults which are not detectable by any of the technologies previously mentioned.

All electricians are familiar with them, but do they know all the details? ABB’s Florian Krackhecke takes a brief excursion into the wider world of miniature circuit breakers (MCBs).

Eaton’s Pedro de la Horra Calomarde reflects on dangers associated with arc faults and considers how installers can help safeguard businesses and homes against fire.

John Robb, Segment Leader Commercial Buildings EMEA, Eaton considers the threat of arc fault incidents to commercial buildings and the methods of protection that can be implemented to reduce the risk.

in my experience when i did projects on new ships i found that most leccies had no idea about curcuit breakers time after time i came across new build and modern ships with instantaneous trip breakers feeding fans and motors and no amount of explaining to them that the reason for tripping was wrong rating i got opposition and argument one outfit even got another electrician to vet my statements from two hundred miles away.

I,m all for safety but this could be a real headache when upgrading a new consumer unit in an already tight physical space. If say the new AFDD devices are fitted to each circuit any new consumer unit will double in size.

I can see UK boards following the direction of some European ones where both live and neutral are controlled by a (double) MCB and are in 2 banks, each RCD controlled and 1 above another rather than our side by side.

AFDDs will provide an extra area of safety on older existing networks where there may be excessive use on the system on older cables.

I’m all for protecting people and property / premieres, what is the cost of these devices?, I know money versus lift is no comparison, but there is still a cost implication.

As a large user of Eaton RCDs ,RCBO,s and 1Pole+N MCBs ,in our agricultural fan panels the additional fire protection from these new devices I am sure would be welcomed by our clients insurers , however I note that the lowest current device is 10A whilst we are using 4A and 6A MCBs for our fan circuits

The correct selection of any protective device is paramount but this only comes with correct design and consideration to the connected load. AFDD will play it’s part but there are other devices like the PIP. But with high quality installation and good design what are these protecting the fixed installation or poor quality, badly maintained appliances.

Great video guys and a great help for explaining it to students. One gripe tho, please don’t call an Overload an Overcurrent

It was informative and appreciate that, but as stated by TOMM 986 size may be the issue with consumer units possibly not yet designed to house the AFDD width, and having just bought a new board, I will price some up and see if its practical. come on EATON give us all 50% off

Very interesting and informative however, space may be at a premium and persuading clients that the additional cost is worthwhile may not be easy. That doesn’t mean we shouldn’t try just that it may be an uphill struggle.

Please publish all test results on these items ?. How tested, where, in which country ?. Were they tested by third party assessed test house ?. What is the reliability figures for these devices ?. Are they going to be produced in stand alone versions ?.

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If you find tracking down a trip fault can be time consuming wait until you’re looking for an arc induced trip on an AFDD ;-)

I realise the comments above are from mid 2018, but now that AFDD’s are, I am reliably informed, on the verge of becoming a requirement of BS7671: 2018 (I’m told they are likely to be the subject of the next amendment), it’s worth pointing out that they are projected to cost circa £200.00 each! Doubtless discounts may become available, but try selling that to your average client.

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