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RCDs and MCBs

A Residual Current Device (RCD) is a device like a switch, which monitors the current flowing between live and neutral. If it detects a difference of between 20 and 30 milliamps (20/30 thousandths of an amp) it determines that this current is going to earth somewhere (possibly through you!) and it will disconnect the circuit or circuits being monitored. It cannot be stressed enough that this device can save lives either directly through electrocution, or indirectly from fire.

A Miniature Circuit Breaker (MCB) is the modern equivalent of a rewirable fuse, only much more efficient, switching off an overloaded circuit usually within 100 milliseconds. If a fault occurs causing this to happen, once the fault is rectified the MCB is simply switched back on. No more fiddly fuse wire!

From 1st July 2008, BS7671 (17th edition) came into force and now all circuits that have cables in walls that are less than 50mm deep and are not mechanically protected, must be RCD protected. As it is contrary to BS7671 to have an RCD protecting the whole installation, this means that all new consumer units will need at least two RCDs. All circuits in bath/ shower rooms must also be RCD protected, regardless of depth of cables in walls.

Installations that have any circuits that are not RCD protected (i.e. smoke detectors/ security alarms) and are supplied via overhead cables (and therefore need an earth rod) must also have a 100mA time delayed RCD as the main switch on the consumer unit.

RCBOs are MCBs and RCDs combined into one breaker. Fitting these ensures that just that particular circuit trips if an earth fault occurs and so does not interfere with any other circuit. But like all good ideas, they cost more. Installations that may have circuits that are not RCD protected (i.e. smoke detectors/ security alarms) and are supplied via overhead cables (and therefore need an earth rod) must also have a 100mA time delayed RCD as the main switch on the consumer unit. From 1st January 2016 it became mandatory for all new or replacement domestic consumer units to be non-combustible (i.e. metal) or enclosed in a metal cabinet. Existing plastic consumer units can be retained for alterations or additions unless they show signs of thermal damage.

SPDs & AFDDs

Most domestic properties now have varied electronic equipment, such as smart TVs, computers, etc., that are sensitive to voltage power surges. If a lightning strike hits a local substation, it can cause a surge, or transient along the power cables to the local houses, damaging this equipment. From January 2019, BS7671 recommends that all new installations are fitted with Surge Protection Devices (SPDs). Although this is only a recommendation, if they are not fitted, the fact that the customer declined must be noted on any certificate issued.

Arc Fault Detection Devices (AFDDs) can be installed as a means of providing additional protection against fire caused by arc faults. These faults can occur when there is loose connection in a terminal. The 18th Edition of BS7671 recommends that these are fitted, but it is not mandatory.

For more information contact Brian Coleman on:
Mobile: 07956 510307
Tel/fax: 01322 613185

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Evergreen Electrical Ltd is an NICEIC approved contractor, which means that you have the security of knowing that all our work is to the highest standards, as laid down in BS7671: 2018.  Not only that, All our work is guaranteed for Ten Years! (not insurance backed)
Registered Office: 48 St. Mary’s Road, Swanley, Kent, BR8 7BU
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