FEATURE CIRCUIT PROTECTION


CORRECT SELECTION is vital for protection


Phil Williams of power management company, Eaton, provides useful guidance on the bewildering array of choices when it comes to specifying final circuit protection


he 17th Edition of the IET Wiring Regulations underlines several points that have to be considered when


T


configuring consumer units. These are: ●


Socket outlets not exceeding 20A for


general use must be provided with additional protection by means of 30mA


RCCBs - (Reg. 411.3.3). ●


All circuits in locations containing a


bath or shower including low voltage circuits must be provided with additional protection by means of 30mA RCCBs -


(Reg. 701.411.3.3). ●


All unprotected cables buried less


than 50mm deep in a wall, ceiling or floor must be provided with additional protection by means of 30mA RCCBs -


(Regs. 522.6.100-103). ●


The installation should be sub-divided


to reduce risk, inconvenience and minimise nuisance tripping etc - as


detailed in Reg. 314.1. ●


Separate circuits should be provided


for parts of the installation that need to be separately controlled to ensure that they are not affected by the failure of other circuits - (Reg. 314.2).


WAYS TO COMPLY There are a variety of methods to ensure compliance, with the fitting of every outgoing circuit with its own 30mA RCBO being the most expensive but most comprehensive option.


Above: Eaton’s Phil Williams


Right and below: the wide range of options can make choosing the best final circuit protection seem complicated. This article provides a helping hand but it is also important to refer directly to the relevant standards and regulations


the upstairs power is fed from one RCCB and the lighting is fed from another, to comply with the first four points. A composite unit with two or more RCCB protected zones, as mentioned, but with some individual ways fed directly off the main isolator switch and used to power individual circuits via an MCB or an RCBO, can be configured to provide compliance with the first four areas and with the final point where separate circuit control is required. When choosing the protection devices,


the basic options are MCBs (miniature circuit breakers), RCCBs (residual current circuit breakers) and RCBOs (residual current circuit breakers with overcurrent protection). MCBs provide overcurrent and short circuit protection only and don’t detect earth leakage. This means RCBOs are often a better option, but MCBs are still widely used. BS EN 60898 defines several types of


MCB. In consumer units, Type B MCBs are usually used, with occasional Type C devices on high inrush circuits. Type B MCBs react quickly to overloads, and are built to trip at between three and four and a half times normal full load current. They are suitable for protecting incandescent lighting and socket outlet circuits in domestic and commercial environments, where there is little risk of current surges. Type C MCBs react more slowly, and are


recommended for applications involving loads with high inrush currents, such as fluorescent lighting installations. Type C MCBs are built to trip at between five and ten times normal full load current. The breaking capacity of the MCB must


also be considered, and this must be greater than the prospective short circuit current (PSCC) at the point where the MCB is installed. Devices with 6kA breaking capacity are usually chosen for domestic applications.


The most popular choice is an isolator


controlled dual RCCB consumer unit. This allows the outgoing circuits to be divided across two separate 30mA double-pole RCCB zones ensuring that, for example,


26 MAY 2014 | ELECTRICAL ENGINEERING


AVOIDING CONFUSION RCCBs protect only against earth leakage and are also available in various different types designated by letters. 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 most common types of RCCB are Type AC, Type A and Type B. Type AC RCCBs are sensitive to AC currents and are suitable for most domestic and commercial applications. Type A RCCBs are sensitive to AC currents and to pulsating DC currents. They should be used where there are a lot of ‘electronic’ loads. Type B RCCBs are sensitive to AC, pulsating AC and steady DC leakage currents and may be needed in PV solar energy installations. RCCBs are available with various


sensitivity ratings. The IET Wiring Regulations provide detailed guidance on sensitivity, but for most domestic applications 30mA devices are used. RCBOs combine the functions of an


MCB and an RCCB in one device. It would be possible to produce a huge range of RCBOs with different combinations of MCB and RCCB characteristics, but manufacturers limit themselves to a few combinations - typically a Type B or Type C MCB characteristic combined with either a Type AC or Type A RCCB characteristic. For other combinations, separate MCBs and RCCBs must be used. The wide range of options can make


choosing the best final circuit protection seem complicated. When specifying, designing or installing protection systems, always refer directly to the relevant standards and regulations, and seek additional guidance.


Eaton www.eaton.uk.com T: 0161 655 8900


Enter 222 / ELECTRICALENGINEERING


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