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Voltimum UK Managing Editor James Hunt briefly introduces Residual Current Devices (RCDs), together with a few of the issues:
Moreover, even non-fatal shocks can cause severe and permanent injury, and shocks from faulty equipment can lead to falls from ladders or work platforms, causing injury or death indirectly. Not only that, people using electricity may not be the only ones at risk, as poor electrical installations and faulty appliances can lead to fires which can also cause death or injury to others. Think safety and the proficient electrical contractor automatically thinks of RCDs. These devices have been around for a long time now, and are designed to protect people from the effects of electric shock.
Current-operated RCDs protect against electrocution by detecting current flowing to earth. Normally, any earth current should be very low, so RCDs can be made very sensitive - typically 30mA. RCDs must also trip very fast, unless of the time delayed type. This sensitivity also gives it the ability to detect other faults such as partial live/earth breakdown, which may cause a fire. RCDs are limited to these functions, as they cannot protect against live-neutral shock or overloads. For this reason, RCDs are often used in conjunction with MCBs or fuses.
The correct choice of an RCD, and its proper installation and regular testing greatly reduces the risks.
It's not just in the home either. There have, for example, been a number of cases where entertainers have been injured or even killed while using faulty electrical equipment. The high profile case some years ago of Elizabeth Handy's son, killed whilst playing his electric guitar, comes to mind. A RCD would, almost certainly, have saved him.
BS 7671 defines an RCD as 'a mechanical switching device or association of devices intended to cause the opening of the contacts when the residual current attains a given value under specified conditions'. As such, an RCD is a protective device used to automatically disconnect the electrical supply when an imbalance is detected between live conductors. There are a number of different types of residual current device for various applications, and for more information on these, and on how to select the correct device for any given job, please refer to the various attachments that are part of this VoltiBULLETIN newsletter.
However, it is worth mentioning one type of residual current device that, although it has been around for some time, still finds new applications - and opportunities for properly accredited electrical contractors and installers. This is the combined miniature circuit-breaker (MCB) and residual current device (RCBO). Eaton was early with RCBOs, and its 110V RCBOs use the MEM RCD 'pod' system, thought still to be unique. This can be used to protect 110V building sites and the like. Eaton commented a few years ago that demand for 110V RCBOs will increase as more and more authorities, conscious of their duty of care, look for something more than the basic protection offered by a 110V centre-tapped power supply.
This turns out to be correct, though - in fact - use of both RCDs and RCBOs has increased significantly because of the 17th Edition of the Wiring Regulations. Introduced last year, the important part from an RCD point of view concerns socket-outlets in the domestic environment. Additional protection is now required for all socket-outlets - for automatic supply disconnection. Specifically, in AC systems, such additional protection is to be provided by using an RCD (I?n not exceeding 30mA). Such RCDs will be provided for the following:
- Socket outlets not exceeding 20A rating for use by ordinary persons and intended for general use, and mobile equipment not exceeding 32A rating for use outdoors.
- Mobile equipment not exceeding 32A rating for use outdoors.
Therefore, essentially, every domestic room now requires RCD protection for each socket, though there is debate about the best way of achieving the required protection. The only exceptions are socket-outlets for use under the supervision of skilled or instructed persons (in certain commercial / industrial locations), or suitably identified, socket-outlets provided for particular items of equipment. Another exception is deep freezers where nuisance tripping could ruin foodstuffs.
Testing requirements of RCDs are unchanged. Note that the 17th Edition (Chapter 41) has a new Table 41.5 for RCDs, and that for Tables 41.2 to 41.4, the previous 240V is now 230V.
Safety in RCD testing
Testing an RCD is very important. For a start, every RCD has its own test button, which should be pressed and reset regularly. However, more fundamental testing, after installation especially, but also later on, is crucial. Electrical contractors and installer now have a battery of modern test tools to help them out.
Even if they are inadvertently connected to a live three-phase supply, today's RCD testers should remain undamaged, and present no safety hazard to users. All such test instruments should now comply fully with the requirements of IEC61 01 0-1 for Category III 300V applications.
Test instrument manufacturers can usually provide a range of RCD testers, so ensuring that users can choose a model that exactly meets their needs.
Low-cost entry-level models may typically be suitable for use on supplies between 100V and 280 - 300V with various tests covered, including DC tests. It should also be possible to test standard and selective RCDs, from 30mA to 500mA.
More expensive models may cover a wider supply voltage range, allowing use with centre-tapped 110V supplies, ideal for building site use. Ramp testing for RCD tripping current measurement is often included. One manufacturer, at least, includes an auto sequence test feature, which saves time by allowing the tester to conduct all the RCD tests, and store the results, while the operator stands by the RCD, resetting it as appropriate. The ability to measure AC supply voltage and frequency may be included too. Typical operating current ranges cover l0 to I,000mA devices. Top of the range models now often feature facilities for testing of programmable RCDs.
Increasingly, test result storage, plus downloading via an integral USB port, is becoming a normal facility.
Such RCD testers today will be pretty compact, and be easy to use and handle. The bodies will usually be made from a tough impact-resistant plastic, often with a rubber overmoulding for extra protection. All models should be water and dust resistant, with an IP54 ingress protection rating at least. Displays will, more often than not, be LCDs, but - ideally - these should be backlit to ensure reasonably easy use where light levels are low.
When connected to a live supply, RCD test instruments are often designed to default to voltmeter mode.
More on RCDs:
To read far more about RCDs, their modes of operation, selection, installation and testing, please use the other downloads in this VoltiBULLETIN:
ECA - The 17th Edition and RCDs - The ECA has produced a useful Technical Fact Sheet entitled 'The 17th Edition and RCDs'.
www.voltimum.co.uk/news/11939/consult.experts_hottopics.rcds/ECA---The-17th-Edition-and-RCDs.html
Electrical safety in the home - Increasing electrical safety in the home has been a prominent theme of late, due in part to the Building Regulations, and the revisions to the IEE Wiring Regulations. Alan Roadway of ABB discusses how electrical installers can help ensure safe electrical practise.
www.voltimum.co.uk/news/11927/consult.experts_hottopics.rcds/Electrical-safety-in-the-home.html
Arranging RCDs and RCBOs in consumer units to meet 17th Edition requirements - Phil Williams, Product Manager - Consumer Unit Products at Eaton, examines consumer unit arrangements to meet the 17th Edition Wiring Regulations.
Why choose earth leakage protection? - Two useful PDF documents by Schneider Electric examine the reasons for using RCDs in the first place, and also look in detail at many aspects of operation, such as discrimination.
RCD, and other relevant definitions, taken from the EIEMA RCD Handbook - The BEAMA RCD Handbook - entitled the 'EIEMA Guide to the Selection and Application of Residual Current Devices' - provides some useful definitions for RCDs and related equipment.
Residual current protection - special devices - With the imminent introduction of the 17th Edition of the IET Wiring Regulations, more emphasis than ever will be placed on residual current protection. This is Schneider Electric's brief take on the subject, concentrating on a special form of RCD.
Special locations and the 17th Edition - The 17th Edition of the IEE Wiring Regulations gives particular attention to protection in so-called 'Special Locations'. These include, for example, swimming pools, marinas and other basins, as well as caravan and camping parks. In general, the requirements for these areas dictate the almost universal provision of residual current protection, typically in the form of an residual current circuit breaker (RCCB), also known as a residual current device (RCD).
Testing RCDs and meeting the standards - Residual Current Operated Devices (RCDs) are fitted in electrical installations to provide additional protection. An RCD detects fault currents flowing to earth that are too small to trip over-current protection devices (such as fuses), but are still sufficient to cause a dangerous electric shock or an electrical fire. Verifying their operation is critical for safety.
Multifunction installation testers are ideal for RCD testing - Fluke's 1650B Series of multifunction installation testers test and verify domestic, commercial and industrial electrical installations - the 1651B, 1652B and 1653B models.
SiE earth leakage protection in aggressive environments - Protection of people and equipment and guaranteed optimum safety at all times is one of the main specialities of Schneider Electric and has been so for many years. The earth leakage protection function has thus continually evolved with an aim to improving not only its inherent capacities but also that of its adaptation to all kinds of environmental constraints. The SiE is the outcome: