Getting to know RCDs
As the industry awaits the latest changes to the British standards for electrical installations (BS 7671) Alan Roadway, Product Manager for ABB’s Low Voltage Distribution Products business, explores the new mandatory use of Residual Current Devices (RCDs) and the consequent effects expected within the industry.
RCDs in all their different forms have been around for many years. While professionals in the industry are aware of their capabilities, there is still much confusion surrounding the different terminology used, the types of RCDs available and knowing exactly where and how the technology should be installed. The 17th edition of the IEE Wiring Regulations requires electricians to consider the use of RCDs in a wider variety of new applications, than previous standards have required to ensure building protection.
The basic function of an RCD is to trip and disconnect the supply of electricity in the event of an earth fault. It
does this by detecting the imbalance of currents between the live and neutral lines caused by a fault to earth. Drawing attention to the importance of using RCDs in the right context and with the right equipment, the IEE Wiring Regulations will provide guidance on how RCDs should be incorporated into new builds.
One example is the requirement for cables concealed in a wall/partition at a depth of less than 50mm. Cables concealed in walls in domestic installations at a depth less than 50mm will need to be protected by an RCD rated at 30mA or below even if they are in a safe zone. This has implications for lighting circuits, which under the current 16th edition of the wiring regulations have no requirements for RCD protection.
The only exception to this rule is if the cables are enclosed in an earthed metallic covering/conduit or equivalent protection capable of resisting nails, screws or any sharp objects. This is also the case for
cables installed in metal-framed walls – a modern and popular construction technique.
However, the technology is now available in several forms, including:
....Portable adapters
....RCDs incorporated in socket outlets or plugs
....A stand alone unit for fixed wiring installations
....A modular unit for DIN rail mounting, for example, in household consumer units
....A modular unit combined with RCD/Miniature Circuit Breaker (MCB) units.
In general, RCDs are not designed to break the high short circuit currents, which may occur in household wiring installations and must be used with an over-current protection device such as a fuse or MCB in the installation. If you are unsure on what the best device is for your household, the simple answer is to
consult a professional.
Many of the changes to the current wiring regulations are part of a harmonisation process to create commonality of installation standards throughout Europe and the rest of the world. While the 17th edition of the IEE Wiring Regulations will see an increase in
the use of RCDs, it is unlikely that the
products themselves will change as they
are designed and manufactured to the
‘normalised’ European and worldwide
standards.
With more regulations to now
consider, the IEE Wiring regulations
encourage communications across the
industries and enable a more harmonised
approach towards electrical protection in
buildings. Now, builders, electricians,
designers, manufacturers and any other
parties involved in building design or
construction will all need to be able to
discuss the practices required to meet the
new regulations and ensure greater health
and safety for any building’s inhabitants.
Within the revised regulations, all socket outlets rated at 20A or below within a domestic building will
require 30mA RCD protection. This means that all ring main circuits from consumer units will either need to be fed from the RCD side of a traditional split load board or have individual RCBOs (Residual Current Breakers with overload protection). An RCBO combines the functions of an MCB (Miniature Circuit Breaker) and an RCD in one unit. They are used to protect a particular circuit, instead of having a single RCD for the
whole building. Because the RCD is sensitive to current imbalances, it is not practical to fit just one device to protect an entire house. If a fault develops within one circuit, all circuits would be switched off, losing all power immediately. By using circuits with individual RCD protection as well as groups of circuits protected by one RCD in combination with one another, the required protection can be provided while maintaining continuity of supply to other non-affected zones within the installation. This does mean that for architects,
builders and electricians much more detail must be put into the design and implementation of any new building supply. Now utility rooms and storage areas will require specific design and consideration based on the devices installed. Bathrooms are under particular scrutiny, particularly with the increased popularity of en-suites. The new regulations will require that RCD protection be implemented for all circuits supplying electrical equipment within Zones 0, 1 and 2 (Zone 3 is being removed). In addition, all cables buried in
walls surrounding bathrooms must have 30mA RCD protection – regardless of the points they are supplying. This could have significant implications on the routing of cables to avoid crossing these zones within ceiling voids and adjoining walls.
Educating the Consumer
However, it is not just the industry that needs to adapt to the new IEE wiring regulations. Consumers and
homeowners must be educated about the technology, as there is a limited knowledge on the operation of RCDs and the additional protection that they can provide for any property. By encouraging a higher level of
understanding of the devices, RCDs could help reduce the accident statistics and improve everyday protection against electrical faults.
Control and monitoring are also increasingly important. The most advanced RCDs on the market are capable of using add-on accessories allowing the supply of electricity to be disconnected remotely. Whilst this may be of little use to residential properties, for commercial properties this provides a simple yet effective tool in reducing potential hazards.
RCDs are recognised by the Department for Trade and Industry (DTI) as a valuable contribution to home safety. In the DTI October 1997 consumer research guide to RCDs, it was estimated that electrical currents caused over 200 hospital-related injuries and 25 deaths each year. Combine this with the 5,000 fires that
are started due to electrical fault resulting in 20 deaths and 500 casualties and the new regulations
demonstrate how manufacturers, wholesalers and electricians can achieve the full potential of RCDs by providing protection against death by electrocution and in reducing the incidences of fires.
RCDs – the range available
Many people think the term RCD applies to just one device, when actually the term refers to a range of devices with different functions and capabilities. Homeowners need to consider the hazards and the protection required before deciding on the technology to invest in. There are currently two main types of RCDs:
....RCCBs – Residual Current Circuit Breakers – which provide earth fault protection only
....RCBOs – Residual Current Circuit Breakers with Overload protection, which provides earth fault, overload
and short-circuit protection.
However, the technology is now available in several forms, including:
....Portable adapters
....RCDs incorporated in socket outlets or plugs
....A stand alone unit for fixed wiring installations
....A modular unit for DIN rail mounting, for example, in household consumer units
....A modular unit combined with RCD/Miniature Circuit Breaker (MCB) units.
In general, RCDs are not designed to break the high short circuit currents, which may occur in household wiring installations and must be used with an over-current protection device such as a fuse or MCB in the installation. If you are unsure on what the best device is for your household, the simple answer is to consult a professional.
Many of the changes to the current wiring regulations are part of a harmonisation process to create
commonality of installation standards throughout Europe and the rest of the world. While the 17th edition of the IEE Wiring Regulations will see an increase in the use of RCDs, it is unlikely that the products themselves will change as they are designed and manufactured to the ‘normalised’ European and worldwide standards.
With more regulations to now consider, the IEE Wiring regulations encourage communications across the
industries and enable a more harmonised approach towards electrical protection in buildings. Now, builders, electricians, designers, manufacturers and any other parties involved in building design or
construction will all need to be able to discuss the practices required to meet the new regulations and ensure greater health and safety for any building’s inhabitants.
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