To rcd or not to rcd, that is the question? Whether it is better to fit and forget, or to understand and correctly select and apply the correct device?
With apologies to William Shakespeare, these are very real questions requiring very real answers.

The use of electricity is so much a part of everyday life that it is often taken for granted. The risks associated with its use at home and work are underestimated or misunderstood. In a typical year, 19 people die from electric shock in the home and a similar number suffer the same fate in other buildings. Fire brigades are called to 10 000 incidents attributed to electrical faults every year, of which half are in the home. These domestic fires result in about 600 serious injuries and 23 deaths.

Residual current devices (rcds) are electrical products that can provide the highest degree of protection against the risks of electrocution and fire caused by earth faults. While they are not a panacea for all installation problems, it is important to understand what protection they can offer.

With different types of rcd available, their selection can be confusing. It can result in the wrong choice or application, leading to the misconception that rcds are more trouble than they are worth.

Advice on hand
Three years ago, members of the Electrical Installation Equipment Manufacturers’ Association (EIEMA), as it was known at that time, set about preparing guidance that would answer the fundamental questions about rcds and many more queries concerning the selection, application and fault finding. This work has culminated in the imminent publication of The rcd handbook – a BEAMA Installation guide to the selection and application of residual current devices.

The 60-page RCD handbook is considered to be the most comprehensive guidance on the subject ever produced. It is intended for use by specifiers, installers and end-users. The RCD handbook gives clear guidance on the selection and application of the wide range of rcds available. Information is also provided on the installation and maintenance of rcds, including many of the installation conditions that cause unwanted tripping. The publication features studies demonstrating the benefits of fitting rcds and the consequences of failing to do so.

BEAMA Installation is very aware that not everybody is an expert on rcds. The publication is therefore presented with explanations and guidance for the non-specialist and end-user as well as the experienced specifier or installer.

BEAMA Installation believes that the Handbook’s guidance will, when read in conjunction with BS 7671: Requirements for electrical installations (The IEE Wiring Regulations, 16th Edition), contribute to safe, reliable installations. Much support and constructive input was provided by the HSE, IEE, ECA and NICEIC for the Handbook.

The following extracts give an impression of the Handbook’s content and scope, from descriptions of the operation of an rcd, through electrocution risks, to the complex formulae for establishing the effects of cable capacitance on rcd performance.

Risk of electrocution
Appropriate protection against serious injury or death calls for disconnection in a fraction of a second: 40 ms or 0·04 s at 230 mA. For lower values of shock current, longer disconnection times may be acceptable, but if disconnection takes place within 40 ms, fibrillation is unlikely to occur.

There are basically two different types of electrocution risk. The first type of risk occurs if insulation, such as the non-metallic covering around cables and leads, is accidentally damaged, exposing live conductors.

If a person comes into contact with the live and earth conductors there is a more serious risk because the current flowing to earth will be insufficient to operate the fuse or circuit-breaker. This is because the human body is a poor conductor of electricity. Consequently, fuses or circuit-breakers provide no protection at all for people against contact with live conductors.

If an rcd is installed in this situation, the current leaking to earth through the body would cause an imbalance and the rcd would trip. While not preventing an electric shock, the speed of operation of the rcd will minimise the risk of electrocution.

The second principal risk of electrocution occurs when the metal enclosure of electrical equipment or any metal fixture, such as a domestic sink or plumbing system, accidentally comes into contact with a live conductor. This contact would cause the metalwork to become live.

In the UK, a fuse or a circuit-breaker normally provides protection against this risk because all exposed metalwork is connected to earth. In a correctly designed installation, the current flowing to earth will be sufficient to blow the fuse or trip the circuit-breaker.

Mains-borne transients and surges
Although the overall reliability of residual current devices is excellent, in some cases conditions can occur within an installation that can cause an rcd to trip when no apparent fault condition can be found.

This type of unwanted tripping is often incorrectly referred to as spurious or nuisance. It can be a source of considerable frustration for a contractor who attempts to trace this elusive fault.

Two main causes of unwanted tripping can be identified:

  • transient surge currents between phases or between phase and neutral within the installation;
  • a combination of supply network transient overvoltages and capacitance to earth within the installation.

Once the reasons for unwanted tripping are understood and it is realised that it is attributable to the installation conditions and not the residual current device, a methodical course of action will overcome the problem with minimum effort. This is fully explained in the RCD Handbook.

RCDs in the future
There is no doubt that residual current devices give protection against electrocution and can reduce the risk of fire arising from insulation failure in an electrical installation. This level of protection can never be equalled by circuit-breakers or fuses alone.

The effect on safety, measured by the fewer number of electrocutions and fewer fires occurring, means that rcds are here to stay and their use is likely to increase greatly.

BEAMA Installation’s RCD Handbook provides the extra knowledge necessary to ensure that the correct selection of a residual current device is made appropriate to the application. The rcd is not a solution for all installation problems, but by a better understanding of its features and benefits the risk of misapplication is minimised.

Residual current devices: The basics

What is an rcd?
A residual current device (rcd) is a product designed to provide protection against electrocution or fires by cutting off the flow of electricity automatically or actuating an alarm when it senses a leakage of electric current from a circuit. How does it operate?
With an rcd, the live (brown) and neutral (blue) conductors pass through the core of a sensitive current transformer (see figure 1, above), the output of which is electrically connected to a tripping system. In a healthy installation the current flows through the live and returns through the neutral conductor. Since these are equal and opposite, the core remains balanced. When a leakage of electric current occurs (see figure 2, above right), the live and neutral currents are no longer equal. This results in an output from the transformer, which is used to trip the rcd and disconnect the electrical supply.

Source

Electrical and Mechanical Contractor