Residual current devices

In the UK, the electricity supply is connected to earth. It is this system that enables earth faults on electrical equipment to be detected and the electrical supply to be cut off automatically. This automatic cut-off is performed by fuses or automatic circuit breakers if a fault occurs the fuse will blow and break the circuit. Although they do not eliminate the risk of electric shock, danger may be reduced by the use of a residual current device (RCD) designed to operate rapidly at small leakage currents. RCDs should only be considered as providing a second line of defence. It is essential to regularly operate the test trip button to maintain their effectiveness. 

To prevent people from receiving an electric shock accidentally, all circuits contain protective devices. All exposed metal is earthed fuses and miniature circuit-breakers (MCBs) are designed to trip under fault conditions, and residual current devices (RCDs) are designed to trip below the fatal level as described in Chapter 4. 

This is the type of supply more often found when the installation is fed from overhead cables. The supply authorities do not provide an earth terminal and the installation s circuit protective conductors (CPCs) must be connected to earth via an earth electrode provided by the consumer. lET Regulation 542.2.3 lists the type of earth rod, earth plate or earth tapes recognized by BS 7671. An effective earth connection is sometimes difficult to obtain and in most cases a residual current device (RCD) is provided when this type of supply is used. The arrangement is shown in Fig. 3.36. 

Functional test - Carry out additional protection testing (e.g. operation of residual current devices, RCDs). 

X- 110-volt centre tapped to earth supply X- residual current device (ROD) in 240-volt supply X- means to discharge static electricity charges... 

Additional back-up protection can be provided by residual current devices that ensure that in the event of an earth fault the current is cut off before a fatal shock is received. This form of protection works on the principle of monitoring any differential between (i) the current entering a circuit to supply power to the portable apparatus and (ii) the current returning to the supply point. For normal safe operation this current differential is zero but if there is a fault, such as leakage to earth, a differential current occurs which the device rapidly senses, tripping its contacts to cut off the supply to the apparatus. 

All electrical appliances to be connected via a residual current device (RCD). 

Electrical hazards are covered in detail in Chapter 14, and all the control measures mentioned apply on a construction site. However, due to the possibility of wet conditions, it is recommended that only 110 V equipment is used on site. If mains electricity is used (perhaps during the final fitting out of the building), residual current devices should be used with all electrical equipment. Where workers or tall vehicles are working near or under overhead power lines, either the power should be turned off or goal posts or taped markers used to prevent contact with the lines. Similarly, underground supply lines should be located and marked before digging takes place. 

If electrical equipment must operate at mains voltage, the best form of protection against electric shock is the residual current device (RCD). RCDs, also known as earth leakage circuit breakers, monitor and compare the current flowing in the live and neutral conductors supplying the protected equipment. Such devices are very sensitive to differences of current between the live and neutral power lines and will cut the supply to the equipment in a very short period of time when a difference of only a few milliamps occurs. It is the speed of the reaction which offers the protection against electric shock. 

The majority of electric shock injuries occur when the body acts as conductor between line and earth. A general level of protection against such shocks is provided by the inclusion of a current sensitive earth leakage circuit breaker in the supply line. A typical example is shown in Figure 4.4.2. Residual current devices (RCDs) are discussed further in section 4.4.15. 

Additional back-up protection can be provided by residual current devices that ensure tirat in the event of an earth fault the current is cut off... 

British Standards Institution, BS 7071, Specification for portable residual current devices, BSI, London (1998)... 

Protection for nominal 240 V supplies can be obtained by the use of residual current devices (RCDs) either in the supply circuit or on the connection to the particular equipment. In addition, on construction sites nominal 240 V supplies should be carried by armoured, metal sheathed or other suitably protected cables. 

Reactive airways dysfxmction Residual current device... 

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