Electricity 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. 

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

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. 

Electrical installations, whether temporary or permanent, together with electrical equipment and apparatus, should be protected by a range of measures including residual current devices (RCDs) which protect human beings from electrical shock, fuses and miniature moulded case circuit breakers (MCCBs) which provide protection to apparatus from overheating and short circuits. Arrangements should be in place to ensure that all electrical work to installations and/or electrical equipment is undertaken by only trained, qualified and competent electricians who are familiar with the systems involved. 

Safety management responsibilities include definition of site temporary electrical requirements and loads, including the arrangements for design, testing and installation of circuits and their protection by fuses and by residual current devices or their equivalent. 

Electrical supplies should be protected by 30mA/30ms residual current devices. 

The electricity supply company has one of its conductors solidly connected to the earth and every circuit supplied by the company must have one of its conductors connected to earth. This means that if there is a fault, such as a break in the circuit, the current, known as the earth fault current, will return directly to earth, which forms the circuit of least resistance, thus maintaining the supply circuit. This process is known as earthing. Other devices, such as fuses and residual currertt devices, which will be described later, will also be needed within the circuit to interrupt the current flow to earth so as to protect people from electric shock and equipment from overheating. Good and effective earthing is absolutely essential and must be connected and checked by a competent person. Where a direct contact with earth is not possible, for example, in a motor car, a common voltage reference point is used, such as the vehicle chassis. 

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