Residual current circuit breakers

More commonly known as earth leakage circuit breakers (ELCBs) or residual current circuit breakers (RCCBs). They operate on the principle of residual current. 

RCBOs (residual current circuit breakers with overload protection) combine RCD protection and MCB protection into one unit. 

Touring caravans must be supplied from a 16A industrial-type socket outlet adjacent to the caravan park pitch, similar to that shown in Fig. 3.75. Each socket outlet must be provided with individual over-current protection and an individual residual current circuit-breaker with a rated tripping current of 30 mA (lET Regulations 708.553.1.12 and 708.553.1.13). The distance between the caravan connector and the site socket outlet must not be more than 20 m (lET Regulation 708.530.3). These requirements are shown in Fig. 3.77. 

If it is not possible to establish an equipotential zone because, for example, there is a conducting floor (such as a concrete floor) or because equipment is being used outdoors, supplementary or alternative measures must be taken. The use of residual current circuit breakers to provide sensitive earth leakage protection, in addition to the overcurrent protection, is one acceptable option the RCD would detect earth fault currents, including shock currents flowing to earth, and rapidly interrupt the circuit. Indeed, it is general practice to ensure that socket outlets that will foreseeably be used to supply external equipment should have RCD-protection fitted. 

Portable and transportable apparatus should be excluded from the hazardous areas, as far as possible, because the eonditions of use make it, and particularly its flexible cable, a greater ignition and accident risk than fixed apparatus. When it is used, it should be frequently inspected, tested and maintained to avoid trouble. The flexible cable should be of the screened type. The connection to the supply should be by means of an EEx plug and socket, interlocked with the supply switch, to ensure that the plug can only be inserted or withdrawn when the circuit is dead. It is also recommended that the circuit is protected by a residual current circuit breaker or circulating current earth monitoring protection or both. A sound approach is to ensure, via suitable management procedures, that such apparatus is only used in a hazardous area after the area has been proven by measurement to be gas-free - work would then usually be carried out under a permit to work issued by the process specialist who certifies that the area is gas-free. 

Residual current operated circuit breakers, without integral overcurrent protection. General rules ... 

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. 

A residual current operated circuit-breaker with integral overcument protection (RCBO) provides protection against overload and/or short-circuit. RCBOs give the combined protection of an MCB and an RCD in one device. 

Where the RCD has a rated tripping current not exceeding 30 mA and has been installed to reduce the risk associated with basic and or fault protection as indicated in Regulation 411.1, a residual current of 150mA should cause the circuit breaker to open within 40 ms. 

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. 

British Standards Institution, BS EN 61008-1, Residual current operated circuit-breakers without integral overcurrent protection for household and similar uses (RCCBs). Part 1, General rules. BSI, London (1995)... 

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. 

Low-voltage switchgears include miniature circuit breakers, residual current devices, fuses, switches, and relays and contactors. 

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