Supplementary protective bonding

Supplementary protective bonding to pipework will only be required as an addition to fault protection (lET Regulation 415.2) if the immersion heater vessel is in a bathroom that does not have ... 

PVC cables enclosed in heavy-duty PVC conduit are suitable for installations in most agricultural buildings. All exposed and extraneous metalwork must be provided with supplementary protective bonding in areas where livestock is kept (lET Regulation 705.415.2.1). In many situations, waterproof socket outlets to BS 196 must be installed. All socket outlet circuits must be protected by an RCD complying with the appropriate British Standard and the operating current must not exceed 30 mA. 

Note Local supplementary protective bonding may be an additional requirement of the Local Authority regulations in, for example, licensed premises, student accommodation and rented property. 

The lET Regulations describe the need to consider additional protection by supplementary protective bonding in situations where there is a high risk of electric shock (e.g. in kitchens and bathrooms) (lET Regulation 415.2). 

In rooms containing a fixed bath or shower, supplementary protective bonding conductors must be installed to reduce to a minimum the risk of an electric shock unless the following two conditions are met ... 

Supplementary protective bonding conductors in domestic premises will normally be of 4mm copper with PVC insulation to comply with lET Regulation 543.1.1 and must be connected between all exposed metalwork (e.g. between metal baths, bath and sink taps, shower fittings, metal waste pipes and radiators), as shown in Fig. 3.79. 

Figure 3.79 Supplementary protective bonding in bathrooms to metal pipework.

Testing for continuity of protective conductors, including main and supplementary protective bonding (Regulation 612.2.1)... 

There are also provisions in section 601-04 relating to supplementary equipotential bonding. This requires the terminals of protective conductors associated with Class I and Class II equipment in Zones 1, 2 and 3 to be connected together and for them to be connected to extraneous conductive parts on those zones. The latter parts are listed as including metallic items such as service pipes (gas, water and so on), waste pipes, central heating pipes, air conditioning duct work, accessible structural parts of buildings, and baths and shower basins. 

If the earth loop impedance is too high for overcurrent protection to operate within the prescribed time, Regulation 413-02-04(i) permits this time to be exceeded provided local supplementary equipotential bonding is used to meet the requirements of Regulations 413-02-27 and 28 and the fault persistence does not cause damage. Again, the intention is to minimise the potential differences between touchable metalwork items on the occurrence of an earth fault. As an alternative. Regulation 413-02-04(ii) allows the use of RCD protection. 

Test the continuity of the protective conductors including the main protective bonding conductors and supplementary bonding conductors. 

The object of the test is to ensure that the CPC is correctly connected, is electrically sound and has a total resistance which is low enough to permit the overcurrent protective device to operate within the disconnection time requirements of Regulation 411.4.6, should an earth fault occur. Every protective conductor must be separately tested from the consumer s main protective earthing terminal to verify that it is electrically soimd and correctly connected, including the protective equipotential bonding conductors and supplementary bonding conductors. 

Where other potential metallic conductors exist near to electrical conductors in a building, they must be connected to the main earth terminal to ensure equipo-tential bonding of all conductors to earth. This applies to gas, water and central heating pipes and other devices such as lightning protection systems. Supplementary bonding is required in bathrooms and kitchens where, for example, metal sinks and other metallic equipment surfaces are present. This involves the connection of a conductor from the sink to a water supply pipe which has been earthed by equipotential bonding. There have been several fatalities due to electric shocks from live service pipes or kitchen sinks. 

Regulation 551-04-04 addresses protection against indirect contact for static inverters, typically used for uninterruptable power supplies in installations where continuity of supply is crucial. Where the disconnection times of section 413-02 cannot be achieved, supplementary bonding must be used to minimise the risk of a shock between exposed metalwork. A warning is provided in Regulation 551-04-05 about the possible deleterious effects on the operation of protective devices, such as circuit breakers, of direct current generated by the static inverter or filters. 

All exposed and extraneous conductive parts accessible to livestock have to be bonded together by supplementary equipotential conductors. Where there are metal grids in conductive floors, they have to be bonded to the protective conductor. Fire precautions (subsection 605-10) call for heaters to be separated from livestock and combustible material, and for RCD protection operating at not more than 500 mA except where supplies are essential for animal welfare. 

Protection against overcurrent and electric shock is in section 3. It compares the performances of semi-enclosed and cartridge fuses and MCBs for both overcurrent and short circuit protection. It deals with RCDs for earth leakage and electric shock protection. It concludes with Tables 10 and 11 for the sizes of main earthing, main bonding and supplementary bonding conductors. 

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