Extraneous conductive parts

Extraneous conductive parts are the other metal parts which do not form a part of the electrical installation the structural steelwork of the building, gas, water and central heating pipes and radiators,... 

Extraneous conductive parts - the structural steelwork of a building and other service pipes such as gas, water, radiators and sinks. They do not form a part of the eiectricai installation but may introduce a potential, generally earth potential,... 

Extraneous conductive parts - the structural steelwork of a building and other service pipes such as gas, water, radiators and sinks. 

The linking together of the exposed or extraneous conductive parts of an installation for the purpose of safety is one definition of ... 

Extraneous conductive parts Faraday s law Fault protection... 

The linking together of the exposed or extraneous conductive parts of... 

Above 25 V a.c. and 60 V d.c. up to 50 V a.c. and ripple-free 120 V d.c. there is a shock risk, albeit a relatively small one in normal environments, so some additional precautions are required. Direct contact with live parts should be prevented by insulation, barriers or enclosures. No part of the circuit should be earthed, and exposed conductive parts should neither be connected to a protective conductor nor otherwise earthed. This ensures that two faults would have to occur to create a shock hazard. Again, if exposed conductive parts have to be in contact with extraneous conductive parts, the latter must not be capable of attaining a voltage exceeding that of the SELV circuit. In abnormal environments, reductions in the upper voltage limits, relative to the degree of risk, are necessary for the same level of protection. 

The essence of the concept is the prevention of direct and indirect shocks by contact between a hve part or a conductive part made live by a fault and earth. To this end, insulating walls and floors are used with the minimum amount of touchable conductive and extraneous conductive parts. Any that are used and that could become live under fault conditions have to be so spaced as to prevent anyone touching two of them at the same time. The area has to be earth-free, so no protective conductors are employed and conductive and extraneous conductive parts are not earthed. Extraneous conductive parts, such as metal pipes which are in the location and outside it, need protection by insulation, barriers or placing out of reach so that in the event of a fault they cannot transmit a potential, including an earth potential, in either direction. 

As the public supply system employs neutral earthing, the most common method of compliance with this Regulation is to connect together any earth exposed and extraneous conductive parts by means of low impedance protective and bonding conductors. This is the EEBADS technique outlined in Chapter 3. The technique creates an equipotential zone so that when an earth fault occurs these conductive parts are raised to substantially the same potential with respect to the ground (earth). This condition persists until the protective device, be it a fuse or a circuit breaker, interrupts the circuit and clears the fault. Anyone in simultaneous contact with more than one of the conductive parts should not experience a shock because the parts are at about the same potential. 

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. 

In the following text, a direct shock is from contact with a live part which is intentionally live. An indirect shock is from contact with an exposed conductive part or an extraneous conductive part made live from a fault. 

Regulations 413-02-02 and 03 refer only to metallic parts and ignore the risk from contact with other extraneous conductive parts such as walls and floors made of, for example, brick, stone, quarry tiles or concrete which are conductive particularly when damp and which are often in ground contact and may, therefore, remain at earth potential. 

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. 

The more usual description for section 606 is confined conductive locations. These are spaces where freedom of movement is restricted and the body is likely to be in contact with exposed and extraneous conductive parts. This section covers work inside boilers, metal ventilation ducts and tanks, for example, where extensive contact with the metalwork increases the indirect shock hazard. The risk is enhanced if these interiors are wet or so hot that the operator s clothes are soaked with perspiration. Incidentally, although not... 

For protection against both direct and indirect electric shock, the specified supply system is SELV, but the limit of 25 V a.c. or 60 V d.c. has been dropped. Where a functional earth is needed, for certain instruments for example, it may be utihsed provided all exposed and extraneous conductive parts are bonded together and to the protective conductor. Where there is no need for direct electric shock protection and only indirect is required, section 606-04-0 l(iii) allows mains voltage supplies to fixed equipment within the equipotential zone provided the exposed conductive parts are coimected to the extraneous conductive parts in the location. Alternatively, Class ff equipment may be used provided its enclosure is suitable for the location and it has sensitive RCD protection to trip the circuit at a residual current of 150 mA within 40 ms (or in a time of five times the residual operating current when the current is less than 30 mA). For hand-held equipment, portable tools and hand lamps, for example, there is no relaxation and they have to be supplied at SELV. 

Regulation 608-03-04 requires extraneous conductive parts to be connected to the protective conductor to establish an equipotential zone. The special requirements include the provisions of section 601 if there is a bath or shower, so any socket outlets must be not less than 2.5 m from them. The extra-low voltage (usually 12 V) circuit cables to the rear and brake lights have to be separated from the low voltage wiring see Regulation 608-06-04. 

If the installation includes any special facilities such as a test bench where it is necessary to establish and maintain a non-conducting location, the walls and floors must be made of non-conducting material. Their insulation value has to be proved by measuring the leakage current between at least three points on each relevant surface and earth. So a test bench against a wall would require a test at not less than three points on the adjacent wall and three on the floor. If there is an extraneous conductive part in the location, such as a metal window frame or service pipe, one of the test points should be not less than 1 m or more than 1.2 m from it. Check that the extraneous conductive part is separated from any exposed conductive part so that they cannot be touched simultaneously. If they can, the section of the extraneous part, within reach, has to be insulated to withstand a 2 kV a.c. test with a leakage current not exceeding 1 mA. 

The operation of the tester will cause the protective conductors. Class 1 apparatus metalwork and bonded extraneous conductive parts to become live with respect to the ground at a voltage dependent on the earth loop impedance, so during the tests no one should touch such metalwork. 

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