Exposed metalwork

An electrical earth system comprises the provision on the supply system of an earth connection to facilitate earth fault current flow, the connecting of all exposed metalwork within the installation to a common grounding terminal and the connection of this terminal to earth. For a complete installation the principal factors which need to be considered, are ... 

Earth bonding tests apply a substantial test current, typically about 25 A, down the earth pin of the plug top to an earth probe, which should be connected to any exposed metalwork on the portable appliance being tested. The PAT Tester will then calculate the resistance of the earth bond and either give an actual reading or indicate pass or fail. A satisfactory result for this test would typically be a reading of less than 0.1 n. The earth bond test is, of course, not required for double-insulated portable appliances because there will be no earthed metalwork. 

The second element of this protection method is the provision of a means of automatic disconnection of the supply in the event of a fault occurring that causes the exposed metalwork to become live. 

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. 

It should be noted that in this type of situation the earth fault will probably not be apparent until an electric shock is experienced. This is because the appliance will operate as normal, despite the fact that the exposed metalwork, which should be at earth potential, has become live at mains voltage. 

Ensure that exposed metalwork in supplier s works that is not acting as a phase conduetor is earthed, subject to minor exclusions set out in Regulation 8(2). 

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. 

Epidemiological studies relevant to the evaluation of di- and triethanolamine and jV-nitrosodiethanolamine involve occupational cohorts exposed to metalworking fluids. Metalworking fluids can be divided into four broad categories straight oils, soluble oils, semi-synthetic and synthetic fluids. 

According to the 1981-83 National Occupational Exposure Survey (NOES, 1999), as many as 800 000 workers (many of whom were metalworkers) in the United States were potentially exposed to diethanolamine (see General Remarks). 

Tolbert et al. (1992) USA Cohort 33 619 (two of the three facilities in Eisen et al., 1992) 1941-84 Analysis of three sub-groups exposed to each type of metalworking fluid by years of exposure... 

Table 6. Results of epidemiological studies of cohorts exposed to soluble and synthetic metalworking fluids... 

Park Mirer (1996) United States Industry-based proportionate mortality study 1870 decedents employed for > 2 years at two auto engine manufacturing plants 1966-87 1970-89 Population primarily exposed to soluble fluids. Different types of metalworking fluids examined in nested case-control analysis. 

Robbins T. Acute respiratory effects on workers exposed to metalworking fluid aerosols in an automotive transmission plant. Am J Ind Med 1997 31 (5) 510—24. 

Calvert GM, Ward E, Schnorr TM, Fine LJ. Cancer risks among workers exposed to metalworking fluids A systematic review. AmJlndMed 1998 33 282-92. 

Similar processes cause the attack of wrought iron and other metalwork, and accelerate the decay of exposed wood. Even aluminum, which is highly favored for use in exterior metalwork because of its resistance to corrosion under ordinary conditions, is now showing the classic pits and erosion scars of corrosion when used in areas which have severe local air pollution. 

Generally there is far less evidence of stray current induced corrosion from external sources into atmospherically exposed reinforced concrete structures compared to buried structures generally and far less evidence of stray current induced corrosion in buried or submerged concrete compared to buried or submerged steel, as concrete has much higher electrical resistance. However, it can occur and can be tested for in the same manner as for other metalwork. There is discussion of interference testing for stray currents in NACE RP0169-2002 and in BSEN 13509 2000. 

Exposed conductive parts are the metalwork of the electrical installation the conduit, trunking, metal boxes and equipment that make up the electrical installation. 

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