Earthing Portable equipment

Note that in some portable equipment, only a two-wire ac cord is used to connect it to the mains supply. But the spacing requirement is still virtually unchanged, since a user can touch accessible parts on the secondary side and complete the connection through the earth ground. 

When an earth fault occurs at the far end of a cable it is possible that the armouring, cable gland and the frame of the consumer equipment can be raised to a dangerous potential with respect to electric shock exposure to human operators. This subject has been given considerable attention over the last 20 years, and is well documented in for example IEC60364. The international documentation concentrates on low voltage fixed and portable equipment protected by fuses and miniature circuit breakers. See also Chapter 13. 

The principle is that the live conductors are covered by two discrete layers of insulation. Each layer would provide adequate insulation in itself but together they ensure little likelihood of danger arising from insulation failure. This arrangement avoids the need for an earth wire. Double insulation is particularly suitable for portable equipment such as drills. However, safety depends on the insulation remaining in sound condition and the equipment must be properly constructed, used and maintained. 

Some portable equipment requires substantial power to operate and may require voltages higher than those usually used for portable tools, so that the current is kept down to reasonable levels. In these cases, power leads with a separate earth conductor and earth screen must be used. Earth leakage relays and earth monitoring equipment must also be used, together with substantial plugs and sockets designed for this type of system. 

It is important to ensure that all portable electrical equipment is regularly inspected and adequately maintained to minimise the risk of danger to the user. Equipment of the double-insulated or all-insulated types (class II), to the relevant BS , has no provision for earthing and is not earthed. Such equipment should be marked with the s5anbol [ ]. There is no symbol for class I equipment. 

In the case of portable equipment, testing is often carried out using a proprietary portable appliance tester (PAT). One useful test is the measurement of e insulation resistance to confirm that it is sufficiently high to prevent undue current leakage. Additionally, for class I equipment (that must be earthed) it is important to verify that the connection to earth is sound, i.e. its electrical resistance is low, and that the conductor is capable of carrying the sort of high current that may occur under fault conditions. Both the HSE and the lEE have produced guidance on electrical equipment maintenance. ... 

Appendix A Maximum permissible measured earth fault loop impedance is a rewrite of Appendices 7 and 8, and Appendix B - Resistance of copper and aluminium conductors under fault conditions - is a repeat of Appendix 17 of the 15th edition. Appendix C - Minimum separation distances between electricity supply cables and telecommunication cables - tabulates the separation specified for external and internal cables. Appendix D - Permitted leakage currents - gives the figures culled from the relevant British Standards for fixed and portable equipment. 

Portable electrical hand tools and equipment shall be properly grounded and wound to operate on llOV a.c. center tapped to earth supply, and shall only be connected to the system by permanent joints or proper connections. 

Each of these factors may achieve different levels of importance depending on circumstances of use. In a military setting, portability is critically important soldiers cannot carry gallons of water for use as a personal decontaminant. However, in a civilian setting, the ready access to clean water may make water an attractive choice. Similarly, in the military setting a personal decontamination kit that deteriorates rapidly may be less useful than, for example, a pack filled with fullers earth that retains its efficacy indefinitely. In all cases, speed of action and efficacy will be important. It will be understood that corrosive materials are unsuitable for personal decontamination and may also damage delicate equipment thus, a decontaminant suitable for use on the exterior of an armoured fighting vehicle may not be suitable for use either on the skin or on electronic equipment. 

In limestone mines, rock falls are the major cause of deaths and serious injuries, with mobile equipment being the next most significant category. Special electrical precautions (e.g. high integrity earthing and the use of low voltage portable tools) are essential as water percolates into most limestone mines [34.6]. 

When cables are to be sized for a particular project with regard to their short-circuit performance it is necessary to consider the let-through current of the protective device in the circuit e.g., fuse, circuit breaker. It is also necessary to determine whether the consnmer has fixed equipment such as a motor, or temporary equipment such as a portable tool plugged into a socket, because this establishes the minimum time dnration. This aspect is described in more detail when the earth-loop impedance is being considered, see sub-section 9.4.3.6. 

Combined inspection and testing is also necessary on some equipment because some faults cannot be seen by just looking - for example, the continuity and effectiveness of earth paths on class I equipment. For some portable appliances the earth is essential to the safe use of the equipment and, therefore, all earthed equipment and most extension leads should be periodically tested and inspected for these faults. All portable appliance test instruments (PAT Testers) will carry out two important tests earth bonding and insulation resistance. 

A flash test tests the insulation resistance at a higher voltage than the 500 V test described above. The flash test uses 1.5 kV for Class 1 portable appliances, that is, earthed appliances, and 3kV for Class 2 appliances, which are double insulated. The test establishes that the insulation will remain satisfactory under more stringent conditions but must be used with caution, since it may overstress the insulation and will damage electronic equipment. A satisfactory result for this test would typically be less than 3 mA. 

Where plugs and sockets are used for portable tools, sufficient sockets must be provided for all the equipment and adaptors should not be used. Many accidents are caused by faulty flexible cables, extension leads, plugs and sockets, particularly when these items become damp or worn. Accidents often occur when contact is made with some part of the tool which has become live (probably at mains voltage), while the user is standing on, or in contact with, an earthed conducting surface. If the electrical supply is at more than 50 volts ac, then the electric shock that a person may receive from such defective equipment is potentially lethal. In adverse environmental conditions, such as humid or damp atmospheres, even lower voltages can be dangerous. Portable electrical equipment should not be used in flammable atmospheres if it can be avoided and it must also comply with any standard relevant to the particular environment. Air operated equipment should also be used as an alternative whenever it is practical. 

To remove the need for earthing on some portable power tools, double insulation is used. Double insulation employs two independent layers of insulation over the live conductors, each layer alone being adequate to insulate the electrical equipment safely. Since such tools are not protected by an earth, they must be inspected and maintained regularly and must be discarded if damaged. 

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. 

There is only limited use of Class II equipment in the fixed installation because in most cases it has to include the protective conductor for use elsewhere in the circuit to connect Class I equipment and this usage negates the Class II classification. Class II is more prevalent in portable apparatus. Protection by non-conducting location is not much used in the UK except for special locations such as test facilities. Earth-free local bonding, again, is used only in special locations where the necessary precautions can be taken to avoid importing an earth. Electrical separation s main application is also in electrical testing areas. 

The safety philosophy is based on dividing the site installation into two distribution systems the 400/230 V system where there is a comparatively high shock risk because the voltage to earth is 230 V, and the comparatively safe 110 V system where the voltage to earth does not exceed 64 V. The 400/ 230 V system is intended to be installed and maintained only by authorised and competent persons, i.e. those who are electrically qualified. The construction workers role is confined to operating the distribution equipment, which is metalclad, weatherproof and earthed. The portable apparatus which they constantly handle is all connected to the llOV system (see Fig. 11.1). 

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