The term ‘isolation and switching’, as used in BS 7671 and shown in Table 1, embraces four distinct concepts, each with its own particular requirements for safety.
The four concepts are isolation, switching off for mechanical maintenance, emergency switching and functional switching.
In this article only the concept of switching off for mechanical maintenance will be explored.
Purpose of switching off for mechanical maintenance
Mechanical maintenance is defined in Part 2 of BS 7671 as the replacement, refurbishment or cleaning of lamps and nonelectrical parts of equipment, plant and machinery. An example could be replacing the lamp in the luminaire shown in Fig 1 for an LED type.
Switching off for mechanical maintenance is to enable non-electrical maintenance to be performed safely without the risk of burns or injury from mechanical movement. The demands of safety are such that in most cases the same means of preventing unintentional or inadvertent reclosing of the switch must be provided as are required for isolation. The enclosures of electrical parts of equipment are intended to remain in place during mechanical maintenance. Injury such as burns from heating elements or some types of lamp, or from mechanical movement such as might be caused by electrically actuated machinery. For example, the safe replacement of a bandsaw blade in an electric machine should be possible after switching off for mechanical maintenance.
Switching off for mechanical maintenance is not intended to provide protection against electric shock; for this purpose a means of isolation must be provided in accordance with Regulation 462.2.
Unskilled Persons
Switching off for mechanical maintenance is provided for the benefit of persons who are not necessarily electrically skilled and who may not necessarily be competent to avoid the dangers which electricity may create. Great care must be taken, in the design and implementation of measures for switching off for mechanical maintenance, to ensure that safe working conditions are provided, particularly where non-electrically skilled persons are likely to be undertaking the maintenance.
For example, a cooker switch can perform the function of switching off for mechanical maintenance, providing the switch is
located so that it is continuously under the control of the person undertaking the maintenance task. The means of interrupting the supply on load should be readily accessible but the device should not be positioned where, in order to access it, a person would have to reach over the cooking appliance. Consideration should also be given as to whether the position of the device enables it to remain readily accessible in the event of a cooking fire.
Historically, this was generally achieved by locating the cooker control switch within 2 m of the cooking appliance as shown in Fig 2. However, in recent editions/amendments of BS 7671, no regulatory requirements giving such a distance are now stated.
Regulation 537.3.2.2 requires that devices used for the function of switching off for mechanical maintenance shall be inserted in the main supply circuit and be capable of cutting off the full-load current of the relevant part of the installation. Where the duties of an isolator are to make and/or break a circuit under load, it is usually marked with a symbol to indicate its switching capability.
A similar situation arises where, typically, a double-pole switch is installed next to an immersion heater as shown in Fig 3. The switch disconnects the supply from the heater element and gives the person working on the water heater control of the supply. As with the cooker switch, this ‘plumbers’ switch must be situated such that it is under the continuous control of the person carrying out the work.
Mounting heights
BS 7671 requires that a socket-outlet on a wall or similar structure is mounted at a sufficient height above the floor or any working surface to minimize the risk of mechanical damage to the socket-outlet or to an associated plug and flexible cord during insertion, use or withdrawal of the plug (Regulation 553.1.6 refers). The purpose of the requirement is to avoid damage such as might result from the flexible cord being too tightly bent between the plug and the floor or working surface.
Part M of the Building Regulations, which applies in England and Wales, requires reasonable provision to be made for people to gain access to, and use a building and its facilities. In particular, Section 1.18 of Approved Document M, which applies to new dwellings, includes the objective of assisting people whose reach is limited to use the dwelling more easily by locating wall-mounted switches, socket-outlets and the like at suitable heights. A way of complying with this is to provide switches and socket-outlets for lighting and other accessories in habitable rooms at appropriate heights between 450 mm and 1200 mm from finished floor level.
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