T E C HN I CAL
device provided for isolation or switching shall comply with the relevant requirements of Chapter 53 (Section 537). Part 2 of BS 7671 defines an isolator as a ‘mechanical switching device which in the open position complies with the requirements specified for the isolation function’. An isolator, which is otherwise known as a disconnector, can be, and often is, an off-load device2. Isolating switches (switch disconnectors) are on-load devices , which may also serve as an isolating device (Table 2 refers).
Apprentice Corner The focus of the apprentice corner in this issue of Connections is isolation and switching, and how the requirements of BS 7671 are applied to its four distinct functions. This will enable a switching device to be correctly selected for a particular function, and enable informed decisions to be made when carrying out an inspection of such installed devices. This will be complemented using a scenario followed by some multiple-choice questions
Isolation
Introduction The term ‘isolation and switching’, as used in BS 7671, relates to four distinct functions as shown in Table 1, each with its own particular requirements for safety. The four functions are: • isolation, • switching off for mechanical maintenance, • emergency switching, and • functional switching
18th Edition of BS 7671 In this edition, there is a new Chapter 46 which deals with non-automatic local and remote isolation and switching measures for the control of circuits or equipment. However, Regulation 461.1 requires that where appropriate, every
1 Regulations 464.1 ‘Means of switching off shall be provided where mechanical maintenance may involve a risk of physical injury.’ 2 Off-load – no current is flowing in the circuit. 3 On-load – current may be flowing in the circuit
Every circuit shall be provided with a means of isolation (refer Part 2 definitions for ‘Isolation’) from each of the live supply conductors except as detailed in Regulation 461.2 (Regulations 422.3.13 and 462.2 refer). A neutral conductor falls within the definition of a live conductor. A group of circuits may be isolated by a common means (Regulation 462.2 refers). However, where such an arrangement is being considered, the requirements of Regulation Group 314 relating to division of an installation must be taken into account. A common example of a single device used to isolate a group of circuits is the main switch of a consumer unit (see Fig 1). A means of isolating and switching the supply on load shall be provided as near as practicable to the origin of every installation. The preferred means of isolation and switching is a main linked switch or a linked circuit-breaker, but single-pole devices situated adjacent to each other are not excluded (Regulations 462.1.201 and 537.2.6 refer). 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. Table 2 indicates how such symbols appear and their application.
Table 1: Function and purpose for each type of isolation and switching category Function
Purpose
Used by
Isolation
i) To enable electrical work to be carried out on a circuit or to isolate equipment ii) Shall be provided as near as practicable to the origin of every installation
Electrically skilled or instructed persons (Group 537.2) (Section 462) The user of the installation 462.1.201
Regulation
Switching off for To enable non-electrical work to be mechanical maintenance carried out on a system
Non electrically skilled persons1 (ordinary persons)
537.3.2 (Section 464)
Emergency switching
An operation intended to remove, as quickly as possible, danger, which may have occurred unexpectedly
Skilled or un-skilled persons
537.3.3 (Section 465)
Functional switching
To enable proper functioning and control of current-using equipment
The user of the installation
537.3.1 (Section 463)
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