8A Provision of Isolation and Switching (1) General installation
(2) Appliance, equipment or luminaire (3) Unguarded moving parts
(4) Electric motors
(5) Switching off for mechanical maintenance (6) Emergency switching
8B Requirements of Isolation and Switching Devices (1) General
(2) Isolating devices
(3) Devices for switching off for mechanical maintenance (4) Devices for emergency switching
Code 8
ISOLATION AND SWITCHING
8A Provision of Isolation and Switching (1) General installation
(a) An installation must be provided with a main switch or circuit breaker and a means of isolation to cut off all voltages. These two functions may be incorporated in a single device. The main switch or circuit breaker should interrupt all live conductors (i.e. phase and neutral conductors) and be capable of cutting off the full load current of the installation from supply. For a 3-phase 4-wire a.c. supply, a linked switch or linked circuit breaker may be arranged to disconnect the phase conductors only and in such case, a link should be inserted in the neutral conductor and securely fixed by bolts or screws.
(b) For an installation serving more than one building, the installation inside separate buildings should be treated as a separate installation, and a separate main switch or circuit breaker and a separate means of isolation for each building are required as in subparagraph (a). (c) Every circuit or group of circuits must be provided with:
(i) a means of isolation; and
(ii) a means of interrupting the supply on load.
(d ) Where a standby generator is installed, electrically and mechanically interlocked 4-pole changeover devices should be used for interconnection between the normal and standby sources to ensure that any neutral unbalance and fault current return to the correct source of supply.
(e) Under no circumstances must a means of isolation or a switching device be provided in a protective conductor.
(f ) A circuit breaker used to receive supply direct from the electricity supplier’s transformer should normally be of draw-out type. An isolator may be used in conjunction with a fixed type circuit breaker provided that it is mechanically interlocked with the circuit breaker. (2) Appliance, equipment or luminaire
(a) An appliance, equipment or luminaire, other than that connected by means of a plug and socket outlet should be provided with a means of interrupting the supply on load.
(b) For electric discharge lighting installation operating normally at an open-circuit voltage exceeding low voltage, an effective local means for isolation of the circuit from supply (such means being additional to the switch normally used for controlling the circuit) should be
provided for the isolation of every self-contained luminaire, or of every circuit supplying luminaires at a voltage exceeding low voltage. (3) Unguarded moving parts
An installation where it is necessary for a person or persons to enter or work in close proximity with normally unguarded moving parts which may constitute a hazard should be provided with a means of isolation near the moving parts in addition to any emergency switching provided. (4) Electric motors
Electric motors except exhaust fans having rated power not exceeding 50W and servo-motors should be provided with:
(a) means of isolation suitably placed and so connected that all voltages may thereby be cut off from the motor and all apparatus including any automatic circuit breaker used therewith. If this means of isolation is remote from a motor, an additional means of isolation adjacent to the motor should be installed or alternatively provision should be made so that the means of isolation can be secured against inadvertent operation as required by Code 8B(2)(g);
(b) means for starting and stopping, suitably placed for ready operation; and
(c) means to prevent automatic restarting after a stoppage due to drop in voltage or failure of supply, where unexpected restarting of the motor might cause danger. Such means is not required where the failure of the motor to start after a brief interruption of the supply would be likely to cause greater danger, e.g. motors driving a ventilation fan of a fire protection system. This requirement does not preclude any arrangement for starting a motor at intervals by an automatic control device, where other adequate precautions are taken against danger from unexpected restarting, e.g. various sequential drives in an automatic processing plant, use of safety equipment such as guards, barriers, etc.
(5) Switching off for mechanical maintenance
(a) Means of switching off for mechanical maintenance must be provided where mechanical maintenance may involve a risk of burns or a risk of injury from mechanical movement. These include every circuit supplying:
(i) an electric motor;
(ii) electrical heating equipment;
(iii) electromagnetic equipment which may cause mechanical accidents;
(iv) luminaires (lamp replacement and cleaning are included as mechanical maintenance); and
(v) any other electrically energised equipment from which possible mechanical or heat hazards can arise from the use of electrical energy.
(b) Suitable means should be provided to prevent electrically powered equipment from becoming unintentionally reactivated during mechanical maintenance, unless the means of switching off is continuously under the control of any person performing such maintenance.
(c) Where electrically powered equipment is within the scope of BS EN 60204, the requirements for switching off for mechanical maintenance of that standard apply.
(6) Emergency switching
(a) Means should be provided for emergency switching of any part of an installation where it may be necessary to control the supply to remove an unexpected danger.
(b) Means for emergency switching should be provided for the purpose of cutting off the appropriate supply to the circuit concerned as rapidly as possible in order to obviate or to remove a dangerous condition as soon as it becomes apparent. A typical example where means for emergency switching should be provided is a machine driven by electrical means which may give rise to danger.
(c) Where electrically powered equipment is within the scope of BS EN 60204, the requirements for emergency switching of that standard apply.
8B Requirements of Isolation and Switching Devices (1) General
Where a common device is used to perform one or more of the following functions:
(a) means of isolation;
(b) means of switching off for mechanical maintenance; (c) means of emergency switching,
the arrangement and characteristics of the device should satisfy all the requirements of this Code 8 for the various functions concerned.
(2) Isolating devices
(a) An isolating device should be capable of:
(i) opening and closing all live conductors (i.e. phase and neutral conductors) of the circuit under no-load condition provided that, for a 3-phase 4-wire a.c. supply, a linked isolator may be arranged to disconnect the phase conductors only and in such case, a link should be inserted in the neutral conductor and securely fixed by bolts or screws;
(ii) carrying the normal circuit current;
(iii) carrying for a specified time abnormal currents which may occur during an overcurrent condition (i.e. overload or short-circuit). (b) The position of the contacts or other means of isolation should be either externally visible or clearly and reliably indicated. An indication of the isolated position should occur only when the specified isolation has been obtained in each pole.
(c) An isolating device should be designed and/or installed so as to prevent unintentional or inadvertent closure.
(d ) The following devices satisfying subparagraphs (a), (b) and (c) are acceptable as means of isolation:
(i) isolators (disconnectors), (ii) fuse switches and switch-fuses,
(iii) links, including fuse-links and fuse cut-outs, (iv) plugs and socket outlets,
(v) cable couplers,
(vi) circuit breakers, including miniature circuit breakers (MCB), moulded case circuit breakers (MCCB) and residual current circuit breakers (RCCB).
(e) Emergency stop push button and semiconductor devices, such as a ‘touch control switch’ or ‘photo-electric switch’, must not be used for isolation.
(f ) Where an isolating device for a particular circuit is placed remotely from the equipment to be isolated, provision should be made so that the means of isolation can be secured in the open position. Where this provision takes the form of a lock or removable handle, the key or handle should be non-interchangeable with any other used for a similar purpose within the premises.
(g) Each device used for isolation should be clearly identified by position or durable marking to indicate the installation or circuit it isolates.
(3) Devices for switching off for mechanical maintenance
(a) A device used for switching off for mechanical maintenance should: (i) require manual operation;
(ii) have visible or clearly and reliably indicated open and closed positions of the contacts;
(iii) be designed and/or installed in such a way to prevent inadvertent or unintentional switching on;
(iv) be capable of cutting off the full load current of the relevant part of the installation; and
(v) be readily accessible for operation.
(b) The following devices satisfying subparagraph (a) are acceptable as means for switching off for mechanical maintenance:
(i) switches, (ii) circuit breakers,
(iii) control switches operating contactors, (iv) plugs and socket outlets.
(4) Devices for emergency switching
(a) Means for emergency switching should act as directly as possible on the appropriate supply conductors. The arrangement should be such that one single action only will interrupt the appropriate supply. (b) Means of interrupting the supply for the purpose of emergency
switching should be capable of cutting off the full load current of the relevant part(s) of the installation.
(c) The means of operation should be capable of latching or being restrained in the ‘off’ or ‘stop’ position, unless both the means of operation for emergency switching and for re-energising are under the control of the same person. The release of an emergency switching device should not re-energise the relevant part of the installation.
(d ) Means of operating (handle, push-button, etc.) a device for emergency switching should be:
(i) clearly marked;
(ii) preferably coloured red; and
(iii) installed in a readily accessible position where danger might occur and, where appropriate, at any additional remote position from which that danger can be removed.
(e) The following devices satisfying subparagraph (d ) are acceptable as emergency switches:
(i) switch in the main circuit (e.g. fireman’s switch for high voltage discharge lighting installation),
(ii) push button and the like in a control or auxiliary circuit (e.g. emergency stop for machinery).
(f ) A plug and socket outlet or similar device should not be selected as a device for emergency switching.
(g) Fireman’s emergency switch should:
(i) be coloured red and have fixed on or near it a permanent durable nameplate marked with the words ‘消防員開關掣 FIREMAN’S SWITCH’ (the nameplate should have a minimum size of 150 mm by 100 mm and the lettering should be easily legible from a distance appropriate to the site conditions but in no case less than 13 mm high);
(ii) have its ‘ON’ and ‘OFF’ positions clearly indicated by lettering legible to a person standing on the ground at the intended site, with the ‘OFF’ position at the top;
(iii) be provided with a device to prevent the switch being inadvertently returned to the ‘ON’ position; and