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)(e);
(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.
(5) Switching off for mechanical maintenance
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:
(a) an electric motor;
(b) electrical heating equipment;
(c) electromagnetic equipment which may cause mechanical accidents;
(d ) luminaires (lamp replacement and cleaning are included as mechanical maintenance); and
(e) any other electrically energised equipment from which possible mechanical or heat hazards can arise from the use of electrical energy.
(6) Emergency switching
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.
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);
(iv) providing sufficient isolating distances between contacts or other means of isolation when in open position not less than that determined for an isolator (disconnector) in accordance with IEC 60947-3;
(v) giving isolating distance between contacts of the device that should be visible or be clearly and reliably indicated by “off” or
“open” marking. Such indication should only occur when the
isolating distance between open contacts on each pole of the device has been attained; and
(vi) not being unintentionally or automatically reclosed.
(b) The following devices satisfying subparagraph (a) 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).
(c) Emergency stop push button and semiconductor devices, such as a
‘touch control switch’ or ‘photo-electric switch’, must not be used for isolation.
(d ) An isolating device should be near to the equipment it controls.
Where the isolating device is not near to the equipment it controls, provision should be made so that the means of isolation can be secured against inadvertent reclosure during the operation for which it is intended. The main isolating device must be so arranged that it can only be closed by an electrical worker who will ensure that it is safe to close before doing so. Such a main isolating device must be capable of being locked at ‘ON’ or ‘OFF’ position, or must have a removable handle. The key or the handle must be non-interchangeable with those used for a similar purpose within the installation to prevent unauthorised operation.
(e) All isolating devices should be readily accessible and clearly identifiable to indicate the circuit or equipment which they isolate.
(3) Devices for switching off for mechanical maintenance
(a) A device used for switching off for mechanical maintenance should:
(i) be capable of being manually operated;
(ii) provide clearance between open contacts of the device that should be visible or be clearly and reliably indicated by “off” or
“open” marking. Such indication should only occur when the
“off” or “open” position on each pole of the device has been attained;
(iii) be selected or installed in such a way to prevent unintentional or automatical reclosure;
(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 be operated by a single initiative action only to cause the removal of the danger by cutting off 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 of the installation.
(c) Emergency switches for operating the devices should be:
(i) clearly marked;
(ii) preferably coloured red; and
(iii) installed in a readily accessible position where danger might occur.
(d ) The following devices satisfying subparagraph (c) 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).
(e) Plugs and socket outlets are not acceptable as means of emergency switching.
( f ) 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
(iv) be arranged to facilitate operation by a fireman.