Switching off for mechanical maintenance 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.
Fig 1 Isolation arrangement at the origin of a typical domestic and non-domestic installation Linked
462.1.201
Double-pole isolator
Main Switch
Single phase supply
Emergency switching In domestic installations, there is generally no requirement to install devices such as emergency stop buttons. However, installers should be aware that even though BS 7671 does not permit BS 1363 plugs and socketoutlets to be selected as a means of providing emergency switching (see Table 537.4 and Regulation 537.3.3.3), in an emergency such as a fire involving deep fat fryers or where someone is in contact with a live source for example, the user may use a BS 1363 plug and socket-outlet to disconnect the equipment. When installing socket-outlets that may, in an emergency, be used to disconnect equipment such as that previously described, the installer should ensure that the socket-outlets are positioned so that when the equipment is in use, the socket-outlets are readily accessible to the user. A firefighter’s switch is a particular type of emergency switch typically used for low voltage circuits supplying outdoor lighting installations operating at a voltage exceeding low voltage, and indoor discharge lighting installations operating at a voltage exceeding low voltage. They are also used in certain licensed premises such as petrol filling station forecourts (Regulation 537.4.2 refers).
537.2.6
L1
L2
L3
N
L1
L2
L3
N
Triple-pole isolator
Triple-pole and neutral isolator
This arrangement for TN-S and TN-C-S
This arrangement for TT only
Table 2: Symbols used for some of the more common means of isolation Isolating
Making, breaking and isolating
Disconnector
Switch-disconnector
Disconnector-fuse
Switch-disconnector-fuse
Fuse-disconnector
Fuse-switch-disconnector
Functional switching
Incidentally, this is the marking commonly found on the main switch of a typical consumer unit.
A functional switching device should be provided for each part of a circuit that needs to be controlled independently of other parts of the installation and shall be selected in accordance with Table 537.4. Functional switching devices are generally required to be capable of switching the load ‘on’ and ‘off ’. All functional switching devices are required by Regulation 537.3.1.2 to be suitable for the most onerous duty intended, and should be selected to have a utilisation category appropriate to the type of load being switched. For example a fuse-combination unit feeding a 400 V AC circuit of mixed resistive and inductive loads which would need to be operated frequently, would require a minimum utilisation category of AC-22A (Table 2 of BS EN 60947-3 refers).
Scenario Your company has been asked to design and install a control arrangement for providing power to six bays in a college workshop. All bays are protected by a 30 mA RCBO. The existing supply is 3-phase 400 V AC and the earthing system is TN-C-S. A simple outline drawing (see Fig 2) has been provided by the college. The bay lighting is not required to be included in the new design.
Contractor’s considerations 1. Is the adequacy of the existing equipment at the intake position in the workshop, sufficient to satisfy Regulation 132.16 for the client’s additional loading of the bays? 61 S U MME R 2 018
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