Distribution switchgear to BS EN 60947-3 defines the requirements for low-voltage switchgear and control gear, specifically switches, disconnectors, switch-disconnectors and switch-fuse disconnectors. This applies to equipment used in distribution circuits and motor circuits up to 1000 V AC.
Types of Distribution Switchgear
Switch-fuse-disconnector
These integrate a switch-disconnector and fuse holders, allowing load switching, overcurrent protection and safe isolation.
Functions: Making and breaking load current, overcurrent and short-circuit protection via fuses, and safe isolation with visible or verifiable separation such as positive indication of contact position.
Characteristics: Typically fitted with NH or BS88 fuse links, lockable in the off position for maintenance safety, suitable for AC-22 or AC-23 duties depending on rating, and capable of providing discrimination through fuse selection.
Switch-disconnector
These combine the functions of a switch and disconnector capable of load switching and providing safe isolation.
Functions: On-load switching, subject to utilisation category, and isolation of circuits for safety and maintenance.
Characteristics: Rated according to utilisation categories AC-21, AC-22 or AC-23. They may incorporate visible isolation or mechanically linked contacts, lockable handles are common, and they are typically installed upstream or downstream of protective devices.
Switches
These are mechanical switching devices capable of making, carrying and breaking current under defined circuit conditions. Depending on their utilisation category, they may or may not be suitable for on-load switching.
Functions: Routine switching of circuits and operational control.
Characteristics: Load breaking capacity depends on utilisation category. A switch does not itself provide isolation, so not all switches are suitable for breaking inductive loads.
Disconnectors
Designed for circuit isolation. A disconnector must provide positive indication of contact position or a verifiable air gap so circuits can be safely worked on.
Functions: Safe electrical isolation and protection of personnel during maintenance.
Characteristics: Not designed to make or break load current, provides visible or verifiable isolation, typically rated AC-20, and often lockable to prevent inadvertent operation.
Utilization Categories to BS EN 60947-3
BS EN 60947-3 defines utilisation categories that describe the type of load and duty under which a switch, disconnector or switch-disconnector is designed to operate. These categories are critical for correct product selection and compliance with BS 7671.
AC-20A / AC-20B: Isolation only. Devices are not intended to switch load current and should only operate off-load.
AC-21A / AC-21B: Switching of resistive or slightly inductive loads, such as heaters, incandescent lighting and low inductance circuits.
AC-22A / AC-22B: Switching of mixed resistive and inductive loads, including moderate overload conditions. This is the most common category for main switches in low-voltage installations.
AC-23A / AC-23B: Switching of highly inductive loads, including motor loads. This involves high inrush currents and significant arcing during breaking.
Meaning of suffixes: A indicates frequent operation under normal service duty. B indicates infrequent operation or emergency duty.
Selection guidance: Isolation only AC-20, resistive loads AC-21, distribution and mixed loads AC-22, motors and inductive loads AC-23.
Note
BS 7671 462.1.201 requires that a main linked switch or linked circuit-breaker shall be provided as near as practicable to the origin of every installation as a means of switching the supply on load and as a means of isolation. For selection, refer to BS 7671 table 537.4.
Written by Martin Plumbridge – TMIET, Doepke Technical Manager.