Apparatus
These are the classes of assembly that fall within SIC 27120, and the questions each raises when it is specified.
Main switch panels
The assembly at which the incoming supply lands and is first switched and protected. It carries the main switch or incoming breaker, the metering position, and the outgoing ways feeding the rest of the installation. It is sized by the incoming supply rating and by the prospective short-circuit current the supply can deliver, which determines the fault rating every component within it must withstand.
Distribution boards
Sub-assemblies dividing power further into final circuits, with a protective device on each. Specification is a matter of the number of ways, the rating and type of each device, the busbar rating, and whether residual current protection is required and at what sensitivity.
Motor control centres
Assemblies containing the starters, contactors, overloads and drives for a plant’s motors, grouped so that they can be maintained and isolated individually. The starting method matters: direct on line, star-delta, soft start or inverter drive each impose a different demand on the supply and a different mechanical shock on the driven machine.
Control panels
Panels housing the control system rather than the power path, relays or programmable controllers, instruments, indication and operator devices, usually with interlocking so that plant cannot be operated in an unsafe sequence.
| Item | Why it governs the build |
|---|---|
| Rated current | Sizes busbars, devices and the thermal design of the enclosure. |
| Prospective fault current | Every device must be rated to make, carry and break the fault the supply can deliver. |
| Form of separation | Determines how far busbars, functional units and terminals are segregated from one another. |
| Ingress protection | Set by where the assembly stands: a clean plant room and a wash-down area are not comparable. |
| Ambient conditions | Temperature and ventilation derate equipment; an enclosure that runs hot ages and trips. |
| Physical envelope | Access, cable entry, maintenance clearance and how the assembly is to be got into the room. |
Enclosures
The enclosure is not merely a box. It carries the ingress rating, provides the mechanical protection, dissipates the heat the equipment inside generates, and determines whether that equipment can be worked on safely and reached at all. Cable entry position, gland plates, door swing and internal access are settled at design stage, because none of them can be changed afterwards without cutting metal.
Labelling and documentation
An assembly is only maintainable if the person who opens it in five years’ time can tell what everything is. Circuit identification, warning and rating labels, a schedule of ways, and as-built drawings are part of the product rather than an extra.