Surface mount plastic pattresses for socket and switches, and plastic backboxes for flush mounting in plasterboard walls.
Surface mounting plastic boxes for switches, sockets and other electrical accessories.
These are available in similar sizes to the flush mount steel boxes. Standard sizes include single and double, with various other less common arrangements from certain manufacturers.
Thread size and spacing is identical to the steel versions - 3.5mm metric, 0.6mm pitch. Any socket or switch which fits onto a steel backbox can be used with a surface mounting plastic one.
Standard depths are 16mm, 25mm, 35mm and 47mm - identical to the steel flush versions.
However some manufacturers depths do vary slightly, with 29mm and 45mm being some of the options.
16mm are only useful for light switches. 25mm is suitable for most other accessories, with 35mm and 47mm being used where the wiring is of larger size, or there are more wires to accommodate behind the accessory.
Although any manufacturer's accessory can be fitted onto any other manufacturer's pattress, there may be cosmetic differences. Some boxes have square corners, which may look odd if a rounded edge accessory is fitted to it.
There is no electrical problem in doing this - purely an issue of appearance.
Most boxes are made from a hard, brittle plastic such as urea formaldehyde. Multiple points are provided around the edge where a section can be broken out to provide a hole for wiring to enter. Drilling holes in these boxes is not recommended as they break and shatter very easily.
Surface mount items are usually associated with lower cost installations and are typically used with wiring fixed directly to the surface or in plastic mini trunking.
For installations with surface PVC conduit, PVC boxes are used. These have several round 20mm knockouts and can be drilled easily without shattering.
These are designed for use in plasterboard walls. The box fits into a suitably sized hole in the plasterboard and is secured in place by two plastic tabs, one on each side. These tabs contain the accessory fixing screws and will clam against the plasterboard as the screws are tightened.
As depth isn't normally an issue (the wall is hollow), most boxes are around 35mm deep and can be used with virtually any accessory. Some are available in 47mm.
Thread spacing and thread size are the same as the flush steel and surface plastic types.
As with the other boxes, various specials are available such as two singles in the same moulding. These are the only type which is available in a circular box, which may be useful for installing smoke alarms or pull cord light switches on a ceiling.