A metal back box is a metal enclosure used to house electrical sockets, switches, and other electrical fittings. They are installed within walls and provide a secure and stable mounting for the electrical components.

Common sizes for metal back boxes in the UK include:

• Single gang: Typically 86mm x 86mm, used for single switches or sockets.
• Double gang: Typically 146mm x 86mm, used for double switches or sockets.
• Depths: Usually range from 16mm to 47mm, with common depths being 16mm, 25mm, 35mm, and 47mm.

Yes, metal back boxes must be earthed to ensure electrical safety. They typically have an earth terminal to connect the earth wire from the electrical circuit.

Metal Back Boxes: More robust, provide better protection against fire, and are often required by building regulations for certain installations.

Plastic Back Boxes: Easier to install in drywall or plasterboard, non-conductive, and do not require earthing.

Knockouts are pre-scored sections on the back or sides of a metal back box that can be easily removed to allow for cable entry. They are designed to be punched out using a hammer and screwdriver or pliers.