Helpful video reference. John Ward (channel: jwflame) covers the earthing requirement for metal back boxes in this short UK tutorial "Do I need to earth a metal back box? - light switches and sockets". He works through the regulations clearly and demonstrates exactly where the flying lead connects. Worth watching before you lift the plate.
1. Understand the regulation
BS 7671 (the IET Wiring Regulations, 18th Edition) requires that all exposed conductive parts be connected to the circuit protective conductor. A metal back box qualifies as an exposed conductive part because a live conductor inside it could touch the metal shell, putting the box at a dangerous voltage. The flying lead makes sure that voltage is quickly cleared by the protective device.
Plastic back boxes have no exposed metalwork, so this does not apply to them. If your box is plastic, you are done before you start.
2. Identify the earthing terminals
On a metal back box there is a small threaded boss, usually near a corner, with a screw terminal. Some older metal boxes have a plain threaded hole rather than a dedicated terminal - you fit a machine screw with a loop of cable beneath it.
On the accessory (the switch or socket faceplate), there is also an earth terminal, often marked with the symbol ⏚ or simply "E" or "Earth". The flying lead runs between these two terminals.
3. Check whether a flying lead is already present
When you ease the faceplate away from the box, look for a short green and yellow conductor connecting the box terminal to the plate terminal. Many accessory boxes already have this fitted from the original installation.
If there is one, check it is secure at both ends and not frayed or damaged. If it is fine, refit the plate and you are done. If there is no flying lead, move to the next step.
4. Check whether the circuit has an earth at all
Open the consumer unit (or use a continuity tester) to confirm the lighting or ring final circuit has a circuit protective conductor (the bare or green/yellow conductor in twin-and-earth cable). In homes wired before the 1960s there may be no earth conductor at all - two-core rubber-insulated cable with no CPC was common.
If there is no earth in the circuit, you cannot add a meaningful flying lead because there is nothing to bond to. This is a C2 defect that needs a rewire of the affected circuit - call an electrician.
5. Add the flying earth lead
Cut a short piece of 2.5 mm or 1.5 mm single-core conductor (matched to the circuit's CPC) and sleeve it entirely in green and yellow PVC sleeving - bare copper inside a live back box is asking for a short circuit. Make the lead just long enough to reach comfortably between the two terminals with a small loop of slack.
Connect one end to the earth terminal on the back box and tighten firmly. Connect the other end to the earth terminal on the faceplate and tighten firmly. If the terminal is a clamp type, make sure the conductor is fully inserted before tightening the screw.
6. Check earthing on a metal-fronted accessory
Some accessories - brushed steel switches, antique-finish sockets, industrial surface-mounted socket outlets - have a metal faceplate as well as a metal back box. The metal faceplate itself needs earthing too. Confirm there is either a direct path through the mounting screws (some designs rely on this), or a dedicated earth terminal on the plate that is connected back to the circuit earth.
7. Verify continuity, then refit
With a continuity tester or multimeter set to resistance, confirm you have a low-resistance path from the circuit earth conductor, through the back box terminal, and through the flying lead to the faceplate terminal. A reading of well under 1 ohm is what you expect for a short flying lead.
Fold the conductors gently into the box, refit the faceplate with both screws, restore power, and test the accessory normally.
When to call us
Adding a flying lead to a metal back box that already has a circuit earth is a quick job for a careful homeowner. If the earth is absent from the circuit altogether - or if the wiring is old enough that you are not sure what you are looking at - that is the point to call. Richard covers small jobs in Sandwich at £10 per 10 minutes, and an EICR will find every missing earth on the whole property if you want a full picture.
Earth checks in Sandwich
Richard can inspect the earthing on individual accessories or carry out a full EICR across the property.
Contact Richard