A one-way lighting circuit is a circuit that uses a single switch to operate a single or multiple light fittings.
In a typical UK home wiring situation, lighting circuits usually incorporate the lighting needs of all of the rooms and areas of a single floor, incorporating one-way, two-way and intermediate switching circuits as required. Generally these circuits are protected by a 6 amp miniature circuit breaker (MCB) and are wired using 1.5mm2 'twin and earth' cable. Previously this might have been a 5 amp MCB or fuse, and 1.0mm2cable.
The overriding limitation of the circuit is the maximum wattage that can be supplied. Without taking into account any de-rating factors, the circuit can supply a theoretical maximum of 230 volts x 6 amps = 1380 watts. In general practice, lighting circuit wiring should be planned in such a way as to have a load of 1000 watts or less. In this way there is a small margin left for third party addition or lamp changes which might affect overall load levels.
The circuit
We can see from the picture above that when the switch is closed, electricity can flow from the live supply connection to the lamp and pass through the lamp filament to the neutral supply connection, causing the lamp to illuminate. When the switch is opened, the circuit is broken, causing the lamp to extinguish. This of course is how the circuit would look if it was done as a bench exercise.
Wiring circuits
In practice, a lighting circuit would supply more than one switch and one light fitting. We are merely looking at a single segment of a typical circuit. To achieve the same desired effect as the circuit drawing above, there are three ways in which a domestic lighting circuit could be wired using UK 'twin and earth' cable.
1. Loop-in at the light.
This is the most favoured method, as it keeps most of the permanently live 'mains feed' cabling in the ceiling void or loft space out of harm's way. A cable is run from the consumer unit to the first light fitting. Another is run from the light fitting to the switch. Another from the light fitting to the next light fitting, or light / switch combo, until the circuit is complete. This wiring method is easy to identify due to the number of cables at the light fitting.
It can be confusing to the DIY enthusiast, especially if they have disconnected the light without marking up the connections beforehand, since there could be as many as four twin and earth cables at the first fitting in a room. In this instance, colour-to-colour wire replacement at the light means that the light illuminates immediately when the mcb is reset, then when the switch is closed (to try to switch off the light), the resulting short-circuit welds the switch closed permanently and trips the MCB. Best practice is to identify the switched live wire clearly at the switch and at the light fitting before taking the old fitting down. These circuits have often led to call outs for electricians when DIY efforts have gone wrong.
NOTE: Switching off the light at the switch DOES NOT isolate mains electricity from the light fitting or switch - always isolate mains power at the consumer unit before starting work.
2. Junction Box.
This method of wiring lighting circuits is often used where open loft spaces allow easy access to the junction box, or containment conduits have been installed prior to cabling. A 'mains feed' cable is run from the consumer unit to the junction box, which is usually placed centrally within the loft or floor area. A cable is then run from each light fitting and from each switch back to the junction box.
This method uses a little more cable than the previous method, but allows for easy addition of further lights or switches. This wiring method is slightly harder to identify than the one above, however it becomes apparent if both light fitting and switch are inspected, as only the minimum required cables will be present at these locations. Cable identification at the junction box is paramount using this method, as switches and light fittings could be easily confused or jumbled, causing unwanted faults or mis-operation.
NOTE: Always isolate mains power at the consumer unit before starting work.
3. Loop-in at the switch
This is the least favoured method of wiring a lighting circuit, since it means there are permanently live 'mains feed' cabling runs within walls to each switch. Also, neutral and earth connections may become squashed inside the switch accessory box (especially the shallow ones), causing circuit faults.
With this method a cable is run from the consumer unit to the first switch. Another is run from the switch to the light fitting(s). Another to the next switch / light combo, until the circuit is complete. This wiring method is easy to identify due to the number of cables at the switch. It requires only that the live and switched live are identified at the switch.
NOTE: Always isolate mains power at the consumer unit before starting work.