Junction box wiring method (4-terminal)

The 4-terminal junction box does the same job as a loop-in ceiling rose, but is used when you cannot fit a rose at the loop point — for example a recessed downlight or a wall light with no room for loop-in cables behind it. The box lives in the ceiling void and the mains loops through it instead. It has four terminal blocks: L (live/line), N (neutral), E (earth/CPC), and Sw (switched live). Four cables connect: • Mains in: supply from the CU (or previous light) • Mains out: continues to the next light on the circuit • Switch wire: permanent live down to the switch; switched live returned • Light drop: switched live and neutral to the light fitting BS 7671 Reg 526.3: the junction box must be accessible (or use a maintenance-free type if concealed).