Inside a 4-Terminal Junction Box

A 4-terminal junction box is the central hub in junction box wiring. Understanding its terminal connections is essential before opening one.

Terminal Layout

1Mains L2Mains N3Switch4Earth

What Connects Where

Terminal 1 — Mains Live:

  • Brown (L) in from consumer unit
  • Brown (L) out to ceiling rose
  • Brown (L) out to next junction box (if daisy-chaining)

Terminal 2 — Mains Neutral:

  • Blue (N) in from consumer unit
  • Blue (N) out to ceiling rose
  • Blue (N) out to next junction box

Terminal 3 — Switch:

  • Brown (switch live) out to wall switch
  • Blue, sleeved brown (switch return) in from wall switch

Terminal 4 — Earth:

  • All earth conductors from all cables
  • Earth of rose feed cable
  • Earth of switch cable

Key Point

Terminal 3 is where the switching action happens. Only live conductors connect here — there is no neutral at the switch. The switch simply breaks or connects the live path back to the rose.

Safety

Always check the junction box is accessible and properly labelled. A fully enclosed JB cannot be embedded in a wall or ceiling — it must remain accessible for inspection.