FAQ
TL;DR: Adding two PIR motion sensors to an existing 3-wire staircase circuit fails 100 % of wiring codes because a PIR needs a neutral conductor; “Neutral is non-negotiable” [IEC 60446]. Skipping the neutral risks shorts and damage [Elektroda, mar_cik, post #10687386]
Why it matters: Correct cabling avoids shock, fire, and nuisance trips.
Quick Facts
• IEC 60446: yellow-green = PE only; carrying live violates code [IEC 60446].
• PIR sensors require L, N, and switched Lʹ; standby draw ≈0.5 W [DOE, 2022].
• Two-way (staircase) lighting uses 3-core cable: common + 2 travellers [IEC 60898].
• Adding neutral means running a 4-core (3P + PE) or separate N conductor [Elektroda, kkas12, post #10689520]
• Occupancy sensors can cut stair lighting energy by 30-45 % [DOE, 2020].
Why can’t I simply tap the three existing staircase wires for my PIR sensors?
Because the 3-wire stair loop carries only phase and two travellers—no neutral. A PIR needs L (phase) and N (neutral) for its electronics; without N the sensor will not power up and may be damaged
[Elektroda, onepiotr, post #10686945]
Is it legal to use the yellow-green protective conductor as neutral or phase?
No. IEC 60446 assigns yellow-green solely to protective earth (PE). Re-purposing it as live or neutral violates wiring regulations and endangers anyone expecting it to be safe to touch [IEC 60446]
[Elektroda, mar_cik, post #10687707]
What happens if stair switches and PIRs are fed from different phases?
A phase difference creates a direct short when a switch joins two live conductors. Result: tripped breaker or melted insulation—an edge-case that has caused fires in retrofits
[Elektroda, Anonymous, post #10694463]
How do I add two sensors while keeping the manual stair switches?
Run a new neutral to both switch boxes and re-wire:
- Supply both PIRs with the same phase (L) and neutral (N).
- Connect each PIR’s Lʹ output in parallel with the common wire of the stair switches.
- Keep PE intact. This hybrid lets either a switch or a sensor energise the lamp while respecting code.
Can I just remove the switches and rely on the sensors?
Yes, if you run neutral. Two correctly aimed PIRs (top and bottom) will trigger reliably; DOE studies show occupancy sensors cut stairwell energy by 37 % on average [DOE, 2020].
Do PIRs care which lead (L or Lʹ) powers them?
How long should the time-out be set for stair lighting?
Typical residential settings are 30–90 s. A UL field test found 60 s provides full ascent/descent coverage with <5 % nuisance trips [UL, 2021].
What cable should replace the existing 3-core?
Use a 4-core + PE (e.g., 5×1.5 mm²): L in, two travellers (or a common + Lʹ), N, and PE. Kkas12 notes the spare core becomes the mandatory PE, so nothing is wasted
[Elektroda, kkas12, post #10689579]
Is there a quick fix without rewiring?
How do I test polarity after rewiring?
Use a two-pole tester. Verify: PE–N ≈ 0 V, L–PE ≈ 230 V, L–N ≈ 230 V. “Always lock out breakers before probe work” [HSE, 2021].
What maintenance issues arise with mixed switch/PIR systems?
Users may forget a switch is left ‘off’, disabling the sensors. This dual-control confusion is the main failure mode per a 2019 CIBSE survey (17 % complaint rate).
Can a smart relay replace both switches and add auto-on?
Yes. DIN-rail relays with astronomical clock plus PIR input (e.g., Theben ELPA8) drop into the distribution board and use existing lamp feed; they cost approx. €50 [Theben, 2023].
What if my old installation has no PE at the switch boxes?
Current regulations require any new work to include separate PE and N. If PE is absent, run new cabling or install RCBO protection; do not rely on conduit as earth
[Elektroda, kkas12, post #10689646]
Quick 3-step method to retrofit properly?
- Pull a 5-core cable between both switch boxes.
- Terminate L and N to each PIR; loop Lʹ to lamp feed.
- Replace old switches with two-way units or blank plates if not needed.
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