Outdoor Lights Not Working: PIR Sensors, Photocells, IP Ratings & Earthing Faults

Summary

Outdoor lighting faults are one of the more common callbacks for domestic electricians. The outdoor environment is hostile to electrical equipment — UV, temperature extremes, rain, frost, and insects all attack fittings and connections. A fitting that worked for 5 years may fail because the IP gasket has degraded and water has entered, corroding a terminal.

The diagnostic approach for outdoor lights follows the same logic as all electrical fault finding: confirm supply, then confirm control, then confirm lamp. The additional outdoor-specific steps are: check the IP rating of the fitting (is it appropriate?), check for moisture ingress in the fitting or back-box, and check the RCD has not tripped on a ground fault.

Understanding how PIR (passive infrared) motion sensors and photocells (light sensors/dawn-to-dusk sensors) work is essential because these controls are common on outdoor lights and their failure modes are specific.

Key Facts

• IP rating for outdoor use — Minimum IP44 for wall lights (sheltered position), IP65 for exposed positions (rain directly striking the fitting), IP67 for partially buried or ground-level fittings
• IP breakdown — First digit (dust): 4=dust protected, 6=dust tight. Second digit (water): 4=splash-proof, 5=jet-proof, 6=immersion-proof, 7=temporary submersion
• PIR sensor detection — Detects change in infrared radiation (heat movement). Range typically 10–12m, angle 90–180°. Adjustable sensitivity, time, and range on most units
• PIR false triggering — Caused by: warm objects in range (cars, animals), direct sunlight/heat sources in field of view, radiated heat from heating outlets, or electronic interference
• PIR not triggering — Dead battery (wireless), obstructed lens, lens contaminated with spiders/debris, sensitivity set too low, range direction misaligned
• Photocell (dusk-to-dawn sensor) — LDR (light-dependent resistor) that switches the light on when ambient light falls below a threshold. Fails if contaminated, covered, or the component ages
• LED driver failure — LED fittings have a driver (transformer/converter) that fails more often than the LED chips. Symptoms: light doesn't come on, flickers, or dims over time
• RCD tripping — Earth fault from water ingress causes RCD to trip. If outdoor circuit RCD trips intermittently, check all outdoor fittings for moisture in the backbox

Quick Reference Table

SYMPTOM: Outdoor light doesn't come on at all
     |
     ├── Other outdoor lights working?
     │   No → Outdoor circuit RCD/MCB tripped? Reset and test.
     │   Yes → Fault is at this fitting only ↓
     ├── Check supply at fitting terminals (multimeter)
     │   No 230V → Trace back through outdoor socket/spur FCU
     │   230V present ↓
     ├── Bypass PIR or photocell (connect direct to live supply for test)
     │   Lamp lights → PIR/photocell failed — replace
     │   Lamp still doesn't light ↓
     └── Remove lamp and test in a known-good fitting
         Lamp works → Fitting or driver faulty
         Lamp doesn't work → Lamp (or driver) faulty — replace

SYMPTOM: PIR light doesn't trigger reliably
     |
     ├── Check time-of-day: is it daylight? (if photocell-linked, won't trigger in daylight)
     │   Yes → Normal operation
     │   No ↓
     ├── Walk in front of PIR at close range (2m)
     │   No trigger → Check PIR power supply, clean lens, check sensitivity setting
     │   Triggers close → PIR range set too low — increase sensitivity ↓
     ├── Does PIR trigger on animals/cars but not people?
     │   → Sensitivity too low or lens obstructed — adjust and clean
     └── PIR triggers for no apparent reason (false alarms)
         → Adjust aim to remove heat sources from field of view
         → Reduce sensitivity
         → Check for spider webs on lens

Detailed Guidance

Moisture Ingress Diagnosis

Outdoor light failures are frequently caused by moisture entering the fitting or junction box. Signs of moisture ingress:

• Corrosion (rust, green oxidation) on terminals or lamp contacts
• Water visible in the fitting body or back-box when the fitting is removed
• RCD tripping after rain (water creates a ground fault path)
• Intermittent operation that correlates with wet weather

Fixing moisture ingress:

1. Remove the fitting and back-box
2. Dry out thoroughly (hair dryer on low heat, or leave in a warm dry environment)
3. Identify the ingress point — often a failed gasket, cracked fitting body, or cable entry not sealed
4. Replace the fitting if the body is cracked or the gasket is perished
5. Ensure cable entry is sealed with appropriate gland or silicone
6. Check the IP rating of the replacement fitting is appropriate for the mounting position

PIR Sensor Testing and Adjustment

Testing a PIR:

1. Confirm power at the PIR terminals (230V between live and neutral)
2. Cover the PIR lens with tape (to simulate darkness)
3. Walk in front of the PIR — if properly functioning, it should switch the output on within a few seconds
4. Uncover the lens — the PIR should switch off after the set hold time

Adjustment settings (on most PIRs):

• Sensitivity (LUX): How much ambient light is present before the PIR will respond. Set lower in areas with bright street lighting
• Sensitivity (SEN): Motion detection sensitivity. Turn up if it's not detecting, down if it triggers falsely
• Time (TIME): How long the light stays on after motion stops. Range typically 5 seconds to 10 minutes
• Range: Physical angle is fixed on most sensors, but sensitivity adjustment effectively changes the detection range

PIR not detecting:

• Clean the lens with a dry cloth (dust, spider webs, and dried insect residue on the lens dramatically reduce sensitivity)
• Ensure the PIR is aimed correctly — the detection zone is in front of the lens, not to the sides
• Check the PIR is not covered by vegetation or a structural element
• Confirm power to the PIR

PIR wiring (4-wire type): Live in, live out to lamp, switched live from the lamp switch, and neutral. The 4th wire (switched live) allows the PIR to be bypassed from a wall switch.

Photocell (Dusk-to-Dawn) Faults

A photocell switches the light on at dusk and off at dawn. It consists of an LDR (light-dependent resistor) and a switching relay.

Photocell runs all day (light on in daylight):

• LDR has failed in the "night" state (no light detected even in daylight)
• LDR is covered or in shadow — reposition
• LDR has been painted over (spray paint obscures the lens)

Photocell doesn't activate at night (light off):

• LDR failed in "day" state
• LDR aimed at a light source (adjacent street light or window) that keeps it in permanent daylight detection
• Photocell switch/relay has failed — test by bypassing

Replacing photocell: Most external photocells are standard Schuko-type plugged into the top of the fitting (twist and remove). Universal replacement photocells (B22 or GU10 types, or external screw-cap type) are available from electrical wholesalers.

LED Driver Failure

LED lights use a driver (power conversion unit) to convert 230V AC to the low-voltage DC required by the LED chips. Drivers fail — typically after 3–8 years depending on quality. Symptoms:

• Light doesn't come on despite power present
• Light flickers constantly (not just at switch-on)
• Light gradually dims over 6–12 months before failure
• Driver body visibly deformed or smells burnt

Testing: With power isolated, disconnect the driver. Apply 230V AC to the input terminals. Measure DC voltage output — should match the specification (typically 12V or 24V DC). No output = failed driver.

Replacement: Many fitting-specific drivers are not interchangeable. Check the output voltage, current rating (mA), and LED array connection type before ordering a replacement. Some fittings are more economically replaced as a complete unit.

IP Rating Check for Installation Location

Using a fitting with insufficient IP rating in an exposed position is a guarantee of early failure and potentially a safety hazard. If in doubt, use IP65.

Frequently Asked Questions

The outdoor RCD keeps tripping after rain. What do I do?

This indicates a ground fault — water is creating a conductive path between live conductors and earth in one or more outdoor fittings. To trace: isolate each outdoor circuit branch one at a time and test for insulation resistance (IR) between line and earth with a Megger. The fault is in the branch that fails the IR test. Check every fitting in that branch for moisture ingress.

My PIR triggered reliably for 3 years and now it won't detect people but still detects warm cars. Why?

The PIR lens is most likely contaminated. Spiders build webs across PIR lenses — the fine filaments absorb warmth and the PIR responds to the warm web instead of a person. Cars have much greater thermal mass and will trigger even through a moderately contaminated lens. Clean the lens with a dry cloth.

Can I install outdoor lighting myself without being an electrician?

If you are replacing like-for-like (same fitting position, same wiring), this is generally not Part P notifiable. If you are adding a new outdoor circuit or spur, this is notifiable (special location — outdoors) and must be done or certificated by a registered electrician or notified to Building Control.

Regulations & Standards

• BS 7671:2018+A2:2022 — IET Wiring Regulations: IP ratings for outdoor fittings, RCD protection (mandatory for all outdoor circuits)

• BS EN 60529 — IP classification standard

• Approved Document P — Notifiable electrical work in special locations including external areas

• IET Guidance Note 7: Special Locations — Outdoor electrical installation requirements

• ELECSA Outdoor Lighting Guidance — Part P and competent person scheme guidance

• Electrical Safety First — Garden Electrics — Consumer guidance on safe outdoor electrical installations

• outdoor electrics — Full outdoor electrical installation guide

• bathroom zones — IP rating zone classification (similar principle to outdoor)

• rcd tripping — RCD tripping diagnosis procedure

• pir sensors — PIR sensor wiring and selection guide