Introduction
You fit new LED downlights and, bang, the RCD trips when the client hits the switch. Or it holds today but nuisance trips that evening. It’s not just annoying — it burns time, dents confidence, and eats margin. Here’s what’s going on, why it matters, and how to fix it once and for all with field-tested steps you can repeat on every job.
Table of Contents
Key Takeaways
- Nuisance tripping is usually a mix of earth leakage plus LED driver inrush, not “bad fittings”. In general, LED drivers can pull many times their nominal current for a few milliseconds at switch-on.
- Device choice matters. Commonly, B-curve MCBs trip at roughly 3–5x In, while C-curve at about 5–10x. Type A RCBOs handle DC components better than Type AC and often reduce nuisance trips.
- Quick pre-tests (insulation resistance, loop, leakage checks) take around 15–25 minutes and can prevent hours of callbacks later.
- Many contractors find splitting a large lighting zone into two circuits and sequencing switching cuts nuisance trips by half.
- Capture circuit details and driver specs during survey. Many contractors report saving 30–60 minutes per quote using voice capture and sending clear, signable proposals.
The Downlight Tripping Problem
Nuisance tripping happens when the protection device sees too much instantaneous current (inrush) or cumulative earth leakage. With legacy halogens swapped for LEDs, the circuit load is lighter, but the electronics add behaviour traditional devices don’t love.
Why It Matters
- Callbacks on “simple” lighting can quietly drain profit. It’s common for small jobs to lose a few hours in return visits if the cause isn’t nailed first time.
- Clients equate tripping with “unsafe”. Confidence drops fast even if the installation tests fine.
- According to common trade experience, cumulative leakage across multiple LED drivers and appliances can add up to 10–20 mA on a dwelling — closer to 30 mA RCD thresholds than many expect.
Real-World Signal
Many contractors report that the worst offenders are large open-plan areas with 10–16 downlights on one switch and older Type AC RCDs upstream. Everything passes tests individually, but trips on first energisation or at random times.
Diagnose Before You Replace Anything
Start with facts, not guesses. A tight, repeatable diagnostic pass prevents part-swapping.
Quick Checks That Find Most Issues
- Visual and make-up
- Confirm polarity and CPC continuity at each fitting.
- Check junction boxes and connectors are maintenance-free where concealed.
- Insulation resistance (IR)
- Test line–neutral, line–earth, neutral–earth with fittings disconnected if needed. In general, you want stable readings that don’t wobble when wiggling cables.
- Loop= impedance (Zs) and prospective fault current (PFC)
- Verify disconnection times are achievable with the existing protective device.
- Earth leakage snapshot
- If you carry a clamp meter capable of mA readings, measure leakage on the final circuit and at the RCD. It’s common to find a few mA per driver adding up across a big zone.
- Inrush behaviour
- Where available, use a meter with inrush capture. In general, LED drivers may pull 10–50x rated current for a few ms; grouping many on one switch can spike beyond B-curve thresholds.
Example: Kitchen/Family Room Zone
- 14 downlights on one switch, B6 MCB, upstream Type AC RCD.
- IR is stable; Zs is fine; leakage on that lighting circuit peaks around 7–9 mA when all drivers energise.
- First-on after a mains dropout trips the RCD; subsequent toggles often don’t — classic inrush plus cumulative leakage case.
Fix The Protection And Switching
Match your protection to modern loads and control the switching profile so you’re not hitting everything at once.
Choose The Right Device
| Option | Typical Instant Trip Behaviour | Where It Helps | Considerations |
|---|
| B-curve MCB | ~3–5x In | Traditional resistive loads | Can nuisance-trip with LED inrush |
| C-curve MCB | ~5–10x In | Circuits with moderate inrush | Ensure Zs still meets disconnection times |
| Type A RCBO | RCD element tolerates DC components; MCB per curve | LED drivers, EV chargers, appliances with electronics | Usually reduces nuisance vs Type AC; one device per circuit |
- In general, moving from a B6 to a C6 or using a Type A RCBO on the lighting circuit reduces false trips where inrush is the culprit.
- According to common UK practice under BS 7671, Type A is preferred over Type AC for most modern electronic loads.
Split And Sequence
- Split large zones into two circuits (e.g., cooking area vs seating area). Many contractors find this alone halves nuisance trips.
- Use two-gang switching or staggered relays so not every driver energises at the same millisecond.
Example Fix
That 14-downlight zone becomes two 7-point circuits, each on a Type A RCBO, with separate switches. The upstream RCD sees a lower combined inrush and leakage. Tripping stops, client is happy, and you’re not back at 8pm.
Reduce Leakage And Inrush At The Source
Don’t just treat the symptom. Tame the drivers and the wiring.
Pick Drivers And Fittings That Behave
- Specify quality drivers with soft-start or inrush control. In general, these tame the spike that trips B-curve devices.
- Avoid mixing driver types on the same switch; different inrush profiles can stack unpredictably.
- Use IP-rated fittings in damp areas; moisture ingress can increase leakage over time.
Wiring And Connections
- Keep neutrals tidy. Shared neutrals between RCD-protected circuits are a classic trip trigger.
- Use maintenance-free connectors in concealed locations; poor terminations can leak or arc under transient load.
- Consider slightly upsizing cable on long runs to keep impedance within comfortable limits for C-curve disconnection.
Leakage Budgeting (Simple, Practical)
- Aim for a lighting circuit leakage “budget” in the low single-digit mA range. It’s common to see roughly 0.5–1.0 mA per driver, so plan the number of fittings per circuit accordingly.
- If a dwelling’s total standing leakage is already high from appliances, avoid adding another big electronic load behind the same 30 mA device.
Example Specification Note
“Kitchen lighting split across two circuits; use Type A RCBOs; only XYZ-series downlights with integrated soft-start drivers; maximum 7 fittings per switch bank; IP65 over sink and hob zones; maintenance-free junctions.”
Build It Into Your Proposal And Handover
If you sell the fix clearly, you avoid price-only comparisons and set expectations.
What To Put In The Proposal
- The problem: “Existing protection and driver inrush are causing nuisance trips.”
- The fix: “Split circuits, Type A RCBOs, soft-start drivers, tidy neutrals.”
- The benefit: “Fewer trips, safer discrimination, cleaner test results.”
- The proof: include basic test snapshots (IR ranges, Zs, and a brief leakage note).
Many contractors find that spelling this out reduces back-and-forth by half and positions you as the pro who solves root causes.
Capture Fast, Send Fast
- On site, speak the circuit details, counts, and test notes into your phone and attach a couple of photos. With Donizo, your voice, text, and images convert into a clean proposal fast.
- Send a branded PDF with clear scope, then get a digital e-signature so the client formally accepts the method. After acceptance, convert to invoice in one click to keep cash moving.
Commonly, this saves 30–60 minutes per quote and shortens the decision cycle, especially when clients compare multiple bids.
Frequently Asked Questions
Why Do LED Downlights Trip RCDs More Than Halogens?
LED drivers contain electronics that create brief inrush currents and small but continuous earth leakage. In general, a bunch of drivers switching on together will spike current and can nudge a 30 mA RCD if cumulative leakage is already high. Halogens were mostly resistive, so they didn’t behave this way.
When Should I Use A C-Curve MCB Or A Type A RCBO?
If inrush is the main issue and Zs still satisfies disconnection times, a C-curve can help. Where DC components and leakage are contributing (which is common with modern electronics), a Type A RCBO per circuit is often the cleanest answer. Many contractors report nuisance trips dropping significantly after switching from Type AC to Type A devices and splitting circuits.
How Many Downlights Per Circuit Is Sensible?
It depends on driver type and inrush, not just wattage. In general, 6–8 quality fittings per switch bank behaves better than 12–16 on one gang. If the space needs more points, split across two circuits or use drivers with soft-start.
What Tests Should I Do Before Quoting A Fix?
Run insulation resistance (lines to earth and neutral), Zs at the farthest point, and a quick leakage snapshot with a clamp that reads milliamps. These checks usually take 15–25 minutes and tell you whether you’re dealing with inrush, leakage, or wiring faults.
Do I Need To Change The Main RCD?
Not always. Often the best move is to use Type A RCBOs on the lighting circuits and tidy the neutrals. If the main RCD is old or overly sensitive, replacing it with a modern Type A unit can help, but verify the whole-board leakage first.
Conclusion
Downlight trips aren’t random. They’re predictable once you look at leakage, inrush, protection type, and switching. Diagnose quickly, specify the right devices, split and sequence larger zones, and choose drivers that behave. Capture the solution clearly in your quote and you’ll avoid callbacks, protect your margin, and keep clients confident.
Want to move faster? Speak your site notes and photos into Donizo, generate a professional proposal, get a legal e-signature, and convert to invoice when the client says yes — without retyping a thing. That’s time back in your week and fewer evenings stuck at the desk.
