Troubleshooting & maintenance

Why the RCD trips when you switch the lights on

The whole group trips the RCD when switched on together, but not one at a time. Each driver contributes earth leakage current, and a simultaneous close adds a transient on top. Sizing formula for a 30 mA RCD, three ways to split the circuit, a six-step location procedure and four factors that make it worse.

Last updated：2026-11-02

A whole group of luminaires trips the residual current device when switched on together but not one at a time. In most cases nothing is wrong with the drivers: every switched-mode supply contributes a small earth leakage current, and with enough units the sum matters - plus a transient on top when they all close at once. This article gives a size limit you can compute, three ways to fix it, and a six-step procedure to locate the cause.

The one-sentence rule: design to one third of the rated residual current, not to the rating

• On a 30 mA RCD, design to 10 mA, about one third of I delta n
• The remaining two thirds absorb transients, ageing and damp-season insulation drop
• At a typical 0.5 mA per unit: about 20 units per circuit. At 0.75 mA: about 13

Where the leakage comes from: every unit contributes

A switched-mode supply has EMI suppression capacitors (X and Y) across its input and parasitic capacitance in the transformer primary, and together they form a small path to earth during normal operation. That current:

• is normal operating current, not failed insulation
• grows with power rating, larger units leak more
• exists on constant-voltage units H01, K01 and S01 and on constant-current unit H19 alike - take the figure from the leakage current line of the datasheet, never from a rule of thumb

Computing how many units fit

n_max = (I_delta_n x 0.33) / I_leak_per

I_delta_n     rated residual operating current of the RCD (commonly 30 mA)
0.33          design factor, leaving room for transients and ageing
I_leak_per    leakage current of one driver (from the datasheet, typically 0.25 to 0.75 mA)

For a 30 mA RCD on a 230 V 50 Hz system:

Why one at a time is fine but all at once trips: the transient layer

Closing units one by one spreads the inrush over time. Closing them together means:

• every input EMI capacitor and bus capacitor charges at the same instant, so inrush and earth-capacitor charging current add up
• the peak lasts a very short time, milliseconds, but its amplitude is commonly several times the steady-state sum (a field observation, not a standard value)
• a circuit that sits at only 8 mA steady state can therefore cross 30 mA during closing

That is why the leakage meter reads low and the breaker still trips. The test is simple: if closing one at a time holds but closing the main trips, it is transient summation, not an insulation fault.

Three ways to fix it, in order

1. Split the circuit (first choice): divide one RCD way into two or three, each inside the design limit. It costs a few modules in the board and it shrinks the failure zone - one way trips, the rest stay lit
2. Get the real per-unit leakage and recount: ask the supplier for the measured leakage current in the datasheet and recompute with the 0.33 factor. Sometimes a lower-leakage model holds the existing circuit, which is cheaper than reworking the board
3. Change the protection scheme: a time-delayed (selective) RCD upstream with ordinary RCDs or MCBs downstream, so only the layer nearest the fault trips. This is distribution design: it must be done by a licensed electrician to local rules, and it has to be confirmed that the higher rated residual current is permitted at that location

Four things that make it worse (check these before touching the board)

1. Neutral and earth crossed or earthed twice: part of the working current returns through earth and the residual current is artificially raised
2. Driver cases bonded to large metalwork: luminaires or trunking increase parasitic capacitance to earth; an insulating shim helps
3. Long output cable run against earth or metal: same effect, very visible on outdoor facades
4. Insulation drop in the damp season: rainy seasons and condensing basements are the peak season for nuisance tripping; readings fall again once dry - design for the worst season, do not accept on a dry day

Six steps to locate it

1. Tell leakage from overcurrent: RCD only, MCB holding means leakage; both tripping or the MCB alone is an inrush current and breaker sizing problem
2. Close one way at a time: find which way, shrinking the search from a whole floor to one circuit
3. Clamp the residual current: clamp line and neutral together on a single-phase circuit; the reading is the residual current. Compare against the 10 mA design value
4. Halve the load: disconnect half, close again, and narrow down each round
5. Inspect wiring and mounting: neutral and earth crossed, case bonded to metalwork, output cable routing
6. Get the datasheet figure and recount: with the real leakage current, decide between splitting the circuit and changing the model

Acceptance checklist

1. Per-unit leakage taken from the datasheet, not estimated
2. Units per RCD way at or below (I delta n x 0.33) divided by per-unit leakage
3. Main switch closed three times, no trip
4. Residual current re-measured in the damp season, or after a water test, still 15 mA or less
5. A trip affects only that way, not the whole floor
6. The final protection type and rating comply with local rules, with a signed record from the electrician

Leakage figures for constant-voltage units H01, K01 and S01 and constant-current unit H19 must come from their own datasheets. Circuit current and power conversion runs through the LED driver sizing calculator, and outdoor or damp locations add the condensation and insulation requirements in IP rating and outdoor selection.

© 2026 YFX. Figures are for selection purposes. Always follow the datasheet and local wiring regulations for final installation.