Fit the wrong dimmer on a run of LED downlights and you'll be back on site within a fortnight sorting a buzz nobody mentioned at quote stage.

What's Actually Different Between Trailing Edge and Leading Edge?

Leading edge is the older, simpler technology. It cuts the start of each half-cycle and was designed around resistive and inductive loads like halogen and some incandescent fittings. Most leading edge modules need a minimum load of around 40W to switch cleanly, which a single LED lamp rarely provides on its own.

Trailing edge dimmers are electronic, and they cut the end of each half-cycle after the zero-crossing point instead. BG Evolve's trailing edge single dimmer module handles loads from as low as 3W up to 200W, which is the gap that actually causes most of the flicker complaints we see when someone's fitted a leading edge dimmer onto an LED circuit.

Why Does the Wrong Dimmer Make LEDs Flicker or Buzz?

Most LED retrofit lamps sit well under the minimum load a leading edge dimmer needs to switch properly. The dimmer's circuit ends up hunting for a load it can't find, and that shows up as flicker, buzz, or both. We had exactly this come in from a sparky in Sheffield last month: a full kitchen of LED downlights buzzing on a leading edge dimmer that had been sitting on the same circuit since the halogens it originally controlled. Swapping to a trailing edge module fixed it without touching a single lamp.

Trailing Edge vs Leading Edge: Quick Comparison

Does BS 7671 Amendment 4 Change Anything for Dimmer Circuits?

BS 7671:2018+A4:2026 widens where Arc Fault Detection Devices need to be specified, effective from 15 April 2026 with the transition period running to 15 October 2026. Electronic dimmers generate electrical noise on the circuit, and leading edge modules tend to generate more of it than trailing edge ones, which matters because noisy circuits are exactly what AFDDs are watching for.

BG Evolve dimmer modules are built to BS EN 60669-2-1, the relevant switch and dimmer product standard, but the AFDD compatibility question sits at the installation level rather than the product standard. It's worth speccing trailing edge BG Evolve modules ahead of leading edge ones on any circuit you know will carry an AFDD, simply because they run cleaner, and pairing them with matching BG Evolve switch plates keeps the finish consistent across the room. Check the IET's BS 7671 guidance and the AFDD manufacturer's own notes if you're unsure on a specific job.

For the wider Amendment 4 picture, our BS 7671 Amendment 4 guide covers what's changing across the rest of the installation.

Which BG Evolve Dimmer Module Should You Spec?

Unless the job is pure halogen, trailing edge is the safer default even at the extra few pounds. The callback you avoid costs more than the price difference.

Frequently Asked Questions