LED bulbs that emit a faint glow when the switch is in the off position are one of the most common LED retrofit complaints in UK homes – especially in bedrooms where any light at night is intrusive. The phenomenon has a name: LED ghost glow (sometimes phantom power or ghosting). It is almost never dangerous, but it is annoying and the fix is usually cheap and quick.

This guide explains what causes ghost glow in UK installations, how to identify which cause applies to your circuit, and the most reliable fix – a small bypass capacitor at the light fitting.

What ghost glow looks like

Step into the affected room with the lights switched off and let your eyes adjust to the dark for a minute. If you have LED ghost glow, you will see a faint, steady (or slowly pulsing) glow from the bulbs themselves. The glow is much dimmer than the bulb at full brightness – just visible in a properly dark room, often missed in daylight or with curtains open.

Other signs:

• Some bulbs in the circuit glow, others do not (driver sensitivity varies between bulb models)
• Bulbs glow at slightly different intensities
• The glow is more obvious when the room is completely dark
• The glow remains the same whether the dimmer is fully off or in any position when the main switch is off

Why ghost glow happens

An LED bulb has electronic driver circuitry inside that converts mains voltage to the low DC voltage the actual LEDs need. The driver is sensitive: even a very small AC current through it can deliver enough energy to light the LEDs at a fraction of their normal brightness.

In a UK lighting circuit there are three common sources of that small current reaching a switched-off bulb:

1. Leakage current from a dimmer or smart switch

By far the most common cause. Modern dimmers and especially no-neutral smart switches need a tiny current to power their own electronics even when the load is “off”. That current has to flow somewhere, and the only path available is through the load – the bulb. Incandescent and halogen bulbs absorbed this current invisibly; LED bulbs do not.

This is the cause if:

• The affected circuit has a dimmer or a smart switch
• Replacing the dimmer/smart switch with a plain switch fixes the glow
• The neutral wire is not present at the switch back box (the standard UK loop-at-the-rose topology)

2. Capacitive coupling from parallel cables

Where cables run long distances in parallel – typically in conduit, trunking, or bundled in joists – the switched live conductor can pick up a small induced voltage from the adjacent permanent live conductor. The LED driver harvests enough energy from this induced voltage to glow faintly.

This is the cause if:

• The cable run from switch to light is unusually long
• Several cables run together in conduit, trunking, or a tight bundle
• The glow appears on circuits with no dimmer and no smart switch

Less common in domestic installations than cause 1, but worth knowing about for longer commercial or outdoor runs.

3. Illuminated (neon-indicator) wall switches

Some older wall switches – the ones with a small orange neon dot that lights up when the switch is off so you can find it in the dark – work by passing a tiny current through the lamp at all times. That current illuminates the neon indicator. With incandescent bulbs this current was too small to produce visible light; with LED bulbs it lights them up faintly.

This is the cause if:

• The affected circuit has an illuminated switch (the switch face has a neon glow when off)
• Removing or replacing the illuminated switch fixes the glow

The fix: a bypass capacitor at the light fitting

For all three causes, the standard UK fix is the same: a small bypass capacitor wired in parallel with the lamp at the light fitting. The capacitor presents a low-impedance path to the small leakage current at mains frequency – it effectively “absorbs” the trickle before it can reach the LED driver. With no current reaching the driver, the bulb stays fully off.

Fitting is straightforward: the capacitor goes across the live and neutral terminals at the ceiling rose, or directly across the lamp holder terminals. One capacitor per affected light fitting is usually sufficient – it covers every bulb on that fitting. The capacitor draws no measurable power itself, so it does not affect the lamp brightness or the electricity bill.

How to fit the bypass capacitor

1. Isolate the lighting circuit at the consumer unit and prove dead at the light fitting with a GS38-compliant two-pole voltage tester.
2. Access the light fitting – typically the ceiling rose or the lamp holder of a pendant. For downlights, access from the loft or via the recessed fitting.
3. Identify live and neutral at the lamp connection point. In a standard UK ceiling rose, live is the switched live coming from the wall switch; neutral is the neutral loop.
4. Wire the capacitor across the lamp’s live and neutral terminals in parallel – one capacitor lead to live, the other to neutral. Polarity does not matter; these capacitors are non-polarised AC types.
5. Tuck the capacitor into the rose enclosure so it is not touching the lamp or fouling other conductors. Secure with a tie-wrap or push-in clip if needed.
6. Reassemble the fitting, restore power, and verify the bulb fully extinguishes when the switch is off.

If you have multiple light fittings on the same circuit and only some are affected, fit the capacitor only to the affected fittings – there is no need to install one at every fitting.

Other options

Try a different LED bulb. LED bulb sensitivity to leakage varies enormously by model. Quality bulbs from established brands (Philips, Osram, LIFX, and similar) are generally less prone to ghost glow than the cheapest unbranded options. If ghost glow appeared after a bulb change, swapping back to the previous brand may resolve it without a capacitor.

Remove the neon indicator from illuminated switches. If the cause is an illuminated wall switch, replace it with a standard non-illuminated switch. This is often the cheapest fix and produces a permanent solution.

Replace the dimmer or smart switch. Some dimmers and smart switches are designed for very low residual current and produce less leakage than older models. If yours is an early-generation no-neutral smart switch and you do not want to use a bypass capacitor, a newer LED-rated dimmer may reduce the glow without further intervention.

When ghost glow is not just a nuisance

Ordinary ghost glow is dim, steady, and harmless. It does not warm the bulb, draw measurable power, or affect bulb lifespan in any practical way. If your symptoms are different from the standard description above, treat it as a wiring issue rather than a leakage issue.

Smart switches and ghost glow

No-neutral smart switches are the most common cause of LED ghost glow in modern UK homes. They need a small standby current to power their wireless radios, and that current flows through the lamp. The SM323 smart dimmer range is designed to minimise this residual current while maintaining wireless connectivity. Where ghost glow is still visible after fitting an SM323 dimmer, the bypass capacitor above is the standard remedy.

• SM323 Zigbee Dimmer – Zigbee mesh; Home Assistant, Philips Hue Bridge, SmartThings, Hubitat
• SM323-WF WiFi Dimmer – direct WiFi; Tuya / Smart Life, Alexa, Google Home
• SM323-MT Matter Dimmer – Matter over WiFi or Thread; Apple Home, Alexa, Google Home