I have some strange symptoms with my house's lighting circuit, which I am trying to isolate.

There are two lighting circuits in the house, upstairs and downstairs (although some downstairs lights seem to be on the upstairs ring - which I find strange)

I had 3 GU10 bulbs in a light fitting in the kitchen - two halogen, one LED. This was working fine, until one of the halogen bulbs blew.

I replaced all 3 bulbs with LED bulbs, for consistency and since then I have had a persistent problem that the bulbs are very dim until you turn on several other nearby lights (all on the same ring from what I can tell)

Whilst the bulbs are glowing dim, it also causes interference with our TV reception, which is probably due to a booster unit being wired up to the lighting ring.

My question is: can bulbs cause this strange behaviour? We have had other similar issues, which have been resolved by replacing all bulbs in the house with non-LED/non-CFL bulbs, which seem to cause problems. Or perhaps it is one or more light fittings at fault?

The breaker never trips, which is something I guess.

  • What country are you in? Are you sure the circuits are rings and not branches? When was the wiring installed? Feb 2, 2018 at 13:53
  • UK, 1996 (approx). Not sure on ring vs branch, I was basing my assumption of "ring" on the breaker having two separate switches for lighting
    – james246
    Feb 2, 2018 at 13:56
  • Do you have access to a voltmeter? Measure the voltage across the LED fixture under the various conditions. Feb 2, 2018 at 14:01
  • UK 6A lighting circuits are usually radial circuits. Only the 32A socket circuits are rings. Having separate lighting circuits per floor is normal. Feb 2, 2018 at 14:07
  • 1
    I had to remove my misleading comment since overvoltage from a failing neutral occurs only with split single phase supply as commonly used in the USA. Feb 2, 2018 at 14:27

2 Answers 2


It's your old dimmer, motion sensor, lighted switch, smart switch etc.

It isn't made to play with LEDs.

A dimmer, smart switch etc. is an active machine which requires power to operate itself (i.e. always-hot and neutral). Many old installations use what we Yanks call a "switch loop", containing always-hot, switched-hot, sometimes earth and never neutral. (Our Codes now require provisioning of a neutral).

Therefore older active switches (and some new ones) power themselves by "placing themselves in series" with the load. When the load is off, they depend on a unique characteristic of incandescents that an unlit incandescent is a lot like a dead short. The active switch can leak small amounts of power through the incandescent without lighting it up.

LED and CFL have switching power supplies which do not like that (unless they are specifically designed for that). When you removed the last incandescent, you placed the active switch and LEDs in combat with each other, hence the RFI.

You did not state which special ability the switch has, so it may not be important to you. Swap it for a regular switch and done. Or, put one incandescent back in. Or get a modern LED-rated version of that thing and matching LEDs.


can bulbs cause this strange behaviour?

Faulty bulbs might, but in general, it is probably more accurate to say that LED bulbs tend to show up other faults/defects/incompatibilities elsewhere in the circuit.

In your description there are indications that something is amiss elsewhere

some downstairs lights seem to be on the upstairs ring

Like you, I find this very strange. Any downstairs location is likely to be nearer to the downstairs lighting circuit than to an upper-floor lighting circuit. It suggests some changes may have been made to the wiring after construction. Perhaps by an amateur. Perhaps they introduced some fault which your LED behaviour is indicative of.

Whilst the bulbs are glowing dim, it also causes interference with our TV reception

This suggests the LED bulbs may have insufficient protection against EMF emission. The internal driver circuits sometimes use a miniature switched-mode power-supply design - these operate at high frequencies. Alternatively, your TV signal booster may have insufficient filtering on it's power input circuitry. However many LED drivers use a passive voltage-dropper which I don't think would be as likely to cause interference to a TV booster.

I'd try inserting one standard B22 LED bulb from a major maker (e.g. Phillips, GE, etc) and see if those symptoms recur.

I think I'd also try unplugging the signal booster to see if a LED bulb then behaves more normally.

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