Modern LED lighting is great. Bright, clean and a fraction of the running cost of incandescent lighting such as halogen or tungsten lamps. That said, one oddity is that they sometimes glow when off. I can think of four reasons for this phenomenon and I have a potential quick and dirty fix for the most common...

 
I first noticed the 'glowing when off' issue after I fitted three LED lamps in my home's entranceway back in 2005. LED lamps were in their infancy back then and these cluster lamps aren't great, but they're only lighting a small space and at a paltry 1.3W each they've already saved a sack load of energy over the 11W CFL fitting that was there before.


I did find it curious that when switched off, some of the LED elements in these lamps continued to glow dimly. There have been no ill effects over the past nine years and these lamps continue to work just fine. I came across the problem again today however after converting an R50 fitting from incandescent to LED operation. This time the effect was more noticeable, and because it was in a bedroom, less desirable.


In this case, the cause was stray capacitance on the switch wiring, and this is likely to be the issue for most readers. The problem is worse on older lighting circuits and longer switch runs such as in two or three-way switching arrangements and relates to how lighting and switch wiring is routed... or used to be routed. On older properties, lighting wiring was often in 'singles', so the line wire feeding the light ran separate to the neutral wire and there was often no circuit protective conductor. The switch drop then consisted of two singles or a twin cable to provide the feed in to the switch and the switched line back out to the lamp. For two or three-way switching, you'd tended to have a twin cable plus a single, again often with no protective conductor.


When a voltage is applied to a wire, an electric field is created around it. Where a line wire is coupled with a neutral and/or a circuit protective conductor (earth), this electric field is dampened, but when a line wire runs as a single or in an unearthed two-core cable as in many switch drops, then there is nothing to dampen the field. As a result, the live feed wire going to the switch has an electric field around it which pervades the switched wire coming out. When the switch is off, a capacitor is effectively created between the two wires; a capacitor being two conductive plates separated by an insulator and our switch wire being two copper cores separated by plastic and air. The switch wire which should be non-energised becomes energised by an induced voltage, and a small current passes across the electric field between both wires. This current then passes through your LED lamp to get to neutral.

roguevoltage
Case in point: Here I'm reading 15.4 VAC at 50 Hz on this isolated lighting circuit because of capacitive coupling on the switch wire.


This has always been the case with your house wiring, but that tiny current was too small to have any affect on incandescent or fluorescent lamps you may have used before. LED lamps are different though and in some cases that current may just be enough to actuate the circuitry in your lamp and cause it to glow, albeit dimly when the light switch is off.


Short of rewiring your lighting, one solution is to stick something in the circuit to mop up this unwanted voltage. Using a CFL or incandescent lamp in one fitting (if a multiple lamp/downlight configuration) will soak it up, but use of an inefficient lamp isn't ideal.


Another way which should work without mixing lamp technologies is to obtain a neon indicator such as this example on Amazon. Such should only cost a couple o' quid or so and one prewired with flying leads as pictured will save some messing about. Install this between line and neutral at the fitting or at the end of a string of downlights and that may absorb the unwanted current at an annual cost of nothing at all. The neon will glow instead of the LED lamps, dimly when off, more brightly when on, but it can be hidden behind the fitting and out of sight. I've used this trick before to get out of trouble.

neon


Just make sure the neon you select is rated correctly for your territory, i.e. 220-240V for UK/Europe/Australia, 110V for the US, 120V for Canada and so on. Incidentally, old fridges, freezers, electric showers and kettles often have these neon indicators on their control panels or switches, and if you're handy with a screwdriver it doesn't hurt to nab 'em off any such equipment when it's reached end-of-life. Just make sure it is a self-contained mains-rated neon and not a 2V LED indicator if rescuing one from the scrap heap! Good neon indicators can burn away for decades, losing brightness over time but functioning nonetheless, and can find a new lease of life in a situation such as this.


A second reason for nuisance lighting are electronic dimmer switches as these can also be a cause of trouble for LED lamps even if they are supposed to be LED compatible. A dimmer may have a minimum rating in order to function correctly, and your LED lamps may not reach this rating. One particular Hager switch gave me some problems when trying to dim twelve Sylvania lamps, and continued to put out 30v even when 'off'. Hager's solution was to recommend I install a 10W resistive load in order to mop up the stray voltage. Companies such as Danlers actually make such a product which is bulky and expensive. It seems daft however to install energy efficient lighting and then to employ some energy gobbling resistance to worsen the efficiency for the sake of the dimmer switch! In the end, I sourced an alternative dimmer with a lower minimum load requirement.

wmresloadThe Danlers WMRESLOAD. This adds 10 Watts of resistive load to "pull up" a circuit that is drawing less than the minimum rating of an upstream device such as a dimmer switch.


I've also found some dimmable LED lamps to work better than others, and sometimes mixing in an odd lamp from a different brand can have positive effects on its neighbours.


A third issue are the new smart switches such as WiFi enabled, illuminated and touch sensitive models (Lightwave RF, Wemo, Philips Hue et al.) and the fact that in most switch drops here in the UK there is no neutral; just a live feed in and a switched live out. So how does a smart switch with intelligent circuitry operate without a neutral return path? Well, the answer is that there is a neutral return path, and if you look at the diagram below, you'll see that the path is through the light bulb...


smartdim

Yes folks, the clever circuitry in such a switch operates by passing a small current through the lamp(s) and back to neutral. This current passes all the time in order to keep the switch ticking over even when the lights are off. These kind of switches need power 24/7, often because they have touch controls, indicator lights or communicate with a WiFi bridge and need to stay alert for any on/off/dimming or colour changing commands. While passing a small current through the filament of an incandescent lamp will not be enough to cause it to glow, all bets are off when you pass any current through the electronics of a LED lamp as the myriad array of different makes, models and circuit designs within such lamps means some will react to this passing current while others will not. If you've invested in smart light switches from any manufacturer and find your LED lamps are all aglow when the switch is 'off', this is likely to be the problem, and the solution will be to install whatever LED lamps the switch manufacturer says are compatible.


Finally, problems from unwanted voltage on your wiring may be not through capacitive induction, but because of poor insulation resistance from adjacent wiring. If the insulation between two line wires on separate circuits is poor or compromised, then a voltage can arc across from one circuit and into another. If your wiring is old or hasn't been checked lately then it may be worth booking an inspection to see how your circuits are behaving. Tracing and repairing insulation faults is one for the specialists.


Unfortunately the bottom line is that the issue may be down to the lamps, the switch or the wiring and frigs such as the neon or resistive load may resolve the issue, but if it is driving you up the wall and you're not confident around electricity and screwdrivers, then look up a reputable electrician in your area who can assist with diagnosing and correcting the issue.


This article is also available in video form with the usual bad language thrown in: