Design voltage of incandescent lamps

As is well known, the nominal or declared voltage of UK low voltage mains was reduced from 240 volts down to 230 volts, some years ago. Nothing much actually changed though and the measured voltage still tends to be 240 volts most of the time in most places.

But what  is the design voltage of mains voltage incandescent lamps for the UK market ? is it 230 volts or 240 volts. Is the light output and service life measured at 230 volts or at 240 ?

If a lamp designed for 230 volts is burnt at 240 volts the life will be significantly reduced.

If a lamp designed for 240 volts is run from an actual 230 volt supply, then the light output will be significantly reduced.

Incandescent lamps are now much less used, but there is still a substantial market via a number of loopholes. Traffic signals still use incandescent lamps.

  • As far as I know, for supply to CENELEC countries including the UK, manufacturers will design for 230 V, with expected variations as per BS EN 60038 at the point of utilization (i.e. 230 V + 10 % / - 15 % for general public supplies across CENELEC).

    Occasionally, for UK only products, the design is for 240 V + 6 % / - 10 %, e.g. some showers.

  • Over the permissable range it doesn't make a huge difference with incandescent's as they're NTC devices - the hotter the fillament the higher its resistance so they effectively regulate themselves. When I was a kid I built a Wien bridge oscillator which actually exploits this in its feedback loop.

    However if you put an incandescent bulb on a dimmer then you lose this effect which can be a little annoying for e.g. mood lighting when the grid voltage isn't stable or the electical installation dosn't meet vd requirements.

  • And you need to rate the semiconductors in the dimmer for the cold inrush current - as when very dim, the filament resistance never rises to the final value. For tungsten lamps figures  of 8:1 to 10:1 come to mind,  so a 60watt lamp (1/4 amp, ~ 1000 ohms when hot) has a cold resistance of about 120 ohms and needs at least a 2A triac.  The life shortening is not negligible though as high efficiency filaments run quite close to failure - 90% volts is almost double life expectancy and 120% is near instant failure.
    As this graph illustrates.

    From Vaughn and Hughes 'Lamps and Lighting'


  • One of my friends at uni went on to work in broadcasting.

    He told me that the stage lighting is dimmed down to imperceivable lumens for this very reason – stage lights are very expensive, especially if one fails during filming and the director notices. Keeping the lamp warm is terrible efficiency but means that during filming the failure rate is almost zero.

    This is also why bulbs are rated where they are. If you look at that chart you might wonder why people don't deliberatly drop the voltage to get a longer bulb life. Well it would cost you more in energy than the cost of the bulb. Contrary to popular opinion it was never a conspiracy by bulb manufacturers to sell more bulbs - it was a very sensible energy efficiency measure.

  • And photoflood lights were run at the white hot end of the curve and had a full power life of tens of hours.

    There is a whole science of this sort of lighting slowly being forgotten and going the way of the gas mantle (which oddly you can still buy new).


  • Interesting debate

    230v nominal well that is for the maths part.  Now speak to someone in Yorkshire and they will tell you they get 247 to 250v on a regular basis.

    As for the Manufacturer who make Traffic signals using incandescent lamps they need to speak to their R&D and get working on the LED version.  The cost savings for whoever pays for the street sign / Traffic signals pence/kWh should be a convincing factor.  100w lamp vs 15w LED Lamp.

    I think they started to swap out the lights in London around 2014 from incandescent to LED lamps

  • As for the Manufacturer who make Traffic signals using incandescent lamps they need to speak to their R&D and get working on the LED version.  The cost savings for whoever pays for the street sign / Traffic signals pence/kWh should be a convincing factor.  100w lamp vs 15w LED Lamp.

    The failure mode for most LED lamps is often to a stroboscopic output as opposed to off.

    I’m sure there are LED traffic signals – I presume they’d need a bespoke driver so can’t be easily retrofit. I’ve got a client who manufactures railway signals with LED’s, and a former one who manufactures LED runway lamps. Both are subject to very stringent requirements so that they fail to a known safe state.

    Incandescent lamps are a lot easier on this front – their failure modes are more obvious. If the controller detects an open circuit then it knows there’s a fault and can degrade into the safest state which in most cases is to trip all signals such that motorists will interpret as caution and give way. 

  • To me it seem quite obvious that LED used for outdoor like the following

    Traffic signals

    Runway lamps

    Railway signals

    should fail to safe. If they don't then there is common sense lacking and the design team need to be re-educated as to what the failing are.  As you say Strobe, which is annoying enough on a garden PIR floodlight but it is absolutely dangerous for a traffic signal or a run way at night in the rain (just adding risk to emphasise the point for future readers) I would then also go 1 step further

    Get that unit to report back to central office for traffic signals or rail signal or control panel at an airport of its failure.  Designers need to make kit that is fit for purpose and fit for peer review.
    UK traffic lights could be fitted with IoT technology quite easily.

    The airport would also get further enhancements like secondary/redundancy light sets.   

  • It's all very well that a person knows the difference between these states.

    But you've got do design a machine that knows, and meets a SIL rating. What happens if the visible front of a lamp appears off, but the monitoring equipment which can only see current and voltage doesn't see this? That's not too difficult if you're on a clean sheet design, but if you've got an existing mature design then it's not something you can just change.

    Safety engineering is very hard work unfortunatly.

  • A formal answer to you question.

    BS EN 60432-1:200+A2:2012 is the standard for incandescent lamps.

    2.2.1 Mandatory markings requires that lamps are marked with their rated voltage or voltage range [this raises obvious question's which aren't addressed in the standard but let's assume nominal within range], and the rated wattage.

    But there's a national departure in the BS version "The rated voltage marking for lamps intended for use on United Kingdom supply voltages may be 240 volts or 240 V." so you need to check the datasheet.

    This standard is for domestic and similar general lighting. Specialist lamps will have their own standards. But I think it's particularly interesting on bulb life - I have a reading lamp which needs an incadescent because LED's don't pull enough for the dimmer. As I can no longer buy bulbs at retail I've been getting them of a popular website named after a river in South America - and they don't last two seconds probably rated for 230V. I think I might try using a variac.