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Monday, 2 December 2013

Street Lighting

My local council is considering dimming or even switching some of the county’s street lights to save money. A position that I don’t think they have thought through.


All that happens in a simple dimming system is that you exchange lots of light and heat for less light and loads more heat. This defeats the object of saving power.

An ordinary resistor is a piece of material that doesn't conduct electrical current well -- it offers a lot of resistance to moving electrical charge. A variable resistor consists of a piece of resistive material, a stationary contact arm and a moving contact arm.

As the charge works to move through the resistor, energy is lost in the form of heat. When you put a resistor in a series circuit, the resistor's energy consumption causes a voltage drop in the circuit, decreasing the energy available to other loads (the light bulb, for example). Decreased voltage across the light bulb reduces its light output.

The problem with this solution is that you end up using a lot of energy to heat the resistor, which doesn't help you light up the room but still costs you. In addition to be being inefficient, these switches tend to be cumbersome and potentially dangerous, since the variable resistor releases a substantial amount of heat.

TFE note: There are no, dimmers fitted in my area as there was never considered  any justifiable cost.

Switching off:

Apart from the obvious dangers of  the running down of pedestrians on a more regular basis, there may be legal ramifications. Consider the 30 MPH speed limit in towns, villages and cities. There is more than just the signage to consider.


The Beak: “Mr Filthy Engineer. You were stopped for exceeding the 30 MPH speed limit. How do you plead?”

TFE: “Not guilty”.

(Stunned silence in court as everyone rushes out to get popcorn.)

TB: “Please state your reason for your plea as we have the statement from two Officers that you were at the time exceeding the limit”.

TFE: “I didn’t know there was a limit, your honour. I’ve never driven through that area before”.

TB: “Didn’t you see the sign”?

TFE: “Maybe that was when I had a sneezing fit.”

TB: “But didn’t you see the street lights?”

TFE: “There were no street lights”.

TB: “How can you say that? They are clearly visible”.

TFE: “Not when it’s raining, dark, and it’s a cloudy night. AND the bloody things are switched off”.

TB: (Consults clerk) “Case dismissed”.



  1. Is there such a thing as an efficient dimmer?

  2. It's not that simple, I'm afraid FE. Tungsten filament lamps haven't been used for street lighting for decades - they don't last long enough and cost too much to run. Variations of low and high pressure Sodium, and high pressure Mercury vapour or Metal Halide are the main ones used up to now. None of these can be dimmed, either by simple resistive or electronic dimmers.

    Fluorescent lamps can be dimmed, by thyristor "chopper" circuits, but need extra transformers to keep the cathode filaments warm. This old fashioned system is used in my friends chicken sheds. The modern way is electronic ballasts which can be used with full sized tubes or compact types such as PL or 2D. These would be one of the 2 candidates for street lighting, the other being LED's which also require electronic control gear. In both cases efficiency is high as the conversion is done at high frequency, but it will cost a hell of a lot to retrofit all the existing street furniture, or even replace them completely.

    I'll leave you to consider the cost/benefit figures involved to save "nasty" CO2 from being emitted by the power stations still needed to provide base load. But then it's only the taxpayer footing the bill after all...

    1. The filthy engineer3 December 2013 at 10:13

      That was just an example of how expensive it would become to retrospectively fit street lights with dimmers.

  3. Didn't the lack of street lights in London during WW2 cause a lot of casualties?

  4. MD beat me to the technical bit....

    This year they held a "consultation" before changing all the lights (pillars and all). This was done on PFI, just to make sure that it was as big a waste of tax-payers hard-earned as possible,

    I would have preferred them to just turn the lights off after midnight, because in this village the only people out are all in vehicles, all of which have headlights.

    These are also white streelights, so as a driver I've lost useful information about whether there is another car around any corner I'm approaching.

  5. If the lights go out, all the snooping cameras will have to converted to infra-red, just to spot the burglars.

  6. I wondered about this speed limit thing myself. Fairly recenty driving at night (can't remember where it was, poss Nottingham) I came across an official sign that said the street lights on that road were not in use. Does that mean the speed limit is cancelled?

    1. You are correct - see some .GOV site for details. It states that if the lighting in not in use after "lighting up time", the 30mph limit cannot apply by itself (but does still apply if "30" signs are visible). However, in daylight, the presence of street lights indicates the 30mph limit is in force.

  7. Thanks Ed, there was a slightly logical side to my question and what you say makes sense. However if I got a ticket I would still argue that if the street lights are officially not in use (didn't say 'switched off' but said 'streetlights not in use') that must also cancel their other 'use' as speed limit notifiers - even in daytime - because the sign declares them to have no uses?

  8. Many years ago, a favourite activity was to park in a position which afforded a good view of the urban sprawl and watch the street lights progressively go out at about 1:30am. It was such a romantic moment that possibly 15% of the population were conceived very shortly after. Many years later I found that the lights were controlled by a separate circuit from each transformer and a timeclock. Later this circuit was replaced by photo electric cells in each light. The local councils generally pay for street lighting (at least here) and that is calculated on the type of light and the number of hours it runs on average. Further developments could be a time switch to turn the light off when there is minimal traffic and/or motion detectors to turn the lights on when needed. These devices are not expensive and can be installed progressively when lights etc. are changed.

    As far as tickets go, you're responsible adults and should know and be comfortable with driving within the prevailing limits and road conditions or stick to public transport.


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