Why do street lights flicker in snowy weather?

I don't think they ever did. They did a planned maintenance thing which was replacing all the bulbs whether working or not. I saw a private contractor doing that the other day.

Many photocells, particularly older ones, are thermal. The CdS cell controls current flowing through a piece of resistance wire wound around a bimetal strip. When sufficient light falls on the cell, the strip is heated, it bends and opens the contacts. This inherently creates a delay. Electronic photocells are designed with hysteresis for the same reason, normally the delay is around a minute.

Another characteristic of SOX is that the lamp voltage (and power consumption) rise rather significantly over the life of the lamp. This causes the same issue, eventually the voltage is higher than the ballast can supply and the arc will become unstable and flicker. These things usually fail due to either sodium depletion or the electrode seals fail from the corrosive action of the heated sodium, but an older lamp coupled with colder weather, especially if the ballast is a choke rather than a leak autotransformer, they can do this instead. HPS lamps will not restrike hot, so when they reach end of life they normally will cycle, but SOX will usually restrike even hot, so they can flicker.

What most often happens is that the lamp shuts off completely, and then restarts, which takes it just as long to pump up as before because the run temp is far higher than the shut off idle temp, even if after only a couple minutes.

I used to make a joke and said that I shut them off 'with my aura' all the time because we often saw them shutting off as we approached while out getting a buzz, back in 'the day'.

"Sam Wormley" wrote

[snip]

Well, if they are all guilty, you wouldn't see it very often

Jeremy Parker

Demand is always almost equal to capacity. If it would be more, it would be a waste, if less there would be serious stability issues. That's the problem with wind turbines, you just don't know when the wind blows, and you have to cover each MW of wt with at least of 700 kW conventional reserve, because when the wind stalls what? Stop everything?

Yeah that's what I was saying with HPS (metal halide and mercury behave similarly). The higher pressure in a hot arc tube raises the strike voltage beyond what is available from the ballast. It has to cool until the pressure reduces to a point that the arc will strike.

SOX lamps are much lower pressure to begin with and will usually restrike hot.

----------------------------- Please check the definitions of demand and capacity - as the above is nonsense.

Ideally the capacity should exceed the demand by some optimal margin but as adding and dropping on line capacity is in blocks corresponding to the capacity or rating of individual generators, and demand is up to the customers (predictable but not controllable) the capacity will normally exceed the demand by a fairly large margin at times- there is no "stability" problem. As for waste by having extra on-line generation- economic dispatch optimization is a common procedure. If demand exceeds capacity, then problems can occur- not necessarily stability problems. As for the wind turbine reserve, you are being a bit over optimistic. You are assuming 30% availability of wind capacity. In practice, from recent data it appears that 10-15% is a better figure and this is a statistic based on an annual average, which means nothing if wind fails. In other words. reserve capacity must be available for the worst case situation- 100% failure of any generation source, concentrated as in a 500MVA fossil plant or distributed as in 500-1MVA wind units in a region where wind diversity is small.

---------------- I have seen many such lamps flicker but have never associated this with temperature. I have not seen any more excessive flickering in cold conditions than in warm conditions By cold conditions, I mean central Alberta, Canada, -30 to -40C is not all that uncommon and windchill down to -50C equivalent. However, there could be differences in design that affect this. As an example we don't run exterior water or sewer pipes:)

Most of the generators have variable power output, not simply on or off.

Frequency drops below nominal, and conversely when supply exceeds demand, frequency increases above normal. There's a requirement in the UK for frequency to average out correctly long-term (so things like synchronous clocks don't drift long-term), consequently, supply has to exactly match demand long-term. However, since the demand and supply can't exactly track each other short term due to inherent lags, there are periods of both demand exceeding supply, and supply exceeding demand. These are both inevitable due to the supply lag with different types of plant and unexpected plant failures on some occasions, and deliberately forced to correct for earlier drifts on other occasions.

There is contingency reserve in addition to the supply - additional plant spinning sychronous online ready when needed due to either an increase in demand or an unexpected loss of supply, and yet more plant offline ready to run up and cut in with a bit more notice.

A longer article I wrote on this some years back, with some examples of how it was applied in the UK to some specific historic events, and how it went wrong on one occasion...

The BBC did a programme about the wind power in Denmark, one of the highest users of wind power. In spite of installing lots of turbines and being able to point to all the power they get from them, they haven't been able to spin down any conventional generating station, because they need them when the wind stops. When the wind blows, they have an excess of conventional electricity which they sell, but for their neighbours, it's effectively as unreliable as the wind, since its export stops as soon as the wind stops, so it only commands a low price as an unreliable source. This combined with a failure of a transmission circuit, plunged much of central Europe into darkness a couple of years ago when supply suddenly fell well short of demand, and emergency load shedding was initiated.

Shit. I knew I wrote that badly. I'm talking about physical capacity.

/BAH

| In the UK we currently have cold winds and snow. | | A radio phone-in got a lot of people talking about flickering street | lights. | | Is there a real connection between bad weather and flickering street | lights. If so then how does it work?

What kind of lammps do you have, carbon arc, sodium vapor, mercury vapor, incandescent, LEDs?

It makes a difference.

Vapor lamps are sensitive to the cold, as they require a narrow range of pressure internally to function. Cold reduces the presssure, making operation less reliable.

Cold can also shorten wires. If you have a break internally in an insulated wire, cold could shrink the metal, weakening the connection at the break and raising resistance, which heats the wire which expands again, which drops the resistance, which lets the wire cool down again, ad nauseum. That is the mechanism for flashers in Christmas lights and automotive turn signals.

Tom Davidson Richmond, VA

What kind of lammps do you have, carbon arc, sodium vapor, mercury vapor, incandescent, LEDs?

It makes a difference.

Vapor lamps are sensitive to the cold, as they require a narrow range of pressure internally to function. Cold reduces the presssure, making operation less reliable.

Cold can also shorten wires. If you have a break internally in an insulated wire, cold could shrink the metal, weakening the connection at the break and raising resistance, which heats the wire which expands again, which drops the resistance, which lets the wire cool down again, ad nauseum. That is the mechanism for flashers in Christmas lights and automotive turn signals.

Tom Davidson Richmond, VA

From a practical standpoint, these days the only lamps that are sensitive to cold temperatures down to ambient temperatures of about 0 F. (-18 C) are fluorescent lamps. Even so, special fluorescent lamps are used routinely at -20F (-29 C) in commercial freezers, outdoor signs and exterior building lighting. High pressure sodium, high pressure mercury, metal halide, LED, cold-cathode ("neon") and, of course, incandescent all work just fine. Low pressure sodium lamps have a "thermos bottle" construction so they may just take a little longer to warm up. Discharge lamps do require somewhat higher voltages to initiate the arc at low temperatures, so in applications where cold temperatures are expected, so-called "minus 20" ballasts are specified.

The only discharge lamps that I've ever seen flicker in cold temperatures are CFLs and unprotected or reduced-wattage ("watt miser") fluorescent types which have been mis-applied. Rarely, a high pressure mercury lamp will flicker due to a faulty arc tube and there's an odd type of high pressure sodium lamp designed to work on mercury ballasts that sometimes flickers near end-of-life. I've heard that high pressure sodium lamps operated on old series (constant-current) streetlighting systems will flicker; but, if so, all of the lamps on the circuit will flicker, not just one. I've never seen that phenomenon, however.

Terry McGowan

Oh sure, carbon arc street lights... with gilded hand-carved capitals, chromium plated motors and a Rolls-Royce gearbox.

use 'em in our refrigerators too, striking an arc when the door opens; it saves having a little man inside to operate a switch.

Street lamps are usually sodium vapor some old installations may be mercury vapor. It's simple economics.

Absolutely. Tadchem wins the poster who actually has a brain prize! When cold high pressure sodium or mercury vapor lamps will flicker or repeatedly restart if they have lots of hours on them. As someone noted they are usually just replaced en mass at a certain age so by that time it's common for many to be at the flicker to not work at all stage.

Possible, but not very likely. More likely is that because of snow being white and reflective (as opposed to black pavement) people just tend to NOTICE the flickering more which was really there all the time.

What is this verbal diarrhea? You are just babbling on about nothing at all. NONE of it is from a "practical standpoint". Moron.

Great bunch of information all leading to the totally WRONG conclusion. Never met a mercury vapor or sodium vapor lamp in person did you? Hey, I've got one that lights my parking lot. And when it gets cold and when it has lots of hours on it, the damn thing flickers, Buster! I take it you came here to pontificate and show everyone how much you know and all you did was show the world how little practical knowledge you actually have.

It's simple. Streetlights these days are sodium vapor for economic reasons. The one I've got produces an AMAZING amount of light for consuming 35 watts. Street lights are somewhat though not extremely larger. As they age the arc gets harder and harder to strike. Also as it heats up the arc may not sustain leading to restarting. In this mix there is also flickering as the gas pressure begins to creep out of the stable arc range as the bulb ages. These conditions are much exacerbated by low temperatures. Hence a lamp that was working fine in warmer weather will suddenly turn cantankerous if the temperature suddenly drops. And finally snow reflects the light making the flickering much more obvious. And THAT kiddies is the real story about flickering street lights. (Even if Terry has never seen it. Judging from his bluster I take it he probably works for the government designing street lights that flicker)

TKM comes from sci.engr.lighting. He worked in the lighting industry. His answer is typical of the long, technical answers from the very sharp people on sci.engr.lighting. His "babbling" is all from a "practical standpoint".

[snip]

As Terry said, cold ambient temperatures effect only fluorescent lamps.

Fluorescent lamps operate at wall temperatures of 40C or about 313K in a 25C ambient; and the mercury pressure is a strong function of wall temperature. (The mercury pressure, in turn, has a strong impact on the lamp's electrical characteristics.)

Metal halide lamps operate at wall temperatures of about

1000C or about 1275K in a 25C ambient, while HPS lamps operate at wall temperatures of about 1225C or 1500K. A reduction of 40C in ambient temperature has a much greater relative effect on the relatively low wall temperature of fluorescent lamps than on the much higher wall temperature of metal halide lamps or HPS lamps.

In metal halide lamps the mercury dose is fully vaporized when the lamps are at operating temperature. Therefore, even a moderate decrease in wall temperature can change the lamp color, by reducing the metal halide pressure, but will not significantly reduce the mercury pressure, so the electrical characteristics will remain substantially unchanged.

In HPS lamps the mercury and sodium are typically not fully vaporized. A lower than normal wall temperature will reduce the mercury and sodium pressures, and can impact the electrical characteristics. However, the very high temperature of arc tube makes even HPS lamps relatively immune to changes in ambient temperature.

We use 'em in our refrigerators too, striking an arc when the door opens;

The first electricity in Middletown, Ohio was generated for one large Carbon Arc light mounted on a tower. According to the town's history, people cold read a newspaper over a mile from the tower. From there, electricity was provided to the downtown area, then the suburbs. Later, the system was sold to CG&E.