My community uses all HPS street lights, however I have noticed more than once that a single street light in a cluster will have a light output that looks like Mercury Vapor or Metal Halide - the blueish whitish light. These lamps will usually be cycling. In one incidence, I noticed this one day, and when I drove by the light the next, it had a normal HPS lamp in it. What would cause this? Did someone put the wrong type of bulb in the fixture? How exactly would an MV or MH lamp behave when installed in an HPS fixture?

Thanks!

The one lantern with the bluish white light had a cycling HPS lamp that lost the sodium and the mercury took over the discharge, so it glowed a mercury color.

The exact case you describe is likely what Dor123 said.
But note, then HPS and MH lamps for markets with 230V mains are electrically compatible, so you may use HPS lamp in MH fixture and MH lamp in HPS fixture would work as well. However for code compliant MH use the fixture has to have some safety features not necessary for HPS: Namely hot lamp fragments (after explosion) retaining cover and protection against lamp rectification (thermal protection or properly rated electrical fusing)

Very interesting! How does the lamp "lose" the sodium, exactly?

Sodium gas consist of very small particles, what can quite easily diffuse trough the arctube wall. This happen trough the whole life till all the sodium leaked away.
This gradual sodium loss is the reason, why most HPS are designed using saturated vapor concept: The vapor pressure is controlled by the temperature of the reservoir and so it is not much dependent on it's total amount in he arctube, So as it gradually leak away, it is "refilled" from the reservoir. In the moment there is no sodium in the reservoir anymore, the pressure start to drop and it mean end of life (oe of HPS failure modes).
And it is this saturated vapor concept, what cause the lamp being thermally unstable when overheated, because the pressure, so the arc voltage (so power taken from the rather constant current ballast) is strongly dependent on the temperature, what cause the positive feedback in the system, leading in fact to thermal runaway till the lamp extinguish (due to too high arc voltage), what manifest as cycling (after extinction it cool down, restart and the runaway repeat). This runaway is usually triggered by larger light and IR absorption of the blackened arctube and it is another (and these days most frequent) HPS EOL failure mode.
The thermal stability is strongly dependent on ballast characteristics (as the ballast is the element what define, how the real lamp power respond to the change in arc voltage).So if the electronic ballast keep the power independent (or even dropping) when the voltage increase, the setup would be always stable, so the runaway would not occure, so the sodium loss remain the dominant EOL mechanism.
Or in case the CWA (popular in the US due to it's line regulation properties) is really a constant current source leading to the positive feedback being more severe, so lamps start to cycle sooner.
For these special lamps were developed what drop the saturated vapor concept, but use rather the accurate small dose, what fully evaporate, so keep the sodium vapor unsaturated. In this way the pressure is not dependent on the temperature anymore, so the positive feedback is broken. This approach was possible due to advances in arctube manufacturing, what reduced the rate of sodium loss so, it does not have to be "refilled" from the liquid reservoir. But the sodium still leak out, so the sodium loss is the major EOL failure mode with these lamps. These lamps were able to just reach the lifetime of the saturated vapor classic, but mostly do not exhibit the cycling.

Since PSMH and HPS both using a starting ignitor, wouldn't the two be electrically compatible in theory? Using an open-rated lamp as an added precaution, would a PSMH work in an HPS fixture of equal wattage? I've been Googling and keep getting mixed answers.

Medved, you are absolutely BRILLIANT on these things!!! 

Since PSMH and HPS both using a starting ignitor, wouldn't the two be electrically compatible in theory? Using an open-rated lamp as an added precaution, would a PSMH work in an HPS fixture of equal wattage? I've been Googling and keep getting mixed answers.

In Europe generally are (but there are some exceptions), the reason is the 230V mains and the desire to work with simple series choke, so the arc voltage should not exceed much about 115V. And this lead to virtually all lamps having arc voltage about that level (saturated vapor types, like HPS, have arc voltage initially about 70..90V, as it rise trough the life, unsaturated, like MV, are designed for the 110..140V; higher arc load on higher wattages have more stable arc, so allow higher arc voltages).

In US all lamps (except low wattage types) need autotransformer anyway, so the arc voltage of HPS was chosen only according to the best efficacy of the lamp itself, what lead to higher arc voltages.
And as the pulse start MH was spread originally in Europe (as even the probe start required ignitor, so using simpler lamps without the probe was finally easier, even with the higher voltage ignitor), arc voltage standards for these lamps were more build around European needs. And as they spread around the same time as HPS and principally require the same type of gear (choke ballast plus HV ignitor), they were designed rather for the same ballast.

So in US HPS and pulse-MH need each type diferent ballast.

Quote from: gramirez2012 on December 29, 2010, 06:08:05 PM

Since PSMH and HPS both using a starting ignitor, wouldn't the two be electrically compatible in theory? Using an open-rated lamp as an added precaution, would a PSMH work in an HPS fixture of equal wattage? I've been Googling and keep getting mixed answers.

In Europe generally are (but there are some exceptions), the reason is the 230V mains and the desire to work with simple series choke, so the arc voltage should not exceed much about 115V. And this lead to virtually all lamps having arc voltage about that level (saturated vapor types, like HPS, have arc voltage initially about 70..90V, as it rise trough the life, unsaturated, like MV, are designed for the 110..140V; higher arc load on higher wattages have more stable arc, so allow higher arc voltages).

In US all lamps (except low wattage types) need autotransformer anyway, so the arc voltage of HPS was chosen only according to the best efficacy of the lamp itself, what lead to higher arc voltages.
And as the pulse start MH was spread originally in Europe (as even the probe start required ignitor, so using simpler lamps without the probe was finally easier, even with the higher voltage ignitor), arc voltage standards for these lamps were more build around European needs. And as they spread around the same time as HPS and principally require the same type of gear (choke ballast plus HV ignitor), they were designed rather for the same ballast.

So in US HPS and pulse-MH need each type diferent ballast.

Interesting, I have been experimenting with mercury lamps on high pressure sodium lamp gear in the past few weeks, heres what I`ve found.
I had an old, (used) GEC Trulite mercury on a 250watt son ballast, and it ran with lamp voltage at 149V, and current of 2.2A.
A GE WDX mercury, (new), ran at a lamp voltage of, 135V and current of 2.4A, on the HPS gear.
The old Trulite on a proper mercury ballast ran at a lamp volts of, 151V and current of 2.0A.
Some halide and SON lamps do run at 100V, with a lamp current of 3.0A, but it seems to me that HPS ballasts in the UK will match MBF, MBIF or SON lamps when run on them.
Rich

@LinearSLI/H:
250W MV is specified as 2.15A nominal (130V arc), what yield 0.3H (assume loss-less inductor for 240V mains)
250W SON is specified as 3A nominal (93V arc), what yield 0.235H (assume loss-less inductor for 240V mains)
Note, then the inductance is valid only for the specified AC current, ferromagnetic stuff is nonlinear, so the inductance is current dependent

MV and SON are not ballast choke compatible, the SON ballast would overdrive (up to ~25%, depend on the ballast linearity) the lamp rated for MV ballast. But MV's are quite tolerant to such small overdrive (expect higher efficacy, but a bit higher failure rate) and underdriven HPS (in case they are operated at MV ballasts) would only have lower efficacy and CRI (it is within the allowed dimming range), so the "misballasting" would not have as tragic consequence. But the larger stress with MV run at SON ballast may cause the lamp to completely fail (leak,...), so i would be careful with collection-piece lamps.

Worn out lamps tend to have higher arc voltage, what lower the lamp current (as less voltage remain on the choke).
This effect is sometimes (mainly with SON ballasts) on purpose exaggerated (by letting part of the core cross-section to saturate), the reason is to suppress the real lamp power dependency on the exact arc voltage, in order to suppress the thermal instability close to the lamp EOL (when the arc voltage steeply rise with delivered power), so prolong the useful lamp life before it start to cycle. But this feature bring in a consequence in stronger lamp power dependency on mains voltage (the arc current is dependent on choke drop, what rise more steeply with mains), so not all ballasts are equal in this matter (each type may be tailored to different optimum), so i would not rely on it, when looking for ballast for MV.

And for the voltage values (Generally known, but very frequently forgotten to take into account):
As the voltage on the arc is nearly "square-wave", it is important to know, how exactly your meter deal with AC. All are calibrated to display the rms value for sinewave, the reading become wrong on different waveform shape, if the instrument does not really measure the rms (so it is not marked as "true-rms"). Most non-"true-rms" measure in reality the "rectified-average" value and then apply a correction factor of 1.11 (=Pi/(2*sqrt(2)) ), so with these you have to divide the result by this number when dealing with "square-wave" (so discharge arc voltages), as the "rms" and "rectified-average" values are equal for square wave shape.

@Medved: (expect higher efficacy, but a bit higher failure rate)
Thanks for that mate, so in effect the MBF on SON gear will behave more like a Halide lamp, i.e, as you say, higher efficancy, shorter lamp life.
So say you didn`t have a Halide to hand at the time of re-lamping, you could `sub` it with an MBF lamp?
The ballast I ran it on was a Thorn GF 53523.4, with a G 53455 ignitor and 30mf PFC.
Just a test I caried out after our maintanance team at work put an 80watt MBF in a 70watt SON flood light
Rich

@Linear:
You cannot directly put MV into HPS/MH fixture, as the ignitor will destroy the lamp (after few hot restarts). When speaking about using SON ballast with MV i mean only the choke, but without the ignitor.
But all spoken before was meant pure technically.
And there is other, more important issue, mainly with commercial installation: When using lamp and ballast combination, that is not rated to work together, it is violation of the electrical code and legally considered as unsafe. So when something (accident, injury,...) happen and somebody blame lighting, you can not defend and would be held (at least partially) liable for that. Because you would not have any certificate, then that combination meet requirement.
If the same happen under correct combination, you are "covered" by the fact, then you respect manufacturer's instructions, so you can not be blamed for anything wrong with that...

Good point, our maintenance team at work seem to know nothing about HID lighting.
Most of the lights in the building are 400watt Halide, that they had a contracting firm install.
I remember them being put in, about 5 years ago now, but maint don`t seem to change them until, A, they explode or B, the ignighters burn out!!!!!
The lamps are in enclosed fittings, but it seems to me, still bad practace, and when you complain about how `dull` they are, we just get told, "well, their very expensive to buy, so we only change them when they go out"!!!!!!
However it cost them more to, not only replace the lamp, but then the whole gear tray!!!!!,
but who am I to tell them their job?
Rich

I found out that the ballast in my fixture is an R-NPF ballast. From what I found, PSMH uses CWA or HX-HPF ballasts. 

The PulseMH need an OCV about 220..250V and an inductive impedance (to control the lamp current; so lag type - to ensure the ringing after current zero cross sum with the nearly peak OCV at that point help to reignite the arc), so it depend on the mains voltage, what concept the ballast may use:
- CWA is not a good type, as it use capacitance as the ballasting impedance (it is the lead type). That mean, then the arc is not able to restart immediately after current zero-cross, capacitor does not allow so large ringing. Consequence is early cycling.
- HX autotransformer: Universal in terms of available mains voltages (as it is a transformer - so may convert anything to the required ~230V OCV) and the output impedance is inductive (= leakage inductance)
- Serial reactor (coil): Most efficient and the smallest type (as it have to implement only the impedance), but feasible only for the mains voltage in the required OCV range (only 240V for US/Canada and standard 230V mains in "230V world")

For the HX autotransformer and the serial reactor it does not matter, if it is NPF or HPF, as the difference is only in the presence of the PF compensating capacitor parallel to the mains side (on HX usually between the highest voltage tap and the neutral, it's effect is then correctly "transformed" to the used voltage level). It does not influence the behavior towards the lamp (unless it form the current path for the ignitor pulser - e.g. for SN57-like ignitor circuit)

I found out that the ballast in my fixture is an R-NPF ballast. From what I found, PSMH uses CWA or HX-HPF ballasts.