@James
Wow, as always, that was a lot of good information. Thank you so much.

I know nothing about AFCI breakers, but I googled it and found out that it could be your long wire run's cable capacitance, electrical noise, damaged wiring, and a defective or overly sensitive AFCI. I discounted overloads because it only happened with a very light load.

The above comments are correct, but it should also be noted that the sodium does not actually react with the aluminium oxide arc tube to any great extent.  Wikipedia is completely wrong in this respect.  The sodium does react slightly with some dopants in the crystalline structure of the alumina, notably the magnesium oxide and calcium oxide components which are present at the grain boundaries.  This gradually makes the arc tube wall more porous during life.  Sodium atoms are then able to diffuse along the grain boundaries and leak into the outer bulb, where they react with the glass bulb and cause additional darkening which reduces luminous flux and traps more heat inside the lamp, further exacerbating the voltage rise caused by arc tube blackening.

There is also extensive reaction between sodium and the glassy frit-seals at the arc tube ends, and with the barium tungstate emitter coating of the electrodes.  In that case the sodium does not leave the arc tube but becomes chemically bound with those components.

As Dez mentioned, the primary ageing mechanism of standard HPS lamps is the changing amalgam ratio.  HPS lamps reach peak efficiency and luminous flux when the delta-lambda distance between the two spectral peaks either side of the sodium resonance radiation is about 120 Angstroms.  But as the sodium-mercury ratio increases and the arc tube temperature rises due to blackening, the red-wing of the sodium spectrum is broadened and the d-line is also broadened.  More red and infrared radiation is produced by the plasma, so the luminous flux must decrease.

Note that these failure mechanisms are entirely different in the higher performance Unsaturated Vapour HPS lamps.  These are actually characterised by a falling lamp voltage during life, since there is no excess of sodium and the whole dose is vaporised.  Therefore no longer so dependent on the cold spot temperature and its changes during life as a result of blackening.  USV lamps are only feasible when the sodium-sinks are reduced or eliminated.  So the alumina arc tubes tend to be doped with zirconium or erbium oxides in addition to magnesium oxide, and with reduced calcium oxide impurity, which reduces sodium reactions at the grain boundaries and hence the rate of sodium loss.  Of far more importance though is the change of electrode emitter material, usually one of the biggest sodium sinks.  The traditional barium tungstate emitter is changed to something less reactive - in the case of the Sylvania lamps that was BSY2, barium strontium yttrate combined with a special sintering procedure.  Some companies also used more resistant frit sealing glasses.  Incidentally just before the USV lamps fail and the last of their sodium is consumed, there is a sudden rapid rise in voltage back up to something close to the original level.  This characteristic ensures that there is no end-of-life cycling.  Lamps turn completely blue when the sodium is gone, and continue to burn as a pure mercury discharge.  But then of course the plasma temperature increases, and since the ceramic arc tube is not chemically stable enough to withstand a pure mercury arc (sodium or metal halides are required to protect it), the ceramic eventually disintegrates and leads to complete lamp failure. 

If you want to learn more about this I can highly reccomend the book of one of my former colleagues, Sjef de Groot at Philips Eindhoven, who wrote "The High Pressure Sodium Lamp".  There was a copy going cheaply on Ebay for a long time that seems not to have sold but now I cannot find it.  That book is rather old and does not cover the newer developments like the USV, deluxe/white, retrofit, high xenon pressure, mercury-free lamps etc.  The subject was brought just about fully up to date in a comprehensive IEEE paper written in 1993 by my old boss at Sylvania, Rudy Geens, and our American colleague Elliot Wyner.  See https://digital-library.theiet.org/doi/abs/10.1049/ip-a-3.1993.0070  I can send a copy if you do not manage to find it via the usual scientific literature sources.

Thank you!
Aha! I found the document that I was talking about! I hope this will be of help.

One of the very first metal halide lamps from Westinghouse was actually thallium dosed.  It was marketed as a super high output mercury lamp, but was unpopular because of the terrible colour rendering and was quickly superseded.

Good question. I have never found a comprehensive chart that lists all types, so have compiled my own based on a combination of multiple different references.  I hope the attached file may be useful.  If you are able to fill in any of the gaps that would also be appreciated!  It's been a very long time since I made this sheet and perhaps in the mean time other references may be available, and enable an update.

Hello everyone,


I had a semi-fascinating/unusual story I wanted to share with everyone regarding the “ghostly” nuisance tripping of an AFCI circuit breaker.  For some background, in late 2022, we re-wired much of our house and had AFCI circuit breakers installed on all the new circuits.  And, per my request, we also had our electrician place a two-pole AFCI breaker on a vintage (original) multi-wire branch circuit (MWBC) for added safety.  They are all Square D Homeline AFCI breakers, and were all manufactured around the same time.  All of the AFCI breakers on the new circuits work perfectly, and not even one has ever experienced a “nuisance trip.” 


However, on the old MWBC, we’ve now had two “ghostly” nuisance trips.  By “ghostly,” I say this because the circuit isn’t even in real use when these trips occur, with the exception of maybe some lighting.  The first time, (a couple of years ago) it tripped one morning with nothing even turned on (except maybe a light).  However, it never tripped again after that, until a couple of days ago.  The circuit also wasn’t in heavy use; only a griddle on a low setting was plugged into one of the kitchen receptacles connected to it, and the outdoor low-voltage lights (also on this circuit) were on as well.  However, even the cumulative current draw of these loads wouldn’t have been nearly enough to cause it to trip, at least not so quickly.  Thus, I’m a bit perplexed as to what may have been causing it to trip.  The griddle has never caused any issues before, and neither has the transformer for the low-voltage outdoor lights. 


The only thing I can really think of is that there was some sort of unusual frequency or “noise” on the line that the breaker was picking up on and seeing as a “hazard,” especially given how long the cumulative length of the circuit is.  It not only serves the lights in the garage, but also a four-plex outlet in the garage, a countertop outlet in the kitchen, and all the bedroom, dining room, and bathroom lights.  I should mention that the circuit has actually been inspected before, and nothing is crossed/shared in an unusual way, with the exception being it is a shared neutral (multi-wire) circuit (which the two-pole AFCI breaker it is on was specifically made for).


Finally, we currently don’t have any plans to have the breaker replaced, as it only very rarely trips and doesn’t otherwise seem to have problems during “regular” (everyday) use.  It doesn’t seem to have any issues with motor loads (blenders, mixers, the Nespresso machine), or any thermostatically controlled heating appliances.  But, it still just sometimes trips for no apparent reason.  Go figure.


If anyone has any thoughts, ideas, or other input they’d like to share, I’d love to hear!

I've an Aliexpress 5000K MH lamp which have Na-Sc-Dy-Tl.

I get good results using catalog numbers for ballasts and ANSI codes for lamps.

I realized that the green spectrum could theoretically enhance the efficiency of NaSc metal halide bulbs. So why wasn't it ever used? I get it; the CRI would probably be worse, but it would still be fine for outdoor lighting. However, Thallium was never used in NaSc lamps.

Attached below is what the spectrum of a NaScTI lamp could look like.