Recently Harry Nicholls mentioned that I am producing some railway and trolley light bulbs. These bulbs range in just two sizes so far, A19 and A21, Medium base. The original focus was on 56 Watt,A-19, 130 volt, DC with super long life filaments to which I added some gasses and tested them here at Ridgewood before we sent them on to Muni in SF for some 24/7 testing.

John Holt of Bay Area Western Ry Museum raised a reasonable question about my use of Nitrogen in a vacuum (short form... the comments were some longer). Part of the assembly of a light bulb is sanitation or washing the residual stuff out of a new glass globe and to do that most shops use Nitrogen. Heat is a constant with natural gas jets running at all stations to keep the glass at the right melt temperature as things from filaments to electrodes are added.

When some gas jets are eliminated the glass cools a little and a vacuum begins to form. Vacuum is measured at the base and one to three more heated gases are added. Our current project is 30 volt DC PCC type bulbs (Clear glass) without the self shorting bar.

These gas additions are hoped to provide extreme life in the PCC bulbs and some ease in finding the bad bulbs by the use of clear glass. FYI the gasses added are usually Argon, and small amounts of Kripton. Nitrogen actually helps snuff the DC arc on failure. Argon to keep the filament running cooler and Kripton to slow the flow of electrons to the globe (the bulb blacking effect). Filaments due here this month. Testing next month and if all goes well, production in late August.

Paul Class

That is excellent!

I'm curious what the differences are between a DC bulb and one intended for AC. I understand interesting things happen (!) when an ordinary 120V AC bulb is used in a 5 bulb series string on 600 VDC, it burns out, and the momentary arc at the new gap is exposed to the full 600V.

Are there also issues with a low voltage bulb?

I think it's been the subject of much armchair speculation on just why an ordinary 120V bulb (170Vpp) does *not* strike an ongoing arc when it burns out.

Steve

I recall reading in some old (1940s?) manufacturer's literature on lamp bulbs that there were two special features in the "Street Railway" line of lamps, 36, 56, 94, 101, 201, and 301 watt rating, 120V nominal voltage. One was the use of vacuum in place of any gas filling. The other was insulation in the base that enabled the lamp to successfully withstand the 600VDC (or more in some cases) that the lamp would see when the filament failed in a series string. The vacuum was said to prevent an arc from striking inside the bulb and the insulation was to prevent an arc forming inside the lamp screw base under the same conditions. There was also a statement cautioning users against the use of any other design of lamp on the 600VDC circuits because of the risk of arc damage inside the base in particular. I know some museums have used "rough service" lamps in this service, but that practice may carry the risk or arcing or fire. Are the current efforts at preserving production of "Street Railway" lamps taking these matters into account?

"Are the current efforts at preserving production of "Street Railway" lamps taking these matters into account?"--David H. Hamley

That comment brings to mind that there is a firm in the Portland area (and also in Seattle) called Rejuvenation Lighting, which specializes in reproduction lighting from the 19th century into the mid-20th century; their products include button switches (which now meet UL standards, and are also available with dimming features, something the old ones didn't have), and Edison carbon filament bulbs (you've got to love their advertising on these--"one third the light for ten times the price of standard bulbs").

Anyway, it might be pricey, but might this firm be one to check with for special bulbs like this?

http://www.rejuvenation.com/

http://www.rejuvenation.com/catalog/lig ... c9a90e04b4

http://www.rejuvenation.com/typepageRep ... c9a90e04b4

I have seen some serious arcs develop when an AC 120 volt "rough service" bulb failed in a 600 volt DC string in a street railway vehicle. The arc burned along the filament support, then burned into the bulb base, and then into the socket. Remaining bulbs in the series string provided ballast resistance to limit current in the arc that prevented blowing a fuse.

Most streetcars show evidence of several socket fires in the past when taken apart for repair or restoration.

So beware of using other that "Street Railway" bulbs in electric railway vehicles.

From the high prices, I would guess that Rejuvenation Lighting is retailing what any Museum can get from the manufacturer, Aamsco.com, at wholesale prices.

The low voltage Street Railway bulbs, as used in P.C.C. cars, had to have the shorting feature to bypass or shunt an arc because they were gas filled. Direct Current arcs are easier to form than Alternating Current arcs in a gas. Street Railway bulbs above 94 Watts had surpassed the limit for vacuum and had to be "Arc Resist". There was something special about the filament construction to try to prevent an arc while in the presence of gas. Maybe it was like an explosion proof bulb, the legs of the filament spring apart if they break.

Mr. Cass seems to have found a mixture of gasses that do not encourage an arc on 600 volts direct Current.

600 volts is a pretty low voltage to form an arc. This online calculator for arc in air gives .003 inches as a maximum.
http://www.cirris.com/testing/voltage/arc.html
If you really want to get into this there’s always Paschen’s Law to calculate breakdown voltage.
The wire in a conducting circuit such as a 5-lamp string in a trolley car will however induce a higher voltage when broken.
Especially with DC when the circuit opens magnetic fields around the wire collapse and produce a “fly back” impulse, like an ignition coil, that will induce a spark at the break point. Once started the plasma of the arc itself then conducts much better than air.
May be if the bulb itself is designed to suppress the arc the closest gap at the base becomes the point of discharge.
Newer light bulbs are made with such flimsy bases I could see how a flash over inside an aluminum screw base could burn out of control pretty fast.

Hi,

The SFMTA has been purchasing these bulbs from Gales Creek and have not had any issues to date. These are in daily service on the MUNI"s famed "F" line.

I have to say they are better than the last remaining batch of bulbs marked as "Street Railway" (56 watt) from either GE or Sylvania which approx one bulb out of the car burned out every 8 hours. These have lasted a long time. And yes, a few have burned out here and there, and no, no cars have yet burned to the ground, either a PCC or the older Vintage cars.

Frankly , there are not a lot of choices out there. The SFMTA is switching to all the cars over to low voltage (30 volt) bulbs ever so slowly, and eventually not be in need of the 56 watt bulbs.

Cheers

Karl J.
Acting Superintendent, Beach Yard (formally Geneva Yard)