I just bought a 1968 airstream. It is my first. I got it from a family that did not know much about it, and honestly either do I. My first challenge when I got it home is when I plugged the external power cord it into my outlet at my house (through the converter plug that went from the big campsite style plug to the standard house plug) the only thing that happened is the red polarity light came on and none of the other electrical worked.
Does anyone have any advice for a novice on this? Is there a manual I can read for simple things like plugging your airstream into "shore" power.
The polarity light is wired between the 120V neutral wire and the ground wire. If the previous owner(s) did some maintenance and replaced the shore power cord or the plug on the end of the cord, it's possible he/she reversed the hot and neutral wires. If the hot side of the 120 volt line is applied to the neutral wire in the trailer, this lights the polarity light--the polarity light is really a "wrong polarity" light. Or, per GORANSONS post, the recepticle in your house is reversed.
For the inoperative electrical system, first thing I'd check is the converter (do you have a converter?)--is it plugged in? Are the 120V circuit breakers "on?" One of the four breakers acts as a mains breaker. Is the main fuse in the converter OK? Are the other fuses OK? Is it humming (if you still have an old converter, you will be able to hear it's 60/120 cycle hum if you're close to it--the new solid state ones are silent)?
Do you have any 120V appliances, like the Air Conditioner? Sometimes it's hard to tell if you have 120V without having something to turn on. Plug a trouble light into one of the recepticles.
A reversed polarity will not shut down your 120V system. As far as I can tell from the maintenance manual for 1975, the neutral circuit is not tied to the ground circuit in the main breaker box, so you shouldn't have a hot shell, despite the reversed polarity.
The danger of reversed polarity is that older two-prong recepticle appliances can have one side of the power tied to their chasis (old TVs and radios, for example), so if there is a knob missing or some other access to the chasis, you're just one touch of the shell away from a shock.
The polarity light is wired between the 120V neutral wire and the ground wire. ... As far as I can tell from the maintenance manual for 1975, the neutral circuit is not tied to the ground circuit in the main breaker box, so you shouldn't have a hot shell, despite the reversed polarity.
The danger of reversed polarity is that older two-prong recepticle appliances can have one side of the power tied to their chasis (old TVs and radios, for example), so if there is a knob missing or some other access to the chasis, you're just one touch of the shell away from a shock. Zep
Hi Zep:
I'm quite a few electrons short of being a wiring wizard, but doesn't the reverse polarity light bulb itself connect the neutral to the shell, a path to make the shell hot? Isn't a lit reverse polarity light actually a facilitator, as well as an indicator, of a hot shell? If the reverse polarity light bulb is removed, wouldn't that then break the seemingly only remaining connection between neutral and ground?
My questions are based on observations made by Bud Cooper about his 1975 Airstream on pages 32 & 33 in the 4th Quarter 2003 issue (Vol. 9, No. 4) of The Vintage Advantage, where he suggests the 1/3 watt incandescent reverse polarity light bulb itself be removed for safety's sake. He also hinted that some type of neon light might make a better reverse polarity light bulb, but didn't state why. Coop also stated a GFI outlet better serves the purpose today of avoiding hot shell.
I'd like to see further discussion on this topic. Thanks!
__________________
Fred Coldwell, WBCCI #1510, AIR #2675
Denver, Colorado - WBCCI Unit 24
Charter Associate Member FCU
Vintage Airstream Club Historian
Airstream Life "Old Aluminum Adventures"
...doesn't the reverse polarity light bulb itself connect the neutral to the shell, a path to make the shell hot? Isn't a lit reverse polarity light actually a facilitator, as well as an indicator, of a hot shell? If the reverse polarity light bulb is removed, wouldn't that then break the seemingly only remaining connection between neutral and ground?
My questions are based on observations made by Bud Cooper about his 1975 Airstream on pages 32 & 33 in the 4th Quarter 2003 issue (Vol. 9, No. 4) of The Vintage Advantage, where he suggests the 1/3 watt incandescent reverse polarity light bulb itself be removed for safety's sake. He also hinted that some type of neon light might make a better reverse polarity light bulb, but didn't state why. Coop also stated a GFI outlet better serves the purpose today of avoiding hot shell. ...
Hey, Fred, good morning.
You ask some great questions. First, the light bulb. Theoretically the answer is no--the bulb does not provide a path to the accidentally hot neutral. If that bulb lights up, there should be 120 volts "dropped" across the bulb, eg, it's "consuming" the voltage as the electrons move from the hot wire to ground. Howerver, because of a problem called "ground loop," which is where the ground has some significant resistance (usually due to length of wire) from the ground point in the shell to a real ground point (presumably at the main breaker box at the house/RV park/etc., which can be some many feet away), you could potentially provide a better ground path if you're standing in salt water and touch the shell. This would be a very low voltage problem, maybe 5 volts or so for a really bad ground loop, so you might detect that funny little tingling/roughness as you move your hand over the shell.
The real danger would be if the power cord to the shore power outlet, or the shore power outlet itself, was missing a ground. Then the light would not come on, but the shell would be hot with full 120v (because there's no current flowing through the bulb). Now when you stand in the aforementioned salt water and touch the shell, the power goes through the bulb and then through you to ground. Some of the voltage would be dropped across the bulb, some across you, and some across your contact (as good or poor as it may be) with the ground. You wouldn't like it but you probably wouldn't twitch much. On the other hand and in rare circumstances, you could be mort.
The neon light would help a little because they are very low current devices. Most of the voltage would be dropped across the neon, and if your resistance, plus your contact resistance with the ground, were sufficient, the neon might not light at all and there would be no current. But you/ve still got a hot shell and don't know it.
A GFI might solve this problem, but you need it across the mains to solve the problem for the whole trailer, so it has to carry the total input current. Maybe they make 30 amp GFI breakers, but I'm used to seeing 20 amp models. You might resolve this by putting the polarity lamp on the downstream side of a GFI on one of the distribution circuits. The 70s Airstreams have a 4 breaker box, where one is the mains, one is GFI and the other two are normal. I'll have to look in the manual and see if the polarity bulb is on that GFI circuit.
A GFI works by comparing the current in the hot and neutral legs of the circuit. If it's imbalanced, it trips, as well as tripping at the maximum. GFIs can detect very small imbalances, maybe something like 20 milliamps (anyone know the real number?). That's why they're such a pain when you have a very small leakage current and it keeps tripping for no apparent reason--you've got some wet mouse stuff on a recepticle, etc., and you can't find it.
In sum, if the RV park has a badly wired outlet (with hot and neutral reversed), I don't think you can count on the ground being good. Without a good ground, the polarity light doesn't work. So there is some hazard due to the light providing a path from neutral to shell, yet it doesn't always have the ability to warn you of a problem. My opinion is I'll keep mine. I need to make it more obvious--I don't think I see it when I plug in.
My polarity light came one at one time when I plugged into my garage outlets. I knew it wasn't the trailer circuit because that had not happened before. Garage?? Nope. It was the new sealed store bought extention cord!
Neil.
__________________
Neil and Lynn Holman
FreshAir #12407
Thanks for the thorough explanation! Coop's situation occurred on a wet beach in Mexico when he was plugged into the old trailer park's two prong outlet (no ground on shore power). Coop's polarity light did not come on, and thanks to your explanation I now know that was because there was no separate ground leg to complete the polarity light's circuit and light it.
The camp ground's two prong shore power outlet was wired incorrectly, and when Coop's wife Bettye stepped onto the wet sand while holding on the trailer for stabiity, she was shocked enough to be knocked down. The trailer had a hot shell, and Bettye became the new path to ground when she stepped onto the wet sand. It was that rare instance when the polarity bulb filament provided path to ground through Bettye.
So lessons learned are to always check the campground two prong or three prong shore power outlet with a small polarity checker before plugging it; remember that the trailer's built in polarity light does not work on two prong electrical outlets or if the three prong ground is bad; and be extra cautious in wet weather (wear rubber golashes?) .
I like old blue highway and backwoods campgrounds, where the electrical wiring may have been done by the local handyman and not double checked. So to avoid becoming a Reddy Kilowat, before plugging in I'll check those trailer park shore power outlets with my portable polarity checker.
__________________
Fred Coldwell, WBCCI #1510, AIR #2675
Denver, Colorado - WBCCI Unit 24
Charter Associate Member FCU
Vintage Airstream Club Historian
Airstream Life "Old Aluminum Adventures"
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