Disclaimer: I don't work in the trade, and have no field experience. This blog post exists only for the purpose of informing what a Carter System is, and how to protect yourself if you have one -- any electrical work requires a qualified electrician.
The previous diagram shows a wacky (and dangerous) way to bring power from one end to another using three way switches with a light in between. It uses the travelers to carry one neutral and one hot. With this, the two commons can have 4 states:
- Hot - Neutral (lamp turns on)
- Neutral - Neutral (lamp turns off)
- Neutral - Hot (lamp turns on, with reversed polarity)
- Hot - Hot (lamp turns off, and the socket remains hot)
States #3 and #4 are dangerous, as anyone changing a lightbulb who happens to touch the socket gets an electric shock. An electrician can also have problems changing the light fixture thinking it is off, while it's not necessarily the case. There is no way to know. The only thing that can be of any indication is if the tradesman that initially installed this used both red wires to connect to the light as illustrated above.
I did some research and found out that this is called a "Carter System" or "Chicago Three-Way". I have no clue how local electricians call this over here, but I'm sure they have a name for this. The examples I've seen on the net show that it can be used to power a garage or shed with 3-conductor cable, with an outside light toggled at both ends. Oh yes, and I can't see any relation to President Carter since by the time he was in office, the NEC had outlawed the practice for over 50 years.
In my case, I've seen this inside my circa-1950 house and thus why I blog this here. There are many houses like mine in Montreal, with a long passageway using three-way switches, and I suspect that a cable routing like this must have been installed in more than one place around here.
Can it be fixed? Not without rewiring. You could remove one of the switches, replace the other one with a standard two-way switch, then bring the neutral to the light fixture from the other side of the circuit. But then you'll end up with a light that can be turned on only from one far end of the house, which is probably against some building code as well. Assuming this Carter System passed inspection and was allowed at the time of the installation, it can probably stay there as long as you don't modify the circuit. But if you remodel, it would be a good thing to have it fixed.
To my knowledge, a Carter System doesn't seem to pose any fire hazard, but something as trivial as changing a light bulb can result in an electric shock. And if you're really, really unlucky, the shock can be fatal... As an example, just imagine someone unlucky enough to have a weak heart, unlucky enough to be bare feet on an aluminium step ladder, unlucky enough to have that ladder grounded by one way or another, and unlucky enough to have the switches set to state #4 above. That's a really stupid death that could have been prevented, and a case for primetime news.
Lesson learned: If you live in an old house, wear rubber gloves when changing bulbs, or turn off your breakers first.
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