The US and Mexican method for wiring three-way light switches is different. In the US, both switches are wired to make or break the line (black) side of the light circuit using two “travelers” (red) as shown below.
Note that the neutral is not switched and connects to the screw base of the light(s).
In Mexico, there are two other methods commonly used.
In the following figure, the two switches connect to the two sides of the light bulb(s) and switch between the line and the neutral conductors. If either switch is high and the other low the light is on, but if both are high or both are low the light is off.
What this also means is that there are two different conditions that leave the light bulb off. One is that both center and screw sides of the base are low (neutral) and the other is that both sides of the bulb are high (line). It works just fine, but you need to be aware that there is a 50% chance that the bulb is “hot” even when it is off. This is dangerous, so you need to use care not to touch the base of the bulb when changing it. Actually, you should be careful not to do this even on a simple light circuit since many light fixtures in Mexico are wired with the line side connected to the screw base.
The second common method shown below connects one side of the bulb (hopefully the screw base) to the neutral and switches the line side of the circuit (black) through a single runner (red) connected between the two switches. This is far safer than the circuit marked Mexican Method #1, but it is also fairly easy to convert to the NEC standard used in the US. An advantage to the US standard is that it lends itself to adding an additional (4 way) switch, while neither of the Mexican circuits do.
Rewiring these circuits is possible, but it is not always practical. For now, use caution when changing light bulbs.
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