SPDT as transfer switch for single phase?

Thread Starter


Joined Apr 8, 2005
I'm putting in a PLC based home automation system. One of the things I'd like to do is wire up an outlet in the basement where I can plug in an X10 signal strength meter for diagnostic purposes. Given that I also need to put in a coupler/repeater and a whole-home surge suppressor, I figured I could install everything in a nice enclosure and leave the meter plugged in as a monitor (and for diagnostic event logging). However, I want to be able to switch the meter from one leg of my power to the other (like almost all residences, I have split single phase).

I initially looked at an SPDT toggle switch with a center-off for this (Leviton 1281 for example) - essentially a single pole transfer switch since the neutral is common. However, while slightly cheaper than DPDT switches, they're still a fortune (at least for ones that fit in a standard switch box w/ a normal cover - $30 for a switch is a bit steep). So, I'm considering a 3-way switch for it - as they're essentially an SPDT switch.

My dilema is this - since 3-way residential switches do not have a center-off, is there any chance that when flipping from one side to the other, the switch actually closes both contacts? Even if it were for only an instant it would be bad (it would essentially short across 220V).

Any input much appreciated.


Erin G.

Joined Mar 3, 2005
What is a PLC based home automation system?

You need to find out if the the seitch you're interested in buying is a "make before break" type. Most off the shelf, three way switch are "break before make", which is what you're after. As the name implies, it's supposed to break the contact it's on before it makes the next contact. But all things man-made tend to eventually break, expecially with repeated use.

The fact that there is a PLC in this opens a door that you might not otherwise have available. I'm not sure about the purpose of your PLC, but if you're wanting to monitor current and log events, why not buy a couple of CTs? Put one on each of your hots in the panel board and feed them to an analog input card on the PLC.