Hi Guys,

I have a problem/question for you. I am an air conditioning freak, I love a nice cold room. I have trouble sleeping if the room is not cool so I figured that I would rig up an X10 device to turn my AC on an hour before I normally go to bed, and off again after I leave for work.

I could not figure out how to tell what kind of power my AC was drawing, but I figured since it was a normal 3 prong plug that it would use a normal appliance module. The heavy duty module's seem to take different plug configurations (ie the T prong). I plugged the AC into the module, and everything seemed to be working fine.

That night I was experimenting with it, and it would turn on with no problems. When I turn it off, I saw a blue "glow" out of the corner of my eye. I took a closer look and it looks like when the relay is turning off my AC, it arc's and produces a big blueish spark. I was not sure what to do so I stopped using it. I would really like to controll my AC with X10, but not so much that I burn down my house. :) I am using a cheap Radio Shack Appliance Module, I ordered some other one's but they did not have the ground prong so I did not use them. It claims that it can be used with AC units, but only if they draw less than X current (or was it voltage?). Either way I was not sure what my AC was doing so I don't know if I am "in spec" or not. It is just a normal ~2 year old small room AC.

I spoke with a friend and he suggested that the blue spark is expected behavior with a relay, and if I look at it logically, I would agree. I was wondering if anyone had any input on the subject. Stuff like: Is it safe? Should I get a different modle that has sheilding? Should I stick it on an extension cord and put the module in a fire-proof box...:) Any suggestions or info would be greatly appreciated.

Thanks!

Will
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Electricty obeys the same general equations as mechanical things. That is why you can build electrical analogues of mechanical systems to test them out. Mechanical things with mass posess something called inertia, which to put simply is a masses resistance to change in velocity. Electricity has a similar attribute. When you open a circuit like the power to an AC unit, which has a motor in it and therefore a high inertial component (I am simplifying here), the electricity does not want to stop its flow instantaneously, in fact it can't. The voltage builds up at the break point in the circuit (the relay contacts) until it exceeds the breakdown voltage of the resistance which is stopping the flow. In this case the air bewteen the relay contacts. The blue spark you are seeing is the arc as the elctricity ionizes the air. It is bad for the contacts, not particularly dangerous since it is inside the relay module, and "natural" when turning off heavy loads. Why it doesn't happen when turning them on is a bit more complicated but basdically there is not a sudden change in state but a more gradual one which is able to be absorbed by the system without arcing.

Hope that helps