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[robertebrown]

When an X10 device loses power, once the power is restored how is the device supposed to respond?

Does it
  1) Return to its prior state?
  2) Return with an 'on' state?
  3) Return with an 'off' state?
  4) Return with a random state?

Or isn't there any standard at all???

  I ask this because my house recently experienced a very brief (a second or two) power outage.
I had four X10 devices in use, all in the 'off' state.  When power resumed, three of the four
switched to the 'on' state; one remained in the 'off' state.

  What's going on???

[Brian H]

Brief power glitches can cause random states.
In a power failure long enough for the modules power supplies to discharge.
Ratchet style switch types that do not dim should return to the state they where in.
Dimmers usually stay Off.

[Dan Lawrence]

Usually in a Power Failure, the PC with AHP shuts off, X10Nets.exe stops working so module status does not work, but the CM15A will run on battery power depending how long the power is off.  Since AC power is off, timers and macros won't work either since there's no power for X10 signals to be carried to modules.   In short, NOTHING X10 will work.  Where I live (Baltimore, Maryland we rarely have power failures,  Two power failures in the last 20 years.

[Noam]

It all depends on the modules.
Some of the Smarthome ones return to their previous states.
I've also seen modules that default to the ON state after power is restored.

for the past few years, I've had a Powerflash module plugged in, with the contacts shorted. After a power outage (lasting more than a few seconds - long enough to discharge the internal capacitors), it sends an "M1 On" command, which triggers macros in the CM15A to turn on the right lights, based on the day and time.
It has worked pretty well for me.

Dan - You're really lucky that you've only had two outages in 20 years. I'm in PEPCO's service area, so outages seem to be more common these days than they used to be.

[dave w]

When an X10 device loses power, once the power is restored how is the device supposed to respond?
Or isn't there any standard at all???

  I ask this because my house recently experienced a very brief (a second or two) power outage.
I had four X10 devices in use, all in the 'off' state. 

It's the nature of the beast, and like all have said, impossible to predict. This used to be a huge problem for X10 Appliance and Lamp modules back in the 1980's but X10 has made improvements. In my experience a brief and "dirty" power outage frequently causes Appliance Modules to trigger ON. By "dirty" I mean power does not come back cleanly. Flickers, and "brown-ups" occur while the power is trying to come back. Lamp Modules are slightly less suseptable. There is no fix other than having a home automation system which will restore all your modules to a "desired" state after an outage.