response 12 of 19:

Oct 6 18:34 UTC 2006

Re resp:11: Yes, that's exactly what it is.  The secondary on the 
transformer is center-tapped, and the center tap is the neutral.  It's 
also grounded (or "earthed," for you British types.)  This gives a 240V 
system where no conductor is more than 120V above ground potential.

If it's a very new house, you might find NEMA 14-30 sockets.  Until 
relatively recently NEMA 10-30 and 10-50 sockets were more common.  
They're a bit of a throwback as they don't have a separate safety 
ground conductor.  10-30 was usually used for dryers and 10-50 for 
stoves.

response 14 of 19:

Mar 30 18:59 UTC 2013

Let's see if anyone is still around here.....

This is only somewhat "Electrical Wiring" related, but there is no other more
relevant item, so I'll try it here.

I have a household X-10 light control system that sends activation codes
to switch and outlet units at programmmed times to turn lights on and off.
Over the past year the system became more and more erratic, until hardly
any control worked. The control xmttr has a LED pilot light that also, over
the past year, began to flicker. This is attributed on one discussion group
to line noise produced by CFLs. The situation became so bad I have replaced
several X-10 switch units with plug-in timers. However these can't be used
for outside lights, which stopped being controller.

Two days ago we arrived home after dark and discovered *half* of our 
house power had failed. We have a three wire service, with two 120v hot 
wire branches 180 degrees out of phase. This produces 240 volts for the 
electric stove. All the 120 volt outlets and switches are distributed 
about equally between the two branches. The power had only gone out on 
one of the 120 volt branches. We've had many power failures in the past, 
but they had always been total, not just on one branch.

The full power came back on after about 30 minutes, so no harm done. But 
we subsequently noticed that the X-10 controlled lights were working 
again: in particular, the outside lights went on in the evening and out 
at morning. I finally noticed also that the X-10 Xmttr LED pilot light 
no longer flickered!

Apparently power line noise much have been developing somwewhere outside 
the house and got fixed during the 30 minute single-branch blackout. It 
also means that CFL noise was not the culprit for the problem with X-10 
control.

Has anyone else ever observed single-branch power noise and eventual 
failure on a three-wire service, and know what in the power distrikbtion 
system might cause this?

I've also submitted an inquiry to DTE.

response 16 of 19:

Mar 31 04:00 UTC 2013

I suspect that also - but WHAT "something"? 

While the problem persisted, there was no dimming or fluctuations 
observable from the lights on the separate branches. Also, I have 
couplers (bridges) between the two phases as the X-10 control signal is 
injected into only one of the branches. So "noise" at the X-10 carrier 
frequency (120kH) would be found on both branches. (Of course I COULD 
have isolated the two branches and put a scope on each to find out which 
was the sources, but I mostly felt because of the online conference 
comments that the cause was CFLs. It turns out, they were wrong.).

response 18 of 19:

Aug 28 04:10 UTC 2013

You're probably right. I could not get anyone at DTE to tell me what had 
happened. I dind't ask the neighbors if they had a power outage as I 
just assumed they did. Now you mention it, I wonder if any noticed the 
one-sidedness of the outage.