I have an edge to edge stud finder with a power line detector. It found the stud but at the same the power detector lighted up. Is it safe to drill that stud. I'm installing a wall mount for a 39" flat tv.
Power lines can be run either through a hole drilled through the stud (i.e. horizontally) or stapled along the edge of the stud (i.e. vertically).
Horizontal runs are generally used to connect multiple sockets in the same room. For normal outlets at about 1' above floor level, the wiring will usually be a few inches above the outlets, but you could have them at other heights too, like if you have a kitchen or bathroom on the other side of the wall, or wall sconces for lighting.
Vertical runs of wire generally bring power into a room from wire runs (that eventually lead back to the service panel) in the attic, crawlspace/basement or the space between floors.
If it's running through the stud, there should be a metal plate attached to the front of the stud to prevent you from driving nails or screws into the power line.
Of course, things aren't always done the way they should be, so you can detect if you have these plates with a strong magnet.
My current house may have me a bit paranoid about these sorts of things, but honestly, I have seen things that astounded me and also nearly killed me with this home.
One possible thing that this could be that happened in my home when I was remodeling the bathroom, was that one of the previous owners ran electric line from the wall switch to the mirror vanity lights, but not up, over and down like it should have been done. Instead to use several less feet of electric line, they gouged out a channel in the plaster wall, ran the line through this, unprotected, and then proceeded to plaster over it all.
I didn't find out what happened until I started hitting at the wall with my claw hammer and the power shut out suddenly. Fortunately the GFCI outlet tripped and saved me from a not so pleasant situation.
Oh and on a side note, it wasn't even 12 gauge copper like the rest of the house either, it was 16 gauge ALUMINUM, I was shocked that the house didn't burn down well before this!
The point I am trying to make is that unless the walls are down or you cut a small view hole, you NEVER know what could really be happening behind that wall, so it is ALWAYS best to play it safe just in case.
When drilling in the area where a power line is detected, always shut off the breaker feeding that line. If unsure switch all of the breakers.