In article G1osk.668$Ro1.545@trnddc04,
"Ivan Vegvary" wrote:

I own a "Circuit Alert" made by GB Instruments. (had it for over 6 years,
has new batteries). These are the probes that you place in the vicinity of
a live circuit and the instrument 'chirps' in response.

I just discovered that it does not detect current, or voltage, when you wave
it around a 240 volt cable. While it 'chirps' happily within 3-4 inches of
a 120 volt wire or cable, it will not do so for 240 volts. Won't detect my
main house service (6 gauge, bundled in sheathing), air conditioning feed
(10 gauge, bundled in sheathing) or any other 240 volt bundled source. Of
course, if you go to the end of the feed and wave the tool near either of
the 120 volt legs, it shows positive.

I'm doing a lot of demolition work around the house and am quite surprised
by the above results. I suppose one leg of the 240 feed is out of phase
with the other and they cancel each other? Any comments.

You hit the nail on the head. Typical 240 volt single-phase service has
one wire going positive while the other goes negative, the average
always being zero. The voltage from either wire to ground is 120.

I bet a 3-phase feeder will also escape notice by this circuit alert.

The Circuit Alert gadget sounds dangerous.

Joe Gwinn