Yep, noticed the same issue. A probe that goes crazy when working within
a couple feet of knob and tube wiring is sensitive within 6" of a 120 NM
cable but is insensitive to 240 line with no neutral.

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.

Be careful our there.

Ivan Vegvary