While trouble shooting a motor circuit today I happened to hold my Fluke volt stick to the wires in the switchbox and it strobed. The motor was off on thermal OL at the time and the switch was on. It was a 110V capacitor start single phase two speed motor (1-1/3HP). Running thru the switchbox and conduit also were two circuits feeding flourescent lights with a common neutral. My question is what caused the voltstick to strobe like that? Was it the flourescent circuits or maybe a power quality issue with the motor circuit? This was in the last few minutes of the day and the motor problem has not been diagnosed yet-it's on the list for tomorrow. Any help appreciated,
Brian

Most non-contact voltage testers will show a wire as hot even when it is off, if it is in a common conduit with other hot wires. The energized wires will induce enough voltage into the "dead" wire for the tester to detect. These types of testers often give a false positive (indicating voltage where there is none), but in some cases will do the opposite. The most common case where the tester will show no voltage where there really is voltage is NM cable where the paper within the jacket has become wet. This wet paper in contact with the bare EGC creates a shield and many times the tester will show no voltage even when the wire is live.
Don

Don:

As much as a non-contact probe is a great safety tool, there should be a warning printed on a bright-colored card to the effect of what you stated. False positives can be a real pain, but false negatives can be an even bigger, um, pain.

I still commend those who use a voltage probe as an extra cautionary means.

Although the non-contact testers are a great preliminary tool that can be carried around like a pencil in your pocket, don't use them to assure that a circuit is dead.
I still carry my old SqD Wiggy. The impedance is low enough to not give false readings...S

Yes I am familiar with the false readings with these testers both positive and negative and I never stake my life on the reading. It's just a convienent tool always in your shirt pocket for peliminary checking but I have a solenoid tester in my bag too.
I understand why it lights up on a wire with induced voltage (magnetic field) but I had never seen it strobe continuously like that and I was just wanting to understand why. The best I can figure is 1.the ballasted lighting circuit in the same pipe was causing it or 2. maybe the motor start capacitor was leaking down?
Thanks, Brian

Leakage into the "Test" Conductor, caused by Currents flowing in the active circuit being coupled to everything via Inductive and / or Capacitive Coupling effects.

Grounding the "Test" Conductor out would likely remove the high level reading (unless the line was actually a circuit path, then either sparks will be released or ground loops created with continuing high readings from proxy non-contact tester).

Scott35

Non-contact AC testers are not perfect, and a skilled user should be well aware of that. It is a simple go/no-go test but it's easy to get a feel for their capabilities and limitations after some regular use.

The same concept as with the $18 testers is used all the time up to 800kV. They are not foolproof, and should be backed up with a contact-type tester if there’s any question of their indication.

It is foolish not to be able to also readily and independently prove a dead circuit with another type of backup device, based on your best judgment.

Aside—stay away from testers that do not have both reliable audible and visual indication suitable for the environment in which it’s used. It’s your butt…