Yes they work, on powered lines
Tony,
I was a working maintenance supervisor at a manufacturing company over near you, on McGinnis Ferry Road. We purchased a Major brand name circuit breaker locator. You must read and always follow directions for the desired results. Sometimes, we experienced an erratic read and it would indicate on two or three breakers. We would lock out all breakers (Please use "Lock Out devices, and if it's industrial Tag out as well) That way no one can come by the breaker panel and re-energize a circuit you may be working on.
Please use a voltage detector (Small pocket model, just a bit bigger than a large pen as a final check for voltage present, use the audible & visible mode, available at Lowes and HD. The few dollars spent is well worth it.
Lock Out - Tag Out kits are available for safety equipment suppliers, not only for breakers, light switches, and disconnects (lever).
I have a no name one for use at home. They work.
Charlie