What is a Neon Glow or "Phase" Tester?

A neon glow tester, sometimes called a "phase" tester is a device used to detect mains voltages. The neon capsule in the device is usually sensitive to voltages from 90 volts upwards. Contact must be made with a live terminal for the neon bulb to glow. When doing electrical maintenance work, a neon tester can be used to double check power is turned off after switching off a breaker. Neon testers are commonly available in the form of a flat screwdriver whose blade is sized to suit the screws in electrical fittings.
Newer type hand held electrical testers such as the Fluke VoltAlert use an electronic detection circuit which activates an LED when the tip of the tester is in close proximity to a live terminal or power cable, actual contact is not necessary. These testers are useful for detecting presence of voltage e.g in a power flex or on the output of a breaker. However if the tip is too far away, the LED may not light up, giving the false impression that a circuit is not live. So they should never be used to test for absence of voltage. A neon tester should be used for absolute certainty when testing (and the tester should be pre-checked by touching it against a live terminal , e.g. the outlet of a socket to ensure it is working properly).