A low-cost type of test lamp only contacts one side of the circuit under test, and relies on stray capacitance and current passing through the user's body to complete the circuit. The device may have the form of a screwdriver. The tip of the tester is touched to the conductor being tested (for instance, it can be used on a wire in a switch, or inserted into a hole of an electric socket). A neon lamp takes very little current to light, and thus can use the user's body capacitance to earth ground to complete the circuit.
Screwdriver-type test lamps are very inexpensive, but cannot meet the construction requirements of UK GS 38. If the shaft is exposed, a shock hazard to the user exists, and the internal construction of the tester provides no protection against short-circuit faults. Failure of the resistor and lamp series network can put the user in direct metallic contact with the circuit under test. For example, water trapped inside the screwdriver may allow enough leakage current to shock the user. Even if an internal short circuit does not electrocute the user, the resulting electric shock may result in a fall or other injury. The lamp provides no indication below the strike voltage of the neon lamp, and so cannot detect certain hazardous leakage conditions. Since it relies on capacitance to complete the circuit, direct-current potential cannot be reliably indicated. If the user of the screwdriver is themselves isolated from ground and capacitively coupled to other nearby live wires, a false negative may occur when they test a live circuit, and a false positive when they test a dead circuit. False negatives may also occur in brightly lit areas which make the neon glow hard to see.