A hand-held test lamp necessarily puts the user in proximity to live circuits. Accidental contact with live wiring can result in a short circuit or electric shock. Inexpensive or home-made test lamps may not include sufficient protection against high-energy faults. It is customary to connect a test lamp to a known live circuit both before and after testing an unknown circuit, to check for failure of the test lamp itself.
In the UK, guidelines established by the Health and Safety Executive (HSE) provide recommendations for the construction and use of test lamps.Probes must be well-insulated, with minimal exposure of live terminals, with finger guards to prevent accidental contact, and must not expose live wires if the test lamp glass bulb is broken. To limit the energy delivered in case of a short-circuit, test lights must have a current-limiting fuse or current-limiting resistor and fuse. The HSE guidelines also recommend procedures to validate operation of the test light. When a known live circuit is not available, a separate proving unit that provides a known test voltage and sufficient power to illuminate the lamp is used to confirm operation of the lamp before and after testing a circuit.
Since energy to operate the test lamp is drawn from the circuit under test, some high-impedance leakage voltages may not be detectable using this type of non-amplified test equipment.