Why can't a continuity tester be used on a live electrical circuit?

A continuity tester is designed to check for the presence of a complete circuit, meaning it can identify if there are any breaks in the wiring. When used on a live electrical circuit, it can interfere with the normal operation of the circuit and potentially damage the tester itself or the circuit components. The reason that a continuity tester should not be used on a live circuit is because it typically has its own power supply. When it is connected to a live circuit, the power from the circuit can conflict with the tester's own power supply, leading to inaccurate readings and possibly causing harm to the tester's internal components.

Moreover, continuity testers are not built to measure the potential voltage present in live circuits; they are designed to verify continuity by sending a small current through the circuit. When connected to a live circuit, the tester essentially sends out its own current which can lead to misleading or erroneous readings due to interference from the live voltage. Therefore, understanding that continuity testers operate independently of the circuit's live power supply underlines their appropriate use in circuits that are turned off and isolated.