When the probes of a voltage tester are connected to two points in an electrical circuit (usually one end to a suspected live conductor and the other to a ground source), a closed circuit or loop is established. This facilitates the flow of electrical current from the higher potential to the lower potential.
Internally, the current passing through the voltage tester traverses a known resistance. The voltage drop across this internal resistance is then gauged and displayed on the tester's meter or screen, offering the user a voltage reading. In simpler testers, this voltage might merely activate a light or sound signal to denote the presence of voltage, rather than providing a precise numerical value.
Voltage testers come in varied designs. For instance, a non-contact voltage tester detects voltage through insulation, employing capacitive coupling between the wire carrying voltage and the sensor tip. When brought near a live conductor, the electric field influences the electric charge in the tester, prompting an indicator—usually an LED light or a sound—to activate.
Designed with safety in mind, voltage testers are typically encased in non-conductive housings to shield users from accidental electrical contact. Moreover, they usually don't require the circuit to be operational to function, making them versatile tools that can be employed in a wide array of scenarios to ensure the safety of your work environment.