The Extech RT30 uses a pager-sized, non-contact voltage detector transmitter which clamps onto wiring mid-run and transmits a "voltage present" signal to a pocket-sized receiver carried by the user. Clamped onto a live wire, the transmitter signals the user at the breaker panel with LED and audible alerts on the receiver (up to 100 m range). The alerts stop when the correct circuit breaker is switched and work can then be performed safely.