TEL 1 Earth Leakage and Wiring Tester TEL1 is a Test Instrument utilized to Verify Electrical Wiring Connections and the Functioning of Earth Leakage / Residual Current Detectors devices by forcing the ELCB/RCD to trip. This verifies that the protection device open the mains power supply circuit when a current higher than a certain amplitude circulate into the ground/earth wire (generally around 15mA).