ALLOSUN AC DC Voltage Tester Ohm Polarity Check Continuity RCD Checker Multi Tester with LED Buzzer Power Supplied by 1 piece 9V battery,6F22 or equivalent.(Battery not provided by seller) Continuity check: Connect the test leads accross the circuit to be tested. If the circuit is complete, the built-in buzzer will sound and the LED will light up. Resistance test: Connect the test leads accross the resistor to be tested. If the resistance is less than about 10k5k, the buzzer will sound and the LED will light up. RCD test: Connect one test lead to Earth/Ground and the other test lead to live of the socket which is protected by RCD. If the RCD's tripping function is good, it will trip. DC voltage / Polarity check: Connect the red test lead to the positive terminal of the circuit to be tested, and the black test lead to the negative terminal of the circuit. The buzzer will sound and the LED will light up. If the connections are reversed, the tester will not give any indication.