Kewtech Advanced Socket Tester with loop check, mains polarity, RCD test where a mains socket tester with loop check, mains polarity and RCD test. It includes a standard socket test (checks the wiring configuration at the socket), a loop test (the standard socket test checks the presence of the earth connection, the 107, more importantly, also checks the quality), a mains polarity (check on the correct polarity coming into the building) and a 30mA RCD check.