How to Find Incorrect Supply Polarity Testing and Fault Finding Live Testing Single Phase 230 Volts How to find incorrect supply polarity with an approved voltage indicator. The first live test in BS 7671 is recheck supply polarity it is so important and in this video Marcus demonstrates how to prove supply polarity of a single phase 230 volt AC supply in a domestic dwelling. Using a proving unit and a voltage indicator we show step by step how to carry out proving polarity of the supply test. �