First Live Test - Check Polarity of 3 Phase Supply, Using an Approved Voltage Indicator AM2 & AM2S First live test according to BS 7671 is check polarity of the supply with an approved voltage indicator. For AM2 and AM2S the 3 phase distribution board will need testing to prove the incoming 3 phase supply polarity. For this test I used a voltage indicator and proving unit I explained in the video my thinking behind my use of the proving unit as it is not necessarily required. As always preparing