Martindale vt28 voltage tester and pd690 proving device martindale vt28 the martindale vt28 is a tough compact two pole tester with both lcd and led displays for measuring acdc voltages from 12v to 690v additional capabilities include continuity and phase rotation testing plus single pole voltage detectionthe large digit lcd display of the vt28 gives actual voltage readings in addition to the led voltage ranges the acdc voltage detection continuity and phase rotation functions all have