Martindale vt25 voltage tester and pd690 proving device martindale vt25pd safe isolation procedures state that voltage indicators should be proved against a known source both before and after use in response to this requirement martindale supplies this kit that includes both the vt25 voltage indicator and a matching pd690 proving unit all housed in a tc70 carry case with neck strap making it easy to carry and use the equipment on sitethe vt25 is a two pole led voltage indicator for use