These insulation resistance testers are the recognized standard in the industry. They have proven so reliable and popular that they have been reintroduced, generation after generation, with circuitry upgraded to complement the familiar design.True analog movement remains a popular feature, especially where diagnosis is involved. The trained eye can get much more than just a M reading from an analog meter. Similarly, a guard terminal adds a valuable extra dimension when more than just pass/fail testing is desired. With commonly four test voltages, extra range to the resistance measurements, continuity and k functions for proofing and troubleshooting, they address the widest range of applications and are the most general models.