To measure the insulation resistance of a rubber covered electrical cable, which instrument is appropriate?

The appropriate instrument for measuring the insulation resistance of a rubber covered electrical cable is a Megger. A Megger, or insulation resistance tester, is specifically designed to apply a high voltage to a circuit and measure the resistance to ground or between conductors. This is critical in evaluating the integrity of the insulation within cables.

Rubber insulation can deteriorate over time due to various factors such as temperature changes, wear and tear, and exposure to chemicals, making insulation resistance testing essential. A Megger provides a clear indication of whether the insulation is still effective or if it has degraded, which could lead to electric shocks or equipment failures.

Other instruments like an ohmmeter are typically used for low voltage resistance measurements and are not suitable for evaluating insulation health. A voltmeter measures voltage in a circuit but does not provide information about insulation resistance. An ammeter measures current, which does not relate directly to insulation integrity. Therefore, a Megger is the best choice for this specific testing requirement.