A device and method for detecting faults in a shield of an electrosurgical instrument is described. The device has a relay configured to selectively interrupt power to the electrosurgical instrument, monitoring circuitry configured to monitor a shield in the electrosurgical instrument, control circuitry to control the relay, and a battery power source. The monitoring circuitry has an envelope detector and a detected average shield current detector. The monitoring circuitry is configured to compare a shield current peak value to a shield current peak threshold value, and to compare a detected average shield current value to a detected average shield current threshold value. The device is further configured to operatively couple an active electrode of an electrosurgical instrument and a return electrode to an electrosurgical generator.