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 an 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 an detected average shield current value to an 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.