Abstract: A tingle voltage filter, comprising a saturating reactor connected between the neutral and the local bonding to ground of an electrical service so as to reduce ground currents, is associated with a device to determine whether the potential, referred to a remote ground, at the bonding significantly exceeds that at the neutral, also referred to a remote ground, thus indicating a wiring fault which may make desirable removal of the impedance of the filter from the local connection between the neutral and the bonding. Upon detecting such a condition, the device closes a switch to bypass the filter. The device also contains indicators of elevated potential of the local ground bonding relative to the remote ground, and elevated potential of the neutral relative to the remote ground, to provide further indications of abnormal conditions and of proper operation of the filter.