Abstract: A device for detecting cable insulation faults while laying underground cables. The device is adapted for attachment to the cable reel, having one lead connected to the metal shield of the cable or to selected conductors and the other lead to the metal reel. A bridge circuit within the device applies a voltage potential between the two leads, with the resulting resistance value serving as one leg of the bridge circuit. Two high impedance amplifiers are connected across the bridge circuit to act as buffers between the bridge and the operational circuitry. A differential amplifier is connected to the outputs of the two high impedance amplifiers to provide an output proportional to the difference between the two high impedance amplifiers. This differential amplifier's output is adjusted to provide an alarm trigger signal when its two inputs differ by a predetermined amount. Preferably the alarm signal is transmitted to a remote location for providing the alarm.