Abstract: An in-circuit contact monitor for providing reliable and safe detection of the position of fail safe, or vital, circuit contacts in railroad signaling and inter-locking control circuits comprises an impedance network arranged in a bridge configuration, a low-power, high-frequency signal source, an isolation transformer, and a pair of DC blocking capacitors. The low-power, high-frequency AC source provides energy for an isolated circuit which is used in detecting the toggling of an individual circuit contact. The circuit is configured such that the vital circuitry, which is connected to the circuit contact, is completely isolated from the DC components generated by the low-power, high-voltage AC source. The isolation transformer acts to isolate the monitor system from the in-circuit contact monitor circuitry and thus the vital circuitry.