Abstract: A security system of the bridge circuit type has in one of its arms a conducting element whose impedance is to be monitored and in its other arms reference and/or compensating elements, for detecting an unbalance of the bridge resulting from a sudden change in the impedance of the conducting element. At least one of the reference elements has a variable impedance. The phase shift of the signal obtained at the output of the bridge circuit is detected, due to the drift of the bridge circuit, with respect to the A.C. supply frequency. The detection signal is then used for compensating the drift by acting on the reference element having a variable impedance in order to bring the bridge circuit back to balanced condition.