Abstract: By introducing into an operating 3-phase transmission line system through a transformer having the common point of its "Y" or "star" connected primary grounded and its ungrounded secondary coupled in open "delta" configuration to a source of a measurement voltage having a second frequency significantly lower than the first or operating frequency of the transmission line system, detecting the current flowing in the transmission line system at the second frequency and multiplying the measurement voltage injected into the transmission line by the measurement current produced thereby, a d.c. component is obtained which is representative of the insulation resistance's magnitude, permitting calibration of the apparatus in terms of insulation resistance. At the same time a fault alarm system may operate with part of its circuit in common with the circuit of the resistance measuring apparatus.