PURPOSE: To monitor the presence of fault occurrence by detecting automatically a voltage fed by the phantom system in an ISDN with the feeding monitor connecting to a signal line.

CONSTITUTION: Each potential on R line 3a and a T line 3b forming a signal line 3 is detected by using each potential detection transformer and a difference voltage between neutral potential levels of windings of each of the transformers is detected as a feeding voltage. Then the detected voltage is monitored in terms of software for a very short period, the detection voltage read for a preceding period and the detection voltage read by this period are compared and when different, the presence of the fault to the detected voltage this time is analyzed and occurrence of a fault voltage and occurrence of a power failure are detected.