Open circuit two wire voltage fault tester - uses two probes to indicate all faults in one test

摘要

The fault tester detects all voltage faults in an open circuit two wire telephone line in one test. A high threshold probe (SF1) determines the voltage between a combination of both wires (a and b) and earth (E). The low threshold probe (SF2) measures the voltage between the two wires (a/b) and between the single wire and earth (a/E, b/E). The thresholds are determined by the negative feedback loops (R5/R6, R7/R8). The high impedance voltage divider (R1, R2, R3, R4) has a symmetric earth (E) and conducts sufficiently to neutralise the line. R2 and R3 are of low impedance compared to the probes and the voltage divider. The threshold indicator (A) displays a voltage fault.