| 摘要 |
PURPOSE:To improve the accuracy of detecting earthing restoration resistance by providing a normal feeding circuit and a feeding circuit at protection to a feeding circuit section and selecting any of the circuits depending on the presence of earthing. CONSTITUTION:A feeding circuit for a subscriber circuit is provided with a feeding resistor 52 whose resistance is higher than a feeding resistor of the feeding circuit 10 and a constant current source 51 supplying a low current and selects the constant current with a current supplied from a high resistance feeding circuit. A comparator 13 generates an output with a hysteresis characteristic and a switch 14 is provided, which selects either the circuit 10 or the circuit 5 in response to the output. Normally the comparator 13 controls the switch 14 to connect the circuit 10 to feeding terminals R, T via feeding resistor pairs RFT, RFR. In the case of earthing, the comparator 13 detects the earthing state when an output voltage of a current detection circuit 12 exceeds a higher threshold level. The switch 14 is changed over by the output of the comparator 13 to connect the circuit 5 to the feeding terminals R, T via feeding resistor pairs RFT, RFR. |
