JPH0411443A - Protection circuit for subscriber circuit - Google Patents

Abstract

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.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a protection circuit for a subscriber circuit, and in particular to a protection circuit for a subscriber circuit that can improve the accuracy of a ground fault recovery resistor in the event of a ground fault in the power supply circuit of the subscriber circuit. The present invention relates to a protection circuit for personal circuits.

[Conventional technology]

Conventional ground fault protection circuits for subscriber circuits of this type include a current detection circuit that detects the current flowing through a pair of power supply resistors connected to a pair of power supply terminals, and a comparator with hysteresis characteristics whose input is the output of the current detection circuit. A device having a circuit and a switch section that switches between a normal power supply circuit section and a high-resistance power supply resistor using the output of this comparator circuit has been used.

FIG. 3 shows a protection circuit for a subscriber circuit according to this prior art.

In the conventional example shown in FIG. 3, a pair of power supply terminals (T,
When the terminal (R terminal) on the power supply voltage V□ side in R) is grounded with low resistance, a ground fault current flows from the ground toward the R terminal. For example, power supply resistance 200 [Ω]
, power supply voltage (VAS) 48 (V), ground fault resistance 200
In the case of [Ω], the ground fault current is 120 (mA). The current detection circuit 12 detects this current and generates an output.

The comparator 13 functions to detect a ground fault condition when the output voltage of the current detection circuit 12 exceeds the higher threshold (VTHI).

The output of the comparator 13 switches the switch 14, connects the resistor 52, and enters a protection state in which high constant resistance power supply is performed. In other words, ground fault conditions typically last for a long time, so low power is required;
In order to keep the current low, the power supply resistance value is increased.

When the ground fault condition is restored and the ground fault resistance increases, the ground fault current decreases. The comparator 13 has hysteresis characteristics, and has a threshold value (V
T)+2) or below, it is detected that the ground fault condition has recovered, and the switch 14 is switched to the power supply circuit 10 side, changing the high constant resistance power supply state to the normal power supply state.

(Problems to be Solved by the Invention) The conventional subscriber circuit protection circuit shown in FIG. 3 detects restoration of a ground fault based on a current value.

The ground fault current IG is expressed as Ic=VIIB/(Rc+Ri+). Here, Ro is a ground fault resistance, and R8 is a power supply resistance.

In order to suppress the ground fault current Ic during ground fault protection, the sum of the ground fault resistance RG and the power supply resistance R6 must be increased. In the event of a ground fault, the power supply resistor R8 has a high resistance value, so the slope of the change in ground fault current due to the change in ground fault resistance is small, which causes the problem of large variations in the ground fault recovery resistance value that recovers from the ground fault state. be.

The present invention aims to solve the problems of the prior art, and improves the accuracy of the ground fault restoration resistance value in a protection circuit for a subscriber circuit in which the power supply resistor has a high resistance value in the event of a ground fault. It is an object of the present invention to provide a protection circuit for subscriber circuits that can provide protection for subscriber circuits.

[Failure to solve the problem]

In the present invention, there is provided a power supply circuit that supplies power from a pair of power supply terminals to a pair of subscriber lines via a pair of resistors inserted in series, a protection hourly rate circuit that supplies power from the power supply circuit with a high constant resistance,
A current detection circuit that detects the current value of the resistor pair, a comparator that generates an output with hysteresis characteristics in response to a change in the detected current value, and switches between the power supply circuit and the protection hourly type circuit in accordance with the output of the comparator. It has a switch. Of these, the protection hourly rate circuit is
It has a power supply resistor with a higher resistance than the power supply resistance of the power supply circuit and a constant current source with a low current, and always selects the power supply that has a lower current by comparing this constant current value and the current during high constant resistance power supply. It is structured to do this. This aims to achieve the above-mentioned purpose.

[Function] In the subscriber circuit protection circuit, normally, the power supply terminal pair T, R is connected through the resistor pair RFT + RFR inserted in series.
During protection, power is supplied from the power supply circuit to a pair of subscriber lines with a high constant resistance, and the resistance pair RFT, RF
The current value of R is detected, a change in the detected current value is detected, an output is generated with hysteresis characteristics, and the power supply circuit and the protection hourly type circuit are switched according to this output.

At this time, in the protective hourly type circuit, a power supply resistor with a higher resistance than the normal power supply resistance and a low current constant current source are installed, and the constant current value and the current when the high constant resistance power is supplied are compared. Power supply is selected.

In this way, since the power supply during protection is switched between constant current power supply and high resistance power supply according to the external load (ground fault resistance), the accuracy of ground fault restoration resistance detection is improved.

〔Example〕

FIG. 1 is a block diagram showing one embodiment of the present invention.

The same parts as in FIG. 3 are designated by the same numbers.

A power supply circuit for a subscriber circuit according to the present invention includes a power supply circuit 10 that supplies power to a pair of subscriber lines from a pair of power supply terminals 'r, R via a resistor pair Rya, R□ inserted in series, and this power supply circuit. 1
A protection hourly type circuit 5 that supplies power with a constant resistance higher than 0, and a current detection circuit 1 that detects the current value of the resistor pair RF a+RFjl.
2, a comparator 13 that generates an output with hysteresis characteristics in accordance with changes in the detected current value, and a switch 14 that switches between the power supply circuit 10 and the protection hourly circuit 5 in accordance with the output of the comparator 13. .

At this time, a power supply resistor 52 having a higher resistance than the power supply resistance of the power supply circuit 10 and a low current constant current source 51 are provided in the protection hourly type circuit 5, and ``this constant current value and the current during high constant resistance power supply are set. In comparison, a low current power supply is selected.

In normal operation, the comparator 13 controls the switch 14 to connect the power supply circuit 10 to the power supply terminals R and T via the power supply resistor pair RF T +R□.

At the time of a ground fault, the comparator 13 detects a ground fault state when the output voltage of the current detection circuit 12 exceeds the higher dark value (■A M+).

The switch 14 is switched by the output of the comparator 13, and the protection hourly circuit 5 connects the power supply resistor pair RFT+.
It is connected to power supply terminals R and T via RFII and enters a protected state.

The protection hourly type circuit 5 includes a constant current source 51 and a high constant resistance power supply circuit having a power supply resistance 52 higher than that of the normal power supply by the power supply circuit 10 .

It has a function of selecting the smaller current between the ground fault current determined by the constant current source 51 and the ground fault current determined by the external load. Therefore, when the ground fault resistance is small, a low constant current is provided, and when the ground fault resistance is close to recovery, a high resistance constant resistance power supply is provided.

FIG. 2 shows a comparison of the power supply characteristics of the present invention and a conventional protection circuit for subscriber circuits.

In FIG. 2, (a) shows the characteristics of the ground fault current IG before protection. .

In the conventional circuit, when the ground fault resistance R6 becomes small due to the occurrence of a ground fault and the output voltage of the current detection circuit I2 exceeds the higher threshold (■A M+), the comparator 13 detects this and The switch 14 is switched, and the ground fault current takes a value determined by the sum of the resistance 52 and the ground fault resistance R0. As the ground fault resistance R increases, the ground fault current IG decreases according to characteristic (b), and when the output voltage of the current detection circuit 12 becomes lower than the lower dark value (■A, □), the comparator 13 By detecting , the switch 14 is switched again and the earth fault current is
The value is determined by a). In the conventional circuit, the characteristic (G)
) has a gentle slope, so the comparator 13 has a threshold value V
The variation (C) when detecting 781 was large.

In contrast, in the circuit of the present invention, when a ground fault occurs, the ground fault current characteristic initially becomes the characteristic (d) determined by the constant current source 51, and when the ground fault resistance RG becomes large, the resistance 52 and the ground fault resistance RG A high resistance constant resistance power supply characteristic (e) is determined by the sum of When the comparator 13 detects that the threshold value VTH□ is below, it recovers and returns to characteristic (a). However, since the threshold value VTH2 is selected to be a smaller value than characteristic (d), ground fault recovery is impossible. Characteristic (e) is the threshold ■A
2, and the variation in ground fault recovery detection (
f) becomes smaller.

As described above, in the present invention, the protection hourly type circuit is configured to switch between constant current power supply and high resistance power supply in accordance with the external load (ground fault resistance), thereby improving the accuracy of ground fault restoration resistance detection.

[Effects of the Invention] As explained above, according to the protection circuit for a subscriber circuit of the present invention, a normal power supply circuit and a protection hourly type circuit are provided in the power supply circuit section and are switched between them depending on the presence or absence of a ground fault. Furthermore, since the protection hourly type circuit switches between constant current power supply and high resistance power supply according to the magnitude of the ground fault resistance, it is possible to improve the accuracy of ground fault restoration resistance detection. Furthermore, since the current during earth fault protection is determined by the current value of the constant current source, it has the advantage that it can be smaller than the current during protection in conventional protection circuits.

[Brief explanation of the drawing]

Figure 1 is a diagram showing an embodiment of the present invention, Figure 2 is a diagram comparing the power supply characteristics of the present invention and a conventional protection circuit for subscriber circuits, and Figure 3 is a diagram showing a conventional protection circuit for subscriber circuits. It is a diagram showing a circuit. 10-Power supply circuit, 12-Current detection circuit, 13-Comparator, 14-Switch, 5-Protection power supply circuit, 51 Constant current source, 52-Power supply resistor, T, R-Power supply terminal pair,
RFf+ RFN- resistance pair. Applicant Nippon Denki Co., Ltd. Agent Patent Attorney Isamu Takahashi Figure 2 (Konden Electric) Figure 1 (4 rows of loquats) 5. Power supply circuit 14 during protection - changeover switch Figure 3 (sub-panel) Elements

Claims (1)

[Claims] (1) A power supply circuit that supplies power from a pair of power supply terminals to a pair of subscriber lines via a pair of resistors inserted in series, a protection power supply circuit that supplies power from this power supply circuit with a high constant resistance, and a pair of resistors inserted in series. a current detection circuit that detects a current value of the detected current value, a comparator that generates an output with hysteresis characteristics according to a change in the detected current value, and a switch that switches between the power supply circuit and the protection power supply circuit according to the output of the comparator. In the protection circuit for a subscriber circuit, the protection power supply circuit includes a power supply resistance having a higher resistance than the power supply resistance of the power supply circuit, and a constant current source with a low current, and the power supply circuit has a constant current value and a high constant resistance power supply. A protection circuit for a subscriber circuit, characterized in that it is configured to select a low current power supply by comparing the current at