Telephonic communications – Diagnostic testing – malfunction indication – or electrical... – Monitoring
Patent
1990-09-19
1992-05-12
Schreyer, Stafford D.
Telephonic communications
Diagnostic testing, malfunction indication, or electrical...
Monitoring
379 26, 379324, 379412, 379413, H04M 322, H04M 1900
Patent
active
051134268
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a ground fault detector for long distance subscriber circuits accommodated in a small capacity digital electronic exchange (private branch exchange) and particularly to a ground fault detector having a function to protect the long distance subscriber circuit by stoppage of DC/DC converter when a ground fault occurs in the subscriber line.
The subscriber circuit to be used for the digital electronic exchange is required to provide a so-called BORSCHT function covering the battery feed (B), overvoltage protection (O) ringing (R), supervisory (S), CODEC (C), hybrid (H) and testing (T), but the present invention relates to a circuit for realizing ground fault detection which is the one of the supervisory function (S).
A ground fault is usually generated when the telephone line not insulated is errorneously in contact with the grounded metal or the ground during the constructon. If the ground fault is generated, a current of the battery feed circuit is doubled and requires the power of four times that of the ordinary one, resulting in dangerous condition such as heat generation.
On the other hand, a large capacity digital electronic exchange (office exchange) generally feeds the power to the subscriber lines using a battery of -48V but a small capacity digital electronic exchange (PBX) generally feeds the power using a battery of -24V because the telephone line connecting the exchange and telephone set is comparatively short. However, if the telephone line becomes longer, the battery of -24V cannot supply sufficient speech current and therefore a higher voltage, for example, -48V is necessary for the long distance subscriber lines. In order to obtain such higher negative voltage, the -24V voltage is boosted up to -48V by utilizing a DC/DC converter into the battery feed circuit for the subscriber lines. Since the long distance subscriber lines are placed under severe circuit conditions, the battery feed circuit formed by resistors and transformers provides some advantages.
The advantages of power feeding by resistors includes that dynamic range which is required for the battery circuit by transistors is no longer necessary to be considered and a current can be supplied in bothways without distortion. Namely, a method of protection circuit through constant current which is often used in the battery feed circuit by the transistor circuit is difficult to be used in the battery feed circuit for the long distance subscriber lines.
BACKGROUND
A circuit structure of conventional ground fault detector for subscriber circuits utilizing a DC/DC converter into the battery feed circuit is shown in FIG. 1. In FIG. 1, the reference numeral 1 designates a DC/DC converter; 2, a comparator; 3, transformer for power feeding; 4, a subscriber telephone set.
A DC/DC converter 1 is a DC voltage converting circuit for boosting -24V voltage (system power supply of exchange) up to -48V and a comparator 2 is a circuit for detecting ground fault through comparison of voltage with the reference voltage and sends a supervisory signal to the controller of exchange. If a ground fault is generated on the subscriber lines, a battery feed current I is doubled in case the line resistance is zero. Accordingly, in case a voltage of DC/DC converter 1 is fixed to -48V, a battery current increases. Moreover, the ground fault detection voltage V.sub.E becomes the ground potential, the comparator 2 is inverted, thereby the signal "1" is sent to the exchange controller and an alarm is issued. Even if a ground fault occurs, the battery feed of -48V still supplies -48V continuously, the DC/DC converter 1 requires the battery feed resistors R.sub.T and R.sub.R which are resistive to four times of power dissipation and is also required to have a structure which is resistive to the doubled current. As a result, the DC/DC converter 1 results in not only rise of cost but also increase in size of circuit elements and restriction on mounting of elements.
SUMMARY OF THE INVENTION
Therefore, it is a pr
REFERENCES:
patent: 4748652 (1988-05-01), Nagai et al.
patent: 4897872 (1990-01-01), Siligoni et al.
Kinoshita Kazumi
Takato Kenji
Tojo Toshiro
Fujitsu Limited
Schreyer Stafford D.
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