Method and device for connecting an apparatus to that terminal of the electricity mains which has the lowest impedance to earth compared with the other terminal

Abstract: The invention relates to a method for connection one side of an apparatus, connected on the other side to an earthed system, to one of several terminals of a single-phase or polyphase electricity mains, which terminal has the lowest impedance to earth compared with the other terminal or terminals; the invention furthermore relates to a device for carrying out such a method. The method according to the invention is characterized ▪ in that each of the electricity mains terminals is coupled to the earthed system; ▪ in that the current which flows between the electricity mains and the earthed system is detected for each of the terminals; ▪ in that the terminal for which the current is a minimum is detected; ▪ and in that the terminal for which the current is a minimum is connected to the apparatus. The device according to the invention is further characterized by a current detection device (R3/D) which is connected, on the one side, to one or more electricity mains terminals and which is connected, on the other side, to the earthed system, and moreover by a selection device (S) which can be controlled by the current detection device and which is connected, on the one side, to the said apparatus (T) and, on the other side, to the electricity mains terminals, which selection device connects, under the control of the current detection device, that electricity mains terminal, to the apparatus, for which the detected current is a minimum.

Description: A. BACKGROUND OF THE INVENTION 1. Field of the invention The invention relates to a method for connecting one side of an apparatus, connected on the other side to an earthed system, to one of several terminals of a single-phase or polyphase electricity mains, which has the lowest impedance to earth compared with the other terminal or terminals. The invention furthermore relates to a device for carrying out such a method. In particular, the invention relates to a method and device for connecting a telephone subscriber call duration meter to the neutral terminal (neutral phase) of the public electricity mains. 2. Problems In the Dutch telephone system, a telephone subscriber call duration meter is controlled from the telephone exchange by means of an alternating voltage pulse (75 volt, 50 Hz) which is applied between, on the one hand, the a and the b wire of the telephone connection and, on the other hand, earth potential. In this way, the counting pulse is inaudible to the telephone subscriber. If a subscriber wishes to use a call duration meter, for example to monitor his telephone expenses, it is necessary for the telephone terminal of said subscriber to be provided with an earth terminal so that the call duration meter can be connected between, on the one hand, the a and b wire (via an impedance) and, on the other hand, the earth terminal. Although in many cases said earth terminal is in fact present it is not, however, usually terminated in the telephone wall outlet, If a telephone subscriber purchases a call duration meter, this makes it necessary for an engineer from the telephoen company (PTT) to investigate whether the subscriber terminal is in fact provided with an earth terminal. If no earth terminal is present, it then has to be effected. B. SUMMARY OF THE INVENTION In order to encourage the use of call duration meters, the invention provides a method for obtaining an earth terminal via the public electricity mains. The invention furthermore provides a device for carrying out such a method. In general, the method according to the invention is characterized in that each of the electricity mains terminals is coupled to the earthed system; in that the current which flows between the electricity mains and the earthed system is detected for each of the terminals; in that the terminal for which the current is a minimum is detected; and in that the terminal for which the current is a minimum is connected to the apparatus. This method therefore determines, by means of measuring the respective currents between the terminals and the telephone system (the a and b wires of which are connected to earth via impedances), that electricity mains terminal whose voltage (and therefore the impedance) is the lowest with respect to earth. The device for carrying out the abovementioned method is preferably characterized in that the said apparatus is a call duration meter and in that the said earthed system is a telephone system, one or both subscriber wires of which are connected to earth. The current detection device therein can either measure the various currents simultaneously (multiple current detection device) or do this sequentially (single current detection device), in which latter case the current detection device has to be connected, by means of a selection device, consecutively with the various terminals of the electricity mains. In accordance therewith, the device according to the invention is preferably further characterized by a current detection device (R3/D) which is connected, on the one side, to one or more electricity mains terminals and which is connected, on the other side, to the earthed system, and moreover by a selection device (S) which can be controlled by the current detection device and which is connected, on the one side, to the said apparatus (T) and, on the other side, to the electricity mains terminals, which selection device connects, under the control of the current detection device, that electricity mains terminal, to the apparatus, for which the detected current is a minimum. In particular, the said earthed system is a telephone system, of which one or both subscriber wires are connected to earth (via an impedance). C. REFERENCES GB-A-2 005 111 discloses a general telephone charge pulse receiver circuit. D. EXEMPLARY EMBODIMENTS Figure 1 shows a preferred embodiment of the device according to the invention. The device comprises two resistors R1 and R2 and two capacitors C1 and C2 via which the device is connected to the a and b wire of the telephone system. It further comprises two capacitors C3 and C4 via which the device is connected, by means of a contract plug P, to the electricity mains. The device comprises, moreover, a resistor R3 with a voltage detector D in parallel with it and an (electronic or mechanical) switching device S. The switching device S can be controlled by the voltage detector D. The figure shows, moreover, a call duration meter T which is connected to either side of resistor R3. The a and b wires of the telephone system are each connected via coils L1 and L2 to earth. The operation is as follows. As soon as the contact plug P makes contact with the electricity mains, depending on the position of the switching device S, the detection device D detects whether there is a voltage across resistor R3. If resistor R3 is connected via switching device S to a terminal of the electricity mains on which there is no (or only a low) voltage with respect to earth (position 2 of the switching device S in the figure), the current through R3 is 0 (or small). If this is the case, the voltage detector D does not deliver a control signal to the switching device S, as a result of which the resistor R3 therefore remains connected to said terminal of the electricity mains. Since said terminal has a low impedance with respect to earth, it can be used to act as earth terminal for the call duration meter T which is connected, on the other hand, via R1/C1 and R2/C2 to the a and b wire of the telephone terminal and can therefore receive metering pulses via a metering pulse generator G in the telephone exchange (via a transformer T; the metering pulse generator G is controlled by means of control pulses t). If resistor R3 is connected via switching device S to a terminal of the electricity mains on which there actually is a voltage with respect to earth (position 1 of the switching device S in the figure), the current through R3 is relatively large. If this is the case, the voltage detector D delivers a control signal to the switching device S, as a result of which it switches over to the other position (positions 2). Resistor R3 is thereby connected to that electricity mains terminal which has the relatively low voltage (and impedance) with respect to earth, as a result of which the meter T is connected again to the desired terminal. It is pointed out that the switching device S may also be a multi-position switching device if the device should have to be connected to a polyphase electricity mains. In that case, the resistor R3 is successively connected via the switching device S to the various terminals of the electricity mains until a terminal is found for which the voltage across R3 is zero or has a low value.

Claims: Method for connecting one side of an apparatus, connected on the other side to an earthed system, to one of several terminals of a single-phase or polyphase electricity mains, which terminal has the lowest impedance to earth compared with the other terminal or terminals, characterized in that each of the electricity mains terminals is coupled to the earthed system; in that the current which flows between the electricity mains and the earthed system is detected for each of the terminals; in that the terminal for which the current is a minimum is detected; and in that the terminal for which the current is a minimum is connected to the apparatus. Method according to Claim 1, characterized in that the said apparatus is a call duration meter and in that the said earthed system is a telephone system, one or both subscriber wires of which are connected to earth. Device for carrying out the method for connecting one side of an apparatus, connected on the other side to an earthed system, to one of several terminals of a single-phase or polyphase electricity mains, which terminal has the lowest impedance to earth compared with the other terminal or terminals according to Claim 1, characterized by a current detection device (R3/D) which is connected, on the one side, to one or more electricity mains terminals and which is connected, on the other side, to the earthed system, and moreover, by a selection device (S) which can be controlled by the current detection device and which is connected, on the one side, to the said apparatus (T) and, on the other side, to the electricity mains terminals, which selection device connects, under the control of the current detection device, that electricity mains terminal, to the apparatus, for which the detected current is a minimum. Device according to Claim 3, characterized in that the current detection device (R3/D) and the selection device (S) are inserted in series with each other between the electricity mains and the earthed system and in that the common connection point (x) of the devices can be coupled to the said apparatus (T).

Inventors: BOELHOUWERS HENDRIK CORNELIS, CATIJN PAUL JOHANNES, VAN DER KROGT ADRIANUS ALPHONS, BOELHOUWERS, HENDRIK CORNELIS, CATIJN, PAUL JOHANNES, VAN DER KROGT, ADRIANUS ALPHONSIUS MARIA
Applicants: NEDERLAND PTT, KONINKLIJKE PTT NEDERLAND N.V.
Application year: 1990
Main classification: H04M15
IPC codes: H04M15