This invention relates to a power supply system for telephone line wires.

The traditional telephone exchange battery for powering a handset at subscriber's premises has comprised a 50 volt DC power supply, this value having been chosen with a view to minimising stress on the copper telephone lines and to safety in the case of human contact with the wires. Recently, however the requirement for more efficient use of the telephone network has led to the introduction of line sharing systems wherein multiple subscribers are served by a single line. To enable a corresponding number of telephone apparatus to be powered from the line, it has been necessary to increase the power supplied to the line. This has hitherto been achieved by increasing the exchange battery voltage applied to the line whilst limiting the available current to maintain acceptable levels of safety. This solution nevertheless increases stress on the copper line conductors, undesirably reducing their service life.

The same considerations may apply in cases where redundant telephone line wires are used only for power supply, for example to equipment handling signals transmitted by optical fibres that cannot carrry power.

It is accordingly an object of the invention to provide an improved system for transmittng power via telephone line wires.

The invention provides a power supply system comprising a variable rate power supply, a load coupled to said power supply by a line the resistance of which is significant in relation to that of the load, monitoring means associated with said power supply for monitoring the voltage and/or current of the line circuit in the vicinity of the power supply, monitoring means for monitoring the voltage and/or current in the line circuit in the vicinity of the said load, telemetry means for transmitting output signals from the monitoring means associated with the load to receiving means in the

vicinity of the power supply, and means associated with said power supply for for controlling the power delivered by the power supply in accordance with signals received from the respective monitoring means .

The line coupling the power supply to the load may be a conventional telephone line pair, or a single line feeding one or more circuits having an earth return. The telephone line may serve as the medium for the transmission of the telemetry signals or the signals may be transmitted independently, for example by an associated optical fibre link for which a redundant telephone line serves only as a source of power.

The control of the power supply may serve one or more alternative purposes.

Monitoring only of the current flowing respectively at the power supply and load may be sufficient, for example, to determine leakage from the line and to enable the supply to be shut down for safety upon contact with the line. This may in turn enable the power supply equipment to have a higher permissible maximum current rating than would otherwise be considered safe.

Monitoring of both voltage and current may enable determination of the relative conditions of the line and load, for example enabling the power supply to be matched to the load without the need to take into consideration the line resistance. Where the line is fo supply a group of subscribers, the load may be in the form of a switch mode power supply the duty cycle of which varies in accordance with the number of subscribers being served at any given time. Variation of the resistance of the switch mode power supply as seen from the line will cause a corresponding variation in the voltage and current sensed in the vicinity of the load and thus the power supplied from the exchange battery can be varied accordingly.

In summary, an arrangement in accordance with the invention has the advantages that it can provide a maximum efficiency of power

transfer under long line conditions; can provide only the amount of power required by the load, thus minimising voltage stress on the access network, and can provide safe operation at high terminal voltages without the need for arbitrary current limitation.

One embodiment of an arrangement in accordance with the invention is illustrated by way of example in the accompanying drawings, in which:

Fig 1 is a diagrammatic view of the basic system configuration, and

Fig 2 is a block schematic diagram showing the system in more detail.

Figure 1 shows the basic system configuration. It consists of five component parts, the load which is represented by a resistor, a variable voltage feed supply, a communication channel from the load to the feed supply, two line current monitors and a simulated line resistance.

The nominal value of load is designed such that it is equal to the line resistance. This ensures that for any given feed voltage the maximum power is supplied to the load. A voltage feed is used rather than a current because it results in a smaller change in power delivered to the load for any given resistive fault on the line. The communication channel transmits the line current and the voltage across the load to the feed.

The purpose of the communication channel is to allow the load to demand only the necessary power from the exchange. As the system always operates at the maximum power transfer point the voltage on the line is kept to a minimum. This will reduce the effect of any electrolytic action on the copper pair and terminal blocks.

As the exchange system knows the power that is fed to the line, the power used by the load and the power dissipated in the line, it is

possible to detect any transient fault condition. Steady state line leakage conditions can also be monitored.

If a person should now make contact with the line some of the current from the feed will be taken away from the load. The system can sense this and shut down the feed supply making the system safe. In addition as the line voltage is always kept to a minimum it will inherently be safer than conventional systems.

Figure 2 shows an example block schematic of the system. The power feed and the load referred to in Figure 1 are now shown as the exchange and remote units respectively.

The exchange power feed consists of a dual output flyback switch mode supply. The line voltage is fixed as symmetrical about earth. Line current monitors are included for each wire and the results fed to the EU control circuit.

The transformers T2 and T3 provide the communication channel. This could use the 2B1Q basic rate ISDN line protocol. The voltage and current information would be transmitted using the spare bits in the EOC channel. It is not necessary to use this protocol, any appropriate system could be used provide that there is the spare bandwidth.

The remote power supply is also based upon a flyback switch mode configuration, however the primary and secondary sides are isolated. A voltage and current monitor is provided on the primary side and opto couplers are used to feed the results to the RU controller.

The EU controller knows the voltage across the remote unit -and the feed voltage at the exchange. It adjusts the feed so that the remote voltage is nominally 60% of the exchange. This allows the remote to instantaneously draw more power from the line and give the exchange time to increase the feed to maintain the 60% ratio.

If at any time the three line current monitors do not indicate the sam _* value of current it can be assumed that a line fault exists. When th- fault condition arises slowly it will be due to l ⁱ ne leakage. _A n approπriate threshold can be set above which the system shuts down and raises an alarm. If the fault occurs quickly ⁱ t w ⁱ ll be due to a contact, such as a person touching the line. Under these conditions the system can immediately shut down mak ⁱ ng the system safe.