GB2178915A - Power supply apparatus - Google Patents

Abstract

Electric power is supplied to items of equipment by a circuit having phase and neutral conductors 1, 2 and a residual current conductor 7 connected to earth to receive residual currents from the items. A detector 11 emits a signal indicating the magnitude of the residual current, the signal is received by outlet units 13 feeding the power to the items and the items are disconnected when the residual current exceeds a predetermined value. The outlets 13 may be selectively disconnected in response to the signal. <IMAGE>

Description

SPECIFICATION Power supply apparatus This invention relates to apparatus for use in an electric power supply circuit.

In a conventional single or three phase electric supply system for supplying electric current to current using equipment for household and similar purposes the current is supplied to outlet sockets or fixed electrical equipment through a plurality of conductors one of which connects the earth terminal of the sockets, or the items of current using equipment themselves, to earth in order to avoid danger of electric shocks in the event of an earth to phase or neutral fault occuring.

A shock hazard would exist if the earth conductor continuity were disrupted whilst a fault impedance was present between any normally earthed conductive parts and a phase.

This danger may be avoided or alleviated by providing the supply circuit with a residual current circuit-breaker which isolates the circuit from this power source when the current flow to earth exceeds a given value, for example 30 or 300 mA.

One type of residual current circuit-breaker is sensitive to the vector sum of the currents passing through the phase or phases and neutral conductors of the circuit. This sum is zero if there is no leakage; any earth leakage results in a corresponding difference between the currents passing through the phase and phases and neutral conductors, giving a non-zero vector sum. In one type of residual current circuit-breaker the phase and neutral conductors pass through a transformer, whereas the earth conductor does not, and a non-zero vector sum of the currents in the phase and neutral conductors produces an emf from the transformer, which actuates a relay for disconnecting the circuit from the power source when this emf exceeds a predetermined value.

Some current using equipment now in common use for household and similar purposes, including personal computers, are generally fitted or used in conjunction with a voltage stabilisation device incorporating filters and these devices are designed to themselves pass an appreciable residual current through the protective earth conductor. When a number of such devices are connected to a supply circuit the total of the intentional residual currents can be large and sufficient to cause a residual current circuit-breaker protecting the circuit to trip.

Similarly when a number of such devices are connected to a supply circuit which is not protected by a residual current circuit-breaker, the intentional residual currents can present a serious electric shock hazard if the protective earth conductor continuity is disrupted, as all normally earthed metal work on that part of the circuit subsequently not earthed will rise to phase potential, the prospective shock current being equal to the sum of all intentional residual currents normally passing through the earth conductor in that part of the circuit.

These problems may be reduced or avoided by providing, in addition to the earth conductor, a residual current conductor connected to earth for receiving intentional residual currents from the items of equipment and a residual current circuitbreaker arranged to disconnect at least the phase conductor from the power source when the vector sum of the currents passing through the phase, neutral and residual current conductors exceeds a predetermined value. Using this arrangement the circuit-breaker is not affected by the intentional residual currents generated by the items of equipment and it is only tripped when leakage through the earth, caused by a fault, exceeds a predetermined value.However, with this arrangement the intentional residual current still passes ultimately to earth and in practice it is necessary to cause the circuit-breaker to trip when the intentional residual current exceeds a given value, for example 0.5 A. When a large number of items, such as computers, which generate intentional leakage currents are powered by the circuit there is then a substantial risk of the circuitbreaker tripping even under normal conditions, so that all power fed to the items is lost. Such total disconnection can be very troublesome and may be commercially disastrous when a large amount of information is stored electronically in memories of the items; disconnection generally results in total loss of this information.

The present invention is intended to provide power supply apparatus in which items fed with electric power from a power circuit may be disconnected from the circuit individually in response to a total residual current exceeding a given magnitude.

According to one aspect of the invention there is provided apparatus for supplying electric current to current-using equipment from a power source, the apparatus comprising phase or phases and neutral power supply conductors and a residual current conductor connected to earth for receiving residual currents from the items of equipment, the conductors being connected to a plurality of outlet units each of which is adapted for connection to an item of equipment such that the phase or phases and neutral conductors may supply currentto the item of equipment and the residual current conductor may receive residual currents from the item of equipment and a detector sensitive to the current in the residual current conductor arranged to emit a signal which is a function of the magnitude of that current, each outlet unit being provided with means for receiving said signal and disconnecting at least the phase conductor from the item of equipment connected to the respective outlet unit when the current in the residual current conductor exceeds a predetermined magnitude.

The detector may comprise a thermistor emitting a signal which is a function of the temperature of a resistor in series with the residual current conductor, the tem peratu re of the resistor increasing with the current passing through the conductor. Each outlet unit may comprise a receiver for receiving the signal from the detector, a comparator set to a predetermined signal value and a relay or equivalent device for disconnecting the power supplied through the outlet unit when the signal from the receiver reaches or exceeds the value set in the comparator.The comparators of different outlets fed by the conductors may be set to different values, enabling the items of equipment powered by the circuit to be disconnected selectively; thus the comparators of the outlets connected to items of low priority, such as ordinary typewriters, may be set at a low value so that low priority items are disconnected first when the residual current exceeds the predetermined magnitude and the comparators of outlets connected to more important items, such as computer equipment in which information is stored electronically, may be set at higher values. When one or more of the low priority items is disconnected the residual current will normally fall so that the signal from the detector can fall below that set in the comparators of the more important items, which can continue operation without interruption.

The invention may be applied to a power circuit of conventional type, in which power is supplied by phase or phases and neutral conductors and any residual currents, both intentional and caused by a fault, are conducted to earth by an earth conductor.

In such an arrangement a total intentional residual current exceeding the predetermined magnitude will selectively disconnect the items powered by the circuit, in the manner described above, and a large current through the earth conductor caused by a circuit fault will disconnect all the items.

The invention is also applicable to a power circuit provided with a residual current circuit-breaker and a conductor connected to earth for receiving intentional residual current, in addition to the normal earth conductor, the circuit-breaker and residual current conductor being such that the current in the residual current conductor will not cause tripping of the circuit-breaker. In this instance it is the current in the residual current conductor which is detected by the detector to cause emission of the signal received by the receiving means of the outlets to cause disconnection of the items fed with current by the outlets, whereas a current above a predetermined value in the earth conductor symptomatic of a fault trips the circuit-breaker to disconnect the whole circuit.

The outlets provided with a receiver, comparator and relay may also be provided with a time delay circuit to introduce a delay, for example of about 3 minutes, between reception of a signal exceeding that set in the comparator and opening of the relay.

An annunciator may also be provided to give an acoustic and/or visual warning when the signal received reaches this value. The annunciator, together with the delay, allows an operator to take appropriate action before the item powered by the outlet is disconnected. For example, information stored electronically in that item may be recorded on a disc within the time delay period in order to avoid loss of the information.

A circuit arrangement according to one embodiment of the invention will now be described by way of example with reference to the accompanying drawing (Figure 1) in which the arrangement is shown schematically for a single phase circuit.

As shown in the drawing an electric supply circuit comprises a live conductor 1 and a neutral conductor 2 supplying alternating currents at 240 V and 50 Hz frequency from the mains supply, indicated by source 3. Conductors 1 and 2 extend through a building in which they are connected to the phase and neutral terminals of a number of sockets for feeding power to current using equipment. A third terminal of the sockets is connected to earth conductor 4. The circuit is protected by a residual current circuit-breaker comprising a transformer 5 through which the phase and neutral conductors pass connected to a relay unit 6 arranged to break the supply circuit when a signal from transformer 5 representing the vector sum of the currents in the conductors passing through the transformer exceeds a predetermined value, such as 10 mA.

The supply circuit is provided with a fourth conductor 7 passing through the transformer which is connected to the earth conductor 4 via resistor 9 and capacitor 10 at point 8, on the side of the circuitbreaker remote from the sockets fed by the circuit.

Conductor 7 is connected to a fourth terminal of the sockets or fixed current using equipment which is intended to receive intentional residual currents from devices and appliances receiving current from the circuit. The appliances will be arranged so that any intentional residual current generated during their normal operation will be fed to their residual current output, and not to the earth conductor, while any unintentional residual current arising from a fault in the current-using equipment will be fed to earth conductor 4. By this means, the intentional residual current in normal operation cannot trip the circuit-breaker, as the latter is sensitive to the vector sum of the currents in the phase, neutral and residual current conductors and this sum is zero under normal circumstances.However, unintentional residual currents through conductor 4 will render this sum non-zero and trip the circuitbreaker when it exceeds the predetermined value.

The supply circuit is thus protected from shortcircuits and limited current faults between phase, neutral and earth in the appliances connected to it while allowing intentional residual currents generated by the current using-equipment and associated devices to be accommodated.

Resistor 9 forms part of a thermistor device, shown schematically as 11, which provides an output signal to leads 12 which is a function of the temperature of the resistor, and thus of the magnitude of the current flowing through the resistor; the output on leads 12 increases according to the magnitude of the current in resistor 9.

One of the sockets supplied by the conductors is shown schematically at 13. The socket comprises appropriate terminals connected to conductors 1, 2, 4 and 7 which in turn are to be connected to the device or appliance receiving power from the socket in the manner described above, to provide power from conductors 1 and 2, to earth the device or appliance through conductor 4 and to transmit intentional residual current through conductor 7.

The socket also comprises a receiver 14 which is connected to leads 12 to measure the signal from device 11. The socket is provided with a comparator 15 which is arranged to compare the magnitude of the signal received by receiver 14 with a preset value and to actuate control device 18 which operates relay 19 when the signal received by detector 14 exceeds in magnitude the preset value in comparator 15. Relay 19 is arranged to disconnect the device or appliance from the socket terminals receiving conductor 1, and optionally conductor 2, so that the device or appliance is disconnected from the power supply circuit when the residual current detected by the thermistor exceeds a predetermined value. Disconnection of the device or appliance reduces the residual current emitted by the device or appliance to zero, reducing the total residual current passing through conductor 7.

Conductors 1,2,4 and 7 and leads 12 are connected to a number of similar sockets 13, each socket supplying power to a respective device or appliance, and the comparator of each socket is set to a predetermined signal magnitude so that the device or appliance connected to it is disconnected from its power supply when the total residual current in conductor7 reaches a certain magnitude.

The signal magnitudes set in the comparators of the different sockets neeed not be the same, so that when the residual current increases the power supply through one or more sockets, having the lowest signal magnitudes set in their comparators, is disconnected before disconnection at the other sockets occurs. This feature allows selection of the devices or appliances which are to be disconnected when the intentional residual current in conductor 7 exceeds the permissible value. Disconnection of the selected devices generally produces a reduction in the intentional residual current so that no further disconnections occur.

Socket 13 is provided with a time delay device shown schematically as 16, to introduce a delay between detection of a signal from the thermistor exceeding that in the comparator and operation of relay 19. An annunciator 17, which may give an acousticandlorvisual warning of impending operation of relay 19, is also provided. The delay may be set at such a value, for example 3 minutes, that an operator can take appropriate action when warned of imminent disconnection. For example, if the device or appliance powered through socket 13 contains information stored in an electronic memory which is lost in the event of disconnection, the operator will have time to record the contents of the memory on a disc or other permanent storage medium before disconnection takes place.

The arrangement shown in the drawing relates to a single-phase power supply circuit but the invention is also applicable to multi-phase supply circuits, including three-phase power supply circuits.

Claims (9)

1. Apparatus for supplying electric current to current-using equipment from a power source, the apparatus comprising phase or phases and neutral power supply conductors and a residual current conductor connected to earth for receiving residual currents from the items of equipment, the conductors being connected to a plurality of outlet units each of which is adapted for connection to an item of equipment such that the phase or phases and neutral conductors may supply current to the item of equipment and the residual current conductor may receive residual current from the item of equipment, and a detector sensitive to the current in the residual current conductor arranged to emit a signal which is a function of the magnitude of that current, each outlet unit being provided with means for receiving said signal and disconnecting at least the phase conductor from the item of equipment connected to the respective outlet unit when the current in the residual current conductor exceeds a predetermined magnitude.

2. Apparatus according to claim 1, in which the residual current conductor comprises an earth conductor for conducting both intentional and nonintentional residual currents to earth.

3. Apparatus according to claim 1, in which the residual current conductor is arranged to conduct intentional residual currentsto earth, and the apparatus further comprises an earth conductor for conducting unintentional residual currents to earth, and a residual current circuit-breaker arranged to disconnect at least the phase conductor from the power source when the vector sum of the currents passing through the phase, neutral and residual current conductors exceeds a predetermined value.

4. Apparatus according to claim 1, 2 or 3, in which the detector comprises a thermistor.

5. Apparatus according to any preceding claim, in which each outlet unit comprises a receiver for receiving the signal from the detector, a comparator for storing a signal of predetermined magnitude and a device capable of disconnecting at least the phase conductor from the item of equipment when the signal received by the detector is of equal or greater magnitude than the signal stored in the comparator.

6. Apparatus according to claim 5, in which the comparators of different outlet units connected to the conductors contain stored signals of different magnitudes.

7. Apparatus according to claim 5 or 6, in which each outlet unit is provided with a delay device for delaying disconnection of the item of equipment for a predetermined time after reception by the receiver of said signal of equal or greater magnitude than the signal stored in the comparator.

8. Apparatus according to claim 7, in which each outlet unit is provided with an annunciator to provide warning of an impending disconnection of the item of equipment.

9. Apparatus for supplying electric current, substantially as hereinbefore described with reference to the accompanying drawing.