GB2242084A - Branched supply network earth continuity testing by DC injection into each branch - Google Patents

Abstract

The installation comprises a branched power socket chain connected (at A) to a power source through a control unit B, with a pilot conductor E linking each branch end C3-C6 to the control unit B. A network with (n-1) branching points will have n branch ends, and in the present case n will be between 1 and, Bay, 5. The control unit B includes a current source for driving a controlled DC current through the pilot conductor E into the earth conductor through terminating impedances F1-F4 at each branch end C3-C8, thereby establishing a pilot potential on the pilot conductor E. The control unit B has a model circuit adjustable in steps according to the value of n and generating a reference potential that will normally equal the nominal pilot potential. The control unit B compares the reference potential to the pilot potential and disconnects the supply when the difference exceeds a predetermined amount in either direction. The terminating impedances are preferably equal resistors. The installation may be used with office furniture. <IMAGE>

Description

ELECTRIC POWER SUPPLY INSTALLATIONS, ESPECIALLY FOR OFFICE FURNITURE This invention relates to installations for the supply of electric power to a number of appliances, each drawing a relatively small amount of power; more especially, but not exclusively, it relates to installations for use in open-plan offices and similar environments.

Efficient operation of offices and the like demands the provision of power to large numbers of "small" electric appliances, such as word processors and other computers, typewriters, dictation equipment, lights, telephone and facsimile machines, and fans, the locations and numbers of which may need to be changed at frequent intervals. The power demand of such appliances is small, and a single hard-wired power socket with a capacity of around 15 amps (13 in normal UK practice) can provide adequate power for the typical requirements of up to around twelve appliances. To avoid unsightly and dangerous trailing leads, it is the practice to provide auxiliary power sockets within the office furniture which are interconnected by flexible leads with plug-and-socket or equivalent connections, and with a single connection to a mains socket.

Such installations are often subject to hazard from forces accidentally applied to the flexible leads during re-arrangement of furniture and equipment, as well as spillages or other mishaps, and risk of electric shock can arise if, as a result of such hazards, continuity of the earth conductor fails by wire fracture, broken or improperly connected plugs or other causes, and it is therefore desirable to provide means for monitoring earth continuity and for cutting off the power supply if continuity is in doubt.

One type of installation for this purpose utilises a fourth conductor (hereinafter called the pilot conductor) in addition to the ordinary live (or line), neutral and earth conductors, and in our European Patent Applicaiton 89312230.9 we have described an improved installation of this type. in which the power supply sockets are connected to live, neutral and earth conductors of a power source through a control unit; a pilot conductor is connected via an impedance to the earth conductor at the socket furthest from the control unit and returns to the control unit.This impedance forms part of a potential divider through which a pilot current flows to establish a pilot potential on the pilot conductor; and comparator means (preferably a window comparator) monitors the said pilot potential and disconnects the live pole of the supply sockets from the live pole of the supply if the pilot potential falls either above or below a preset range of values.

This type of installation is now in commercial use and works well provided that the sockets are arranged in a single unbranched chain. In principle, it could be extended to installations with branched chains of sockets, which some users would prefer, by connecting a separate impedance between the earth conductor and the pilot conductor at the end of every branch of the chain with values such that the resultant impedance of the parallel circuit thus formed is equal to the impedance for which the control unit was designed.In practice, the impedances of the individual branches will vary by a few percent owing to manufacturing tolerances, temperature variations and the uncertain number and quality of plug and socket connections, and the resultant of parallel impedances is very sensitive to variations in the their individual values, and a point is very quickly reached at which a comparator window set wide enough to accommodate these variations may be so wide that it will accept the loss of one branch without tripping.The option of providing adjustment facilities for achieving a match between the impedance required by the control unit and the resultant impedance found in the individual installation is unattractive not only because it requires expensive skilled labour whenever the installation is modified but also because it would introduce the danger that the adjustment facility might be used to over-ride tripping due to a real fault.

The present invention provides an installation in which a sufficient, preset, number of branches up (say up to three or four branch points, four or five ends) can be safely accommodated and which, in its prefered forms, will refuse supply if the preset number of branches fails to match the number of properly terminated branch ends actually present.

In accordance with the present invention, an electric supply installation comprises a control unit, a plurality of power supply sockets connected to live, neutral and earth conductors of a power source through the control unit and arranged in a chain with (n-1) branching points and therefore n chain ends (where n is a small whole number); a pilot conductor is connected to the earth conductor via a terminating impedance at the chain end (if n=1) or via respective terminating impedances at each one of the chain ends (if n > 1) and returns to the control unit; and the control unit includes: (i) a pilot current source for driving a controlled current through the terminating impedance or all of them and thereby establishing a pilot potential on the pilot conductor; (ii) a model circuit adjustable in steps according to the value of n for generating a reference potential that will equal the nominal pilot potential if the earth conductor and the pilot conductor are both continuous from the control unit to all of the chain ends; and (iii) comparator means for monitoring the reference potential and the pilot potential and for disconnecting the live pole of the chain of supply sockets from the live pole of the supply if the pilot potential differs form the reference potential by more than a predetermined amount in either direction.

The number of sockets in such a system will not normally exceed twelve and it is customary to make branches at a socket unit (rather than by a separate connector) and to use socket units with two or three sockets rather than only one, the potential for branching is quite limited and it will normally be sufficient to provide for values of n from 1 to 4 or at most 1 to 5.

The terminating impedances are preferably resistors. Preferably they all have the same value, and that value is preferably independent of n.

Preferably the pilot current source is a "constant current" source capable of providing substantially the same total current regardless how many terminating resistors are present (within the range 1 - n), in which case the pilot potential will be inversely proportional to the value of n, or at least approximately so.

Alternatively, a "constant voltage" source could be used to give a nominally constant pilot potential; or more generally, any kind of current source that can be accurately modelled may be used.

Preferably the model circuit comprises a current source designed to generate a current always proportional to, but preferably less than1 the current delivered by the pilot current source , a number of resistors with equal values related to the values of the terminating resistors by the same ratio as relates the model circuit current to the pilot current and switching means for selecting n of these resistors to be connected in parallel with one another across the current source. For many kinds of current source, a capacitance will be required to avoid oscillation and/or filter interference, and this is preferably modelled in to the model circuit so that the pilot circuit and the model circuit have substantially the same time constant, in order to avoid spurious tripping if supply voltage fluctuates.

Preferably the comparator means comprises means for generating two voltages at predetermined levels respectively above and below the reference potential (or the pilot potential) and a window comparator for determining whether the pilot potential (or in the alternative case the reference potential) lies between those two voltages or not.

The invention will be further described, by way of example, with reference to the accompanying drawings in which Figure 1 is an outline diagram of one form of installation in accordance with the invention and Figure 2 is a circuit diagram of a control unit forming a key part of the installation. To facilitate reference, certain groups of components in figure 2 that can be regarded as functional modules have been enclosed by chain lines (references 2-10, applied in clockwise order).

As seen in figure 1, an installation powered from a mains plug A comprises a control unit B and six pairs of power supply sockets C1-C6 connected together by three-core plug leads D. Socket pairs C1 and C2 form branches in the chain of sockets, which thus has four ends at C3, C4, C5 and C6.

At each of these ends, a respective 100 ohm resistor F1, F2, F3, F4, connects the earth wire of the system to a shared pilot wire E which returns to the control unit B.

Coming now to figure 2, the control unit has (at the left-hand side of the figure) live, neutral and earth inputs 11, 12, and 13 respectively for connection to the mains plug A, live, neutral and earth outputs 14, 15, and 16 respectively for connection by a plug lead D to the first socket C1 and thus to the remainder of the system, and a terminal 17 for the pilot wire.

The mains input, shunted by a protective voltage-limiting resistor 18, is taken directly to a transformer 19 in power supply module 2; the transformer output is rectified by diodes 20, 21, and smoothed by capacitor 22 to provide the power requirements of modules 3-8.

The principal components in module 3 are a pair of double-pole relays 24, 24 which, when energised, close contacts 25 and 26 respectively to connect live output 14 to live input 11 and neutral output 15 to neutral input 12; when the relay is de-energised, contact 25 connects the live output 14 via a discharge resistor 27 to neutral input 12 in order to ensure that any capacitive load will be discharged and the voltage on the live output 14 rapidly fall to zero.

Current for the relay coils is controlled by power transistor 28 with associated resistors 29, 30 and is switched by a signal applied to the base of the transistor, as described later; a light-emitting diode (LED) 31 and series resistor 32 are connected across the relay coils so that the LED serves as a "power-on" indicator.

To the right of module 3, diode 33, resistor 34 and capacitor 35 protect the subsequent circuits from switching transients from module 3 and from mains-borne interference.

Zener diode 36 (shunted by capacitor 37) and resistor 38 act as a potential divider to provide a stable reference voltage (in this example + 2.OV) at 39.

Module 4 is a constant-current source for the pilot circuit constituted by pilot wire E and the earth wire of the system, and uses operational amplifier 41 (one of four such amplifiers included in integrated circuit LM324) as a voltage follower. Its first (+) input terminal is connected to the reference voltage at 39 and its second (-) input terminal to the junction between a current-programming resistor 42 and the emitter of control transistor 43 and the collection of power transistor 44.The output of the operational amplifier biasses transistor 43 (via resistor 45) and this in turn biasses the power transistor 44; the characteristics of the amplifier are such that it tends to equalise the voltages at its two inputs, and since the reference voltage at 39 is constant this means that the voltage drop across resistor 42 and therefore the current flowing through it is kept constant (at about 60mA).

Practically all this current is carried by the power transistor 44 and passed to the pilot wire terminal 17 and thence via the pilot wire and parallel resistors F1-F4 (fig 1) to earth. Capacitor 47 (below module 4) prevents oscillation of the current source and gives the pilot circuit a predetermined time constant.

Module 5 is a model circuit in which operational amplifier 51, transistor 53 and resistor 55 are identical in form and function with components 41, 43 and 45 respectively; resistor 52 has ten times the value of resistor 42 in order that the model circuit current is a tenth of the pilot circuit current (ie 6 mA); consequently, a power transistor is unnecessary in the model circuit, and the value of capacitor 57 has to be one tenth that of capacitor 47 in order for the model circuit to have the same time constant as the pilot circuit. Resistors 61-64 model the resistors F1-F4, and since the current is a tenth their resistance values need to be ten times greater, ie 1000 ohms. The resistors 61-64 are selectable by switches 65, and the number in circuit must be the same as the number of terminating resistors (F1-F4).If preferred, the minimum requirement of one of these resistors could be permanently wired, without a switch, but the inclusion of a switch for each resistor avoids the need to apply even the simplest arithmetic in the setting-up operation, as the installer closes as many switches in the control unit as he fits terminating resistors in the pilot circuit.

The effect of the model circuit is that the reference voltage r at 70 will always be substantially the same as the pilot voltage p on terminal 17 provided that the number of resistors F1-F4 and 61-64 actually in circuit are equal (but otherwise differ considerably).

Module 6 is a window generator and comprises the two remaining operational amplifiers 71 and 71 of circuit LM324. Operational amplifier 71 receives the reference voltage r on its positive input and is connected as a voltage follower so as to maintain the same voltage r on its output. It draws current from the positive side of the supply via a resistor 73 and a diode 74; this diode is used as a voltge reference, and resistors 75 and 76 divide the voltage across it to obtain a voltage of r+0.2 at their junction; this voltage is fed via resistor 77 to the second operational amplifier 72 which is arranged (with resistor 78) to act as an inverting amplifier with a gain of 1, relative to its positive input, so that is output is r-0.2 volts.

These voltages serve as window limits and are passed to comparator module 7 comprising two unit comparators 81, 82 out of four provided in integrated circuit LM339 and respective load resistors 83, 84. The r+0.2 voltage becomes one input to unit comparator 81 and r-0.2 the corresponding input to unit comparator 82; the other input to each unit capacitor is the pilot voltage p on terminal 17, and "low" outputs are obtained on both unit comparators provided that r-0.2 < p < r+0.2.

These normally-low outputs are applied to the bases of respective transistors 91, 92 in trip module 8 to hold them in a low-conductivity state and turn 'off' respective LED indicators 94, 95 (with respective series resistors 97, 98). The trip module also includes a capacitor 100 charged from the power supply through a high-value resistor 101 and a relatively low-value resistor 102; comparator 104 relates the voltage found at the junction of the two resistors with that established by a potential divider formed by resistors 105 and 106 and normally outputs (with its load resistor 107) a biassing voltage to transistor 28 (module 3) sufficient to hold it conductive and turn the relay 24 on.However, the junction of the resistors 101 and 102 is also connected via blocking diodes 110, 111 respectively to the collectors of transistors 91, 92 and if the base of either transistor goes 'high' due to switching of one of the comparators 81, 82 this provides a low resistance discharge path for the capacitor: on discharge, the comparator 104 reverses, biasses transistor 28 "off" and trips the relay. The high value resistor 101 gives the capacitor 100 a long time constant on charging (but not on discharging) to avoid rapid switching of the relay in case of an intermittent (or voltage-dependent) fault.

In the event of failure of continuity in the earth conductor (or the pilot wire), or if through error there are more of the resistors 61-64 switched in circuit in the control unit than there are terminating resistors F1-F4 installed in the external wiring, then the pilot voltage p exceeds the reference voltage r by more than the tolerance (0.2 volts) set by module 6, the comparator 81 switches to "high" output, turns transistor 91 to a conductive state, illuminating the LED 94 to indicate a "system earth" fault and rapidly discharging capacitor 100 and tripping relay 24 by the sequence of steps already described.

In the event of a failure of insulation of the pilot wire E causing an earth fault (shorting out the terminating resistors F1 to F4), or if through error there are fewer of the resistors 61-64 switched in circuit in the control unit then there are terminating resistors installed in the external wiring, then the pilot voltage p falls below the reference voltage r by more than the tolerance (0.2 volts) set by module 6, the comparator 82 switches to "high" output, turns transistor 92 to a conductive state, illuminating the LED 95 to indicate a "pilot wire" fault and discharging capacitor 100 with the same consequences as before.

In either case, the system resets automatically to restore supply if and when the fault is cleared and sufficient time has elapsed for the capacitor 100 to re-charge to about its normal potential.

Module 10 is a 'mains earth' monitor working on conventional principles, and will be described only in outline. Components 120 - 124 provide a d.c. supply fo illuminating LED 126 under control of a normally-off switching transistor 127. Components 130-131 and 132-133 would act as an a.c. voltage divider across the mains supply, but are normally prevented from doing so because 132 and 133 are shorted by connection from neutral to earth that exists, at the consumer's supply terminal or upstream of it, in all modern installations (at least in the UK). In case of failure of that connection (usually due to a broken earth wire between the control unit and the mains plug A or a loose earth connection in that plug), a voltage approximating half the supply voltage appears between 131 and 133; the appropriate half cycle is selected and applied to turn on transistor 127 and illuminate LED 126.Photo transistor 140 (forming with the LED an optoisolator to protect the trip module from high voltages in case of component failure) produces an output which is amplified by transistor 141 and applied to the base of transistor 93 which, with associated components.96, 99 and 112 operates in the same manner as transistors 91 and 92 to illuminate LED 96 as a "mains earth" fault indicator and discharge capacitor 100 with the usual consequences.

Module 10 will also refuse supply by the same mechanism if live and neutral inputs are reversed by a wiring error in the plug A or the consumer's installed wiring.

Finally, module 9 is an optional test module providing three test buttons: button 151 inserts a series resistor 152 in the pilot circuit to simulate a (partial) earth continuity failure; button 153 connects resistor 154 between the pilot wire terminal 17 and earth to simulate a pilot wire (partial) earth fault; and button 155 isolates the mains earth monitor module only from earth to simulate a mains earth fault. Each thus trips the control unit and illuminates the corresponding LED indicator.

Claims (8)

1. An electric supply installation comprising a control unit, a plurality of power supply sockets connected to live, neutral and earth conductors of a power source through the control unit and arranged in a chain with (n-1) branching points and therefore n chain ends (where n is a small whole number); a pilot conductor connected to the earth conductor via a terminating impedance at the chain end (if n=1) or via respective terminating impedances at each one of the chain ends (if n > 1) and returning to the control unit; and the control unit including; (i) a pilot current source for driving a controlled current through the terminating impedance or all of them and thereby establishing a pilot potential on the pilot conductor; (ii) a model circuit adjustable in steps according to the value of n for generating a reference potential that will equal the nominal pilot potential if the earth conductor and the pilot conductor are both continuous form the control unit to all of the chain ends; and (iii) comparator means for monitoring the reference potential and the pilot potential and for disconnecting the live pole of the chain of supply sockets form the live pole of the supply if the pilot potential differs from the reference potential by more than a predetermined amount in either direction.

2. An installation as claimed in claim I in which the model circuit provides for values of n from 1 to 4 or 1 to 5.

3. An installation as claimed in claim 1 or claim 2 in which the terminating impedances are of resistors of the same value.

4. An installation as claimed in any one of the preceding claims in which the pilot current source is a "constant current" source capable of providing substantially the same total current regardless how many terminating resistors are present (within the range 1 n).

5. An installation claimed in any one of the preceding claims in which the model circuit comprises a current source designed to generate a current always proportional to the current delivered by the pilot current source, a number of resistors with equal values related to the values of the terminating resistors by the same ratio as relates the model circuit current to the pilot current and switching means for selecting n of these resistors to be connected in parallel with one another across the current source.

6. An installation as claimed in 5 in which the pilot circuit includes a capacitance to avoid oscillation and/or to filter interference, which capacitance is modelled in the model circuit so that the pilot circuit and the model circuit have substantially the same time constant.

7. An installation as claimed in any one of the preceding claims in which the comparator means comprises means for generating two voltages at predetermined levels respectively above and below the reference potential (or the pilot potential) and a window comparator for determining whether the pilot potential (or in the alternative case the reference potential) lies between those two voltages or not.

8. An electric supply installation substantially as described with reference to the drawings.