US3753050A

US3753050A - Sequential switching device

Info

Publication number: US3753050A
Application number: US00221011A
Authority: US
Inventors: D Bowen
Original assignee: Individual
Current assignee: Individual
Priority date: 1971-01-27
Filing date: 1972-01-26
Publication date: 1973-08-14
Anticipated expiration: 1990-08-14
Also published as: CA943191A; GB1368462A

Abstract

A sequential switching device for repeating an input signal such as one representing the opening of a circuit breaker at the far end of a cable, to a local switching means. The device includes a normally active on-off transistor oscillator which stops oscillating in a non-conductive condition to warn of a cable fault, and in a conductive condition to repeat the input signal.

Description

United States Patent 1191 Bowen SEQUENTIAL SWITCHING DEVICE [76] inventor: Dennis Dawes Bowen, 8 Rowan Tree Dr., Sale, England [22] Filed: Jan. 26, 1972 [21] Appl. No.: 221,011

[30] Foreign Application Priority Data Jan. 27, 1971 Great Britain 3,335/71 May 25, 1971 Great Britain 16,787/71 [52] US. CL... 317/141 S, 317/D1G. 10, 317/142 R, 317/146 [51] Int. Cl. Hillh 47/18, l-lOlh 47/22 [58] Field of Search 317/ DIG. l0, DIG. 2, 317/146, 141 S, 142 R, 148.5

[56] References Cited UNITED STATES PATENTS Atkins 317/1316. 10

[ Aug. 14, 1973 Atkins 317/D1G. 2 Atkins 3l7/DlG. 2

Primary Examiner-J. D. Miller Assistant Examiner-Harvey Fendelman Attorney-Harold D. Steinberg et al.

[5 7] STRACT A sequential switching device for repeating an input signal such as one representing the opening of a circuit breaker at the far end of a cable, to a local switching means. The device includes a normally active on-off transistor oscillator which stops oscillating in a nonconductive condition to warn of a cable fault, and in a conductive condition to repeat the input signal.

8 Claims, 2 Drawing Figures PATENTEDMI: 14 ms SHEET 2 OF 2 1 SEQUENTIAL SWITCHING DEVICE Prior application ln Great Britain on Jan. 27, 197i and numbered In Great Britain on May 25, 1971 and numbered The present invention relates to fail-safe sequential switching devices. More especially, the present invention provides means for opening a second switch if a first switch is opened, as used for tripping an auxiliary circuit breaker if a main circuit breaker trips. The invention may also be used in other ways. For example, if a turbine loses oil it is essential that the associated oil pump is switched off. The circuits of the invention are continuously operating and self-checking. They indicate whether the components thereof are correctly operational, and they give an indication or alarm if a fault occurs.

In accordance with the present invention there is provided a sequential switching device comprising a solid state switching circuit having an on state and an off state, and normally alternating between said states to produce a series of pulses, input terminals connected to said circuit such that arrival of an input signal at said terminals holds said circuit in one of said states, means for repeating said input signal to output terminals when said circuit is held in said one state by said input signal, said circuit being held in the other of said states on failure of a component of said device, and means indicat ing said failure when said circuit is held in said other I state.

Preferred embodiments of the present invention will now be described with reference to the accompanying drawings, FIGS. 1 and 2 of which show fail-safe sequential switching circuits in accordance with the invention.

The circuit shown in FIG. 1 is connected to normally closed contact CT] of an existing first switch or main circuit breaker via input terminals, and is adapted to release contact RAl of a relay RA and opening thus to open a second switch or auxiliary circuit breaker via output terminals if contact T1 opens this constituting an input signal. A time delay capacitor C10 is incorporated in the circuit so that the contact of relay RA opens only if contact OTl remains open for a significant period.

Relay RA is. normally energised when no alarm condition is present, from a positive voltage supply via resistor R and diode D7. The relay RA and diode D7 combination is connected in parallel with a solid state switching device consisting of a transistor-thyristor combination S4. lf contact 0T1 opens for more than a significant period set by the delay circuit Cl0-R20- R21, the thyristor-transistor combination S4 is switched on (see later). Relay RA is therefore shunted and released and contact RAl opens.

Under normal conditions, that is, with contact 0T1 closed and hence no input signal received relay RA is initially fully energised and a positive potential occurs at the junction of resistor R15. and diode D7. This potential charges capacitor C3 via a resistor R14, thereby raising the potential of the base of transistor TRC so that it switches on. Current through R16 triggers the thyristor combination S4 on, thereby shunting relay RA. The potential at the junction between resistor R15 and diode D7 drops to zero volts, transistor TRC is switched off, and thus switches off the transistor of the thyristor-transistor combination S4, thereby rendering the thyristor once more non-conductive. Relay RA is thus re-energised. This cycle of pulses repeats at a frequency of several KHz so that relay RA remains oper? ated. It may be rendered slow release for this purpose. Consequently contact RAl remains closed. In addition, provided there is no fault in the circuit or in the thyristor combination S4, an AC voltage appears across capacitor C4 which lights a lamp L2. This lamp indicates, when lit, that the trip relay circuit is fully operational. If now contact OT]! opens for a significant period, relay RA is short-circuited and releases as described above, its conact tripping the second switch or an auxiliary circuit breaker (not shown) via the output terminals. in addition, current is fed via diode D8 and resistor R21 to keep the lamp L2 alight, this current being normally shunted by closed 0T1.

Thus the lamp L2 is lit whenever the circuit is correctly functioning. If any of the components fail, the circuit ceases to oscillate and the lamp L2 is extinguished. Additionally, an audible alarm circuit may be connected to the junction between the lamp L2 and capacitor C4 to give an alarm whenever a fault occurs in the sequential switching circuit.

In a preferred embodiment, resistor R15 has a large resistance compared with that of the relay RA. This is so as to limit the current drawn from the supply when the relay is short-circuited.

A variable time delay may be provided for relay RA; for example, a capacitor may be connected in parallel therewith. If the capacitor went open-circuited, the relay would still fail-safe, but in such a case might be tripped by a fleeting signal from the contact Tl. If the capacitor went short circuit, it would shunt the relay, which is again a fail-safe condition. Any suitable delay means may be used on the relay.

A second embodiment of the invention is shown in FIG. 2, incorporating extra safety features.

The circuit shown in this figure is connected to the normally closed contacts 0T1 of an existing first switch or main circuit breaker and is adapted to open the normally closed contact RAl of a relay RA, and thus to open a second switch or auxiliary circuit breaker when contact OT] opens, this being an input signal. A time delay circuit may be incorporated in the circuit so that contact RAll opens only if contact OTli remains open for a pre-determined period. The double line in the figure indicates the parts of the circuit necessary to control relay RA.

Relay RA is provided with a normally closed contact RAl for the trip circuit, and the relay has one terminal at earth potential and the other terminal connected to a positive supply through a wire-wound resistor R1.

A time delay for release of relay RA can be introduced as by using shorted turns in the winding, or a slug on its core. Relay RA is normally energized when no alarm condition is present, from a positive supply via resistor R1 and diode D1.

A solid state switching device including a transistor T1 is connected across the relay RA, and a diode D1 is in series with the relay. The emitter of transistor T1 is connected to earth via T12. Normally, the base of transistor T1 is at earth potential so that transistor T1 is switched off. The normally closed contact 0T1 forms part of a potential divider including resistors R8 and R5, such that, in the example, less than 3.5 volts ap pears at the point A, and less than 5 milliamps flow 3 through contact T1. The circuit is therefore intrinsically safe. Two Zener diodes ZNl and ZN2 are connected in parallel and the parallel. combination thereof is connected between points A and B. in the circuit shown in the example, each of the Zener diodes ZNl and 2N2 has a rating of 6.2 volts. As two Zener diodes are provided in parallel, the circuit will fail safe should one or other of the diodes become a short-circuit. An open circuit of one Zener diode (which is the least likely mode of failure of a Zener diode) will likewise not affect the normal circuit operation.

if contact 0T1 which is normally closed, opens, resistor R6 is added to the potentiometer, and point A rises to near line potential. Thus, in the illustrated circuit, 5.8 volts appears across resistor R2 and capacitor C1. The value of capacitor C1 is chosen so that a significant time delay may be introduced into the circuit if required. The potential at point B rises and transistor T1 conducts. Point C is hence connected to ground potential (Transistor T12 is also conducting, as is described later). Relay RA de-energises in a time set by its own delay, and the normally closed contact RAl opens. Relay RA is also provided with a pair of normally open contacts RA2. These contacts RA2 close when relay RA is de-energised, and a transistor T2 is switched on. This transistor T2 then provides a path'from point C to earth which is additional to that through Tl.

When relay RA is energised, and therefore contact RAl is closed and contact RA2 is open, the base of transistor T9 is earthed and the transistor is therefore conducting. A further transistor T10 connected to the collector of transistor T9 also conducts, and an indicating lamp L1 is lit. In the example, the lamp Ll may be orange in colour and its function is to indicate when relay RA is energised. lf normally open contact RA2 should close, as would happen if relay RA were deenergized, line potential is connected to the base of transistor T9, thereby turning off this transistor and hence transistor T10. The lamp L1 is then extinguished.

If, after tripping, contact OTl re-closes, point A is returned to its normal potential which is less than 3.5

volts. Transistor T1 is turned off, but relay RA remains de-energised, as point C is still connected to earth through transistor T2. The relay can be re-energised by pressing the accept/reset button which turns on transistor T3, thereby connecting the base of transistor T2 to earth. Transistor T2 is hence tumed off and point C is no longer earthed. Relay RA is therefore re-energised, contact'RAl closes the contact RA2 opens. The lamp L1 is illuminated and-then the reset button is released.

If an open circuit fault appeared between points A and B, or transistor T1 went open-circuit, or point B was short-circuited by any fault to earth, relay RA would be prevented from de-energising upon opening of contact 0T1. The circuit is therefore provided with self-checking means so that a fault in the circuit is indicated whenever it occurs.

A transistor T4 is operated as a known form of relaxation oscillator, the collector of transistor T4 being connected to the base of transistor T]. On switching on, T4 is non-conducting since the base is positive with the emitter, which starts at ground potential whilst the emitter load'resistor charges-up the condenser. When the emitter becomes positive with respect to the base, T4 conducts. Point B rises from ground potential making the emitter more positive and thus driving T4 hard on. The condenser then discharges via the emittercollector path to ground until the emitter again falls below the base voltage when T4 switches off and the cycle again repeats. The values of the components of the oscillator circuit are preferably chosen such that a switching frequency at the base of transistor T1 of approximately 20 Hz is produced. Each time the base of transistor T1 of is switched on, the coil of relay RA is shunted, since point C is momentarily connected to earth. However, a time delay is incorporated in relay RA such that the relay cannot release even though the current through it consists of impulses.

Point C is connected through a capacitor C2 with a diode pump D9 to the base of a transistor T5. Transistors T5 and T6 form a Darlington pair which maintain an indicating lamp L2 at full brightness providing an oscillating potential continues to appear at point C. Preferably the lamp 12 is green in colour to distinguish it from the orange lamp L1, and shows that the circuit is operating correctly. if any fail-danger fault develops in the trip circuit, it will prevent the charging and discharging of the capacitor C2 clue to cessation of impulsing by T1, and then lamp L2 is extinguished, indicating a trip unit fault. If lamp L2 is extinguished, the potential at point Q rises to line potential and this potential can be used to activate an audible fault warning alarm. If contact'Tl opens with the unit functioning correctly, point B rises to a steady DC potential, turning on transistor T1 as explained above, and the relaxation oscillator comprising transistor T4 is switched off. Point C is earthed and therefore the fault lamp L2 would go out. However, since the trip unit has functioned correctly, it is required that the lamp L2 should remain on. This is achieved by restoring DC potential to the base of transistor T5 via contact RA2 which closes when the relay RA is de-energised. The lamp L2 therefore remains illuminated as required.

The connecting cable between the trip contact 0T1 and point A is also monitored by the self-checking facility. It is possible to produce a fail-to-danger fault by either physically short circuiting the cable (as may be caused by a spade), or by the ingress of moisture, creating a significant earth leakage. It will be seen from the figure that point A is held at approximately 3.5 volts when contact 0T1 is normally closed, the whole of the trip cable forming part of a potential divider. Point A is connected to the base of a transistor T7, this being connected to a transistor T8 to form a Darlington pair which illuminates a cable fault warning lamp L3 whenever 35 volts (or more) is supplied to the base of transistor T7. [F contact 0T1 opens, the potential at point A rises to 6.2 volts, which turns the transistor T7 hard on, and so'the lamp L3 remains fully illuminated.

However, if a fault should develop along the trip cable causing either a short circuit or a significant earth leakage, then the base of transistor T7 becomes connected to earth potential and the cable fault warning light L3 is extinguished. Point P rises to line potential when the lamp L3 is extinguished and this potential can be used to operate an audible fault warning alarm. The lamp L3 may be green coloured as is the lamp L2, so that both are immediately recognisable as fault indicating lamps in contradistinction to the lamp L1 which is orange coloured and indicates when relay RA is energised.

The lamps L2 and L3 are each shunted by a resistor, L2 being shunted by resistor R3 and L3 being shunted by resistor R4, so ensuring that an audible warning will still be given in the case of a real fault occurring, in spite of bulb failure in either or both of the lamps.

An open circuit fault occurring in the connecting cable between the trip contact T1 and point A would normally be indistinguishable from the open-circuit produced when contact 0T1 opens. However, although this repersents a fail-safe condition, it does not indicate that a fault has occurred and accordingly the relay may be de-energised unnecessarily. To overcome this, a failsafe fault checking facility is incorporated into the circuit shown.

A transistor T12 is connected in series with transistor T1. The base of transistor T12 is connected by means of transistor T11 to a point near point A. A resistor R is provided between point A and the base of transistor T11, this resistor R5 being additionally connected to contact 0T1. A further resistor R6 is connected across the contact 0T1, and both the resistors R5 and R6 are wire wound.

While current flows from point A to earth, transistor T12 is held ON by transistor T11. When the contact 0T1 is closed, point A is connected to earth by the combination of resistors R5 and R6, and when the contact 0T1 has opened, point A is connected to earth through resistor R6 only. The condition of transistor T12 is constantly checked by the self-checking circuit. If transistor T12 is ON, the pulses appearing at the base of transistor T1 cause transistor T1 to conduct, producing a pulse signal at point C, and therefore the lamp L2 is lit. However, if transistor T12 turns OFF, the pulse signal does not appear at point C, and hence lamp L2 is turned off, thereby indicating that a fault has occurred.

If one or both cores of the trip cable are severed, producing an open circuit condition, current from point A to the contact 0T1 ceases, transistor T12 is turned off, and therefore the lamp L2 is extinguished, and the audible alarm is sounded. However, the relay RA is unaffected and remains energized.

The circuit described above has the facility that both fail-safe and fail-danger faults occurring between contact 0T1 and relay RA do not release the relay RA but give either trip unit or trip cable fault warnings. However, theself-checking circuit ensures that the unique condition of contact 0T1 opening must de-energise relay RA. It is therefore not necessary to use three trip units to guard against fail-safe fault conditions to avoid a non-genuine trip de-energising the relay. Such guard systems are known and operate on a majority basis, but are complicated and expensive.

I claim:

1. A signal reproducing switching device having a plurality of components and comprising a solid state oscillating and switching circuit having a switched-on state and a switched-off state and normally alternating between said states to produce a series of pulses, input terminals and output terminals connected to said circuit and means for holding, upon arrival of an input signal at said input terminals, said circuit in one of said states, means connected to said output terminals for creating a signal at said output terminals corresponding to said input signal when said circuit is held in said one state by said input signal, means connected to said circuit for holding said circuit in the other of said states on failure ofa component of said device, and indicating means connected to said circuit for indicating said failure when said circuit is held in said other state.

2. A device as recited in claim 1 wherein said circuit includes an oscillator which drives said solid state switching circuit into alternately conducting and nonconducting states.

3. A device as recited in claim 1 wherein said indicating means is a lamp normally maintained lit by pulsed current produced under control of pulses arriving at said lamp over a first path, and a second path illuminating said lamp when said solid state switching circuit is held in said one state by arrival of a signal at said input terminals.

4. A device as recited in claim 1 and in which for producing said series of pulses said circuit includes a relay with at least one contact connected in parallel with said solid state switching circuit, said relay remaining operative so long as said switching circuit is non-conductive for at least part of the time.

5. A device as recited in claim 4 wherein said input signal is a removal of a short-circuit across said input terminals, said output signal being produced by the opening of a relay contact by said relay.

6. A device as recited in claim 1 wherein said input signal is derived from a remotely situated contact connected to said device by a cable, said device including determine duration.

Claims

1. A signal reproducing switching device having a plurality of components and comprising a solid state oscillating and switching circuit having a switched-on state and a switched-off state and normally alternating between said states to produce a series of pulses, input terminals and output terminals connected to said circuit and means for holding, upon arrival of an input signal at said input terminals, said circuit in one of said states, means connected to said output terminals for creating a signal at said output terminals corresponding to said input signal when said circuit is held in said one state by said input signal, means connected to said circuit for holding said circuit in the other of said states on failure of a component of said device, and indicating means connected to said circuit for indicating said failure when said circuit is held in said other state.

2. A device as recited in claim 1 wherein said circuit includes an oscillator which drives said solid state switching circuit into alternately conducting and non-conducting states.

6. A device as recited in claim 1 whErein said input signal is derived from a remotely situated contact connected to said device by a cable, said device including means indicating disconnection, short-circuit and earth faults in said cable.

7. A device as recited in claim 6 wherein said means indicating cable faults includes a potentiometer one component of which is located across said contact, and a transistor combination normally maintained conductive by said potentiometer to light a lamp whilst no conductor fault is present.

8. A device as recited in claim 1 including a delay circuit which prevents repetition of an input signal to said output terminals if said signal is of less than a pre-determine duration.