LOAD CURRENT PROTECTION CIRCUIT

Abstract: There is disclosed a device to protect against failure of a switch S and to give an alarm that the switch has failed. It comprises a circuit in parallel with the switch and containing a transistor T2 and diode D1 to cause such voltage drop in the circuit as to prevent current flowing through a second diode D2 when the switch S is closed. If the switch fails open, the potential at B will drop, and diode D2 will allow current to flow to the load L through the circuit. The current flow produces a potential difference across diode D1 to turn on another transistor T1 to cause an alarm to be given. The device may be used to test such switches without affecting the load controlled by the switch.

Description: SWITCH PROTECTION D#7ICE The present invention relates to devices to protect against failure of switches. Such devices may also be used for testing switches without cisturDing the system controlled by the switch. In certain fields, it can be extremely hazardous or costly for a switch to fail without adeate warning or without circumvention being possible. It would of course, be possible to provide two, or even more, switches, in parallel or series depending on whether the switches are normally closed or normally open. Failure of one of these switches would not then affect the system. However such switches are costly and it may be possible for every switch to fail eventually without giving adequate warning. To take an example, in the fields of chemical processes or oil production platforms, there is generally provided a safety system bRy which complete shutdown of the apparatus or platform is controlled by a switch actuated by a control output giving a signal dependant on any one of a number of parameters. Since complete shutdown should be avoided except in extreme emergencies, it is obious that the switch controlling such shutdown must be protected against its failure. It is therefore an object of this invention to provide a device which will obviate such problems. It is another object of this invention to provide a method of testing such switches without disturbing the loads controlled by the switches. According to a first aspect of the present invention there is provided a device to ensure continuation of current supply to a load in the event of failure of a switch, the device comprising a circuit connected in parallel with the switch and comprising means to cause voltage drop thereby to produce a reduced potential in the circuit, and a diode arranged to prevent current flow in the circuit when the potential at the load side of the switch is higher than the reduced potential, and to allow current flow when said potential is reduced tc a value lower than the reduced potential by virtue of the switch opening. Preferably the device further comprises means to detect current flow in the circuit. Advantageously the base of a transistor is connected to a point in the circuit between the voltage reducing means and the diode, the two other connections of the transistor forming part of an alarm circuit which conducts only when said transistor is biased on by current flow in said circuit caused by opening of the switch. The alarm circuit may include a light emitting diode to give visible warning of switch opening. According to a second aspect of the invention there is provided a method of testing a normally closed switch which comprises the steps of: connecting in parallel to said switch a circuit which comprises in series means to cause voltage drop thereby to produce a reduced potential in the circuit, a diode to prevent current flow in the circuit when the potential at the load side of the switch is higher than the reduced potential, and a connection point to the base of a transistor located between said means to cause a voltage drop and said diode, the two other connections of the transistor forming part of an alarm circuit; opening the switch; and checking the alarm circuit output. According to a third aspect of the present invention there is provided a method of testing a normally open switch which comprises the steps of: connecting in parallel with a second normally open switch and the load, a circuit which comprises in series means to cause voltage drop thereby. to produce a reduced potential in the circuit, a connection point to the base of a transistor, and a dummy load; connecting the other connections of the transistor into ar. alarm circuit; closing the normally open first-mentioned switch; and checking the alarm circuit output. Embodiments of the invention will now be more particularly described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram of a switch and switch protection device embodying the present invention; Figure 2 is a circuit diagram to show how any one of such switch protection devices may be used to protect a number of switches; Figure 3 is a circuit diagram to show an application of the protection device in which components are duplicated both in series and in parallel; and Figure 4 is a circuit diagram to illustrate the method of testing normally open switches. Referring now to the drawings, there is shown in Figure 1 a basic circuit comprising a switch S and a device embodying the invention connected in parallel therewith between points A and B. In normal operation, the switch S is closed and the load L draws current from A to B through this switch. The device comprises, connected in sequence, a control transistor T2, connection point C, a diode D1, connection point D, and isolating diode D2. Transistor T2 and diode D1 cause such a#voltage drop as to produce a reduced potential in the circuit whereby current is prevented from flowing through isolating diode D2. Thus while the switch S remains closed there is no current flow from A to B through C and D and therefore transistor T1 does not conduct. Should the switch S fail open, the potential at B will drop and therefore diode D2 will conduct since the reduced potential at D will become greater than that at B. The load L will therefore be held on with current passing from A to B via C and D. This in turn will produce a potential difference across diode D1 which will turn on transistor T1, to illuminate a light emitting diode LED1 and to produce an alarm output. The device of course can still be controlled by means of light emitting diode LED2 and transistor T2, and therefore if it is necessary for the load to be switched off, this can be easily accomplished. If it should be desired, a similar device for protection of a similar switch connected in the return path from the load L can be provided by reversing the diodes and using negative e Wivalents of the transistors. As shown in Figure 2, connectior point D ca. be connected to an auxiliary supply bus ASB, from; which connections can be taken to a number cf isolating diodes, D2(a) and D2tb) for ex2mple, each of which isolating diodes protects a switch S(a#, S(b) respectively. As can be seen therefore, a single device may protect a number of switches, each controlling a load. Each load has two individual control outputs, one in its supply path, and one in its return path. The common part of the supply path anc the common part of the returr. path are each provide: with a separate protection device connected respect- ively tc the auxiliary supply bus ASB and the auxiliary return bus ARB. It can be seen that for the load to be switched off unintentionally, a switch and a protection device on the same side of the load must fail simultaneously. Similarly, for the load to become switched on when it is intended to be off, a switch or a protection device on the positive side of the load and a switch or protection device on the negative side of the load must simultaneously fail. In order to reduce the possibility of such occurrences, each component of the device may be durlicated bcth in series and in parallel, as can be seen from the circuit diagram of Figure 3. The device can also be used to test whether or not the switches are working. In the case of a nonmally closed switch, the switch is opened and the alarm circuit will give an output when the current bypasses the switch to flow through the protection device. The method of testing a normally open switch Sx is shown in Figure 4, wnerein a circuit as described above but with the isolating diode replacec by a durany load is ccnnected in parallel with a second normally cper. switch Sy and the load L, and closing the switch Sx. If the switch is wcrkinc correctly this will cause current to flow through the protection device and the dummy load causing an alarm output to be generated. Obviously since the load L is still connected in series with an open switch Sy, such an operation will not cause the load L to be energised. - This method can be carried out to test switches on both positive and negative sides of the load.

Claims: CLAIMS: 1. A device to ensure continuation of current supply to a load in the event of failure of a switch, characterised in that the device comprises a circuit connected in parallel with the switch and comprising means to cause voltage drop thereby to produce a reduced potential in the circuit, and a diode arranged to prevent current flow in the circuit when the potential at the load side-of the switch is higher, than the reduced potential, and to allow current flow when said potential is reduced to a value lower than the reduced potential by virtue of the switch opening. 2. A device according to claim 1, further characterised by means to detect current flow in the circuit. 3. A device according to claim 2, characterised in that the base of a transistor is connected to a point in the circuit between the voltage reducing means and the diode, the two other connections of the transistor forming part of an alarm circuit which conducts only when said transistor is biased on by current flow in said circuit caused by opening of the switch. 4. A device according to claim 3, characterised in that the alarm circuit includes a light emitting diode to give visible warning of switch opening. 5. A method of testing a normally closed switch characterised by the steps of: connecting in parallel to said switch a circuit which comprises in series means to cause voltage drop thereby to produce a reduced potential in the circuit, a diode to prevent current flow in the circuit when the potential at the load side of the switch is higher than the reduced potential, and a connection point to the base of a transistor located between said means to cause a voltage drop and said diode, the two other connections of the transistor forming part of an alarm circuit; opening the switch; and checking the alarm circuit output. 6. A method of testing a normally open switch characterised by the steps of: connecting in parallel with a second normally open switch and the load, a circuit which comprises in series means to cause voltage drop thereby to produce a reduced potential in the circuit, a connection point to the base of a transistor, and a dummy. load; connecting the other connections of the transistor into an alarm circuit; closing the normally open first-mentioned switch; and checking the alarm circuit output.

Inventors: CLATWORTHY, STUART CHARLES
Applicants: RESTBURY LIMITED
Application year: 1981
Main classification: H02H3
IPC codes: G08B29, H01H47