Patent Number: 047626631
Section: summary

FIELD OF THE INVENTION The present invention relates to a self-testing monitoring circuit and more particularly to a self-testing monitoring circuit for a nuclear power plant, the circuit being capable of assuming artificially created fault states for testing both its own operation and the operation of additional monitoring circuitry. BACKGROUND OF THE INVENTION In many applications in control systems for plants, including nuclear power plants, it is desirable or even necessary to sense the status of switch contacts. The switches may be main control board push buttons or selector switches, or they may be limit switches located on motordriven actuators. In either case, it is crucial that the process control system be able to sense the switch status reliably. One measure used to enhance the reliability of such systems involves the use of paired or redundant contacts, in which a switch has at least two pairs of contacts which are always in opposite states. A switch with form "C" contacts or form "D" contacts is such a switch. In the form "C" switch, there are two pairs of switches, one of which is normally open and the other of which is normally closed. Opening one switch automatically closes the other, and vice versa. In a form "C" switch, the contacts are arranged in a "break before make" fashion; in a form "D" switch, they are arranged in a "make before break" fashion. In either case, the switch may be inferred to be malfunctioning if both pairs of contacts have the same state after a short transition time, that is, both are open or both are closed. Therefore, to test the switch reliably, it is necessary only to provide circuitry which is capable of monitoring the condition of each half of the switch, i.e., each pair of contacts, and which produces an error signal if both pairs of contacts are in the same state. In conventional circuitry, this is accomplished by providing a digital indication of the status of the pair of contacts, e.g., a "0" if the pair of contacts is opened, and a "1" is the pair of contacts is closed. The signal indicative of the state of one pair of contacts is then compared logically with that of the other pair of contacts. If the signals have the same logical value (if the parity of a signal which a combination of the two is even) the switch is detected as malfunctioning, and appropriate corrective measures may be instituted. While such circuitry is sufficient to insure reliable operation of an overall system in many instances, it is often necessary to provide an even higher level of reliability of periodically testing the testing circuitry itself. In the past, this higher-level testing has been simply a maintenance procedure carried out on the order of once every six months or so. Again, while this periodic testing is sufficient to insure a sufficient degree of reliably in many systems, there are advantages to be achieved if testing made automatic so that it can easily be carried out more frequently and even on-line. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a switch testing circuit with a self-testing capability. It is a further object of the present invention to provide a switch testing circuit with circuitry which temporarily disconnects the testing circuitry from the switch being tested, and instead, in effect, connects it to relays which controllably mimic operation of the switch. Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. To achieve the foregoing and in accordance with the purposes of the invention, as embodied and broadly described herein, there is provided a self-testing circuit for monitoring operation of a switch, the switch having first and second pairs of contacts, the pairs having a contact in common, with one of the pairs being open and the other being closed unless the switch is malfunctioning. The circuit has first testing means, connected across the first pair and responsive to a first enable signal and a first select signal, for outputting, in the absence of the enable signal, an output signal having a first value if the first pair is open and a second value if the first pair is closed, and for outputting, in the presence of the first enable signal, an output signal having the first value of the second value regardless of whether the first pair is open or closed. The circuit also has second testing means, connected across the second pair and responsive to a second enable signal and a second select signal, for outputting, in the absence of the second enable signal, an output signal having the first value if the second pair is open and the second value if the second pair is closed, and for outputting, in the presence of the second enable signal, an output signal having the first value or the second value regardless of whether the second pair is open or closed. The output signals are applied to logic means, responsive to said first and second testing means, for producing an error signal if the first and second output signals have equal values. The testing means preferably include relays. The relays can then be used to "inject" invalid logic states into the system. If the injection of an invalid logic state fails to prompt an indication of a malfunction, it may then be inferred that the testing circuit itself is malfunctioning, and corrective measures can be taken. The invention provides means and method for self-testing which are sufficiently simple and easy that self-testing can be carried out frequently, thus enhancing reliability.