Abstract: A measuring probe to detect the presence of electrically conducting liquid or granular material includes a first metal electrode (4) comprising a tube (5) which is adhered to a porous, ceramic insulator (12) adhered in turn to a skirt (10) forming a second metal electrode (6). First electrode (4) is connected to a gas supply (58) supplying gas, for example nitrogen, under pressure which emerges into a space (26) in the probe and which then passes through insulator (12) to emerge from face (20) thereof. A heat resistant cable (46) has its conductor (52) brazed to cap portion (8) of second electrode (6). Cable (46) and first electrode (4) are connected into an electrical circuit (66) in which current flows when a conducting bridge of granular material simultaneously contacts both electrodes (4 and 6). Should the level of the granular material drop below second electrode (6), gas emerging from face (20) blows away any granular material trying to preserve the conducting bridge, and circuit (66) is interrupted.

Abstract: A monitor system for safety critical situations such as burner control receives at inputs 17-21 control information from a programmable logic control and reference information at inputs 12-16 from plant interlocks. This information passes via opto-isolators 40 and buffers 41 to the address bus of an EPROM so as to access information stored therein which normally mirrors the PLC information so as to control relays RL1-5 via drivers 45 to conform to the PLC instructions. The EPROM also contains reset and clock information for use by a counter 48 which allows different areas within the EPROM to be accessed. The reset information is also available to a parity check circuit 49 via oscillator 46 for dynamically testing the monitor for integrity of operation. Failure of the PLC or monitor components will cause access to shutdown addresses of the EPROM and operation of the appropriate relays including lockout relay RL6.

Abstract: A burner control system includes an input interface 10 for receiving burner status inputs. These are passed to address bus 14 and used to access addresses within EPROM 18. Data at the accessed addresses are output on bus 20 to control the burner via output interface 23 with related relays. Oscillator 16 and counter 17 sequence the selected address areas of the EPROM via bus 15 and a start up sequence, for example, can thus be effected. Incorrect status of the inputs from input block 10, flame detector 12 or the internal system checks in blocks 28 or 29 to bus 14 will cause the EPROM to be addressed at locations which contain data in a form that will cause shutdown or lockout via relay 25. Parity check block 22 also actuates lockout if errors are detected. System status can be provided by display 19.

Abstract: A testing circuit for a control system has a two-state input circuit connected across a plurality of switching devices. The input circuit assumes one state when any one of the contacts is closed and the other state when they are all open. An indicator device shows whether the contacts are functioning correctly.