| 摘要 |
A safety relay comprises plurality of parallel relay coils 922-926, coupled in parallel between input nodes 940,942 via respective switches 902-906. Relay contacts 930-934 are series connected in an electrical path between output nodes 944,946, giving tolerance to faults such as welded contacts, and allowing independent testing of each relay under control of a respective switch 902-906. A bypass switch 960 can decouple the relay contacts and provide a second path 964 between the output nodes while relay testing takes place, which may include measuring voltage, current impedance or resistance, in comparison with a value. The relay may further comprise a light emitting diode, a diode in parallel with coils and transistors between the switches and coils (fig 7, 752,772,762). The relay can switch safety-critical process control plant such as valves and actuators. The relay tested and controlled functions can be provided via a communication bus or local are network (LAN) by a controller (fig 1, 120) using program instructions. |
