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I have a problem. I have a sensor in a piece of equipment that needs to switch off that equipment if it detects a dangerous gas. The equipment must remain off until a manual reset is applied. What kind of switch/contactor does that? Is there a special name for it? It should be inherently failsafe.

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    \$\begingroup\$ How about a relay that interrupts power to its own coil when de-energized? Once it's off, it can't come back on. \$\endgroup\$ Jul 6, 2021 at 16:09
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    \$\begingroup\$ When a relay's contact controls it's own coil (latching action) I've heard this called a "seal-in" circuit. \$\endgroup\$
    – rdtsc
    Jul 6, 2021 at 17:54

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"Failsafe" has a variety of meanings in different industries. Presumably the sensor output in this case is configured so that the output "on" verifies that dangerous gas is not detected.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Manual reset. Auto-off if OUT signal or sensor power is lost.

Depending on the risk category you may need to employ redundant circuits. Industrial machinery uses dual channel redundancy and I have explained some of the principles of operation in the posts linked below:

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  • \$\begingroup\$ Just a warning: If your dangerous gas is flammable or explosive, this approach will certainly increase the safety. But, the approach in itself won't be certifiable for operation in a hazardous area unless the design of the switch, the equipment and the sensor all meet other requirements. \$\endgroup\$ Jul 6, 2021 at 21:39

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