I would think two in series would be sufficient for reasonable reliability.Fail Safe is difficult.
Fail resistant could be putting 3 contactors in a row. Assuming the power through them never exceeds the rated value, the chances of all 3 failing at the same moment are minimal. In the event one fails, replace all 3, as the other two have switched the same power the same number of times. Some form of indicator should be used to alert that one has failed.
Depends on how "Fail Safe" the OP desires. Mechanical contacts can't know when they are stuck together, and what to do if they did.I would think two in series would be sufficient for reasonable reliability.
Mercury displacement relays (MDRs) ... their expected life is in the millions of activations, which is far greater than that of mechanical relays. They have no physical contacts to wear out. Their failure mode is almost exclusively OPEN, not closed. This means almost no overfires.
OK. I was assuming that, if it was that important, then there would be a real-time monitor circuit that would indicate if one of the contacts had stuck and this would give an immediate warning to the operator (or shut down the system).Depends on how "Fail Safe" the OP desires. Mechanical contacts can't know when they are stuck together, and what to do if they did.
With two, there's a very slight chance of them failing at the same time, I picked three due to assuming the system may not be monitored or checked often, one check may miss two that are stuck as long as "It's still working" maintenance. If checked often and with indicators to show open state (neon bulb or similar), then two would suffice.
I am looking for a 90 amp 800 volt solid state relay or circuit with 3 to 20 V Ac/DC control. For safety it must fail open. Any help would be greatly appreciated.[/QUOTE
Mechanical relays don't fall within his specs.
Maybe something has changed??
Even with 3, I'd guess that somewhere, some time, people would keep using the system until it fried because "It was made to be fail resistant, and it still works".OK. I was assuming that, if it was that important, then there would be a real-time monitor circuit that would indicate if one of the contacts had stuck and this would give an immediate warning to the operator (or shut down the system).
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