I would like to use 2 SCRs in antiparallel configuration to control the power to a all resistive load (20A) using 240Vac (line to Line). Pretty typical configuration and part of a lot of SCRs datasheets. The application has a ON/OFF switch that controls in part if the SCRs are ON (using zero crossing firing)
The problem to overcome is the failure on an SCR. If an SCR fails short you lose control to the load. So I have added a second set of SCRs for redundant control. One set controls the "Line side" of the load and the "Return-side", such that if one SCR fails the other set of SCRs would allow control of the power.
Yet the problem with that, is the fact that, if an SCR fails short, the load will have power for half a cycle on one end of it, while the other end is effectively disconnected. That causes a potential corner case safety failure if a person "turned OFF" the switch but the load has a potential on one of the legs.
My question is how can you create a FAIL safe SCR. A fuse will not work as the current will be within the limits. My device needs to work at -40C.
Best Answer
You can't make something totally fail-safe. However, you can make failure less likely, and you can make that failure less problematic.
First, make it less problematic. If you have two switches, one on each side of the load, then a failure of one could leave the load connected to the 'wrong' mains pole. That can be mitigated by putting the two SCRs in series in your 'higher reliability' switch, and putting those in the pole you want to switch.
If the failure of any given switch is independent of that of the others (and this may be true for some mechanisms, like a comsic ray strike, and not true for others, like an overvoltage spike, overtemperature, overcurrent or old age), then putting multiple units in series will make failure less likely. If one switch has a failure in 1 in 1000 operations, then 2 switches will be 1 in 1 million, and 3 switches will be i in 10^9.
As you see from my list of what failures would be random, and what related, it's probably better to protect the SCRs from fault conditions, than to simply series up a number of them.