For years, the conventional wisdom, related to SIF design, has been that all safety functions should be de-energize to trip (DET), unless there was absolutely no other way to accomplish the goal of the safety function. I'm curious if that thinking has changed any recently. I was in a discussion recently concerning ETT circuits. The point was made that an ETT SIF has the same safety rating as a DET SIF if, the IO cards have supervised circuits, and there is a UPS. The goal of the ETT function is reducing the spurious trip rate since you do not have power holding the circuit closed. A large number of spurious trips can be linked to coil burnout on solenoids. The argument can be made that the process is in it's most un-safe state, during start-up and shut-down, so spurious trips create a safety hazard and should obviously be minimized without affecting the integrity of the SIF. I know there are lots of ways to reduce spurious trip rate 2oo2, 2oo3, etc, but just wondering what people are thinking about ETT, since the diagnostics and supervision on TMR I/O has improved over the years.
I can't make myself get comfortable with ETT functions yet, but I'm just curious what others in the industry are doing and thinking. It is my belief that 99%+ of the SIFs out there are still DET, but I could be wrong.
Greg
In reply to Len Laskowski: