Steam Turbine Emergency Overspeed Probes Not Sending Signal (2/3) - L14H_ZE

Good morning,

During a recent major outage, we replaced and rewired three primary and three emergency overspeed probes. We had high vibration during startup (another issue altogether) and had one of the emergency overspeed probe wires come loose, producing a 0 signal in the prevote logic. After a few months of running in this manner, with plans to remove the bearing cap and rewire the faulted sensor during the next outage, we had a 2nd emergency overspeed probe wire come loose, and now have a diagnostic alarm L14H_ZE. A little research shows this alarm appears to be active when 2/3 emergency overspeed probes indicate a 0 instead of a 1. Here is my question. What is the result of a loss of the third emergency overspeed probe? Is it a trip, or are we already in a position where a trip will not occur if called for due to 2/3 overspeed probes indicating a 0 logic? Calling CSA on this one, I'm sure he'll be able to steer me in the right direction.

Thanks in advance,
Would be nice to know what control system this was operating on. Also I would think if you had two emergency OS probes fail it’s a big risk not to shutdown to replace them. I believe if it’s two out of three and you lost two then a third won’t make a difference. The documentation I have for an ST on a MarkV says a trip occurs if the median of the three values exceeds the set point, so the x,y,z processors must have to agree on the median speed and compare it to set point.
I don't think I quite get the problem here. I don't know if L14H_ZE is a <Q> or <P> alarm (Process or Diagnostic). I'm really not a steam turbine guy, and in my personal opinion if two of the three emergency speed pick-ups are not sending signals while the unit is running it should be tripped. The <P> core drives the ETR (Emergency Trip Relays) it's supposed to be an independent means of detecting of overspeed and tripping the turbine if it detects an overspeed or dangerous rate of speed change. And, in my personal opinion zero speed on two of three speed pick-up inputs qualifies as a failure to be able to detect an overspeed (or excessive rate of speed change) and therefore the unit should be tripped.

BUT, I don't know how the trip circuit is wired or configured in a Mark V steam turbine panel--I only really know how a TREG card is configured and wired (GT). So, I'm not the best person to comment on this thread.