Chandra R., Kyle R. 

Chandra attempted to re-establish heated GN2 flow through CP4's empty LN2 reservoir this morning (assists bake-out of CP4 currently in progress).  The original SCR had "shorted" after only days of use so we replaced it yesterday with one we removed from CP3's Regeneration Control Panel.  This time the SCR failed (shorted) after only minutes?  Tens of minutes?  

According to the SCR manual, this failure results whenever there is load current detected (sensing coil on one phase) in the absence of a 4-20mA control signal or when the load current is present and not a function of the 4-20mA control signal.  I investigated and found that the three delta-connected heating elements all measured 30 - 31 ohms between each other and Megaohms to ground, i.e. weren't shorted when the low voltage VOM output was applied.  I did not do a High Potential (HiPot) test.  Next, I lifted the CDS wires supplying the 4-20mA control signal and then energized the unit.  Still, even with the input removed, the SCR indicated an "SCR SHORTED" fault.  Long story short, all of my investigating suggested a shorted SCR vs. a problem in the associated external wiring.  

As such, I replaced this second SCR with a third one borrowed from CP5's Regeneration Control Panel and installed it.  This time, I did not land the CDS 4-20mA leads but, instead, left them "hanging".  I energized the Control Panel and did not experience the troublesome fault.  The Measured supply voltage was found to be 498 VAC which is quite high.  I recall that this is a known "feature" from "back in the day..." and seem to remember it was due do the Xformer tap that we use.  In any case, this shouldn't be an issue as the SCR is rated up to 600 VAC.  I also measured 9.4V between the 4-20mA control wires while not terminated.  Hmmmm... the CDS PID control is off.  Should there be voltage across these without a PID output?  

I'll try out this third SCR tomorrow and attempt to do so in small incremental outputs while monitoring as many pertinent metrics as possible.  If this third unit also shorts, we'll abandon the "dream" and concentrate on minimizing air leak losses to the room and adding more heat to CP4's insulated bake-out enclosure.