Wally Bestgen wrote:

You are correct.  It does not preclude mechanical failure of the solenoid.  I does provide and additional layer of safety over the a system that does not have the fault detector. 

I would disagree.  The fault detector does provide the indication of the presence of one of the potential faults.  However, a "no fault" indication does not mean that there is no fault.  Instead it means that there may still be a fault with the system, just not the one improbably one this device is looking for.  A fault detector ought to err on the side of creating false positives.  A fault detector whose "all is well" signal comes with a whole lot of fine print about the likely fault mechanisms that are not covered does not promote safety -- it promotes a false sense of security and that is all to often one of the links in the accident chain.

An open coil is not very likely unless there is poor workmanship or poor design in specifying an incorrect coil (intermediate duty coil for an always-on application for example).  If one is worried about the coil going open (or not getting energized) one ought to address those areas first.

Regards,

Hamid