How to wire a transformer into ELK zone to monitor AC Loss

You may have...  That only trips on fault condition though, not when active.  I'll check out the link you send, thanks again!
Here is the thread I was talking about.  The OP's requirements are a bit different I believe, but this may give you some ideas.
I would place that current donut on both inputs (main and gen) to the transfer switch, that way you can see that:
A. Line Voltage is present
B.  What is giving you that voltage (main or generator) AND that it is indeed providing current (i.e. there is nothing wrong downstream from the transfer switch such as a main breaker to the breaker box tripped.
I'm not sure what a 'FAULT' definition is looking at the manual, but you can detect those contacts as well.
Let's see what others recommend here as well! :)
A pair of RIBS or similar will accomplish all of this and be a heck of a lot easier than a donut on the cable.
A RIB to supervise the loss of AC at the local outlet before the transfer switch and genset kicks in. Easy, the generator is NEVER going to be an instant switchover and start. Tie to a supervision point.
A RIB in a feed from the genset. Hell, if you have multi-phase, put one on each to see where an issue lies.
Leave the fault from the output panel as a general annunciator of issues. Generally a low oil, fail to start, low start battery or similar conditions on most generators.
Yes, however you'll need to split the conductors and provide an enclosure and separation to do this, not to mention the rail to mount them on......and a lot more expensive than a pair of RIB's mounted in knockouts on the related enclosures.
Look at a RIB 2401B vs. a RIBXGA. You can buy a pair of one for the cost of a single donut.