Could you (or someone) post a link to this thing? I have been around
well systems for most of my life and have never seen a pressure relief
valve on a well tank...well, there is the pump control switch that is
usually somewhere down on the tank in/out pipe. Even your description
doesn't ring a bell with me.

Harry K

Here you go:
http://cgi.ebay.com/3-4-WATER-WELL-P...photohost ing

They are usually located on the well head just above ground, I think
the purpose is to protect the pipes between the well head and the tanks
& even the tanks & all other plumbing in case the pressure switch fails
in the on position.