I have a mission-critical device, which involves a micro and mosfet controlling a pump. I want to implement a failsafe, so that if basically anything happens to the micro, or the mosfet blows, the system will engage and turn on a backup pump. The easiest way to do this, it would seem to me, would be to have an always-engaged (NC) relay that is disengaged by a failure, energizing the backup pump. So, my question is, is it a good idea to have a relay on continuously for what may amount to years? The extra 20ma for the coil is negligible in this application, but are there any other consequences to doing this?
Look for a high quality relay that has a rated maximum continuous coil voltage. As long as the voltage driving the coil stays below this value and the environment (temp, humidity, etc) stays within the rated ranges then you should expect to be able to drive the coil continuously for its rated lifetime.
One failure mode to consider is that relay contacts can become "stuck" shut. In this case, the relay does not open even though the coil is deenergized. This happens usually because of sparking across the contacts when they are connected to an inductive or high voltage load when they open. It can also happen if you over heat the contacts by driving too much current though them and getting them hot.
It might be a good idea to periodically deenergize the relay both to exercise and to test it. Ideally, you'd do with without a connected load to minimize wear the contacts.