It depends on what happens if you back heat runs wide open. I always run a max power wide open test (no animals present) to see how bad bad is. I have similar in some ways a rack with hot and cool side belly heat. The hot side maxes out WAY too hot to not use a failsafe (130º and climbing when I shut the test down...) The cool side is the low wattage 17 inch flexwatt it maxs at 87º I ran it maxed out (plugged in the wall no t-stat •again no animals in the rack during the test• ) The hottest it got was 87º and it averaged around 84º over the 12 hour test. Never run a test like this un monitored I never left the area of the rack I could see it and had smoke alarms in the top tub and on the back. I do not run a fail safe on the 17 as if the T-stat fails it is ok not ideal but everything will be alright and take no harm. I am never gone for longer than a single night or maybe two.

I believe this is the only safe way to decide if you need a failsafe if the unregulated temp is higher than 90º yes you need a fail safe. If the unregulated temp exceeds the safe working temp of the product in use. (flexwatt should not be used over 100ºF by the specs) it needs a failsafe unless it fails to reach 100º.

Some of the herpstats (1 and better) have the normal settings alarms and such but also a settings that if the temp gets too cool it shuts down (pulled probe racks seem particularly prone to this) and a mechanical relay that typically will fail close (no power) rather than open. These units it becomes questionable if a failsafe is needed but they are the only units around that have these. I believe the Ecozone has something like a mechanical relay as they do not use a triac system but they are unique in this. They DO NOT have an under heat shut down. The trick they play is the easiest to set up and the best alarms (web set up you just log into your T-stat and use the computer no silly buttons and it emails alerts to you.) The herpkeeper has similar functionality and the VE and helix don't really have anything extra to offer.