I use the standard flexwatt tape, which as you pointed out is a resistive conductor imbedded in mylar film. This is a great product (though not originally designed for this use), it is durable, consistent and can be used in a variety of ways. I am something of an electrician and a controls electrician and here's my angle on your problem.
If there were any deficiencies in the tape or materials it is most likely they would have become evident fairly soon after you powered it up. Here I've got some lengths of 3", 4" and 12" tapes still operating with no breakdowns or problems after 15 years. Some arre terminated with those elaborate crimp-style AMP connectors (without THE TOOL) others are soldered.. actually, the nature of a resistance load (VS a motor load) is such that a poor connection would result in a cool tape, not a fire. . Compare those intallations to a few larger cages I have with drug-store type heat pads, those rigs I replace annually, you can see the deterioration of them and measure how the heat is being output unevenly. So from the viewpoint of an investigator (of sorts) something else contributed to the problem you had.
First, these systems work best with a proportional control system, not a rheostat or an on-off control.. the difference is that a proportional control uses input from the temperature sensor (placed on the tape) to provide only the amount of current (heat) needed to achieve the target temperatures. Whereas a rheostat chokes the current but provides constant power and an on-off thermostat gives shots of full current between off times. The rheostat cannot respond to changes in ambient temperatures.. if you set it up to give 95deg on the cage-bottom in the morning when the room is 70deg., later as the room warms the rheostat keeps giving the same current it was set to provide, very high temps can result. The fault most often seen with on-off thermostats is that the contacts fail, either in an open (or off) position or worse, in a closed (or on) state. Modern programmable proportional controllers are the very best way to go and are a real boost to Boa breeding.
Ok, back to your problem.. as you showed through your investigations the flexwatt tapes in good condition generate heat evenly and hardly enough heat to cause combustion in materials used in most caging. Plug them right in and you get about 120deg (f). And from my experience with the stuff 2 years is not long enough for the tape to fail under any reasonable condition. Other things, however, do change in a typical setup with these under boas or boa cages.
When we first set these up we're careful to position them properly so they are not too tightly wedged between the cage-bottom and the rack or shelf.. this is a prime factor in heat build-up and transfer. And also so the cage doesn't catch or abrade the tape as it is being taken out or placed in the rack. If the tape is under a stand-alone cage this is less of a concern. Over time though perhaps people change the tub and the clearance is diminished or the edges of the new tub are sharp or the form of the tub places more surface directly agaiinst the heat-tape.. also many composite materials (melamine, particle-board, etc.) expand or sag and this causes the clearance to become too restricted, making hot-spots and causing failures.
And lastly but perhaps most often overlooked.. that Boa which was 4 onces or 4 pounds when first placed in the cage is now 2-3 years later 10 times as heavy, and that weight is placed right on the heat-tape... all the while..
Bottom line recommendations:
1) No half-fast rigs with heating systems, best materials, construction, controls and monitoring.
2) Check and recheck everything, all the time.
Hope this helps!
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Gus
A. Rentfro
RioBravoReptiles.com
www.riobravoreptiles.com
"Quality is not an accident. Perfectly healthy animals are a minimum requirement.. everything else is just salesmanship" gus
