Redundant Temp Controls?

[TraconSnake]

I'm wondering. I was researching into the Johnson Controls unit, and in the PDF document from the Johnson Controls site, there is this warning:

IMPORTANT: The A419 Series Temperature
Controls are intended to control equipment under
normal operating conditions. Where failure or
malfunction of an A419 Series Control could lead to
an abnormal operating condition that could cause
personal injury or damage to the equipment or other
property, other devices (limit or safety controls) or
systems (alarm or supervisory) intended to warn of
or protect against failure or malfunction of the A419
Series Control must be incorporated into and
maintained as part of the control system.

The document does state that the unit will shut off in the case of a sensor failure, and will also shut off in case of a program failure.

However, this unit is relay controlled, and relays can stick in the closed position.

So with the Johnson Controls unit, at least, the only way to ensure safety is to use two units - one as the main unit, and a second one set a couple degrees higher so that it will shut down the primary unit if it fails.

I couldn't figure out what the Ranco and HerpStat units use for voltage control. Do either of those use solid state switching as opposed to a mechanical relay switch?

Here's what I found for Ranco:
http://www.icca.invensys.com/uniline/d/d71.pdf

And for HerpStat:
http://www.spyderrobotics.com/produc...ual_online.pdf

Interestingly, HerpStat makes no claim that the unit will shut off in the event of a program failure.


So, any thoughts on redundancy of temperature controls? Considering a BP can live over 20 years, I think one could assume that the risk of a temperature controller failing in that time frame is fairly high. So does anybody think the cost of an extra unit to maintain redundancy is justified, or is it okay if a snake is exposed to a 125 degree strip of flexwat for several hours (the time it may take for you to get home from work and notice the temperatures are way high)