From chats with several resistor manufacturers, it seems that there's a consensus that certain resistor technologies at high impedance ~120Kohm or more will always open and never short. Does anyone have documentation that reinforces this "fact"? Any ideas?
Resistor technologies that guarantee failure mode of OPEN and not short--need docs to prove
I think these resistor types probably fit the bill: -
If you need to research others - there are several brands of wire wound resistors that are regarded as fail-safe when it comes to the design of zener barriers for use in potantially explosive atmospheres.
Here are some other types - try searching "approved safety resistor"
Well, certainly avoid carbon film resistors!
I don't think it is true. I think that the most likely candidate (metal film resistors) have been shown to drop in resistance due to pulse load damage.
Damage Caused By Pulse Loads
More than 80% of the failures attributable to pulse load damage, equivalent to 0.04 ppm overall, displayed helical shorts within the device, causing a large fall in resistive value. A further small number of failures, contributing around 0.01 ppm, displayed a positive change in resistance. These failures arise from irregularities in the ceramic surface that compromise the sputtering process and the formation of the resistive film. Harsh pulses can then cause local hot spots that gradually destroy the sputtered resistive layers and the neighbouring resistive film.
For safety-critical applications you may have to put two resistors in series.
Edit:- Could you use a resistor in series with an approved Y-rated capacitor?
I also note that sometimes UL-listed ESD wrist straps use two resistors in series (one at each end). That's a clear example where a resistor failed short could expose someone to severe risk of electrical shock.