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?

  • \$\begingroup\$ Look into carbon composition (bulk, not film) resistors. Overhauling old vacuum tube gear, the hotter running ones tend to be 130% or more of rated value. Don't have specific documentation, sorry, just saying that's where I'd start looking. \$\endgroup\$ – Brian Drummond Feb 12 '14 at 19:04
  • \$\begingroup\$ Ohmcraft has a line of printed resistors that can withstand pretty surprising pulses. ohmcraft.com/uploads/WP_SuperiorPulseResistorCapability.pdf You might consider giving them a ring and seeing if they can guarantee fail-open. \$\endgroup\$ – Scott Seidman Aug 31 '16 at 12:51

You might be looking for fusible resistors. They are designed specifically for to open when overloaded. There's a helpful discussion on the EEVblog forum.

Examples of fusible resistors are those from Vishay and TE.

  • \$\begingroup\$ Fusible resistors and chip fuses seem to be limted to very low ohmic values and we need ~ 60K-120K ohm. \$\endgroup\$ – user37094 Feb 12 '14 at 21:24
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    \$\begingroup\$ Is there any reason you can't use the fusible resistor in series with the 60k-120k resistor that you need? So what if the 60k fails short, the fusible resistor will open. \$\endgroup\$ – ajs410 Feb 14 '14 at 0:29

Resistor technologies that guarantee failure mode of OPEN and not short--need docs to prove

I think these resistor types probably fit the bill: -

enter image description here

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"

  • \$\begingroup\$ Explored this with TT on a conf call this morning and all agreed that it will likely fit the bill. But the issue remains how to convince UL that a failure that ends up as a short is impossible. Safety resistors seem to have very low ohmic values and we need ~ 60K-120K ohm. \$\endgroup\$ – user37094 Feb 12 '14 at 21:20
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    \$\begingroup\$ Have you tried asking UL what they would consider appropriate? \$\endgroup\$ – Andy aka Feb 12 '14 at 21:27
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    \$\begingroup\$ Yes, trying to. They are being difficult. Hence looking for some concrete docs. Perhaps the better angle is the single-point of failure route raised by @spehro (maybe I need to hire someone to find something that helps in UL916?) \$\endgroup\$ – user37094 Feb 13 '14 at 18:03

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.

  • \$\begingroup\$ From what I've understood, placing even 4 resistors in series will not guarantee an open failure instead of a short. In other words, there could be a cascading failure that will result in all resistors in series shorting. So we're back to the original question: assurance that failure will result in an open and not a short. \$\endgroup\$ – user37094 Feb 12 '14 at 19:50
  • \$\begingroup\$ I mentioned it because it does seem to satisfy UL requirements for some applications- no single point failure can cause a hazard. There are UL recognized fusible resistors, but not 100K ohms AFAIK. \$\endgroup\$ – Spehro Pefhany Feb 12 '14 at 19:54
  • \$\begingroup\$ Funny -- that's been my argument as well. Seems like the no single point of failure argument can fly for much more safety-critical applications, but no for ours!? This is actually for a wall switch, filed under ENERGY MANAGEMENT EQUIPMENT - UL 916. Nothing in the standard seems to deal with single point failure, etc. \$\endgroup\$ – user37094 Feb 12 '14 at 20:27
  • \$\begingroup\$ The particular piece of equipment I have in mind could easily completely take out a single-family dwelling (WMD anyone?). \$\endgroup\$ – Spehro Pefhany Feb 12 '14 at 20:47
  • \$\begingroup\$ Is there a UL doc that describes this single-point of failure point, or is it specific to each standard. UL916 doesn't seem to talk about this either way. \$\endgroup\$ – user37094 Feb 12 '14 at 22:01

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