Keep in mind the ambient temperature ratings of the components you are using as well as the operating load. UL508 only specifies controls up to 40C unless otherwise specified on the manufacturers data sheet. I believe this applies to contactors as well. Also, resistance changes with age and operating temperature. This also applies to fuses, etc. How are your fuses failing, open, failing to open? Are wires melting or how is the wire failing? This sounds like a more serios problem other than MTBF of components. Are the failures only in the hot months?

The 50% rule applies only to failures that have a certain activation energies, roughly between 0.5 eV and 0.65 eV.  However, many electronic components have significantly higher activation energies, which means that they could have significantly higher acceleration rates.

for example, if the activation energy is 1.2 eV, the life reduction is more like 80% for a 10 degree change.

TTFN

To MWMONTY,
Great question!  The responses generated are a learning experience!  I thought I might have a bit of usefull info for you.  In 1990 at IRINFO'90, Mr. Alan C. Pierce of Factory Mutual Research Corporation presented a paper on derating electrical and electrical/mechanical components using tables, a simple equation and temp data.  The report can be obtained from Infraspection Institute at www.infraspection.com , contact Jim or Chris Seffrin.  The derating info applied to your equipment may assist you in convincing the powers to be of what kind of harm is occuring to their equipment.  The Derating information was developed to assist in priortizing maintenace based on thermal excesses to the equipment.  The data used to develop component thermal limits for this report is listed and consist of multiple existing standards.
Best of luck, and to those responding to MWMONTY's question, Thanks!

I wanted to thank all of you for all of the help. I have learned alot from all the the posts. I have only been in the reliability field for 2 years and this RCFA has shown me that I have so much more to learn. I have also found that this discussion board is a vitial resouce with all of it's member and thier technical expertise.

Thanks Again.

Suggestion: Visit
http://www.silicavalley.com/archives/Reliability.pdf
http://www.spie.org/web/abstracts/2700/2780.html
http://www.cem.msu.edu/~cem852/lecture_03_25.pdf
etc. for more info

Interestingly, I first came across this rule in senior high school biology, applied to the decomposition of proteins to amino-acids under heating (frying your steak, f'rinstance - every 10deg rise in cooking temp halves the cooking time (and don't hassle me with comments about rates of heat conduction into the middle! assume ideal cooking of an ideal steak))

So maybe the biologists or bio-chemists can assist?

Bung
Life is non-linear...

Well now I'm curious about something.

I can appreciate that many reactions rates will double for a fixed change in temperature.

But is the amount of that fixed change in temperature always 10C for any material and reaction?  I kind of thought is was different for different materials/reactions,but Bung's discussion makes me wonder.

Electricpete,

I made a quick mathcad template applicable to 150, 185 & 220 oC Insulation for dry type transformer and the 55 & 65 oC for oil filled transformer. Although the result do not show exact loss of life of 50 % with 10 oC increment, the values are close enough for practical purposes. That me suspect that the 10 oC rule is applicable to different materials and medium.

An interesting observation of the Arrhenius equation plotted in a log-log scale shows that this relation behaves as a quasi-straight line within a practical temperature ranges.

This characteristic is perhaps the answer of the consistency of the 10 oC rule for any material and insulating media.

The general Arrhenius equation includes a term that is called activation energy in the semiconductor industry and basically describes the slope of the curve of lifetime as a function of temperature.  The 50% rule applies to certain failure mechanisms that have an activation energy around 0.5 eV.  

There are other failure mechanisms, particularly in semiconductors that hvae activation energies over 1eV up to 1.5 eV, which allows you to test part lifetime using accelerated temperature.

The MIL-STD-883 life test qualification testing uses this principle as the means of certifying that the military IC meets a certin range of reliability and lifetime.

TTFN

Does any one know if the Arrhenius equation applies to loss of life of insulation with the moisture content?

This do not appear to have direct relation with the 10 oC rule, but the following statement from a technical source has so much similarity that call my attention: “Each time the moisture is doubled in a transformer, the life of the insulation is cut by one-half”

Since moisture accelerate the degradation of insulation, I suspect that this chemical reaction could be explained by the Arrhenius equation or a similar mathematical model.

Any reference or Internet link in this subject will be highly appreciated.

Thanks.
 

The Arrhenius relationship is a mathematical model of a rate process.  As such, it's used in chemistry, reliability, etc, WHEREVER, there is a exponential relationship of some process with some independent variable characterized with a single equivalent activation energy parameter.

TTFN