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Construction steels are prone to fatigue failure - they get structurally weaker when exposed to repeated mechanical stress.

Is there any similar process for conductors which makes them less usable once they conduct current for long enough?

In other words, suppose I have an old transformer which has served for say 30 years. It still functions fine (didn't burn down, short or anything like that) but it is rather old already. I expect that magnet wire insulation will get weaker because of being heated for all these years. I also expect that core plates insulation will get weaker.

What about the wires themselves? Will aluminum or copper in them be any different from what I'd find in newer wires?

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  • \$\begingroup\$ Actually, correctly loaded steel (below its Hookean limit) won't fatigue. Aluminium, on the other hand, will fatigue eventually, however low the mechanical stress. (I know of no similar mechanism in conductors, though the heat generated may affect mechanical properties, such as annealing copper, and aluminium alloys have highly complex responses to heating cycles) \$\endgroup\$ – Brian Drummond May 26 '15 at 9:46
  • \$\begingroup\$ It will likely have an oxide layer which no one but some rf guys will care about. \$\endgroup\$ – PlasmaHH May 26 '15 at 10:07
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    \$\begingroup\$ As personal experience, I had until last week a 20-year-old 220V->110V transformer, home use, which was giving 130V on its out. Turns out it had expanded its coil length a bit. So yeah, maybe the wire material itself doesn't "age", but if the assembly can't hold it together indefinitely, you can have some problems. \$\endgroup\$ – Kroltan May 26 '15 at 15:13
  • \$\begingroup\$ Good question to ask. All answers seem useful. | Possible data point: Rats seem to like PVC ahead of other plastics. As TPS cables tend to be PVC this makes them potential food sources for rate. Some other plastics seem to not attract them at all. Not usually an issue with transformers. Some very old transformers tended to use shellac on wires. Crumbles when disturbed but may be OK if not moved. | \$\endgroup\$ – Russell McMahon May 27 '15 at 6:37
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The following is an interesting document about the ageing of conductor cables in nuclear plants.

At pages 20 and 21, inside a table reporting cable components stress factors (stressors), you find possible causes of conductors failures.

You may notice that in no way the conductor material (aluminium, copper) "ages" just by itself, but it develops problems when interacting with other "factors". The simple act of conducting current is not harmful, unless it causes thermal stress or mechanical stress on the cable (not on the material).

As for the material itself, copper used in building conductors is extremely pure, so it doesn't degrade unless exposed to factors that either trigger a chemical reaction (e.g., oxidation, corrosion) or a nuclear reaction (metals exposed for long time to heavy radiations, especially neutrons, undergo a change in their physical properties, since atoms in the crystal trellis may be mutated into other isotopes or even in other elements).

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If the question is about small transformers, the above answers are adequate.

If the question is about large transformers, i.e. oil-immersed power transformers - that is a whole different game.

The metallic conductors in a transformer don't deteriorate with age, but the insulation does. Power transformers are constructed with paper insulation, which has a limited lifetime. The cellulose in the paper degrades with normal heating and becomes mechanically weak; additionally, the degradation of cellulose releases water, which lowers the di-electric strength of the oil.

The rate of degradation is exponential with increasing temperature (see 1 for some discussion.) I believe transformers are usually designed to achieve 25 or 40 year lifetime under rated load.

All of which is to say - the metallic conductors in an old power transformer will be fine - but beware old insulation.

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    \$\begingroup\$ There are no "above" answers... now. \$\endgroup\$ – JYelton May 28 '15 at 20:22
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Not in a transformer - there should be no change unless the transformer was abused.

However, a reasonable load in the context of an IC is considerably higher than a wound coil. At about \$10^6\$ ~ \$10^7 \$ A/\$cm^2\$ (depending on material etc.) electromigration becomes a serious limitation on life.

Thin wires can sometimes deteriorate due to chemicals in the environment- I've seen fine copper coils fail in the presence of (gaseous) sulfur or sulfur compounds getting into pinholes in the insulation (kind of a black plague).

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    \$\begingroup\$ For sewage treatment plants, where there is a lot of H2S (hydrogen sulfide) floating around, all the electronics have to be ordered with conformal coating for corrosion resistance. \$\endgroup\$ – Li-aung Yip May 26 '15 at 10:57
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    \$\begingroup\$ What about thermal cycling due to varying current on timescales of hours or more (the question is not clear on this, but still)? \$\endgroup\$ – Peter Mortensen May 26 '15 at 19:11
  • \$\begingroup\$ @PeterMortensen Coils (especially potted ones) that are exposed to wide thermal cycles (from self-heating or from ambient temperature swings) seem to fail often. \$\endgroup\$ – Spehro Pefhany May 26 '15 at 19:25
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As mentioned above, your transformer should be fine unless it is overloaded. A minor overload, generating excessive heat will degrade the insulation, and potentially lead to turns shorting. A major overload will burn open one of the windings entirely.

The conductors themselves do not degrade in their ability to carry current in any capacity as long as they operate within their ratings. Again, passing 1000 amps through a piece of #30 copper isn't going to work for long, but that is because the heating will melt the conductor.

The primary failure mode of conductors, be they aluminum, copper, or whatever is an enviornmental breakdown of the insulation systems, or in the case of medium to high voltage (15KV to 345KV) insulated cables improper installation, typically not observing the minimum allowed bend radius.

Environmentally, the most common cause of insulated cable failure is exposure to ultraviolet light, with a close second of exposure to oils or other chemicals that break down the actual insulation jacket. Very rarely there can be failures due to water intrusion, but again, that isn't actually a failure of the conductor, but rather the insulation system.

To be complete, there is a possibility of actual conductor failure, but I don't consider it to be a true failure of the conductor personally, from improper installation leading to galvanic action or corrosion (think wires on an automotive battery terminal corroding from the acid, or copper developing it's typical green patina when exposed to salt water). Those failures almost always happen at a connection point, and are the result of poor or improper connections.

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    \$\begingroup\$ Regarding galvanic action - this applies especially when joining aluminium to copper, i.e. at transitions between Al overhead lines and Cu underground cables. Special accessories are needed to make the connection between dissimilar metals. It even gets to the point you have to have two separate abrasive brushes - one for cleaning copper and one for cleaning aluminium - don't mix them up or aluminium particles will get into your copper connection and cause a bad joint! \$\endgroup\$ – Li-aung Yip May 26 '15 at 13:20
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    \$\begingroup\$ Also, I would argue that vermin (rats, termites) are the #1 cause of cable failure, well ahead of UV and harmful chemicals. \$\endgroup\$ – Li-aung Yip May 26 '15 at 13:23
  • \$\begingroup\$ @Li-aungYip Possible data point: Rats seem to like PVC ahead of other plastics. As TPS cables tend to be PVC this makes them potential food sources for rate. Some other plastics seem to not attract them at all. \$\endgroup\$ – Russell McMahon May 27 '15 at 6:39

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