Do conductors age under reasonable electric load?

Question

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?

Answer 1

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).

Answer 2

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.

Answer 3

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).

Answer 4

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.