I know a few things about electronics, but transformers are still an unknown for me. I know AC to DC converters use them to isolate live voltage from the DC output, or at least I think that's what they're used for.

Dumbed down, the transformer is just a bunch of wire coiled over a core, that's usually made of steel (at least for larger transformers) which is very much conductive. Couldn't it ever malfunction in such a way (wire melts or something) that the 2 sides of the transformer become connected and the output becomes live?

Say I have a 240VAC to 24VAC transformer. Is there any chance the output can reach 240V if the transformer overheats or breaks down in any way?

  • 5
    \$\begingroup\$ In just the same was as 11 kV wiring on top of poles can fall to ground level (when the pole it's mounted on catches fire and breaks) and kill someone. Yes, bad things can happen if you take things to extremes. A meteor could destroy the earth. Things can always malfunction but, we rely on a back-up plan. \$\endgroup\$
    – Andy aka
    Jan 14, 2022 at 14:14
  • \$\begingroup\$ Before reading more about transformers, I always thought they were made of 2 coils of wire on an insulating core, but then I found out that the core is usually metallic. So this is the main reason I was asking, because if the coils are separated by enough distance of air, everything should be fine, but if the core touches a live wire, not sure what would happen in that case, hence my question. People seem to think it was a dumb question, but I couldn't find an answer to it online, so I asked... \$\endgroup\$
    – Alex
    Jan 14, 2022 at 14:26
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    \$\begingroup\$ Anything can happen... usually these are designed so that chance is very small. Getting a transformer wet however, will greatly increase that chance. \$\endgroup\$
    – rdtsc
    Jan 14, 2022 at 14:35
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    \$\begingroup\$ @marcelm I would say (a) because of lack of research effort (the main reason if you hover over the downvote button), (b) the naïve description of what a transformer is, (c) because the question is open-ended i.e. is there any chance of x happening is certainly answerable with "yes" and (d) no consideration of safety systems adopted all over the world that prevent danger (again so easy to find in research). It's a poor question. \$\endgroup\$
    – Andy aka
    Jan 14, 2022 at 18:32
  • \$\begingroup\$ Commercially produced step down transformers are usually well protected against that sort of thing, with a pretty substantial plastic sheath between the coils and the core. \$\endgroup\$ Jan 15, 2022 at 19:52

6 Answers 6


Yes, this can be a real problem. And I have seen it happen ... on a 1950's amplifier that had been stored in an attic for years ... possibly slightly damp. Admittedly that was a step UP transformer, apart from the heater windings...

Some transformers have an "inter-winding screen" which you connect to ground by a good solid connection. Apart from reducing electrical noise, if the insulation on the 240V winding does break down, it connects to that ground connection and harmlessly blows a fuse or trips a breaker instead of making your circuit live.

(At least that's the intent : use 3-pin mains leads, make sure you use the correct fuse, and test your breakers at the recommended interval!)

Others are "double insulated" so that if one layer of protection breaks down, the other still provides protection. This may involve the primary and secondary windings being on separate bobbins, for example (in an old C-core transformer) or separate sections on a split bobbin. Such transformers may have the double insulation symbol (box in a box) on the label.

Yet others can be used in a double insulated system where another barrier (such as a plastic enclosure) ensures that even if the secondary becomes live, there is no way to touch the circuitry (such as USB connectors) without tool use to open the box.

  • \$\begingroup\$ Thanks so much for your answer. I was genuinely wondering if their design allows for such failures or not and you really helped. \$\endgroup\$
    – Alex
    Jan 14, 2022 at 14:57
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    \$\begingroup\$ For any specific transformer, you'll need to read its spec/datasheet : this gives a start on what you're looking for. But you are absolutely right to ask the question. The other answers speculating that it rarely happens reflect the success of designs to prevent those failures. \$\endgroup\$
    – user16324
    Jan 14, 2022 at 14:59
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    \$\begingroup\$ This answer is talking about isolation transformers, but other types of transformer, like autotransformers, cannot have the primary and secondary isolated, with a ground screen or otherwise, because they share a single winding. \$\endgroup\$
    – J...
    Jan 15, 2022 at 12:16
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    \$\begingroup\$ I've seen this happen twice on audio equipment - two supposedly isolated 12v windings shorted together in an amp. and a primary-secondary short in an electrostatic speaker's supply transformer which raised our isolated workbench to 5kV! \$\endgroup\$
    – grahamj42
    Jan 15, 2022 at 17:11

In general, of course it is possible.

But it depends a lot on the construction of the transformer how likely it is.

Sometimes the primary and secondary windings are separate sections so there is very small risk, but sometimes the primary and secondary windings are wound on top of each other, with some form of insulation between them, so there might be a slightly larger risk.


Couldn't it ever malfunction in such a way (wire melts or something) that the 2 sides of the transformer become connected and the output becomes live?

Yes that is possible. However, I have never come across a transformer in which that has happened. I don't know what the statistics are for such events, but a better question might be about such statistics. "Possibility" leaves open a very wide range of probability.


Yes, it can happen. But if you keep it in the operating parameters of the transformer, the design should be guaranteed not to happen, the violating one of these two specifications that would probably cause a problem are:

  • Operating temperature
  • Insulation Breakdown or Dielectric Strength of insulation usually listed in kV

If you exceed one of these specs (many transformer cores are saturate so it's unlikely that you could run too much power and heat up the transformer beyond it's ratings, but one should also follow the power\voltage\current ratings), you run the risk of compromising the insulation and shorting the coils together.

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    \$\begingroup\$ The transformer may simply be of low quality. This is how things break even within their specs. \$\endgroup\$
    – fraxinus
    Jan 15, 2022 at 10:23
  • \$\begingroup\$ Good manufactures actually do testing before the products are shipped to customers to make sure they perform to spec. If the transformer has been tested and certified by an NRTL, the NRTLs also perform audits to make sure the device is within spec \$\endgroup\$
    – Voltage Spike
    Jan 15, 2022 at 16:25

Can step down transformers malfunction in such a way that they become live?

Yes. This is possible if insulation between primary and secondary windings fails.

The probability is small, but it isn't zero. The industry recognizes this. There are transformers with double insulation and reinforced insulation. They can handle an insulation fault (a single fault condition) without primary voltage appearing on the secondary side.


I find all these "yes" answers to be a bit suspect. I spent over three years working in a plant which manufactured transformers. These were large units, the sort of thing that gets installed in power plants and substations. My job was to test them, and I expect that I worked on well over a thousand of them during that time. While I saw transformers fail in all sorts of ways in my testing, and we got back a few units which had failed while in operation, I never saw or heard of a transformer which failed in this manner.

Can transformers fail while in operation? Yes, absolutely. Is it possible for one to fail the way you described? While I wouldn't give a definite "no" answer here, I will say that I strongly doubt it. While it is common to talk about a transformer based on the primary/secondary voltages, it is really the current flowing through these windings, along with the number of turns in each winding, which makes the device operate. While such an insulation failure may lead to an increased current flow in part of a winding, it would change the path of that current such that it no longer went through an entire winding. This would most likely lead to a smaller magnetic field being generated (since we're now passing current through fewer turns) and a lower secondary voltage, not a higher one. In such a scenario I would also expect your 240V winding to fail, because now you're likely drawing more current through it than the conductors can handle. After all, all wires are fuses. It's just a question of how much current you need before that fuse blows.

  • 1
    \$\begingroup\$ And yet transformers do fail that way in real life. Manufacturers' tests don't always reflect the stresses that real devices get subjected to. Aging is a significant factor: did you test transformers that were 50 years old? Do you suppose that every transformer is always operated within its maximum rating? \$\endgroup\$
    – John Doty
    Jan 16, 2022 at 19:12

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