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In your experience (and in the literature), how many years is the lifetime of typical step-down autotransformer before the primary winding shorts and the full voltage of the primary gets to the secondary?

I'm concerned about this because the secondary is connected to an important equipment that shouldn't encounter the full voltage of the primary.

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    If the size of the transformer excedes the load value so it is not over heating it should last forever, if undersized and overheating it will fail sooner than forever. In the U.S. homes are connected to the grid by use of step down transformers and with ~100M homes connected you almost never hear about transformer failures. Mechanical damage car hitting a pole , a flood putting the transformer under water or lighting strikes are the main causes for failure but even these are somewhat rare. – Ed Beal Oct 17 '18 at 16:48
  • This guy is a scammer always new objections with no knowledge not 1 up vote for all that have tried to help enough - votes maybe he will go away! – Ed Beal Nov 22 '18 at 2:53
  • Because when I first joined.. I couldn't up vote... beginners at stack exchange can't up vote.. it's only now that I'm aware I can up vote already... and I just upvoted the answers by Ed and others. And I don't know comments should be upvoted too. – Samzun Nov 22 '18 at 2:58
  • No you can always up vote a question you asked you consistently challange at least 3 professionals on this site , with nothing other than time wasting questions I and others have spent a lot of time trying to help so i finally decided to let others know. – Ed Beal Nov 22 '18 at 3:06
  • When I tried to upvote last week. I couldn't because new users need to have certain reputations first before they can upvote. Try asking the system owner. I'm sorry. Can't you forgive me and give me one more chance? – Samzun Nov 22 '18 at 3:09
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The lifetime of an autotransformer is indefinite, and does not necessarily end with a short circuit. Barring a transient on the mains, water leak or some other external event, there is no reason it should fail. However, if the low-voltage part of the winding were to open (as well as if the high side were to short), you'd also be faced with full line voltage across it! If safety is paramount, use a transformer with isolated secondary -- the failure mode is almost always zero-voltage output, rather than full line voltage across it (though it is possible that extreme overheating or over-voltage could burn through insulation).

Another advantage of an ordinary transformer is allowing a ground to be placed anywhere, avoiding likelihood of a ground-loop issue and reducing chance of electric shock.

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There is no typical life of an autotransformer because of wild variations in build quality (if you want reliable, don't buy cheap junk) and usage factor (running the unit near limits < low loading).

Shorting of the 240V primary and supplying 240V to the 120V tap is not a main failure mode. I'm not sure where you got that, except I imagine it is your worst case scenario.

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