I have a bunch of salvaged step-down mains power transformers (220V-240V primary voltage at 50Hz), with powers ranging from a few VA to maybe 100VA. In order to use them safely in my projects I need to specify an appropriate fuse for the primary side.

After reading this article I became aware that it might not be so simple as I used to do, i.e. use a (slow-blow) fuse rated for a current which is about 2 times the normal primary operating current.

It is apparent that the knowledge of the primary current with the secondary shorted is necessary to make a fuse selection that grants reliable operations. To determine that current, in that article they show a method which requires the measurement of the primary impedance using a variac and an isolation transformer.

Since I don't have a variac I thought whether a simpler measurement could be done, i.e. simply measuring the primary current while the transformer is connected to mains and the secondary is shorted for a very short time, say 3-4 seconds, just enough to make a reading with a TRMS DMM measuring the primary current.

I understand that the value I would get could be higher than what I could get in normal operating conditions, due to the primary resistance increasing with transformer temperature. However I thought this would be a reasonable ballpark value that could be derated with a safety factor to account for the increased resistance of the warm transformer.

So, my main concern is whether or not the transformer could handle this brief overload without damage and, if the answer is affirmative, whether there are further caveats for the procedure.


1 Answer 1


In order to use them safely in my projects I need to specify an appropriate fuse for the primary side.

A fuse in this situation, is mainly used to prevent damage to the up-stream (and local) voltage supply infra-structure (feed wires etc.) and not, to protect the transformer. You should rate the fuse to avoid your incoming feed wires becoming burnt and possibly causing a fire. If you are concerned about output current overloads then, it seems more appropriate to me, to utilize a fuse on the secondary (in order to protect the transformer from the down-stream abuse).

If the transformers are not capable of working safely and reliably on a mains power system then, you should choose a transformer that is.

  • 1
    \$\begingroup\$ Yep. I know mains-side fuses are used to protect the infrastructure and avoid fires, etc. I didn't say anywhere I wanted to protect the transformer with that fuse, I said I want to use them safely, i.e. without them burning the house down if a gross overload happens in the circuit they provide power to. Once this part of the design is taken care of, I can of course add further protection circuitry/elements to protect the transformer or the downstream circuitry. \$\endgroup\$ Commented Jan 23, 2023 at 12:10
  • \$\begingroup\$ A transformer should be designed to work safely after all, a lot of safety issues depend on it (such as primary-secondary isolation). If you are trying to reuse salvaged transformers, you might be taking too big of a risk IMHO @LorenzoDonatisupportUkraine --> shorting the secondary is basically testing how well the transformer might survive a down stream situation it seems. \$\endgroup\$
    – Andy aka
    Commented Jan 23, 2023 at 12:14
  • \$\begingroup\$ @Andyaka - a friend told me of an experiment he observed. The idea was to test a high-speed high-current shorting switch for some industrial purpose he didn't specify. A transformer was fed with 132kV , via a circuit breaker, and the 11 kv secondaries were simultaneously shorted, very effectively and quickly, by the device under test. The circuit breaker tripped, but also the transformer, weighing over 1 ton, somersaulted through the roof of the temporary portable building containing it and landed on the ground outside. No-one had thought of bolting it down. \$\endgroup\$ Commented Jan 23, 2023 at 15:48
  • \$\begingroup\$ @MichaelHarvey Impressive! But of course I wouldn't try anything risky with such a power level things. \$\endgroup\$ Commented Jan 23, 2023 at 15:55
  • \$\begingroup\$ @Andyaka The transformers I'm dealing with are quite low-power and were all salvaged from equipment in good state (that is, no junkyard fishing for trafos): they were either obsoleted equipment or equipment whose repair was not economically viable (and the power supply still worked ok). So I could say they are in good shape (anything crappy I would have scrapped rightaway). \$\endgroup\$ Commented Jan 23, 2023 at 16:02

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