I'm looking for a list of ways that mains-powered ELV power supplies with touchable terminals can be made safe in the case of transformer failure with primary and secondary coils becoming connected within the transformer.

One method I'm aware of is a feedback control circuit that isolates the input to the transformer in the case of excessive voltage across the secondary winding, that in part uses an opto-isolator, as described here. However I'm not certain what failure conditions within the transformer that this method can/cannot protect against.

I'm primarily concerned with conformance with CE marking and other EU requirements but non-EU regulations and beyond would be a bonus. I'd like multiple options as I'd rather have overkill (no pun intended) than potentially mediocre protection. Cost is an issue but I'd like to know the options so appropriate optimisations can be made.

Last but not least, references to reputable sources are important.


To establish the ELV you will need isolation. This will probably be a transformer (Linear or Switchmode does not matter)

Transformers can be protected in a number of ways from faults...

  • Self Limiting (Internal resistance limits the available power)
  • The use of Triple Insulated wire (TIW) in the transformer construction.
  • Fuses in the primary or secondary depending on the requirements.
  • Thermal sensor in the transformer (Rare?)
  • Internal thermal fuses
  • Over voltage sensing (Most Switch Mode Power Supply IC do this anyway, it's often how they regulate).

Over voltage sensing can be via opto-isolators with zeners or similar .

In the end testing will probably be required unless you buy a pre-evaluated product.

This depends on your end application. There will be a safety aspects in those standards that relate to transformers and their faults. Look at what the requirements are and this determines your minimum effort/cost. Get your self access to those standards and read them.

The best way is to be taught by one of the more experienced engineers. Ask them to review a design with you. Trying to read standards the first time tends to drive you crazy with forward and backward references ... oh and the language used between Europe and the US can also make life interesting.

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    \$\begingroup\$ Good summary. To add one point re: transformers : you can buy (or have made to your specs) transformers where the primary and secondary windings are on separate partitions in the bobbin : therefore even if one winding loses all insulation and becomes a short circuit, it cannot contact the other. (Unless the bobbin melts or burns : some other protection should cut power before then!) \$\endgroup\$ – Brian Drummond May 11 '13 at 13:20
  • \$\begingroup\$ @Brian Drummond, +1, Good point on the custom transformers and separate partitions. From a Good electrical spacings point of view I prefer partitioned bobbins. But they are rarely required in consumer products. (Correct me if I'm wrong, I tend to deal with industrial related equipment) \$\endgroup\$ – Spoon May 13 '13 at 9:01
  • \$\begingroup\$ You can also get custom transformers with an inter winding screen that you connect to the PE conductor. Useful mainly because they massively reduce capacitive coupling between the windings, but it also makes promary insulation failure take the fuse with it, however it comes at a cost. A good quality transformer should never fail pri/sec short, but if you are feeling paranoid then connecting one leg of the output to PE will make a primary/secondary short a fuse blowing event. \$\endgroup\$ – Dan Mills Nov 20 '17 at 12:36

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