Many "consumer electronics" devices use a plug-in DC power supply that is not earthed.

I bet many of them you could actually touch the barrel connector (especially if it's not plugged into the device.

Surely there's a potential (pun intended) risk that the plug-in converter develops a fault and puts mains voltage onto the output ....

Or am I missing something ?

 

Sounds like a possibility...unsure of the probability.

 

How many people have you heard of who were electrocuted?

 

Surely there's a potential (pun intended) risk that the plug-in converter develops a fault and puts mains voltage onto the output ....

If the isolation transformer in the power supply uses a split-bobbin (below) where the input and output windings are separated by the bobbin plastic, then the chance of that happening is very small.
Not sure though, if they all use that.

 

There is a risk, which is why UL and other certifying agencies have classes and types of insulation defined in their rules. Also, there are different ways of physically constructing a transformer and some of them pretty much guarantee that a transformer fault will not expose the output is the high voltages at the input. The world consumes billions of wall warts eacy hear, and the physics of making them safe are very well known.

Separate from that, not all warts are created equal. There are websites that tear apart clone products and compare them to factory originals. A classic is the little white cube charger that used to come with an iphone. The internal volume is so small that meeting safety certification rules was a significant design challenge. Some of the Chinese knock-offs are in fact dangerous.

ak

 

Make sure any DC supply you purchase is a reputable mains-isolated type. Some cheapo supplies, of dubious origin, may not be, or at least not adequately isolated.

 

No-brand cheap PSUs may suffer this risk

Buy good quality PSUs from reputable suppliers to minimise the risks

 

How many people have you heard of who were electrocuted?

Several in the news over the last few years, from fake Apple power supplies.
Any supply that meets the standards must withstand a 4kV test voltage from mains to the output and in terms of clearance that represents a 5mm gap. It's therefore highly unlikely something could fail and bridge the gap. The critical component is the transformer.

 

Make sure any DC supply you purchase is a reputable mains-isolated type. Some cheapo supplies, of dubious origin, may not be, or at least not adequately isolated.

Sometimes the AC/DC converter comes with the device you've bought, and you have no way of knowing its origin. It wouldn't surprise me if dubious manufacturers have put blatantly false labelling on them ....

 

If the isolation transformer in the power supply uses a split-bobbin (below) where the input and output windings are separated by the bobbin plastic, then the chance of that happening is very small.
Not sure though, if they all use that.

View attachment 291641

Back in the days of 50/60Hz plug-in power supplies they did, but modern switched-mode ones don't.
For a flyback converter the leakage inductances of a split bobbin arrangement are too large. They are still concentrically wound, with several layers of tape between the windings, and with margins on the secondary windings to meet the creepage and clearance distances.
For higher power supplies, the LLC converter uses a split bobbin because the leakage inductance is part of the resonant circuit

 

Sometimes the AC/DC converter comes with the device you've bought, and you have no way of knowing its origin. It wouldn't surprise me if dubious manufacturers have put blatantly false labelling on them ....

More often than not the manufacturer of the product you have bought didn't make the power supply, they bought them in. Sometimes they are tempted by the Chinese suppliers who say "Do you want one that meets the standards or would you prefer the cheaper one?"

 

Several in the news over the last few years, from fake Apple power supplies.
Any supply that meets the standards must withstand a 4kV test voltage from mains to the output and in terms of clearance that represents a 5mm gap. It's therefore highly unlikely something could fail and bridge the gap. The critical component is the transformer.

I've had several of the "wall-wart" supplies apart over the years, and been staggered by the quality of, or lack of, manufacturing and assembly. On one, the wire from the "Live" wall-pin to the (very cheap) PCB had come detached at the PCB end, and was just hanging there, in mid-air, ready to attack anything it might touch.

 

Many "consumer electronics" devices use a plug-in DC power supply that is not earthed.

Try and find any household appliance today that has a GND conductor, I did a brief summary of items here that included steam Iron, air fryer, toaster, three ver of hand mixers.
All had metal outer covers and NO GND conductor..
Many, such as hand tools should be double-insulated.
That was one of the things that struck me when I came to N.A. from the UK, is the amount of appliances with no GND conductor.

 

I've had several of the "wall-wart" supplies apart over the years, and been staggered by the quality of, or lack of, manufacturing and assembly. On one, the wire from the "Live" wall-pin to the (very cheap) PCB had come detached at the PCB end, and was just hanging there, in mid-air, ready to attack anything it might touch.

Again, the standards require it to be "secured" to the pcb so it can't get loose if the solder joint breaks. @MisterBill2 and I had a discussion about suitable ways to do this in this thread https://forum.allaboutcircuits.com/threads/how-to-be-a-good-pcb-design-engineer.192960/