Maybe some are. But I don';t think all are. There was a common wall wart sold over here, maybe it still is. It has 2 slide switches on it, one sets the (unregualted) output voltage (a tap changer o nthe transfoemer), the other reverses the polarity of the output. The output conenctor is a cross-shaped thing with the arms of the cross being 2.5mm jack plug, 3.5mm jack plug, 2.1mm (i/D) coaxial power connecotr and 2.5mm ditto. Hanging off that on shrt leads are a 1.3mm coaxial power conencotr and a PP3-type (9V batter) chip. I had oen of thes plugged in on my bench. The exposed metal contacts of the PP3 clip found one of the arm of the cross-shaped conenctor. The result was that the output was deat short. I noticed this some time later ehrn it was hot enough to smoke and softne the plastic case sufficiently to expose live parts. Of course I disconencted it for the mains at once and after it had cooled I invetigated it. The PCB was charred, the plastic sleeve on the smoothing capacitor had melted and split, but the transformer was still continuous so presumably would have carried on heating. You may think that's a safe design, I do not. -tony