I use both Hulda Clark Zapper and DR. Beck Blood cleaner, I have not ever got a skin burn with Zapper even when the electrodes were not reversed for hours on end.
However on Dr. Beck design if I over soak the cottons I use on the titanium electrodes that I use, then I suffer burns (applying little coconut oil and they are gone in two days time - interestingly using Beck device in other parts of of my body speeds the healing of these burns - I use CS internally perhaps that is contributing factor as well) Again I use Beck device 4 to 6 hours at a time, salt solution could be as irritating to skin as as the current flow - polarity reverses on this design, so this burn is not related to electrolysis just my laziness to fallow instructions (I use this device while working thus I forget about it now and then, that is why I tend to soak it bit too much sometimes, changing location of contact after 1 hour and controlling moisture will solve this problem).
(I clean my electrodes and cottons in clear vodka overnight they become whities again)