This pops up and I think your correct a lot of diyers do not know or have appreciation for a lot of things like volt drop , length of cable runs type of wires used in differant applications. But it's not all diyers . I had a solar company install one of my systems and it was not done correct. But I was issued a lovely certificate to say it was safe. But anyway the list goes on. I my self just had my wrists slapped for taking my cover off my boiler. I had an electrical fault infact a thyristor had failed and all I wanted was the manufacture's readings so I could check it's values, but when I rang the boiler manufacture's they grilled me for just removing the cover. I did not touch anything gas just the electronics. Am I competent or not . Gas don't want anyone touching boilers unless they are trained , electrics in domestic the same. But what about working on your cars which everyone does. Should we now go down the route of not selling parts to the general public. A mechanic has spent years like gas and electricians getting qualified. I personally have seen some bad electrics not all done by a home owner a lot has been the electrician. I normally find a diyer who wants to say install a solar system and battery's will read up on it and normally are aware of issues. If we want to stop all these dangers then the goverment should put in law that only qualified electricians work on electrics, only qualified gas engineers work on gas and the same with mechanics and make it impossible to purchase parts or items for that trade. Outlaw it that will stop it. How many meter people are trained electricians, there not, but they have had training in changing a meter and handling cut out fuses,yet an electrician is not allowed to touch it. I have over the years witnessed 3 meter people not one followed the correct procedure to confirm dead. They did not even torque up the screws.