The fact is that people have been shocked, injured, and even killed by power tools and other things that use line power. This does not happen with a new item that is in proper working condition; no defects. But things do not stay perfect forever. I have seen power tools that have underwent sever abuse and people were still trying to use them. That's human nature.
So the safety guys have come up with the idea that more than one thing should have to go wrong before someone can be hurt by them. I guess the hope is that by the time that there are two or more safety related problems, the tool will have stopped working and the user will have replaced it or had it PROPERLY repaired.
The first way that was done, with tools that had metal cases was to first have the line Voltage insulated from that case and second to have the ground wire that would trip the circuit breaker if that insulation failed. So, in order to get a shock both the insulation and the ground wire would have to fail.
Adapters that had a green lug that would convert a two prong outlet to a three prong one were made available but that green lug had to be SCREWED down with the screw that held the outlet's cover in place. That provided the ground connection and the safety that it provided. Of, course no one ever did that (myself included) but it got you by the code and the safety guys.
Then tools became lighter by using plastic cases, which were non conductive. That removed one of the two things that had to go bad in order for someone to be hurt. There were still metal parts, many of which were accessible from outside of that plastic case so you could get a shock if one of them became energized by an internal failure. And it was not always ECONOMICALLY possible to connect all of them to that ground wire. Besides, the ground wire cost money - look up the price of copper. So the engineers found another way to ensure the idea of TWO failures before harm could come to the user. Yes, there was the original insulation on all the wires and other components that the current flowed through. But the outer, plastic shell was designed so that there were no metal, conductive parts that were in contact with any of the internal metal parts. Now both the original insulation AND the outer protective, insulative shell must both fail before the user can be shocked.
The math of this works through multiplication. If the original tool had a one in a thousand (0.001) chance of harming the user and the second protection method had that same level of protection, then when they are combined those two figures are multiplied to get one in a million (0.001 x 0.001 = 0.000001).
By grinding the key off his tools, I assume you mean that your brother removes the ground terminal from the plugs of the three wire tools. He is removing ONE of the two things that must fail. He has vastly increased his chances of injury as most tools will suffer some kind of failure in their life. This is not a wise thing to do. But again, many have done this. I definitely do not recommend it.
Or perhaps he is making the wider of the two prongs a bit narrower so it will fit either way. Electric devices are designed with a definite idea of which wire is hot and which one is the neutral and therefore not hot. If the device has a fuse or a switch, it will be in the hot leg, not the neutral one. If it has stray current protection, like a GFI outlet, it will disconnect the leg that is the designated hot one. There would be little protection gained from disconnecting the neutral side as the leakage current would still remain. Other protection schemes may exist within a tool. Neither you, he, nor I may know the exact way in which this protection functions, but by allowing the two wires to be reversed and possibly removing it he has made things more dangerous. Again this is not a good idea and I definitely do not recommend it.
Many thousands of deaths have been prevented by this, double failure/double protection idea, dare I say tens or even hundreds of thousands. It is an idea that actually works, that actually saves lives and injury.
Several of my newer tools have polarized two-prong plugs. Is this necessary? When researching this, the common scenario I found is someone unscrewing a light bulb and touching the metal threaded part which is now hot. Ok, I can see that, but with power tools, no part of the circuit (AFAIK
) is exposed that can be touched.
My brother is a professional carpenter and grinds the keys off all his tools, but he tends to play fast and loose with safety and isn't the best resource for information on this.