Inspired by thread such as this http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=231557
Why are 3-pin supplies, giving a safe grounding if anything at all comes loose in your computer/tv/etc, not ubiquitous?
Because we learned as we went along with house wiring.
Learning as we go is still evident when you look at the changes that happen every three years in the National Electric Code. The requirements adapt over time to reflect improvements in technology (such as “double insulated” appliances and tools) and people’s increasing consumption. An example of that would be the fairly new requirement of a dedicated 20 amp circuit for outlets in a bathroom to handle high-powered hair dryers in new house construction.
WRT grounding everyting, the 3rd-wire ground is properly called a “frame ground” for the item. If there’s a wiring fault in it, current will ideally go to the ground, rather than the user, assuming the device has a metal case or frame - such as in power tools. With plastic-cased items, a frame ground isn’t possible, so the ground wire isn’t needed.
Sorry, I was assuming an international attitude…I never realised that 3rd-wire existed in the US apologies…and I guess the 20A req is only because of the voltage (the equivalent at the UK standard 240v 13A would be fine)?
Still I’m amazed that 3rd-wire/Earthing isn’t universal and required, given how obvious the safety advantages are
in many cases i just isn’t necessary. For instance your average clock radio does not have a 3 prong plug because with low power and non-metallic cases it is extremely unlikely that a dangerous charge would build up between the item and earth ground.
As was pointed out the 3rd or “ground” wire came about later after it became clear that such charges do build up in high power items particularly those that use the actual structure of the item as an electrical connection.
Looking around me, there’s five items (computer, monitor, synthesiser, printer, large clock radio) which all have metal cases, and (to be sold in this country must legally) have full earth connections.
The one socket left is my phone charger - double insulated, and therefore the earth pin is plastic (it has to be present to open the other holes on the socket, to stop a kid sticking something in there)
Is the fear of loose connections causing shorts when they’d otherwise kill us an irrational fear? Or is there evidence that 3-pins saves lives?
I’m not sure if there are any statistics on value of grounded systems. However, approximately 1000 people per year die in the US each year from shock or electrical fires, in spite of the widespread use of grounded systems. It would be a lot worse without grounded systems in common use. Electrocutions are less than 500 per year and it appears that not much over 100 of those are caused by plug in applicances.
The logic of why grounded systems are safer is so clear that it would be hard to question. It means that many types of electrical failures have a direct ground causing breakers to trip rather than creating an open circuit (shock hazard) or a high resistance ground (heat generating). The grounding system was created as a direct engineering solution to accidents.
Many parts of the world, especially in Asia and developing countries, do not use grounded systems. They have high rates of accidents and fires.
Here are a number of reports from the US Consumer Product Safety Commisssion on electrical safety.
Because, as is obvious from the thread you linked to, it is complicated and expensive to upgrade older homes. Other than that, new homes and new appliances *are * built with such protection as necessary.
Just a personal opinion (as OP of the linked thread):
The US often suffers from being first implementors of technology. This is seen in the electrical system (2 prong/3 prong mess, 120 VAC while the rest of the world is at 220), as well as cell phones (GSM versus all of our analog/CDMA/other mess) and TV (PAL versus NTSC). Since technology upgrade is expensive and since old, outmoded technology has already been widely implemented and works serviceably, it is grandfathered in. The rest of the world takes the pick of the best, and we are stuck with a number of standards.