I have no idea why. But apple have not earthed the stubbly plug for the macbook pro transformer. So on the short lead the only way the charger is earthed is when I touch it or the macs unibody which gives you a sort of vibrating feeling on your palms.
When I went to the genius bar to complain about it they said that it was completely 'normal'. Where their definition of normal is that every MacBook does it. I have had 4 MacBooks and all have done it.
The genius did say that he though he has seen a few earthed 3pin plugs. So my question is where can I pick one up? or how could we force apple to earth the stubby plugs and recall them?
Best Answer
This is so commonly misunderstood that I shall attempt to provide a short précis, then link to a good explanation which is not overly-technical.
Common misconception - earthing/grounding
Most small household appliances are not, in fact, earthed at all - even if there is a valid earth pin on the mains plug.
They are what is known as 'double insulated' & carry this logo
The 'transformers' on this type of equipment are not the same as in high-power equipment & don't use a step-down coil transformer, linear supply. Instead they use a switch-mode supply.
All PSUs, whether linear or switch-mode leak a small amount of current due to capacitance. In double-insulated devices there is no true ground/earth so the design is optimised to try to prevent this leakage.
Unfortunately, switch-mode supplies run at very high frequencies, so the smaller capacitance in the smaller transformer ends up leaking just as much as the larger capacitance in a larger transformer running at 50 or at 60Hz for example.
The leakage current is usually less than 1mA and is often as low as 10uA, but even at such low current, it can still be present and still can cause problems.
So - the tingle or buzz that you feel is in fact you becoming the temporary ground/earth for this circuit.
It is usually safe, & any reputable manufacturer will ensure their PSUs are well within tolerance.
Cheap copies are cheap because they don't bother with most of the safety circuitry… you know, the stuff that makes the expensive one expensive in the first place ;-)
Have look at http://www.unitechelectronics.com/sparks.htm for a much more detailed explanation. Though it is aimed at hi-fi users & deals in 240v mains, the basic principles are identical.