Stolen from Macintouch
here:
"Posts about Powerbook leakage current were extremely frequent on Apple Discussion boards last summer, and I posted this summary back then. It has since disappeared from the discussions, which is annoying, since the leakage current issue seems perennial. Apple's present policy seems to be to keep less than two month's worth of postings.
Disclaimer: I am an electrical engineer with experience in product safety testing. I have successfully designed medical equipment, tested it, and gotten a CE mark. But... the information provided here is meant to be background material on product safety, and should not be construed to be advice for your particular situation. In particular, if you have the least suspicion your equipment is malfunctioning or unsafe, I advise you to take it to an authorized service dealer for testing or repair. I also explicitly advise you against making your own measurements unless you have been specifically trained to do so - there are lethal voltages present, and you could be injured or killed.
Most office computing equipment falls under a variant of IEC 950. UL has their version 1950, European countries have 60950. You can find some information on the spec at this link.
The issue that is being discussed in most of these posts is leakage current (note, not static electricity). Leakage current, or touch current as it is sometimes referred to, is the result of capacitance between the primary side (house wiring side) of the power supply and the low voltage output side (the part of the computer accessible to your hands on the case). This capacitance is mostly in the transformer, but may also be affected by the filtering that is usually included to! keep transients from going into or out of the power supply. Both of these functions are within the small white square power supply. The transformer has insulation in it, which is what provides the basic safety that keeps the high voltage on the high voltage side of the power supply only, but there is unavoidably capacitance, too. Note that the neutral prong of the power supply is isolated from the low-voltage side of the power supply just as the "hot" prong is, so reversing the plug in an outlet is not going to affect anything (some posts had theorized a prong polarity issue).
It is the nature of capacitors that when the voltage wiggles on one side of the capacitor (say, the high voltage side) some amount of wiggle is transferrred to the low voltage side (where your hands are). That's what capacitance is about - transmitting AC current.
Leakage is a current, so it doesn't make sense to try and measure a voltage from case to ground. The result won't be meaningful. Instead, you measure an AC current, which is going to be small. The 950 spec says that for double-insulated, Class II devices, which the TiBook should fall into with its two prong power supply, the limit is 0.250 milliamp. Can you feel 0.250 milliamp? Well, it depends. Anything under a milliamp of AC current is generally regarded as being hard to detect by feel. The impedance of the human body varies greatly from individual to individual, and also due to conditions like sweat, humidity, etc. From the quantity of the postings on the Apple discussion boards it is likely that some units were out of spec, but the only way to know for certain that a given unit is out of spec would be to use a reasonably sensitive multimeter to measure the actual current (leakage current is not always tested in production for non-medical devices, unlike hi-pot testing). ! For an abbreviated summary of how to perform the test, consult this URL, also from i-spec.com.
Again, I don't recommend doing this test yourself unless you have been trained.
For more backgound, here is a link to download a PDF from a power supply manufacturer: [An113.pdf]
I have occasionally noticed a tingle on my Powerbook 550 DVD, circa February 2002. I measured the leakage current, and found it to be just a little less than 50 microamps, or well below the spec. Yet I can sometimes still feel it. You really can't judge safety by feel, to be certain you have to have it measured."
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