[chx]

Your MacBook then is prepared for ungrounded operations. Now take a laser printer, it uses very high voltage on the inside and if something gets dislodged and touches an outer facing metal part and you touch that in turn, you are dead. That's why the outer parts are connected to a safety ground. Do note Xerox warns you not to use a "cheater plug" http://download.support.xerox.com/pub/docs/4400/userdocs/any... . Laser printers are also a great example of when that C7/C8 connection is a fire hazard because laser printers need a lot of power when heating up their fusers. This is also why you must not connect a laser printer to a UPS.

[spoiler]

It's also possible that op has a socket like this, so the grounding is on the side: http://i.imgur.com/lLM3gQO.png

However, there is also this: http://i.imgur.com/ftPT7sC.png, which grounds from either sides or in the middle for sockets like these http://i.imgur.com/FnXMndC.png

[aianus]

No, I have American sockets.

The charger looks like this: https://support.apple.com/library/content/dam/edam/applecare...

And also comes with a longer, three-prong cable that replaces the little two-prong piece: http://www.ebay.ca/itm/like/172352962036?lpid=116&chn=ps

[sveiss]

The MacBook chargers are designed to be safe without grounding.

The grounding pin does have a use, though: some cheap powered USB devices will put out a voltage relative to ground on their '0v' lines. If you touch a MacBook that's plugged into one of these devices, you can get a small shock. If you're using a 3-pin adapter, this doesn't seem to happen.