Earthing: Is it important for laptops

Solution 1:

Your laptop is almost certainly of a "class 2" design. Meaning it is electrically double insulated. You can Identify this by checking for a symbol which looks like a square, within a slightly bigger square.

Appliance classes

Alternatively, a European GS or VDE standard may have been attached, covering class 2 implicitly.

This covers all Class 2 appliances in europe, and means that the appliance does not need a circuit protective conductor (Green/Yellow earth wire) in order to be used safely.

You will find this on your transformer block between the mains plug and the laptop DC jack. Meaning that both parts are protected from "Direct contact" fault conditions. Now known in Britain as "Basic Protection", as of IET Wiring Regulations 17th edition (2008).

You may find that the earth pin in your plug is there only to facilitate the interface with the socket, and is not actually wired to anything. The cable running from it may also be of a 2-core double insulated design (Phase and Neutral only).

Your laptop is safe. But the building itself is another story entirely. I suggest you avoid having a shower.

I would like to add a comment to Daniel Beck's conversation in the question comments, but do not have sufficient privillages.

The voltage you have measured is the result of a reference point differential, and is unlikely to be capable of carrying a current. This is why the apparent voltage would disappear if you connected the extraneous conductive part of your mac book to the local earth (A grounded reference point).

The metal case of your mac book is separated by two layers of insulating material from any electrical parts by design.

You will also find the Class 2 logo somewhere on your macbook's transformer block.

Solution 2:

The ground or earth is there primarily to protect you from electrocution.

So long as your laptop and charger are in good condition, are of double-insulated design, and are used safely (not in the bathtub or shower) you and they will come to no harm.

If the equipment is designed with the expectation that it be earthed, by connecting it to a non-earthed supply you are running a small additional risk that a fault or damage in the equipment may lead, in the worst case, to something exploding in your face or catching fire while you are asleep. These are the sort of risks of death and injury we take every day when crossing the road.