1 {scene: night, pouring rain, Loch Ness}
1 Jamie: You know it’s dangerous to be on open water in a thunderstorm?
1 Adam: Sounds like a myth to me.
2 Adam: Besides, I brought a suit of armour to wear. It’ll act as a Faraday cage and protect me from lightning!
3 SFX: Kra-booom!! {lightning sunders the sky above them}
4 Jamie: I think that was Thor rolling over in his grave... Or maybe Michael Faraday...

The Mythbusters have played around with Faraday cages before, in their submarine below the surface of Loch Ness.

In theory, fully surrounding yourself with a conductive metal shields you from the effects of electric fields, as electrons in the metal redistribute themselves under the influence of any external field in such a way that it cancels the field inside. If an electrical current passes through a Faraday cage, it will take the path of least electrical resistance, which is through the metal rather than through any less conductive material inside (such as a human body). So in principle wearing a suit of armour could potentially protect you from electrical current if you were to be hit by lightning.

How effective would this be in practice? Many people have asked this question, and the Internet has several discussions of it: StackExchange: Physics, reddit: Ask Science, Quora, Strange History, and StackExchange: Worldbuilding for a fantasy inspired version.

There doesn't seem to be any consensus. Some science sites go so far as to say that if you're caught out in a lightning storm, the safest thing you can wear would be a suit of metal armour. Others point out that while it might sound good in theory, in practice a suit of armour has gaps in the metal coverage which are typically padded with non-conductive cloth, meaning the electric current doesn't have a complete conducting path to follow, and may well divert through your body anyway. And then there are other effects, such as the intense heating caused by the lightning strike, which can reach 30000 kelvins. Touching a piece of metal at that temperature probably isn't a good idea. Then there's the shock wave, which can throw a person bodily through the air, and produce blunt force trauma. Any other items in the immediate area may be blown apart and converted into flying shrapnel (although I suppose metal armour is better than many things for shielding you from flying chunks of wood or rock). And finally there's the elephant in the room: If you're wearing metal armour you're much more likely to be hit by lightning, as the electrical energy will find you a big juicy conductive thing pointing into the sky - i.e. a lightning rod.

Another thing which I haven't seen mentioned is the Lorentz force induced in a conductor carrying a high current, which produces the pinch effect. Basically, if an electrical current is flowing in a conductor, it produces a magnetic field surrounding the conductor. If two conductors are parallel, these magnetic fields interact to produce an attractive force between the conductors. Now imagine a bundle of conductors all carrying electric current in the same direction. These produce magnetic fields that pull all of the conductors together. The larger the current, the stronger the force.

Now imagine a really strong current - such as produced by a lightning bolt - passing along a hollow metal tube. The current around the perimeter of the tube produces magnetic fields that generate an inwards force, pulling the metal into the middle. When the current is large enough, the force can be great enough to physically deform the metal tube. Here's a photo of a piece of copper tube that was used to ground a lightning rod on a building, after the rod had been struck by lightning:

[Image is Creative Commons Attribution-Share Alike 3.0 Unported by Brian James, University of Sydney, from Wikimedia Commons.]

The high current of the lightning strike produced enough magnetic force to completely crush the tube. Now tell me that you'd like to be inside a suit of metal when you're hit by lightning...

Interestingly, the idea of turning yourself into a personal lightning rod inspired a brief fashion trend in late 18th-century Europe. But all things considered, I'd rather prefer not to be wearing a suit of metal armour or any other sort of lightning rod if caught outside in a thunderstorm, thank you very much.

But I will say that the one thing missing from all of the discussion found on the Internet is a complete and utter lack of experimentation. Why is nobody going out and testing this, so we can all know the real answer? If ever there was a job for the real world MythBusters, this is surely it.