I have the following:
A USB phone charger that connects to a plastic project box that holds an arduino. From this I run 3 wires to a metal project box holding a couple of components (around 10 meters away). Those wires are 5V, GND and DATA. However with the last project box being metal, do I have to worry about earthing, or would it be enough to tie the GND to the metal box?
Thank you.
What's "USB EARTH"?
lg, couka
couka:
What's "USB EARTH"?lg, couka
Yes... Exactly.... Whoops my mistake! Ignore that part please... Shall correct the question.
Its not unusual to attach the GND pin to a metal case if that's what you are asking.
pwillard:
Its not unusual to attach the GND pin to a metal case if that's what you are asking.
Yes that's exactly what I'm asking. 
So just to be clear (double negatives aren't my strong point), I should attach the GND to the metal casing...
If the USB charger has only 2 mains prongs on it, you can't earth the metal case through it since the converter is transformer isolated. Attaching the metal case to either lead of the power supply will do nothing useful or harmful.
If I'm remembering my lessons on the NEC correctly, the reason metal cases are earthed is for safety when mains voltage goes into it. If the hot side shorts to the case, it'll trip the breaker instead of leaving the case hot. You're not putting mains voltage into the can, so there's nothing to worry about.
If the metal box only holds sensors or whatever purely powered over your length of wire then
don't connect ground to the case, that'll increase the chance of interference being injected
into the cabling from the box.
10m is a long way to route a high speed logic signal - is DATA a low speed thing?