I have a detached garage that is powered by a circuit breaker in the main house’s breaker box, which is properly grounded. The outlets in the garage have metal junction boxes, fed by metal conduit containing two wires (hot and neutral). There is almost no voltage difference between the neutral wire and the metal box; how can I tell if the box is truly grounded, and not just tied to a common wire somewhere inside the wall?
How to tell whether metal junction box is properly grounded
conduitgroundingneutraltesting
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You piqued my interest and I have a screwdriver.
Here's how a professional electrician in the UK made joins in wiring in my office building constructed 2001 (so as per regulations at that date).
Not all junctions contain joins. Some do.
So yes, use terminal strips (blocks) like these - all DIY stores have them, some ironmongers and larger supermarkets too. Make sure they are rated for the current they'll carry.
As you know, Part P of current UK building regulations apply to DIY work on wiring connected to mains supply.
First off, why is the ground #6? Seems big unless the circuit is over 60A.
Personally, as Tester commented, I'd use a double lug screwed to the box via a machine thread screw, that is if you can find one small enough. The one Tester linked is quite large for the application. Smaller ones might be hard to find though. This one is the smallest I could find: http://www.cesco.com/b2c/product/Burndy-K2A25U-Two-Conductor-Terminal-Lug/159535
Another alternative is to splice the two grounds to a third tail and connect that to a single small lug screwed to the box. http://www.cesco.com/b2c/product/Blackburn-Elastimold-L70-Type-L-Slot-Screw/3978
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Best Answer
Properly assembled metal conduit is an approved grounding conductor.
If the conduit feeding the garage from the house is metallic and connected to the box at the garage, to which the conduits are attached, it's grounded. If there's a proper 4-wire feed (or 3 wires and metallic conduit) the neutral at the garage should be isolated (visibly insulated from the breaker box) and the box itself should be grounded, either to a grounding wire or to conduit (and also to local grounding rods, or concrete encased electrodes.)