when running ground into a metal box,is it enough to just pig tail the grounds and attach them to box,or should you also attach a ground to the green screw on the outlet also.....basically what i did was run an additional outlet in my basement,i pigtailed the grounds and attached them to the metal box,and also ran a short green wire fron the outlet screw into the pigtail.i was told there is no need to ground the outlet because the metal box is grounded....

I added a 15A outlet to my 60A garage sub panel dedicated to a table saw with a 2hp, 220 V motor. I am using the NEMA 6-15 plug on the motor cord and the corresonding outlet in the box and have a 15A, double pole breaker in the box. After running the 12 gauge wire and hooking up the outlet and breaker, I inserted the plug and got a spark from one of the two power prongs and the breaker tripped. Reset the breaker and got the same thing. Checked all the plug connections, outlet connections and breaker connections and everything seemed correct. Tried it again and tripped the breaker in the sub-panel and the house. Anyone have an idea what I am doing wrong? TIA