Thanks for the replies Aussie and PaulUK. So the larger earth pin is to encourage dedicated circuits for higher power appliances. That makes sense.
I suppose in an "ideal world" all sockets in a house would be 15A with a dedicated 20A supply cable to each one and 15A RCCBO protection.
This is economically unfeasible and means the DB would occupy half the garage !
BTW. PaulUK, your Lions Should be arriving home any minute with their tails between their legs. And Aussie 240, the All Blacks will be arriving on your door-step soon. Good Luck !
