Does not have to be copper. I've done several of these using galvanized steel hammered into the ground. We'd hammer in rods of 2 meters length until the resista

Does not have to be copper. I've done several of these using galvanized steel hammered into the ground. We'd hammer in rods of 2 meters length until the resistance was low enough (around 1600 ohms Iirc). This would ensure hpfi tripped below 50mA