cant remember where the idea came from but i made a grounding "probe" last week that is working a treat! basically it comprises the carbon core from a D size battery (removed and well cleaned) which i drilled a small hole into one end to accomodate the end of an earth wire. this was siliconed into place and a plug put on the other end. Hey presto! when placed into the sump the residual charge was gone (could feel it now and again when doing maintanance).