Your getting a larger shock while holding the probe because the whole point of the probe is to give the stray voltage a better path to ground. So essentially when your holding it and it's plugged in to the wall you've made yourself part of the grounding circuit.
Mount it in the sump and then plug it in. Also find the stray voltage culprit and eliminate it ASAP. Even with a ground probe stray voltage can harm a system, its more of a precautionary measure
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