Just because a PN has a 3 prong plug, does not mean it is for wet pick up. A Power head should not be used for wet pick unless specifically made for wet pick up. For example Rinse N Vac carpet cleaning systems, or the old kenmore and bissell big green power teams.
But if you want to try and use a 3 prong power head for wet pickup please Record it and post it to YouTube.
The older D2 rainbow (eureka) heads were grounded due to the PN motor being all metal and The rainbow housing being all metal itself. When Rainbow went to the D3 model some of the very early power heads (rexair) were 3 prong and then they switched to the 2 prong.