For all the obvious advantages of a WON PRO*HEAT , Titanium tube type
heater, I've always had the concern of water breaching the seals and
allowing some very undesirable electrical conduction from either
electrical "leg" through to the water, or within the heater. I have
designed a circuit to check conduction from the heater to either leg
in rectified combination, charging a capacitance, over a timed
interval. I use a thoroughly cleaned stainless steel fork protruding
for about ¾ of the length into the water at the opposite end from the
WON PRO. I find that a 470 microfarad capacitor will gain
approximately ¼ volt for every 1,000 seconds in this special "full
wave" charging from either or both legs mode, which equates to about
0.119 microA.; a very small current indeed, which I attribute to
heater to water capacitance rather than an insulation breach. The WON
PRO has been in service for 6 years and has performed flawlessly.
PS - it's a 45 gallon fresh water aquarium where NaHCO3 (for acid
control) and other dissolved salts (in spite of regular partial water
changes) no doubt provide sufficient conduction for that small test
current.