I built a lake home for some clients and later installed an
underground drop to a pedestal with a 240 volt disconnect
(exactly like an AC disconnect). From there, the people who
built the dock tied into the disconnect to run wire out to the
Then the rains came. The lake rose almost 35 feet in a matter
of weeks. More rain. The lake rose to flood stage and
submerged the pedestal. Yet power remained to the dock. The
pedestal was under about 15' of water, yet did not trip the
breaker at the main service. The owner called me and asked if
this was dangerous and I said yes, turn off the breaker at the
breaker box, which he did.
My question is: why did the breaker not trip when the
disconnect was submerged? It is not waterproof by any means.
Probably the same reason that an open motor column sump pump of mine has
several times been flooded totally underwater and still started and pumped
out the water when power was restored. While fresh water will conduct
electricity, it doesn't do it well and would prefer to travel through our
bodies if they were in the water close to the source.
Had a friend with a very old stone wall foundation, basement, dirt floor.
Panel box in basement. Bad flooding. Basement totally filled. Power still
on in house!
Check this out though. Calls power company. Fills them in. Wanna know what
they told him? Are ya ready? That's right, "Go turn off the main
Yes, grounding has nothing to do with this. Have you ever seen a
simple vaporizer that you use when you have a chest cold? If you
take it apart, it's nothing more than two electrodes, about 1/2"
apart, connected straight to the 120V cord. That obviously doens't
trip the breaker either, because the current flow is not anywhere near
the limit. But it is enough flow to vaporize the water. And in the
case of the lake, I'm sure some current was flowing as well. Exactly
how much would depend on how much uninsulated metal was submerged, how
conductive the water was, etc.
Water isn't a terribly good conductor. There was probably enough
leakage to ground to trip a GFCI breaker, if you had installed one. But
there wasn't the 15+ amps of leakage (either to ground or to neutral)
required to trip a regular 15 A breaker.
Rain water is actually close to distilled water, which is a very poor
conductor. Pure enough water can be treated as an insulator.
Not absolutely pure, but the degree of impurity determines its
conductivity (more or less). The water molecules themselves don't
conduct worth a darn, but other substances dissolved in it can increase
conductance greatly. Well water usually has high dissolved solids,
while rain water has almost none, and lake water tends to be somewhere
in between. In this case, if the lake rose tens of feet in a short
time due to rain, the water was likely less conductive than usual.
In addition, there was likely less than a square inch of "hot" conductor
exposed to the water, and current flow across this interface will be
proportional to area.
Less than usual, sure, but probably not very much less: the lake didn't rise
tens of feet solely from rainwater falling directly into the lake. (That
would have been one hell of a rainfall, no?) The rise had to be due almost
entirely to water falling onto land in the lake's drainage basin, and hence
the water entering the lake was mostly land runoff -- and couldn't possibly
have been pure rainwater.
Doug Miller (alphageek at milmac dot com)
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