water heater shock

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getakey

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my boat has 2 water heaters plumbed in series.
They are stacked one on top of the other
replaced thermostat on lower unit due to corrosion
top unit is "pristine"
After replacement, I turned breakers on. Went to touch tank to see if water was heating up. Touched top tank first and it was "hot", electrical hot!!
WTF? I did not change anything on top tank and all wires/connections good.
Not sure what is happening.
 
Something has gone to gound-either a wire connected to the thermostat or the element has gone bad and is now leaking to the metal cabinet.
 
The electric heating elements consist of an outer metal tube with a filament inside, much like a light bulb. This filament is supported with nonconducting standoffs at set distances apart, to isolate the filament from the outer metal tube.

If the electric power was turned on (or left on) when the tanks were empty, it only takes 20 or 30 seconds for the heating element to burn out if it isn't immersed in water.

Then, if the filament inside the heating element breaks when it burns out, and droops, touching the outer tube of the heating element, you have a direct short to ground. This can result in an electric shock when you touch the outer metal case of the water heater.
 
to test what vic said. disconnect the elements completely and check continuity between the electrical connections and the case. It should be zero.
 
I agree, but the tank must not be grounded properly. If it was, the breaker or overcurrent devise should have tripped. Find the problem, and fix the corroded or missing ground wire.
 
Tank has not been empty. I'm in CA, so no need to winterize.
The resistance across the element is 10 ohms. If I put black lead to one contact on the element and the red lead to tank it shows zero resistance. If I reverse wires from meter, it shows no connectivity.
 
zero is a short. I dont understand the reversing it should show the same.
 
quote:
Originally posted by pdecat

zero is a short. I dont understand the reversing it should show the same.

Click to expand...


I did not understand that either. I checked this 6 or 7 times because I thought that cannot be right.
 
quote:
Originally posted by GSENT

I agree, but the tank must not be grounded properly. If it was, the breaker or overcurrent devise should have tripped. Find the problem, and fix the corroded or missing ground wire.

Click to expand...


Very important. That is the purpose of grounding metal parts of electrical appliances. Someone wired this improperly. Fix it first, then worry about the other stuff.
 
Resistance across the heating element should be 72 +/- 5 Ohms. Replace the heating element.
 
Any stray currents will make all your resistance measurements incorrect. You should try the measurments again with all electrical power totally removed from the boat. Totally removed would include disconnecting your AC power cord and turning off your battery master switch.

Are you checking the heater element resistance to ground directly at the element or upstream of the thermostat?

You should also test the resistance between a solid ground and the water heater tank.

Mike
 
top tank did go empty or at least partially via siphon when the lower tank element was pulled as they are in series. as others have noted the tanks are not grounded if the breaker did not trip and that is a serious concern as the water in the plumbing system is also energized with a faulty ground. very serious safety issue and must be fixed. if you are not 100% comfortable tackling this, bring in a marine electrician, it might save a life.
 
update
vic33 - actually tank did not go empty as I did not replace heating element on lower tank, just replaced thermostat and high limit switch. Did not have to drain.

removed wires from heating element on top tank. There is a short between the white neutral wire and tank which I suspect is shorting to ground somwhere. Black wire is not shorted. The wires are all fully insulated - no problem areas visible. The tank looks to be properly grounded and I tested ground.
The white wire is fully insulated all the way into the 3 strand sheath which is also fully intact.
I don't understand why the breaker doesn't flip when I turn it on
 
yes, element is good, no short with tank
wires were off and neutral had short with ground/tank

Now that I think about it, the short from neutral to ground must be in every circuit, right?
In that case, why don't my GFI's trip
 
quote:
Originally posted by getakey

yes, element is good, no short with tank
wires were off and neutral had short with ground/tank

Now that I think about it, the short from neutral to ground must be in every circuit, right?
In that case, why don't my GFI's trip

Click to expand...


The neutral and ground should not connect together on your boat unless you have a genset or inverter and if you do, only when the transfer switch is arranged for them to power the boat (shorepower disconnected).

If you have no genset or inverter or they are turned off, the neutral and ground are connected together only at the sevice entrance for the shorepower (matina, home, etc.)
 
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