T&T: Debugging an electrical problem

[Greg Bradley]

More information to digest.

I had assumed low voltage as well, I was just afraid to call the marina
and tell them the box was screwed up until I knew for sure it wasn't my
problem.

The probes on my voltmeter won't reach into the receptacle at the dock
box.  I guess I could rig something up to try and get a reading, but
that scares me.  So, I opened the electric panel on the boat and tested
the power coming into the "Ship / Shore" switch on the back of the
panel.  I was expecting to see a low voltage condition, but I didn't. 
It shows 120V hot-neutral on both legs and 240V hot-hot just as I would
expect.  

I guess the next step is to pull the stove and check the voltage at the
back of the stove?

Greg...

--- Sean Welsh <slwelsh+trawlers at gmail.com> wrote:

> Greg Bradley wrote:
> > We we are on shore power (50A), ...  The electric stove will sort
> of power on, but the
> > burners never get very hot and the LED "On" light does not light.
> You don't mention it explicitly, but I assume your 50A shore
> connection 
> is actually 50A, 240V.  If so, it will have three slots/blades and a 
> ground tang on the outer circumference of the connector.
> 
> I am guessing (wildly) from the description of your symptoms that you
> 
> also have a 240-volt stove.  I am further guessing that, up to now,
> all 
> the 50A shore services you have used have been full, 50A/240V/120V
> services.
> 
> I think your current problem is that the shore receptacle you are
> using 
> is either (1) completely wrong, inasmuch as the two "hot" legs of the
> 
> service are both connected to the same "phase" or "leg" of the shore 
> power system.  In this case, you would have only 120VAC available on 
> your boat, and 240-volt appliances would see zero volts (240-volt
> stoves 
> are sometimes wired so that you will end up, in this sort of 
> circumstance, getting half voltage, or quarter power, at the
> burners), 
> while all your 120-volt appliances would work normally.  This is a 
> dangerous condition, because you may well be returning much more than
> 50 
> amps on the neutral wire, which is only rated for 50 amps.
> 
> Or (2), the two hot legs of your power source are actually derived
> from 
> two out of three of the phases of a three-phase supply.  In which
> case, 
> your phase-to-neutral voltage will be 120 volts, and thus all your
> 120 
> volt appliances will work fine, but your hot-to-hot voltage will be
> 208 
> volts instead of 240 volts.  Depending on the model of stove, that
> may 
> be just enough too low for the stove to exhibit the described
> symptoms.
> 
> Get out your voltmeter and measure between each hot leg and neutral,
> and 
> then measure between the two hot legs.  If you get 120
> hot-to-neutral, 
> and 0 hot-to-hot, you have condition (1), if you get 208 hot-to-hot
> you 
> have condition (2).  (You can figure out where on the receptacle to 
> measure from this guide: 
> http://www.marinco.com/docs/guides/Boater%27sGuidetoACElectrical.pdf
> -- 
> figure 4 on page 5.  W is neutral, and X and Y are the two hots.)  If
> 
> you get different readings than these (such as normal readings of 120
> 
> and 240, respectively), then you have a different problem.  But this
> is 
> where I would start.
> 
> -Sean
> http://OurOdyssey.BlogSpot.com
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