T&T: Debugging an electrical problem

[Sean Welsh]

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