T&T: Debugging an electrical problem

[Larry N. Brown]

Excellent guide. I downloaded it for future reference.

Regards,

Larry and Teri
M/V Cigano, 47' Prairie Sundeck Cruiser
Lying: Slidell, LA
N 30 13.28
W 89 48.63




----- Original Message ----- 
From: "Sean Welsh" <slwelsh+trawlers at gmail.com>
To: <trawlers-and-trawlering at lists.samurai.com>
Sent: Monday, July 02, 2007 1:08 AM
Subject: Re: T&T: Debugging an electrical problem


> 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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