T&T: Electrical Problem

[Gary Bell]

Tim Behan asked:

<snip> ...suddenly had no? incoming 120v power on either line. It had rained heavily so I figured they just had problems with their dock power. So I just cranked up my generator... <snip>

... and plugged in my cordsets but could not get a reading on my 120v panel. Acted just like there was no incoming power at all.?I flipped the breakers on and off and tried every combination possible.?I switched cords from one inlet to the other with no luck. Just dead. Again, I cranked up my genny...

At this point I don't know if I have a problem...
 

Reply:
Yep Tim, you definitely have an electric problem.  You implied that 
everything worked just fine when you went to bed, in the morning you had 
the problem, and the problem hasn't changed since.  You seem to have had 
a failure of a single component, under load and in the rain.  Note that 
breakers that have tripped repeatedly usually drop out at significantly 
less than their printed rating, common with guest dock power.  Do you 
have more than one A/C onboard?  If so, once in a while two or more will 
try to start at the same moment, and the combined startup current surge 
will blow breakers.  Logically, if the problem is seen in two or more 
marinas it is more likely an issue with your equipment, from the 
dockside power stanchion outward rather than with the marina services 
(although clearly not impossible).  Since you report that the genny 
works the AC fine, the problem is unlikely to be further 'inward' than 
the genny/shorepower/inverter switching and/or the parallel/split bank 
breaker, as you didn't report any problems running from the genny.  The 
significant risk of fire in these common cases dictates that the very 
first thing you inspect are the exposed plug and socket connections, at 
both ends of your cords, dockside and aboard.  Any darkening is a sure 
sign you need to immediately replace everything that is damaged.  Note 
that if a plug burns, its socket is also suspect.  Next try to inspect 
the wiring from the boatside sockets to the distribution switching or 
power panel.  You didn't indicate having taken any voltage readings, so 
beyond here, if you are not perfectly comfortable working on electrical 
equipment safely, you will need knowledgeable or professional help.  
Your message implied that you are not comfortable with meters and 
electric circuitry, and anyway, I can't assume the risk of instructing 
folks in potentially (pun) dangerous stuff, so I deleted all of the 
tracing voltages and resistance measurements I just wrote.  If the 
source of your troubles wasn't discovered by now, and since you reported 
that exchanging the powercords, dockside and boatside plugs had no 
effect I'm betting that the problem resides with the remaining portions 
that are common to both inputs, namely the common neutral line or the 
switch/breaker that connects the two banks of AC load in parallel for 
running on a single powercord.