Another Marina electrical 'mystery'
Posted by ez_as_31416@reddit | sailing | View on Reddit | 15 comments
I'm in a fairly new nmarina on a 124V 30A circuit. I noticed my reverse polarity faintly glowing the other night. The next day -- no problem. So I started checking. After dark it glows. The voltage between neutral and ground is about 1.4 volts in the daytime, and around 6 volts at night. Neutral and is staying about 124V.
There is a small light on each power pedestal that goes on at dusk and off at daylight. My assumption is that the docklights are the culprit.
I haven't brought it up to the marina because I don't know if it really a big deal or not. Google search results said it's not really enough to worry about. fwiw, my shorepower cord is cool, no scorching at either end.
Thoughts?
Morgan_Pen@reddit
Electrician here. Having apparent voltage between neutral and ground at very low voltages like that is normal due to voltage drop on the neutral line. It can also be caused by unbalanced loads in multi-wire circuits.
Most likely you are seeing the extra difference at night due to people running more systems and if they are using multiple pedestals on the same phase, it can unbalance the load.
indigoinblue@reddit
Side question: is it safe for me to use a Y adapter to power both my a/c and battery charger with one shore power cord? They are separate outlets on my transom.
me_too_999@reddit
Excessive ground current will burn the anodes of every boat on that dock.
A common problem with boats where a wire with worn insulation lays in the bilge.
Plastic_Table_8232@reddit
Are you saying that a bare wire can energize the water around the keel conducting through the keel bolts and the GRP keel? GRP is an electrical insulator as far as I know. The only other scenario that I can logical is the bonding system if connected to the keel bolts and through hulls would allow stray current into the water.
I’m not an electrician by any means and not trying to challenge your comment, just trying to comprehend the logic driving it because it’s counterintuitive to my thinking.
My mate would carry a large anode that he would clip to his rigging for when he was at the dock. Since it was easy to pull up he would monitor it visually. The boat was fully bonded so it made sense to me.
me_too_999@reddit
Almost all boats have a grounding plate under the water.
Propellers, engine or keel.
Plastic_Table_8232@reddit
Correct - but if it’s isn’t bonded or the bilge water isn’t up to the level that they are connecting how is it transmitting energy through bilge water.
me_too_999@reddit
The bottom of the boat looks slightly different between sailboats and motor boats.
Let's do motor boats first.
Anything larger than a dinghy, which usually doesn't have electricity anyway, will have an engine or outboard with metal propellers.
Any metal on board corrodes constantly when wet. Any dissimilar metals form a battery with an anode and cathode. This stray electrical current accelerates corrosion.
To prevent this and for electrical safety, all negatives and neutral for US boats are connected to a common ground buss. The engine block and drive shaft and propeller are also connected to this ground.
For compliance with US electrical code, the common neutral is connected with an interlock switch that opens when connected to shore power, which connects neutral and ground at the main marina utility panel.
When running on generator or inverter, this neutral is connected to ground at the power source.
In addition, any seacocks are usually bronze submerged in both the bilge and the outside water provide a conductive path.
The engine cooling water, heat exchanger, and hoses are also in contact with both the bilge water and the outside water.
Water itself, especially sea water, is a conductor.
All of these provide paths for stray current.
Fiberglass when DRY is an insulator, but is very porous.
When wet, it no longer functions as an insulator.
In sailboats, you have all the above plus the keel bolts and the likely steel or lead keel.
Morgan_Pen@reddit
This is correct, and almost every marina has SOME ground current, but showing a potential under 10 volts is low enough that it can be explained by normal electrical theory instead of leaking voltage 👍
Whole-Quick@reddit
The issue here is voltage on the neutral wire, not the ground wire. Different issue.
ruprectthemonkeyboy@reddit
It’s not necessarily voltage on the neutral. Many reverse polarity indicators are relatively simple and can also trigger based on current difference (between hot & neutral).
In theory, all the current that comes in on the hot should leave on the neutral and there should be zero current on the ground. When there’s a current difference we often call it a ground fault but with boats it could be the stray current is going through the boat’s bonding system and not the ground.
As far as boat anodes go, the problem is usually caused by neighboring boats rather than the marina shore power per se. Since you’re all connected, a boat with no zincs could be robbing from neighbors.
OP could be correct that the source of his reverse polarity indicator is the dock lights, it seems to make a difference where in the power center accessories are connected. I’ve seen stray current readings from electronic meter power supplies and such taking their power from the shore power breaker instead of the bus bar. Unexpected behaviors like that can make tracking down the source of stray current even more challenging.
Disclaimer: Not a marine electrician but I work at a marina and know enough that when the magic smoke escapes you need to replace stuff.
whyrumalwaysgone@reddit
Marine electrician here: Immediately check your zincs. If you are losing a lot (like 50% in 4mo or less) its a problem. This will be the first best indicator if you need to take action.
Normally, any light from the reverse polarity indicator is an issue. Is there any other protection on your boat, like an ELCI or similar?
Some simple tests you can do yourself are try plugging in a different 30a cord, and try plugging into a different dock pedestal to see if the problem persists.
Other than that, all I can say is this isn't normal, and if the dock light is involved than your dock is not wired correctly or safely - it should not be tied to your outlet in any way.
ez_as_31416@reddit (OP)
I didn't have this issue at my previous marina. Next week I'm moving to another marina for a few days and I'll see if the problem moves with me. AFAIK I don't have an ELCI.
FairSeafarer@reddit
Hum, bring it up with the marina. I think they would like to know. Others on here are better than me to tell you if it's a big deal that could do damage. I have no idea.
We stayed 2 weeks and a half in a marina, a very posh marina might I add, but our ladder that was in the water, stainless steel, is now full of eaten speckles of metals. Big deal. So, we'll investigate this properly. I don't know if we were part of the problem, but we've never had this issue in the past. We have yet to dive on the prop and zincs, but I sure hope the ladder took the brunt of it...
overthehillhat@reddit
Meanwhile --
More and more Marinas
Have No swimming warning signs
And those Zincs are being sacrificed
Leaky_gland@reddit
I'd be interested to know the potential difference between your ground and the water at day and night