Guys AC current causes little to no corrosion. There have been multiple studies on this over the years. It is DC that is the most corrosive. AC leaks can however cause DEATH. They are called "Electric Shock Drownings" or if on-board the boat and touching an energized metal item, electrocution..
Stray current corrosion or stray DC current corrosion is the culprit for the most aggressive and fastest levels of corrosion on our boats. About six times out of ten I see it coming from your own boat, not your neighbors.. I have seen thru-hulls eaten to destruction in 24 hours when a live DC cable came in contact with bilge water.
Improper wiring practices are almost always the cause of stray DC current corrosion though bilge pumps have been known to fail and leak current too..
Galvanic corrosion is what happens when dissimilar metals are submersed in an electrolyte, namely salt water. Each metal has its own electromotive potential and when all the metals are immersed in the same electrolyte you create a galvanic cell.
The zinc we use on boats has a electrical potential of about -1030mV to -1070mV (that I've measured). Graphite, like boaters often use in packing materials or PSS shaft seals has an electrical potential of +2000mV to +3000mV. In this scenario we can see that we have a pretty wide voltage spread between the anodic and more active metal zinc, and the more noble less active graphite. The further apart on the galvanic scale the metals are, the more potential for galvanic corrosion there is.
When you plug your boat into the dock the green safety ground wire for the AC system is now connected to every other boat in the marina and all their underwater metals. This is why we use galvanic isolators or isolation transformers to keep our boats isolated from our neighbors. Galvanic isolators protect well against galvanic corrosion but NOT stray DC current. Isolation transformers are the only true "isolation"....
Here is an example of where galvanic corrosion starts:
All that said this has nothing to do with reverse polarity. Reverse polarity can be a serious issue that needs to be corrected ASAP.
It can be caused by the dock wiring. This is easy to have the marina check.
It can be caused by a DIY or hack mechanic miswiring the boat.
Many times the light illuminates when high loads like heaters, water heaters, battery chargers or air conditioning are on. When this happens it usually means a bad connection, corrosion of the receptacle or dirty or corroded cord prongs. For AC to work the current on the white wire is always identical to the current on the black wire. If resistance develops AC is always looking for a path back to the source. It will take the green wire to get there and will illuminate the reverse polarity indicator while seeking its return path.
Actual reverse polarity can exist either on the dock or on-board your own vessel. It does not matter where the reversing point is, on the dock or on-board, both scenarios will energize the reverse polarity indicator and create a potential for shock hazard, electrocution or ESD.
With a faint or flickering RPI light you may not have a real reverse polarity situation but still could so check with a tester first. If faintly illuminated it is usually more likely to be a voltage drop / resistance issue if the dock and polarity tests well.. However if an older neon type light is used then it may just be weak and you could still have a reverse polarity situation. Reverse polarity testers are very inexpensive at Home Depot or Lowes.
The ABYC requirement for reverse polarity indicating lights is that they use minimum 25k Ohm Resistor. This is so they "indicate" at below the 5mA of leakage that a standard US spec GFCI will trip at.
In the old days the indicator lights were neon based and for the last 20 or so years they have been LED but all should have a 25k Ohm resistor in the circuit.. This light sits between the AC white/neutral and AC green/earth ground and responds to a reversal of the ungrounded (black) and the grounded (white) conductors .
Keep in mind that when Muffy and Skippy fire up the brand new Sea Ray, untie the dock lines, and steam away from the dock forgetting about the power cord, it is usually Darrel the dock boy and his other buddy Darrel who re-wire the dock post. Don't trust them and start with a test of the dock post for reverse polarity. If you don't have a tester ask the marina to test it for you and be there to watch it..
It is possible for the reverse polarity indicator circuit to faintly illuminate even though the circuit is properly wired. This most often occurs from resistance but some inverters and even some battery chargers have been known to cause this as well. To test this you can simply disconnect your inverter or charger or other high amp loads and see if the RPI is still faintly lit. Start by turning off all AC branch circuit breakers and only having the main AC breaker on. Does it illuminate? If not then it is likely cased by some high resistance in one of the AC circuits or the dock cord or the pedestal.
Thanks to a guy name Ohm, and his law, voltage is always consumed trying to push amperage through a wire. If the voltage differs between the beginning and end of the run or any two points in-between this is called voltage drop. Voltage drop can be caused by undersized wiring, improper or poor connections / terminations or corrosion. Bad or loose friction fits in shore power receptacles can also cause some high resistance. This difference is what can dimly illuminate a RP indicator light.
If you flip on a high amp draw load there can often be enough resistance along the neutral path to overcome the resistance in the 25,000 Ohm resistor thus lighting it faintly. High resistance is not as dangerous as sucking on live wires, but should be attended to if discovered. High resistance can start fires well before circuit breakers trip!!
Check all connections, use a tester, and make sure your AC input wiring from the dock to the panel are sufficiently sized for your loads. In some cases just trying a different dock pole and shore power cord can eliminate issues on-board and isolate them to the cord or dock post..
What ever you do find out what is causing the RPI to light and fix it..
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