Reverse Polarity (Define Please)

[terry dornan]

In the process of total rewiring of the boat and down to the shore power side. In the process of removing existing components I have come across a reverse polarity alarm. Before everyone jumps to tell me that "anybody knows that", understand the old sailor knows a little bit about electrical systems and controls. Just to start off with we all know there is no polarity associated to an alternating current and that the convention of bonding one side of the secondary transformer tap to ground defines a neutral lead. That is to say that there is no difference in electrical potential from the neutral leg to an earth (water) ground.Question?Are all discussions on this subject, associated alarms, and indicating devices solely concerned with the determination that the boats defined "neutral" lead is bonded to ground? Or is there more to the subject then that? In addition how are un-bonded conditions detected?Terry Dornan s/v stardust

 

[Ron Dague]

There is Polarity in AC

Although technically there is no polarity in AC, the 110 volt AC in the US is arrived by taking one half of a 220 line, and a common ground. That results in a potential of 110 volts, AC. But actually, one plug of your 110 is hot, and the other prong is a common, normally connected at some point to the ground. (I'm leaving out isolated ground circuits used with hi-tech equipment here.) So although you think of 110 volts as 2 wires and a ground, it is really a hot, and 2 grounds, in that the common and ground are joined, either at the outlet or at the circuit breaker box. It doesn't make any difference with a light bulb, or most appliances. However, most home 110, 2 prong plugs, have a polarized plug, so that the ground on the device, is connected to the common, in the event of a short. The problem on a boat, is that the common for the 110, is often wired to the "ground" (assuming you don't have a floating ground circuit). If your shore power polarity is reversed, you are putting a 110 volt live current, connected through your common, and thus leaking current into the water, etc. This will happen through the engine ground, keelbolts, etc. Not a good thing.  A $10 tester from most hardware stores will tell you if your shore polarity is correct or not. It is amazing how much house wiring is reversed! So don't just assume it is correct. Thus, a polarity check or warning on you electrical board. I'm not an electrician, and I'm sure more elequent will come along and explain this better.

 

[Been there]

AC ground & AC neutral often NOT same, on a boat

To protect from stray current electrolysis, prevent cross-over to the DC system, and allow ground fault protection, boat wiring typically separates AC neutral from AC ground. The ground may be the boat's ground, with no direct connection to the incoming ground line, or the two "grounds" might be weakly coupled through a ground isolator.In any case, just because neutral and ground are connected back at the switching box does NOT mean that they are the same on your boat. Do NOT confuse them when adding or changing AC wiring. Calder has a good discussion of this in his book, and shows several different wirings. Yep, sometimes neutral and ground ARE the same. But don't assume this unless you have either verified it or know the wiring on your boat. And in any case, don't interchange them, since at some point down the road, separating them may be key to solving some problem.

 

[Peter Albright]

Polarity of the CONNECTION, not the VOLTAGE

The typical "alarm" light shows voltage between Neutral and Ground. This would indicate that the "Hot" and "Neutral" are reversed. The best option would be to hard wire a 3 light circuit tester into your shore power panel. This will also indicate "open" connections also.Marina "Shore Power" wiring is most often way below code requirements. Additions and repairs are often made by unqualified persons. You can often find things that are miss-wired, have open connections, or have burned off contacts in receptacles.Landside "Electrical Codes" are concerned with the ability of the circuit breaker to operate before a fire can start from a short circuit or overload, and that all metalic parts are grounded to prevent shock.We also have to consider the effects of stray or induced currents.I believe the best protection is a 30 amp "Ground Fault Interrupting" circuit breaker in a waterproof box. This is not a marine device, but will insure that no more than .003 amps are "leaking" from the shore connection.I am rewiring my '77 H30 without a shore side ground connection. This requires a GFI breaker and an isolation transformer. No galvonic isolator is needed with this configuration.Anyone doing AC changes should be familiar with NFPA and ABYC standards. Both have several typical wiring diagrams for shore power installations.Always turn off power before connecting/disconnecting shore power and check the female contacts of all connections.Pete Albright s/v Nancy Ross

 

[goodwitchnow@aol.com]

reverse polarity warning light

polarity warning light is on, checked the dock power and yes,it was reversed....fixed it and checked all power outlets on board and all testing ok....however the light is still on anybody have any ideas?