A/C Wiring Short ?

[Nibiruwayne]

To all the tesla genies out there, if you want a common ground don't you want three wires, black -hot , white - neutral, and green - dedicated ground? Anyone who knows, please explain.

[mitiempo]

That's correct.

The AC main switch opens/closes both hot and neutral, each individual circuit has a breaker that opens/closes only the hot. Both neutral and grounds are common to all. The AC ground (in North America) is connected to the 12 volt negative bus, making all grounds, AC and DC common.

[osirissail]

Originally Posted by Nibiruwayne

To all the tesla genies out there, if you want a common ground don't you want three wires, black -hot , white - neutral, and green - dedicated ground? Anyone who knows, please explain.

Huh? - Basic electric 101 - North American style - black or red is Hot and comes from the power company into your house through a main disconnect (c/b or electric meter).

The white wire is the "neutral" which means it comes off the center tap of the transformer up there on the power pole. In your home electrical panel it is also tied to a grounding stake pounded down into the earth. It is the primary "ground."

The "green" wire is the "safety ground" and doesn't come from or go to the power pole. It goes to the same metal stake pounded into the ground as the "neutral." The green "safety" ground is there in case the white grounded neutral gets cut or disabled. Or if your appliance/whatever develops a short to its case or housing that a human can touch. It allows the line c/b or fuse to trip/blow in case of an overcurrent condition in the black hot supply wire.

In a boat, everything is the same - except - for shore power both the "Hot" and "Neutral" are disconnected by the shore power selector switch. This is done to prevent a reversed hot/neutral wiring problem from shorting out inside your boat possibly causing a fire. A reverse polarity light is normally installed beside the main shore power AC switch to warn you that the power is coming down the wrong wire - before you throw the switch.

In this system you have 2 grounds - the neutral and the green safety ground. In other electrical systems in use around the world they often do not use the "safety ground" wire and only wire things for a "hot" and a "neutral." For this reason, it is very important that your boat's AC green safety ground be connected to the boats grounding system that is in contact with the water/ocean. If it is not then your reverse polarity warning light would not operate and turning on the shore power switch could cause a fire at worst or blow the c/b on shore at best.

[donradcliffe]

Originally Posted by osirissail

In this system you have 2 grounds - the neutral and the green safety ground. In other electrical systems in use around the world they often do not use the "safety ground" wire and only wire things for a "hot" and a "neutral." For this reason, it is very important that your boat's AC green safety ground be connected to the boats grounding system that is in contact with the water/ocean. If it is not then your reverse polarity warning light would not operate and turning on the shore power switch could cause a fire at worst or blow the c/b on shore at best.

You just failed Electricity 101.

1. All modern electrical systems use a green or green/yellow grounding wire.

2. The reverse polarity warning light is a bulb connected across the AC neutral and the AC ground wire. It will light if the neutral and hot wires are crossed in the dockpower system or extensions to the boat. It will operate if the AC ground wire is not connected to the boats grounding system.

3. Reversed polarity will not blow breakers or cause fires UNLESS there is a fault in your electrical system or appliances.

The issue of whether it is wise to connect the AC ground wire to the boats grounding system is contentious and evolving, and has been covered in other threads.

For the OP, the AC equivalent of DC resistance is called impedance, and includes inductance and capacitance, so something that might appear close to a dead short to DC will actually not pull that much AC current. One suggestion would be to energize the circuit and measure AC volts from each connection in the sockets to a known ground. Its hard to understand how your boat is wired, but you may also be able to clamp on the AC ammeter on a hot or neutral wire in various parts of the outlet circuit to see if and where the current is flowing.







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[CharlieJ]

It will operate if the AC ground wire is not connected to the boats grounding system.

That is not correct. The operation of the reversed polarity light is independent of the presence or absence of a AC ground/vessel ground bond.
Charlie

[Maine Sail]

Originally Posted by btrayfors

Note: lightbulbs and motors and similar loads on the circuit could show a low ohm reading as well. Be sure there are none and/or they are switched off.

Been tricked by that one before. Breaker off, lamp turned on continuity between black & white.... Doh'.....

I would suggest unpluging all appliances in the outlet circuit and re-test..

[sigmasailor]

Originally Posted by CharlieJ

Don- Didn't you mean, "It will not operate if the AC ground wire is not connected to the boats grounding system."

Charlie

Guess not; you don't need ground to merely operate an AC circuit; hot and neutral are enough. Now; to operate it safely you do need ground and like other I would recommend ALS (aardlekschakelaar or earth protection switches) protected outlets.

Eric

[osirissail]

Originally Posted by donradcliffe

. . .
3. Reversed polarity will not blow breakers or cause fires UNLESS there is a fault in your electrical system or appliances. . .

Let's see. the ship's neutral is connected to the ship's ground system.

You plug in your shore power cable and because of mis-wiring, the "hot" leg is now on the "neutral" leg wire which is directly connected to ground.

And you contend that when you connect a hot wire to ground you will not allow enough current to pass down the wire to either 1- trip the shore power circuit breaker - or, 2 - the dead short to ground cannot cause the wire to catch on fire?

Nah. don't think so. Give it a try - cross connect your shore power plug to feed "hot" down the neutral wire and turn on your shore power switch . . . just be careful to have a fire extinguisher nearby.

[mitiempo]

Originally Posted by osirissail

Let's see. the ship's neutral is connected to the ship's ground system.

You plug in your shore power cable and because of mis-wiring, the "hot" leg is now on the "neutral" leg wire which is directly connected to ground.

And you contend that when you connect a hot wire to ground you will not allow enough current to pass down the wire to either 1- trip the shore power circuit breaker - or, 2 - the dead short to ground cannot cause the wire to catch on fire?

Nah. don't think so. Give it a try - cross connect your shore power plug to feed "hot" down the neutral wire and turn on your shore power switch . . . just be careful to have a fire extinguisher nearby.

The ship's neutral is not connected to the ship's ground, with the exception of an internal connection in a generator or inverter. When on shorepower there is not a connection between AC ground and neutral on the boat.

As posted the reverse polarity light is wired between the incoming ground and neutral. Normally there is no current between these wires. If the shorepower is wired wrong (hot and neutral reversed) there will be 120 volts between the neutral and ground and the light will be on. The real danger is that since circuits downstream of the main switch only control the hot feed you will think a circuit is off but it will still be hot.

Reverse polarity is not good but appliances will run and a fire extinguisher is not required.

[rtbates]

Are you 100% sure that you're NOT reading across devices on the AC line?

[gjberta]

Just 2 cents worth re the cost of GFCIs; ideally, each AC receptacle is on it's own accessory breaker, but since this is not done very often, keep in mind that if you daisy-chain 2 or 3 receptacles on one breaker, only the first in line need to be GFCI (and downstream of it can be regular duplex).
If you are doing a complete rewire, consider an EFCI double pole main - well worth it for safety of swimmers around your boat if you are in fresh water regions.

[Cheechako]

Wow.... sounds like we have many conflicting "right" answers!
"The issue of whether it is wise to connect the AC ground wire to the boats grounding system is contentious and evolving, and has been covered in other threads" yeah, ran into this before. Some people steadfastly say it's dangerous to connect the neutral to ground. I tend to agree with them...

[mitiempo]

Originally Posted by Cheechako

Wow.... sounds like we have many conflicting "right" answers!
"The issue of whether it is wise to connect the AC ground wire to the boats grounding system is contentious and evolving, and has been covered in other threads" yeah, ran into this before. Some people steadfastly say it's dangerous to connect the neutral to ground. I tend to agree with them...

Neutral should never be connected to ground aboard the boat except internally in a generator or inverter as they are sources. Neutral is connected to ground on shore.

Connecting the AC ground (green) to the boat's grounding system - 12 volt negative bus - is totally different and ABYC calls for it to be done. As you say it is contentious. It does however provide a safe path for a fault in the absence of a shore ground, which cannot always be depended on.

[GordMay]

Originally Posted by mitiempo

The ship's neutral is not connected to the ship's ground, with the exception of an internal connection in a generator or inverter. When on shorepower there is not a connection between AC ground and neutral on the boat.

As posted the reverse polarity light is wired between the incoming ground and neutral [sic 25,000 Ohm resistance recommended]. Normally there is no current between these wires. If the shorepower is wired wrong (hot and neutral reversed) there will be 120 volts between the neutral and ground and the light will be on. The real danger is that since circuits downstream of the main switch only control the hot feed you will think a circuit is off but it will still be hot. ...

Indeed.
See also R.P.I. - Cruisers & Sailing Photo Gallery

[osirissail]

Originally Posted by mitiempo

The ship's neutral is not connected to the ship's ground, with the exception of an internal connection in a generator or inverter. When on shorepower there is not a connection between AC ground and neutral on the boat. . .

Ah! You are making a dangerous assumption that "all" boats are wired in accordance with ABYC's current standards. Donradcliffe at least qualified his response with "All modern electrical systems . . ."

Lots of old boats follow neither ABYC or even any other world standard. I have encountered many old boats with "bonded" (grounded neutral) neutrals and many with no "reverse polarity" light. Quite a few had "reverse polarity" lights that were either disconnected or failed.

Likewise, the assumption that shore power supply systems conform to ABYC or any other system is also a dangerous assumption. From real world experience both inside the U.S.A. in ancient (a.k.a, decrepit or cheap marinas) and in many 3rd World countries you will find shore power supplies with no safety ground (green/green-yellow) wire and many with no bonding to ground on their neutral wiring system. Even some with reversed "hot/neutral" wiring that has been that way for years (especially in the "European" 220VAC, 50Hz 2 wire systems). In these circumstances there is nothing (short of using a portable VOM to test the shore power receptacle) that will alert you to a dangerous situation.

In my post the "Let's see. . ." was a "given" situation. The point being that making flat all-inclusive "book derived" answers can be dangerous in the real world of cruising where many "not-by-the-book" realities exist.