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Old 24-12-2011, 02:23   #1
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Double Pole Breakers on Metal Boats?

A few of my books say that double pole circuit breakers are the best for metal boats.

Does anyone know why double pole breakers are preferred? It's odd, but I have four books that mention it, but none go into any detail except vague hand waving about it being safer.

Are double pole breakers preferred because:
A) if a negative lead short circuits to the hull, this helps isolate the circuit and minimizes the impact
B) to fuse the negative lead for current that would not show up on the positive lead (?? -- maybe a device shorts to the hull, and a device is incorrectly grounded to the hull, and the return path is through the negative lead for the device which then causes a fire?)

My aluminum boat has a floating ground and single pole circuit breakers. Unfortunately, there is not room on the wall to replace 24 single pole breakers with 24 double pole breakers. The panel would have to be twice as tall.

My father-in-law is an electrical engineer, and has offered to help with the "isolate the ground wire" case by making a box with 24 relays. It'd be straightforward to wire up, and these relays would shut the negative leads off on each circuit that is off. But this would not add fusing to the negative wire, if this is the reason double pole circuit breakers are preferred for metal boats.

The other option is to move the entire set of circuit breaker panels to a wall that has room. But this is a big project that is electrically the same as adding relays, if the intent of the double breakers is just to disconnect the negative lead from the battery when the circuit is off.
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Old 24-12-2011, 02:34   #2
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Vague hand waving

I put double switches on the starter and house batteries and on the the Echo charge on Boracay.,

Looking back it seemed like a good idea at the time. I does reduce the possibility of the hull becoming live (I think I got an insulated starter with the engine).

As it turned out nothing is going to beat a "live" bilge pump switch sitting in bilge water (not a good idea), and various circuits need to stay live when everything else is off (refrigeration, solar, bilge pump, and, because I used a car radio, radio).

It does give a nice sense of security to turn everything off at two switches when I leave the boat. If I had an aluminum boat I like it even more.
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Old 24-12-2011, 04:57   #3
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Re: Double pole breakers on metal boats?

if there's an on board 120 Volt AC power source (generator or inverter) wired into the system, there is a DPDT (double-pole, double-throw) switch in the hot (black) and neutral (white) wires that switches the system between shore power and ship's AC power source.
Isolation transformer protected: if neither secondary wires (ship side of transformer) are grounded to Common Ground Point, all circuit breakers are ganged, double-pole type.
You are also talking marina AC issues as well. Counting on them to be correct is a stray current disaster waiting to happen.
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Old 24-12-2011, 05:54   #4
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Re: Double pole breakers on metal boats?

The reason for double pole circuit breakers is to reduce the possibility of corrosion. Stray current corrosion can be very rapid.
If it is very difficult to make the change I would at least fit a "stray current" or electrolysis tester., this will help alert you if there is a path to hull. Ovni fit these as standard.
It is also worth fitting double pole circuit breakers, as a miniumum, to devices that may have an internal path to ground like electric drive autopilots.

How is the engine wired? Is the engine permanently ground?
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Old 24-12-2011, 10:00   #5
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Re: Double Pole Breakers on Metal Boats?

The neutral on a metal boat (ANY boat) should be isolated from the hull and AC ground system so a double pole breaker would provide no protection for electrolysis.

A short to ground from the neutral will not draw enough current to blow a breaker, the Neutral to Ground voltage is usually less than 1 volt although I've seen it as high as 5 volts on a badly wired dock with incorrectly wired boats running neutral currents through the ground.

So the only thing that would blow the low side breaker is a short from the hot side and it already has a breaker.

I cannot see any justification for double pole breakers or disconnecting the neutral on secondary circuits.

For purposes of disconnect safety when doing maintenance, the main breaker and breakers on gensets or inverters should be double pole.

I lived aboard a metal boat for 14 years with about 30 single pole breakers and never had a situation where it was a problem.
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Old 24-12-2011, 10:15   #6
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Re: Double Pole Breakers on Metal Boats?

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Originally Posted by Andina Marie View Post
The neutral on a metal boat (ANY boat) should be isolated from the hull and AC ground system so a double pole breaker would provide no protection for electrolysis.

A short to ground from the neutral will not draw enough current to blow a breaker, the Neutral to Ground voltage is usually less than 1 volt although I've seen it as high as 5 volts on a badly wired dock with incorrectly wired boats running neutral currents through the ground.

So the only thing that would blow the low side breaker is a short from the hot side and it already has a breaker.

I cannot see any justification for double pole breakers or disconnecting the neutral on secondary circuits.

For purposes of disconnect safety when doing maintenance, the main breaker and breakers on gensets or inverters should be double pole.

I lived aboard a metal boat for 14 years with about 30 single pole breakers and never had a situation where it was a problem.
It is double pole DC not AC circuit breakers that the OP is referring to.
The negative side of the DC should (indeed must be) isolated from the hull. A fault, or say carbon build up in an electric motor can provide a path to the hull, with corrosion.

The circuit breaker will not trip if an stray current fault develops, but there is no corrosion when the circuit breaker is off.This is not the case with a single pole circuit breaker.
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Old 24-12-2011, 10:34   #7
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Re: Double Pole Breakers on Metal Boats?

My apology.

Isolating the ground entirely on an aluminum boat is frequently done to avoid corrosion.

Again I don't see where a breaker in the negative side is going to be any advantage. All the negative connections on boat have to be isolated from the hull, a situation that is easy to verify but not always easy to correct.

A breaker in the negative circuit is disconnecting DC negative sources from DC negative loads, none of which connect to the hull. A short from any negative (or positive) DC circuits to the hull will not blow this breaker since the circuits are isolated.

I don't see how corrosion can occr if a breaker in the negative DC circuits is either on or off. There is no circuit to the hull.
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Old 24-12-2011, 10:37   #8
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Re: Double Pole Breakers on Metal Boats?

I have an aluminum boat and all of the circuits on the DC panel are double pole. The reason is they help to reduce the possibility of stray currents which could cause corrosion through electrolysis through the negative side of the circuit. This is true even with a floating DC system, which all metal boats should be wired as such. (Never run negative DC to the hull. All your negatives go back to the battery by wire only.) I have no hull corrosion because I have followed this rule.
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Old 24-12-2011, 11:23   #9
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Re: Double Pole Breakers on Metal Boats?

-DC is connected to the hull of a metal boat in one place, and one place only.
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Old 24-12-2011, 12:15   #10
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Re: Double Pole Breakers on Metal Boats?

Quote:
Originally Posted by Andina Marie View Post

A breaker in the negative circuit is disconnecting DC negative sources from DC negative loads, none of which connect to the hull. .
You are correct, but rather than not connected to the hull a better phrase is you hope they are not connected to the hull. Faults do develop.

An analogy would be seacocks, totally useless if there is no leak, but its prudent to have them, and close the seacock if the system is not needed. Double pole circuit breakers ensure all possibility of corrosion is eliminated when the circuit breaker is off, just like closing the seacock.
Like water leaks, electrical faults are not common, but can be devastating when they occur. Turning the system off, in a metal boat, needs double pole circuit breakers
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Old 24-12-2011, 12:21   #11
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Re: Double Pole Breakers on Metal Boats?

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Originally Posted by DeepFrz View Post
-DC is connected to the hull of a metal boat in one place, and one place only.
There are two systems used in metal boats. One is ground the negative at one place and one place only as you say. The second, and superior system IMHO,is to not ground the hull anywhere.
The former system is common in older, and steel boats, the later, in more modern and aluminium boats, but there is a considerable overlap. Both systems benifit from double pole cuircuit breakers.
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Old 25-12-2011, 07:23   #12
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Re: Double Pole Breakers on Metal Boats?

Your mention of sea cocks brings up an interesting point, Noelex, regarding electrical leakage connections to the hull.

On an aluminum boat you have to use insulated engine mounts and an insulated coupling to keep the ground from connecting to the hull (unless you have an isolated winding starter and alternator).

But you have a grounded raw water pump that is connected to the through hull (or the surrounding aluminum if a plastic seacock) via the salt water in the pipe. I guess for true isolation you should have a plastic ball valve in the raw water line that you shut off when the engine is not running.
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Old 25-12-2011, 07:41   #13
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Re: Double Pole Breakers on Metal Boats?

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Originally Posted by Andina Marie View Post
Your mention of sea cocks brings up an interesting point, Noelex, regarding electrical leakage connections to the hull.

On an aluminum boat you have to use insulated engine mounts and an insulated coupling to keep the ground from connecting to the hull (unless you have an isolated winding starter and alternator).

But you have a grounded raw water pump that is connected to the through hull (or the surrounding aluminum if a plastic seacock) via the salt water in the pipe. I guess for true isolation you should have a plastic ball valve in the raw water line that you shut off when the engine is not running.
No, that is not needed. You do create a path from the engine to ground but not from the engine to the hull. The water inside the raw water pump *is* ground.

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Old 25-12-2011, 07:55   #14
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Re: Double Pole Breakers on Metal Boats?

Correct the water in the pump IS ground.
And all the hull IS ground since it is extremely well connected to the salt water.
And the water pump on the metal engine is also Negative 12 volts, the very thing you have gone to great lengths to ISOLATE from ground.

If you put a platic ball valve in the raw water line you break the connection between water in the pump and "ground" water so now the negative 12 volts is no longer connected to the hull.
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Old 25-12-2011, 08:05   #15
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Quote:
Originally Posted by Andina Marie
Correct the water in the pump IS ground.
And all the hull IS ground since it is extremely well connected to the salt water.
And the water pump on the metal engine is also Negative 12 volts, the very thing you have gone to great lengths to ISOLATE from ground.

If you put a platic ball valve in the raw water line you break the connection between water in the pump and "ground" water so now the negative 12 volts is no longer connected to the hull.
If you want to isolate your engine from ground then you are right but a plastic valve? Why? You already have an insulating hose, so any connection is made by the water flow when the engine is running.

Also, it isn't a real problem to connect the engine block to ground. Most are. And this does not lead to a DC current flowing through the hull.

cheers,
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