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Almost every installation diagram I have come across for a galvanic isolator, has the GI installed in the shore power input line as follows:

GI installation Option 1

I think can understand how this would make sense for a vessel that is conductive, e.g. aluminium or steel hulls.

But as these units can and do fail, and can effectively leave the boat completely unearthed, of which the owner could be completely oblivious.

I can't understand why the following wouldn't result in a safer installation, electrical appliances would have a return path to earth via a guaranteed shore cable connection, and if the electrical appliance did end up shorted to the boats electrical system, the earth current would have a return path via the ocean or through the galvanic isolator.

GI installation Option 2

Why are these units always installed per the first diagram and not the second?

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  • \$\begingroup\$ What even is this "galvanic isolator", for those of us unfamiliar with marine installations? \$\endgroup\$
    – Hearth
    Sep 24 at 4:54
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    \$\begingroup\$ @Hearth Back to back Zener diodes to allow a few V of potential difference to not electroplate away zinc anodes on the boat, yet tested to some IEC norm I forgot to guarantee fail to short to never loose earth. \$\endgroup\$
    – winny
    Sep 24 at 7:57
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    \$\begingroup\$ @winny while there is a failsafe standard very few devices on the market comply with that standard. And it's not a guarantee, it's still possible that these units could fail open, eg after a lightning strike. \$\endgroup\$ Sep 24 at 8:06
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    \$\begingroup\$ @RowanSmith It has been 15 years since I was involved in putting one to market, but we tested it to some IEC norm to fail to short. (Direct) Lightning strikes is one of those cases in the electrical industry where most bets are off since you have bigger issues anyway. \$\endgroup\$
    – winny
    Sep 24 at 8:27
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    \$\begingroup\$ +1 for a question about a marine device I never heard of before despite working on a ship for quite some time. I guess it's only for boats not for ships. \$\endgroup\$
    – Ariser
    Sep 24 at 11:36

3 Answers 3

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The problem that a galvanic isolator addresses is the earthing a vessel's metal hull (not AC-powered devices). Its goal is two-fold:

  1. Prevent killing nearby swimmers, and
  2. Prevent corrosion of immersed metal (hull, propeller)

A picture may help:

Galvanic isolator operation

{Source: my book, "Li-ion Batteries and Applications", volume 2, page 78}

For those who don't know: A galvanic isolator is a pair of back-to-back sets of 2 diodes in series that allow up to 2 V difference between the shore power earth and the vessel's metal hull (or just the metal propeller in fiberglass boats).

schematic

simulate this circuit – Schematic created using CircuitLab

In both of the circuits you have shown, the circuit is the same:

earth --> shore power earth wire --> cable --> galvanic isolator --> engine --> hull --> water -- earth

Therefore, in both case, a failed galvanic isolator will be equally unsafe.

Hence, your solution is not a solution to the problem addressed by the galvanic isolator.

Your solution is actually less safe because it endangers people on board. In case of a short, there will be a voltage difference between the metal of the AC appliance and the metal of the vessel. Current may flow between your hand holding the refrigerator handle and your bare feet on the deck.

Short through hull

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  • \$\begingroup\$ So you're saying it's actually safer to have no PE at all, if the engine/hull is not connected to that circuit? \$\endgroup\$ Sep 24 at 19:35
  • \$\begingroup\$ Gosh, no, I am not saying that. With no earth connection, a short on the vessel will kill a swimmer. \$\endgroup\$ Sep 24 at 20:42
  • \$\begingroup\$ But you said that having the hull disconnect from the PE, per my second option is "less safe" than having the entire vessel disconnect per the first option? Therefore by definition that means it's more safe to have the entire vessel and all AC loads disconnect from the PE than just the Hull. \$\endgroup\$ Sep 24 at 20:54
  • \$\begingroup\$ You asked "So you're saying it's actually safer to have no PE at all". That means that the boat builder never installed an earth wire. That is different from what you stated in your original question: "But as these units can and do fail". That means that the boat builder wired the vessel correctly, but failed. The first one is intentionally unsafe and would jail the boat builder for murder, the second is accidentally unsafe and may fine the boat owner for manslaughter. Big difference. \$\endgroup\$ Sep 24 at 22:20
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    \$\begingroup\$ "if the unit goes fail-open then surely having some PE is better than having no PE." No, because standing bare feet on hull (which has voltage due to a short inside the vessel and touching a grounded refrigerator handle (which is at 0 V) will shock you. Conversely, if the AC products and the hull are connected (even if both have voltage due to the short inside the vessel), there will be no voltage difference between your feet and your hand. \$\endgroup\$ Sep 24 at 23:32
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The key is in the name galvanic isolator. Isolate the boat galvanically.

From Galvanic Isolator:

enter image description here

When boats connect to shore power, the ground electrically connects the boats together. This means that protection on a boat protected with sacrificial anodes is extended to vessels without protection or depleated protection, essentially consuming the sacrificial anodes at a faster rate. Note the flow of current in the image.

Zinc anodes form a galvanic battery to give the boat a -1.05V DC potential. Silicon diodes have a 0.6V to 0.7V potential, so 2 in series give 1.2V minimum. This isolates the boats galvanic protection and allows the protection to work while connected to shore power.

schematic

simulate this circuit – Schematic created using CircuitLab

The isolator must be designed to pass the full ac fault current back to the source (shore power), so there must be 2 sets of diodes in parallel. The isolator isolates the DC sacrificial protection to the boat, while allowing shore powers ac short-circuit protection to operate properly.

The sacrificial anodes must be connected to all components that need protection (shaft, propellor, rudder, etc.). There is no requirement to bond it to ground. But regulations require the ac ground to be connected to the DC ground. Engine is connected to DC ground and the engine is connected to the shaft, so sacrificial anodes are connected to ac ground.

So the second image does not show a connection between DC ground and ac ground. This means there is a path for current flow without a galvanic isolator.

The galvanic isolator must be mounted between shore power connector and the switchboard, in a location that allows testing. Mounted elsewhere means it does not provide isolation and current will probably flow.

Beyond the question: They can fail in open or short. Open no path for short circuit current to flow, which is dangerous. Short and boat's sacrificial anodes extend to other boats.

The attached reference provides a very good explanation of the device.

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    \$\begingroup\$ Very interesting. But how does it address OP's question? "Why are these units always installed per the first diagram and not the second?" \$\endgroup\$ Sep 24 at 18:46
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A galvanic isolator is a pair of back-to-back diodes in the PE line from shore to ship.

Without the galvanic isolator, the PE line permits free flow of galvanic currents that cause galvanic corrosion in the ship.

enter image description here

With the isolator in circuit, the forward voltage of the diodes prevents galvanic current flow and the corrosion that it could cause.

enter image description here

The isolator does not hinder flow of AC leakage current and the protective function of the PE line is still maintained.

A galvanic isolator that's failed 'open-circuit' will disable PE line protection at shore whereas one that's failed 'short-circuit' will disable galvanic corrosion protection.

Here's the recommended installation schematic for a galvanic isolator.

enter image description here

Here's the installation schematic that is based on your suggestion.

enter image description here

Both the schemes are nearly identical and personnel in the boat or in the water are likely to be electrocuted in the event of a ground fault after a galvanic isolator has failed 'open'.

The alternative is a marine isolation transformer.

Marine isolation transformers are used to completely eliminate the risk of galvanic corrosion and to ensure safety for swimmers.

enter image description here

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  • \$\begingroup\$ If you're at sea there is no shore power and therefore no PE. There's only 12v DC. Can you please explain how the AC distribution panel can be disconnected from PE in the second option if it goes open circuit? \$\endgroup\$ Sep 24 at 7:50
  • \$\begingroup\$ Thanks. There is no AC generator. The AC is only used while plugged into the shore for running appliances such as dehumidifiers and battery chargers. Furthermore there would be no risk of galvanic corrosion between two vessels at sea because they are only connected via a single circuit. GIs are only required when connecting to shore power to prevent galvanic corrosion with other boats connected to the same PE. \$\endgroup\$ Sep 24 at 8:56
  • \$\begingroup\$ I thank you for furnishing more details. Yes, I am aware of the galvanic corrosion mechanism. My answer has been edited. \$\endgroup\$
    – vu2nan
    Sep 25 at 8:23

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