Shore Ground NOT Connected to Isolation Transformer

[Pelagic]

I have a 2008 Victron 7000 Isolation Transformer.
At the shipyard in Subic Bay where I am docked, I’m installing a new shore power cable as well as an additional boat side Breaker at engine room junction box. (I feed the shore cable through an engine room vent
The Electrical Supervisor says I should NOT connect the shore ground, which is not what it shows in the Manual for a standard in water connection?
Trying to understand why this would be…. Maybe they have a lot of ground fault issues…
Photos attached shows How I have done it following the manual

Any thought or ideas as to why, before I question his advice??

[s/v Jedi]

The ground from shore should be connected as shown in the installation manual of the isolation transformer. Normally it connects to the terminal for incoming power.

What they mean is that shore ground should not be connected to boat ground. For example, the transformer housing is connected to boat ground. You must verify that with a multimeter.

There is a second shore power? Where does it’s ground conductor connect?

[goboatingnow]

If you connect the boat side protective earth on a supply generated by an isolation transformer to seawater , you are in my view undoing one of the major advantages of such a transformer. This is leaving aside the fact that such a circuit is meaningless as there is no seawater path.

[ranger58sb]

Originally Posted by Pelagic

I have a 2008 Victron 7000 Isolation Transformer.
At the shipyard in Subic Bay where I am docked, I’m installing a new shore power cable as well as an additional boat side Breaker at engine room junction box. (I feed the shore cable through an engine room vent
The Electrical Supervisor says I should NOT connect the shore ground, which is not what it shows in the Manual for a standard in water connection?
Trying to understand why this would be…. Maybe they have a lot of ground fault issues…

We have a factory-installed isolation transformer in our 50A/250V system. Our factory-installed shorepower cord has only 3 wires.

-Chris

[Sailmonkey]

Originally Posted by ranger58sb

We have a factory-installed isolation transformer in our 50A/250V system. Our factory-installed shorepower cord has only 3 wires.



-Chris

So you’ve got two hots and a ground?

[Fishspearit]

What you're talking about is the difference between an isolation transformer (shore grounding not connected to vessel ground) or a polarization transformer (shore grounding connection is connected to vessel ground). There is still debate among electrical experts as to which is best.
The SAFEST is to connect the shorepower grounding wire, but you will need to ensure you have a galvanic isolator installed. The best for preventing stray current corrosion is to not connect it to the boats grounding system, then you have a true isolation transformer, but the risk is that a short to the casing of the transformer could create an electrical shock hazard.
Here is a good summary of the differences written by Capt. David Rifkin. http://qualitymarineservices.net/wp-...le-10-2006.pdf

[Brioche]

Not sure about this particular situation, but in general grounds should be localized. Remember that if everything is working properly there should be little or no current or potential on the ground line. In 120 volt systems AC of course has one "hot" line and another common line that is identical to ground. The ground line might seem a little redundant, but it serves several purposes. If the hot line accidentally connects to something that can carry current -- such as a metal housing, the ground line will carry that current safely back. In the case of a large short, a breaker should interrupt the circuit. In case of a short with resistance involved (such as across a damp surface) the purpose of a ground fall circuit breaker is to detect the diversion of current away from the common line to the ground instead, and to trip at that point. The purpose of the ground line is to safely carry the unintended current from the source, such as a loose or worn wire, to ground. When you have a remote source, such as shore power (or in a house, a situation with a panel and a sub panel), you don't want the excess, unintended current running along an extended ground line back to the source, you want it running to the nearest good ground. A boat in water provides about as good a ground as you can get, so a short in a boat from shore power should run directly into the water rather than being routed back along the shore ground (which could either be broken or overload).

[TonyNA]

The reason you want your boat's ground isolated from the shore ground is faulty shore side wiring. For example, if the shore pedestal's ground is not properly grounded (well connected to copper rods driven into the ground) then your boat could become the ground path for the that entire branch (marina dock) and all things connected to it! Stray current seeking a ground path would find its way to your boat's ground, her anodes and running gear and could quickly waste them away costing you zillions of dollars, not to mention a safety issue for people in the water as well.

[Fishspearit]

Originally Posted by Brioche

When you have a remote source, such as shore power (or in a house, a situation with a panel and a sub panel), you don't want the excess, unintended current running along an extended ground line back to the source, you want it running to the nearest good ground. A boat in water provides about as good a ground as you can get, so a short in a boat from shore power should run directly into the water rather than being routed back along the shore ground (which could either be broken or overload).

That is exactly wrong. Current doesn't just 'go to ground' it always returns to its source. If you have an accidental short to ground you absolutely do want it to run back to the shore pedestal via the grounding wire where it is tied to the neutral at the pedestal. That is the whole purpose of the grounding wire. If you send it into the water it's still going to go back to the pedestal, but while providing 120V or 240V to all of your underwater metal fittings that are grounded or bonded, and through the water where people could be swimming.

The difference with the isolation transformer is that the boat side of the transformer is its own source of power, so a returning fault only needs to return to the boat side of the transformer and not all the way back to the shore pedestal. The shore side of the transformer should still be grounded so a fault on that side (such as at the plug) will return to the pedestal.

[ranger58sb]

Originally Posted by Sailmonkey

So you’ve got two hots and a ground?

I think it's L1, L2, and Neutral. Not sure, though...

-Chris

[goboatingnow]

Originally Posted by Brioche

Not sure about this particular situation, but in general grounds should be localized. Remember that if everything is working properly there should be little or no current or potential on the ground line. In 120 volt systems AC of course has one "hot" line and another common line that is identical to ground. The ground line might seem a little redundant, but it serves several purposes. If the hot line accidentally connects to something that can carry current -- such as a metal housing, the ground line will carry that current safely back. In the case of a large short, a breaker should interrupt the circuit. In case of a short with resistance involved (such as across a damp surface) the purpose of a ground fall circuit breaker is to detect the diversion of current away from the common line to the ground instead, and to trip at that point. The purpose of the ground line is to safely carry the unintended current from the source, such as a loose or worn wire, to ground. When you have a remote source, such as shore power (or in a house, a situation with a panel and a sub panel), you don't want the excess, unintended current running along an extended ground line back to the source, you want it running to the nearest good ground. A boat in water provides about as good a ground as you can get, so a short in a boat from shore power should run directly into the water rather than being routed back along the shore ground (which could either be broken or overload).

This isn’t correct

The purpose of the protective earth wire was to trip fuses in the event of a hot fault. Without such a circuit the first circuit could be via a human touching the hot fault

Residual current circuit breakers provide additional protection as they detect ANY imbalance in returned currents between live and neutral wires , ie fault current is returning to source via some unknown route.

Circuit breakers are there to protect cables , Residual devices are there to protect people.

Currents only return to their isolated sources so it makes no sense to seawater ground the output of a isolation transformer. All that does is open a sneak path for earth reference fault currents.

Most codes insist that local grounds are not provided and that fault currents return via a defined cabled route , this is to ensure that external fault currents don’t use your local grounding point by mistake.

[s/v Jedi]

It is SHOCKING that this is still presented all wrong. How many times has this been discussed?!

The attached diagram. Take it as authoritative. One test that MUST be done is make sure that shore ground is NOT connected to ships ground. Simply test with multimeter in continuity mode, then check with resistance mode to see that there exists a high isolation value.

These are ISOLATION transformers, they prevent shore ground from stretching beyond the transformer.

Ships ground as shown in the diagram is good to go, no need to connect it to anything else except all the outlets and appliances connected to the breaker panel.

[Fishspearit]

Originally Posted by s/v Jedi

It is SHOCKING that this is still presented all wrong. How many times has this been discussed?!

The attached diagram. Take it as authoritative.

No, I won't take it as authoritative. That's how you wire an isolation transformer if that's what you wish to have. There is a very good argument for wiring it as a polarization transformer instead(the pdf I linked to above by David Rifkin).

Sure, most people wire as an isolation transformer so they don't have to worry about stray current corrosion and don't need a galvanic isolator. But there is a safety trade off. When wired as an isolation transformer the metal transformer case is grounded on the boat side. If there is an internal short to the casing from a primary winding of the transformer the only path back to its source is through the boats grounding/bonding system (creating a shock hazard on the boat) and through the water back to the shore ground. This could kill someone.

Now is that a big risk? No, those faults don't happen very often. It's a risk many people don't mind taking. But presenting the wiring of a transformer as something that should only be done one way, that could potentially kill someone, without understanding the risk/benefit of doing it another way is incorrect. That's why this debate will always continue, some people will choose convenience over safety and others will choose safety over convenience. Pick your poison, but both methods are acceptable by ABYC standards, wiring as a polarization transformer (with a galvanic isolator and shore ground tied to boat ground) or as an isolation transformer(shore ground not connected to boat ground).

[s/v Jedi]

Originally Posted by Fishspearit

No, I won't take it as authoritative. That's how you wire an isolation transformer if that's what you wish to have. There is a very good argument for wiring it as a polarization transformer instead(the pdf I linked to above by David Rifkin).

Sure, most people wire as an isolation transformer so they don't have to worry about stray current corrosion and don't need a galvanic isolator. But there is a safety trade off. When wired as an isolation transformer the metal transformer case is grounded on the boat side. If there is an internal short to the casing from a primary winding of the transformer the only path back to its source is through the boats grounding/bonding system (creating a shock hazard on the boat) and through the water back to ground. This could kill someone.

Now is that a big risk? No, those faults don't happen very often. It's a risk most people don't mind taking. But presenting the wiring of a transformer as something that should only be done one way, that could potentially kill someone, without understanding the risk/benefit of doing it another way is incorrect. That's why this debate will always continue, some people will choose convenience over safety and others will choose safety over convenience. Pick your poison, but both methods are acceptable by ABYC standards, wiring as a polarization transformer (with a galvanic isolator and shore ground tied to boat ground) or as an isolation transformer(shore ground not connected to boat ground).

You are wrong. An isolation transformer is the gold standard and a galvanic isolator is a trick for when you can’t afford the transformer. The isolation transformer provides the ultimate protection for the boat as well as the crew. The only thing better is to not connect to shore power at all.

When you look into manufacturers documentation, you will find my diagram.

[Fishspearit]

Originally Posted by s/v Jedi

You are wrong. An isolation transformer is the gold standard and a galvanic isolator is a trick for when you can’t afford the transformer. The isolation transformer provides the ultimate protection for the boat as well as the crew. The only thing better is to not connect to shore power at all.

When you look into manufacturers documentation, you will find my diagram.

By all means, tell me what I wrote that is wrong and prove it. Everything I wrote is in the ABYC Electrical certification study guide and in the ABYC standards. Explain to me how a short from the primary winding of the transformer to the case will not result in an electrical shock hazard on the boat with an isolation transformer.

I'll post this again because it's clear you either didn't read it or don't understand it. http://qualitymarineservices.net/wp-...le-10-2006.pdf

And here's another article by Steve D'Antonio saying all the same things I just wrote. https://www.passagemaker.com/technic...r-transformers