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Old 17-06-2009, 21:07   #1
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One or Two Isolation Transformers?

Folks,

Thanks to all the informative discussion on various threads about isolation methodology, I have decided to purchase and install an isolation transformer on Layla. I do, however, want to verify that I understand this clearly; so here it goes:

I have two 30 Amp shorepower connections:

One of them is the standard shorepower connection with the battery charger, hot water heater, AC outlets, etc (the usual) connections. Based on all the discussions, this is where I should install the isolation transformer.

The second shorepower connection was added when I had twin A/C units installed right after the purchase. This connection feeds the A/C units only and is independent of the boat DC system (I'll follow the wires and double check to avoid surprises.) It seems I don't need to worry about adding an isolation transformer to this circuit.

Am I on the right track? Should I be adding another (or the same?) transformer to the second AC connection?

Also, I have been looking around to purchase a Charles Marine 120/240- 120 type transformer - for added flexibility if I ever make it across the pond... Read the installation manual on-line and the voltage level change seemed pretty straightforward (jumpers) - Any comments/experiences there?

Thanks to all in advance - This forum has been an invaluable resource indeed! (even though you guys made me review some of my projects I thought I had figured out... )

Sailndive
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Old 18-06-2009, 14:58   #2
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Measure the resistance between the green wire on the second shorepower connection (boat side) and the boat DC ground to make sure that there is no connection.

The isolation/stepdown transformer is a good idea if you are going to possibly use 240v power, as its not much more than a simple isolation transformer. Its probably easiest to rewire the system each time you change voltage (don't forget to make and save the wiring diagrams for each configuration) than it is to put a complex changover switch in.
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Old 18-06-2009, 22:26   #3
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An isolation transformer is installed in a shore power line for two reasons: galvanic isolation and polarity insurance.

Galvanic isolation is achieved by not bringing the safety ground wire aboard your vessel and therefore eliminating the electrical connection between your vessel and everybody else's vessel on the dock via the safety green wire.

Polarity (Hot and Neutral) is ensured because the Hot and Neutral are created on board (secondary side) without any functional relationship with the incoming (primary side) polarity. If you plug into a miswired receptacle dockside, all will be right on your side of the isolation transformer.

Quote:
Originally Posted by donradcliffe View Post
Measure the resistance between the green wire on the second shorepower connection (boat side) and the boat DC ground to make sure that there is no connection.
This is not good advice and can be downright dangerous if a fault condition occurs whereas AC gets into the DC system (a failed battery charger, for instance). The DC vessel ground (B-) is required to be connected to the AC safety ground bus by a single conductor (ABYC E-11).

Regarding step up/step down functionality; most of Europe is 50 Hz; we are 60 Hz. Transformers are not capable of modifying the primary side frequency.

One other detail; with two 30 amp services, you must establish and maintain two separate neutral busses and ensure that the N from all the Leg 1 loads return to the Leg 1 N bus and that the N from all of the Leg 2 loads return to the Leg 2 N bus.

Hope this helps.
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Old 19-06-2009, 08:16   #4
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Quote:
Originally Posted by CharlieJ View Post
...
This is not good advice and can be downright dangerous if a fault condition occurs whereas AC gets into the DC system (a failed battery charger, for instance). The DC vessel ground (B-) is required to be connected to the AC safety ground bus by a single conductor (ABYC E-11).

Regarding step up/step down functionality; most of Europe is 50 Hz; we are 60 Hz. Transformers are not capable of modifying the primary side frequency.

One other detail; with two 30 amp services, you must establish and maintain two separate neutral busses and ensure that the N from all the Leg 1 loads return to the Leg 1 N bus and that the N from all of the Leg 2 loads return to the Leg 2 N bus.

Hope this helps.
Many thanks to both CharlieJ and Donradcliffe - this is indeed helpful!

As far as the frequency diff goes, I am quite aware of the limitations I have to deal with (I'm originally from across the pond) - thanks for the reminder! Most of my AC power use will be resistive and not reactive loads (not turning any motors) - I hadn't planned on worrying about running the A/C units (that's the AC circuit I hope to leave alone )

As far as the potential for danger, I assume you mean performing the suggested resistance check while connected to shore power? Since I am only verifying the second AC circuit being isolated from the DC circuit, does it make sense to perform the test while disconnected from dockside shorepower and with the inverter off? This is a relatively simple circuit and I can easily follow the cabling to its terminating point (which is what I had originally planned on doing)

What you pointed out brings up another potential complication: (On the dockside) I use a 50 A shorepower cable from the receptacle to close to the boat, then a (50A - 2x30A) splitter with two 30A cables connecting to the above-mentioned two 30A connectors on the boat. I figured since the dock AC circuit should be on a common neutral anyway, this should not be a factor - am I mistaken with this line of reasoning?

Thanks -again- in advance for any/all input!... Stay well!

Sailndive
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Old 19-06-2009, 08:56   #5
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Quote:
Originally Posted by Sailndive345 View Post

As far as the potential for danger, I assume you mean performing the suggested resistance check while connected to shore power? Since I am only verifying the second AC circuit being isolated from the DC circuit, does it make sense to perform the test while disconnected from dockside shorepower and with the inverter off? This is a relatively simple circuit and I can easily follow the cabling to its terminating point (which is what I had originally planned on doing)Sailndive
No I am not talking about the resistance check. For safety, electrically, and permanently, connect the vessel ground (B-), which has all DC returns attached to it, to the AC safety ground in one and only one place. Said another way, to be safe, you need continuity between the AC safety ground bus and the DC B- bus.

From the current ABYC Standards; E-11
11.17.1.4 The shore-grounding (green) conductor is connected, without interposing switches or
overcurrent protection devices (See E-11.5.5.5.), from the shore power inlet to
11.17.1.4.1 an optional galvanic isolator, and then to
11.17.1.4.2 all non-current carrying parts of the boat’s AC electrical system, including
11.17.1.4.3 the engine negative terminal or its bus.
Regarding the 50/250 split phase into two 30/125 single phase inlets; technically you are correct concerning the neutrals. However, there is a requirement to have a shore power circuit breaker that breaks both the L and the N simultaneously.

There is also a requirement to keep the neutrals separate for multiple shore power inlets.
11.17.1.3 When more than one shore power inlet is used, the shore power neutrals shall not be connected together on the boat.
For a 50/250 split phase arrangement this is accomplished with a three pole breaker; L1, N, and L2. For a conventional 30/125 single phase inlet a double pole breaker; L and N is used. For your hybrid system you would be best served by using two double pole breakers; one for Leg 1 (L1 and N1) and one for Leg 2 (L2 and N2) and then continuing with this scheme for the rest of the system. This would allow you to use two 30/125 inlets safely should you go to a marina with this capability.

Hope this clarifies the issue.
Charlie
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Old 19-06-2009, 09:06   #6
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Definitely do the resistance check with the dock power disconnected and the inverter off!!!

I won't argue with the ABC, but I would still say the second isolation transformer is overkill and you are far more likely to be killed by lightning than by isolating the air conditioning ground from the boat' s DC ground. I'm just a dumb mechanical engineer, but I've sat on enough standards committees to know that their output is influenced as much by commerical interests as by technical reasons. I would ask Charley to explain a bit more about how a fault in an isolated air conditioning circuit is going to cross over to your boat's DC system. You could have reverse polarity issues in the second circuit, but if you have a RP light on that circuit you are no less protected than you are now.

As far as the two neutrals are concerned, you shouldn't have a problem as long as the boat is wired as you describe. If the second circuit is only neutral, load and ground wires to the air conditioning system, you have established a separated neutral bus for that circuit.
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Old 19-06-2009, 10:32   #7
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Quote:
Originally Posted by donradcliffe View Post
Definitely do the resistance check with the dock power disconnected and the inverter off!!!

I won't argue with the ABC, but I would still say the second isolation transformer is overkill and you are far more likely to be killed by lightning than by isolating the air conditioning ground from the boat' s DC ground. I'm just a dumb mechanical engineer, but I've sat on enough standards committees to know that their output is influenced as much by commerical interests as by technical reasons. I would ask Charley to explain a bit more about how a fault in an isolated air conditioning circuit is going to cross over to your boat's DC system. You could have reverse polarity issues in the second circuit, but if you have a RP light on that circuit you are no less protected than you are now.

As far as the two neutrals are concerned, you shouldn't have a problem as long as the boat is wired as you describe. If the second circuit is only neutral, load and ground wires to the air conditioning system, you have established a separated neutral bus for that circuit.
Indeed, the wiring is as you state: There are two separate AC distribution panels (one for each circuit) with the protective circuitry (proper capacity breakers, RP indicator, etc) - The air conditioner 1 and 2, the cooling pump are on their own AC feed with individual breakers... None of this stuff has any connectivity to the first shorepower AC connection that has the charger/inverter, hot water heater, AC outlets, etc; with its own AC distribution panel.

I think I am on the right track here - After checking the wiring to make sure there are no surprises, I'll draw a diagram and keep it for future reference as you gents suggested.

Thanks for the input!

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Old 19-06-2009, 11:27   #8
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[QUOTE=Sailndive345;294355]Folks,

The second shorepower connection was added when I had twin A/C units installed right after the purchase. This connection feeds the A/C units only and is independent of the boat DC system (I'll follow the wires and double check to avoid surprises.) It seems I don't need to worry about adding an isolation transformer to this circuit.

Am I on the right track? Should I be adding another (or the same?) transformer to the second AC connection?


I definitely install an isolation transformer to feeds the A/C units. A/C units through the condensate line can also leaks current back to shore power. Quiet simply any electrical appliances that require to be earthed need to be supplied by an isolation transformer.
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