galvanic isolators

[Mikey]

Sometime back (probably years ago) there was a discussion of the high price of galvanic isolators. In the discussion, as my fading memory recalls, the simplicity of the item was explained and the fact that anyone with a soldering gun and time could easily make one and save piles. Well, time is the enemy and I need to buy a 30 amp isolator. I have searched the usual suspects on line and find them expensive for what I was lead to believe. Have they become more complicated or are the limited number of manufacturers conspiring to get into my wallet. Where do I go from here.

[TailhookTom]

Mikey:

I remember the discussion thread -- something about less than $10 of parts? All I remember was diodes and clears --- I think I might have had a few too many clears, but this is the time for me to do the same thing.

TOm

[Bruce]

You'll need two diodes wired in parallel pointing in opposite directions to stop the ac fault which can flow both ways rated at least 30 amp and a good heat sink. Mount with heat sink grease to dissapate the heat.

You can use diodes out of an old battery charger as long as they are rated for 30 amp or more and they may have a heat sink with them.
You can also use half a bridge rectifier is properly rated.

A good isolator should have two diodes in series in each direction to give a 1.2 volt isolation.

Which begs me to ask a question.

You've gone this far already what's another 150 bucks for an isolator?

Budget you say.
Never saw one of those associated with a boat.

Honest mr. insurance investigator, that melted black box was an isolator, not an incendiary device.
No I don't have ties to save the owls and never contributed any money to Al Gore's global warming movie.

[Kingfish]

Here's a reasonally priced one.


http://www.yandina.com/GalvInfo.htm

[Mikey]

You right, Boss Bruce, You right.
Silly me. Bucks up budget down.

[IRGuy]

Another electrical issue request from me...

Galvanic isolators vs. Isolation transformers...

Why install one vs the other? I have heard isolators are undependable (can't recall who said that or why they did) and isolation transformers are heavy, take up a lot of space, and are expensive.

Comments please?

[Bruce]

Glavanic isolators like any piece of equipment can be damaged by surges and lightning.
Shorted they provide no voltage drop and resistance to the low galvanic currents and open there is no connection to shore ground.

They can be measured with a meter as often as you like to check.

Most all marine isolation transformers are also voltage boost transformers.

The basic transformer operation is to physically disconnect one voltage source from another.

They are heavy depending on the KVA rating and are expensive.
They have no place on a 31 Bertram or most boats under 45'.

The boost function can be very important in places like the Bahamas where low dock voltage can ruin compressors and such. 208 volts is common.

[IRGuy]

Bruce...

Thanks again for your help! Sorry if I am a PITA.

From what you said I assume then with a B33 and two 30 amp shore power lines two galvanic isolators will do the trick.

Where do you measure to verify they are operating correctly?

and...

Do you recommend installing permament meters to monitor them to ensure constant protection?

[Bruce]

Use the diode check function on a multi meter between the two terminals with all other wires disconnected.

Don't need a permanent meter.

[Sean B]

From what you said I assume then with a B33 and two 30 amp shore power lines two galvanic isolators will do the trick.

Frank,

If your B33 AC wiring is the same as it is on my B33, you don't need and can't use two galvanic isolators. Both the house 30A circuit and the A/C 30A circuit share the same ground, which is wired from the shore power cords to the panel, but not (yet) to the panel ground bus. In the panel the two grounds from both shore power lines are spliced together, then routed aft again to the isolator (under the starboard engine cover), and then back to the panel ground bus. Took me a bit to figure that one out, but that is how they did it.

In other words, at the isolator, there is one ground (green) wire for both shore lines. You should get a 60A isolator though, not a 30A or 50A. The rating doesn't matter much unless you get the a good one with the capacitor, the straight diode units are the cheap ones. The 60A-rated isolators are a little difficult to procure but they are available.

You can't really use two isolators the way the boat is wired. Both shore lines share a common ground, as the two lines come into the boat, and also at the bonding system.

If you are replacing your isolator, and using one of the good models that includes a capacitor, there is an "in ground" (shore power side) and an "out ground" (boat side). Make sure you don't mix those up. Disconnect both green wires from the existing isolator, and use a multimeter on resistance setting, with one lead on the shore power ground lug, and check both the wires you disconnected. You will find that one is wired to (either) shore power ground ("in ground" on the isolator), the other is wired to the bonding system ("out ground").

The newer ones also have the now ABYC-required LED waring lights, that tell you if/when the unit is shorted out from a big surge, or if a stray current is running that is over 1.5 volts....in which case the diodes are overpowered and are doing nothing to help you anyway. In the latter case, find the problem quick, and don't go swimming...

[IRGuy]

Thanks Bruce and Sean, I appreciate your help! I am way out of my league in the electrical world, and don't want to make any mistakes if I can help it.

Sean.. I will drop you an email via the B33 forum link.

Frank