Galvanic isolators

[Craig G]

Does anyone have any experience or recommendations when it comes to buying and installing a galvanic isolator. I am about to swap out my shafts due to electrolysis damage and am hoping to prevent that from happening again. Thanks.

[CaptPatrick]

Craig,

GUEST Galvanic Isolators protect your boat from galvanic corrosion and metal loss that can cause thousands of dollars in damage. Isolators meet ABYC A-28 “Fail-Safe” standard for continuous protection, prevent galvanic corrosion due to dissimilar metals between boats and block DC current from flowing through shore power cable. Made with gold plated terminals which provide superior corrosion resistance for harsh environment exposure, each is Ignition Protected and comes with a Five Year Warranty.

Here's a good list of what Amazon.com carries

Galvanic Isolators (courtesy of PETER KENNEDY YACHT SERVICES)


What is a galvanic isolator and why should my shore power system have one?

A galvanic isolator is a device used to block low voltage DC currents coming on board your boat on the shore power ground wire. These currents could cause corrosion to your underwater metals; through hulls, propeller, shaft etc.

Boats in a marina plugged into shore power all act as a giant battery. They are all connected together by the green shore power ground wire, which is (or should be) connected to their DC grounds, engine block, and bonded underwater metals. If the boats are in salt water then that forms an electrolyte and the dissimilar metals connected together act as a battery, causing corrosion.

The galvanic isolator has two pairs of diodes set up so that a voltage of about 1.2 volts is required to cause them to conduct. As most DC voltages caused by galvanic action will be less than this, they are blocked. Good quality isolators also contain a capacitor, which only conducts AC current, as a backup.

Normally no AC current is carried on the shore power ground wire, but it has to be able to carry the full load of the circuit in the event of a fault. Therefore it is important to have a good quality unit that will not overheat when required to carry the rated load. Some heat will be generated by the voltage drop and the unit must be able to withstand this.

As the galvanic isolator fulfills such a key function in the AC circuit it is only prudent to use the best quality unit available.

How to test a galvanic isolator that doesn't have a monitoring system..

Galvanic isolators are a bit difficult to test. Current ABYC standards require that galvanic isolators be self testing. For testing older isolators without this feature there are a number of methods that can be used, the one that follows is one that I found most useful.

Unplug the boat from shore power before starting the test.
Disconnect one lead of the isolator so that you are testing it only. Get a digital multimeter set to the diode test function. Put one lead on one side of the isolator and the other lead on the other side.

As the capacitor starts to conduct current the reading should rise to approximately 0.9 volts. Remove the test leads, short the two wires of the isolator together to discharge the capacitor and repeat the test with the test leads reversed. You should get the same answer.

Interpreting the readings:

If the reading is instantly 0.9 volts then the capacitor is defective or there is no capacitor.
If a voltage of 0.45 volts is observed one of the diodes is shorted.
If there is a reading of 0 volts then both diodes could be shorted.
If there is a reading in excess of 0.9 volts then one or both diodes are open (not conducting) in which case you should stop the test before the capacitor reaches 2.0 volts or you will damage it.

[Craig G]

Thanks Patrick. Is it safe to say, based on your response that you recommend that I get a GUEST galvanic isolator? Also, do most of them come with a monitoring system, and if not, do you recommend getting one that does, or is the self test feature adequate?

Thanks

[CaptPatrick]

Craig,

Guest is a good brand, but certainly not my my only recommendation... ProMariner is also good. I don't think you need a monitoring system, but you do want a Fail Safe model.

The term “fail-safe” in this application is an engineering term that describes the default failure mode of the isolator in the event of a catastrophic event such as a lightning strike.

It’s really important that the integrity of your boat’s grounding system be maintained no matter what, and this system is what ensures that shore power short circuits and ground faults with your onboard AC appliances get back to the source for your shore power on the dock. Ensuring this is what helps minimize electrical shock hazards on board your boat. Galvanic isolators of old could easily fail in an open-circuit mode, and since they are installed in a series with your boat’s green grounding wire, a failure in the open-circuit mode would be the same as cutting the grounding conductor, effectively eliminating the all-important grounding path back to the source of power.

The ABYC electrical committee that oversees the standard that applies here mandated that galvanic isolators be of the fail-safe design five or six years ago. What this means technically is that although the isolator might fail to provide galvanic isolation after a catastrophic event, it will never fail in an open circuit mode, and should maintain its capability of mitigating shock hazards.

One thing is for sure: In the event of a lightning strike to or near your boat, this is a device that should be tested by a qualified ABYC technician to make sure it’s still in good operating condition and can still provide galvanic isolation.

[Whaler1777]

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

[gplume]

I have a guest galvanic isolator...about 2006 vintage. Instaaled the optional monitoring system also.

Boat had a severe issue frying through zincs when i bought it. Rewired ( might have fixed the issue with out knowing it.) installed galvanic islator, and zincs wear down at an an acceptable ......normal level. Monitor light indicating a fault went on after year one, and has never gone out. Tested unit backwards forwards, and any way you can think of...nothing wrong with the function. Was disappointed that i could not get wiring diagram's from guest. Thats my only gripe with the guest unit Works well, but don't know why the fault indicator is always on.

I believe the galvanic isolator is a must have unit. For biiger boats than ours, an isolation transformer sems lie the way to go....but i think over kill for a b31

[Craig G]

Thanks as always for all the excellent info. I'm pretty sure my boat does not have a galvanic isolator unless it is hidden somewhere that I have yet to find. That will be on my list of things to do during or shortly after my upcoming haul out as I will be replacing my electrolysis damaged shafts during the haul out.