Install A Galvanic Isolator

Protect your boat – and your crew – with this inexpensive upgrade to your boat’s power system.

If your boat has an AC power system that’s connected to shore power when docked, it likely needs a galvanic isolator, a low-cost upgrade that provides protection to the vessel and increased safety for the crew. Here’s a look at galvanic isolator basics, from what they do to tips on installation and maintenance.

Why It’s Needed

Galvanic corrosion occurs when two dissimilar metals are immersed in an electrolyte (a liquid that can conduct electricity, such as saltwater) and connected, either by direct contact or an external wire. This creates a natural phenomenon known as a galvanic cell, where electrons will flow from the less noble, more negatively charged metal (anode) to the more noble, more positive metal (cathode). The amount of this low-level DC current flow is determined by the metals involved but is typically less than 1.2 volts DC.

What this flow of electrons means is that the more noble metal will be “protected” by the less noble metal, which will begin to corrode in the process. Any boat with different metals immersed in water (e.g., bronze props, stainless steel shafts, aluminum sterndrives) can act as a galvanic cell and, in turn, be subject to galvanic corrosion.

This localized galvanic corrosion can be controlled by adding an even more active (less noble) metal into the mix, which will then sacrifice itself to protect the other metals. The most common metal used for these sacrificial anodes is zinc, although magnesium and aluminum are also used depending on whether the vessel is in salt, fresh, or brackish water. The surface area of the sacrificial anode determines the amount of protection, while its weight determines how long that protection will last.

For example, if your vessel has a stainless-steel propeller (more noble metal) installed on an aluminum outdrive (less noble), you need to add a sufficiently sized sacrificial anode(s) to the outdrive to protect it.

If the vessel is electrically isolated from other vessels, monitoring and maintaining this system is straightforward. Once the sacrificial anodes are installed, you then monitor your vessel and the condition of its anodes to ensure that the outdrive is always protected against galvanic corrosion.

When your boat is plugged into shore power, however, the AC power system green wire ground connects your boat to the marina power system and every other boat plugged into it, along with any galvanic corrosion issues they may have. This can lead to problems with your own vessel’s protection system, ranging from accelerated rates of deterioration of your sacrificial anodes to corrosion of the underwater metal you want to protect.

In other words, when your boat is plugged into shore power, any other boat plugged in that has insufficient sacrificial anode protection will naturally act as a drain to a boat that is properly equipped with adequate anodes.

Let’s say you have an inadequately protected aluminum outdrive and your dock neighbor’s boat is an inboard with a similarly unprotected stainless-steel shaft and bronze propeller. Connection via the AC power system green wire ground creates a galvanic cell between the two vessels with the potential to generate the low voltage DC galvanic current mentioned above. As aluminum is less noble than both stainless-steel and bronze, this connection means that your unprotected aluminum sterndrive will essentially act as a giant sacrificial anode, protecting the stainless-steel shaft and bronze prop on your neighbor’s boat.

The two methods of addressing this issue (other than not plugging into AC shore power) are installing either an isolation transformer or a galvanic isolator.

• Isolation transformers, which are typically found on larger vessels, are heavy and expensive.
• Galvanic isolators are small and relatively inexpensive, making them the best option for many boats.

What A Galvanic Isolator Does

A galvanic isolator (GI for short) blocks the flow of any low voltage DC current that may be present on your AC shore power cord green wire ground (from other vessels or even the dock wiring itself), preventing it from affecting your vessel. It does this while maintaining the continuity of the ground wire, allowing any AC fault current present to flow back to the dock pedestal, where it can either trip the circuit breaker or flow to ground.

It may seem like a good idea to simply cut this green wire ground and isolate your vessel – but don’t do it!! Without the protection provided by this green wire ground connection, a fault in the AC wiring or onboard AC powered equipment could energize the equipment case, vessel, or surrounding water with deadly results. Safety always takes precedence over corrosion control.

How A GI Works

A galvanic isolator is essentially a blocking diode assembly that prevents the flow of unwanted, low-level DC along the green wire ground mentioned above, galvanically isolating your boat from the dock and any neighboring boats.

A galvanic isolator is installed in series in the green wire ground conductor between the vessel’s shore power inlet and main panel grounding bus. The cut-on point for the GI diode assembly is around 1.2 volts DC, at which point any galvanic DC voltages present are blocked. Most all DC galvanic levels are below this 1.2V DC threshold.

Selection

Choose a good quality marine-grade galvanic isolator, one that has a UL (Underwriters Laboratories) marine rating. UL-rated units are certified as being fail-safe, meaning that should the diodes fail, the safety green wire ground will still be connected, ensuring a pathway to ground exists should an AC fault occur.

GIs are selected based on the amperage rating of the isolator (i.e., the amount of current the isolator can handle under severe fault conditions). The unit you choose will be based on the system amperage of your vessel’s AC circuit, which is typically 30 or 50 amps.

Most galvanic isolators are rated to handle 1.4V DC for stray current protection. Better quality galvanic isolators also include a capacitor and are designed to pass through AC shore power voltages (typically 115V to 230V) and fault currents. AC current is not normally found on the AC shore power green wire ground, however, should an AC fault occur, the ground wire and GI must be able to carry the full load (amperage) of the AC power circuit.

Installation

A GI is installed between the grounding system of your boat and the ground wire of the shore power cable. While you’ll want to follow the specific manufacturer instructions provided with your unit, here are some general recommendations that can be applied to most any galvanic isolator installation.

1. Turn off all AC breakers at the pedestal and on your boat, then disconnect the AC shore power cord from both the boat and pedestal. If your vessel has a DC-to-AC inverter, make sure that unit is also secured (turned off) and disconnected.

2. Securely mount the galvanic isolator inside your vessel on a solid surface near the shore power inlet entry for your boat. (An area near the inlet is usually best because presumably you’ll need less wire to install it.)

The galvanic isolator should be easily accessible, well-ventilated and protected from the environment. (If you have two 30-amp inlets, you will need an appropriately sized GI for each.)

3. Disconnect the ground wire from the rear of the shore power inlet. This wire will be either green or green with a yellow stripe, assuming it is properly color-coded.

4. The GI will have either two terminals or a wired input and output. The labeling will vary among models, but one will be marked “Boat Ground,” “Ship,” or something similar, while the other will be labeled “Shore Ground,” “Shore,” and so on.

For units with terminals, connect the vessel ground wire removed in step 3 to the galvanic isolator terminal designated as the vessel ground. Then install a ground wire between the GI shore ground terminal and the shore power inlet (many units will come with this additional section of ground wire).

For galvanic isolators with wires, connect the green input wire of the isolator to the vacated terminal of the shore power inlet mentioned in step 3. Next, connect the remaining output green wire from the isolator to the vessel ground wire you removed, using a good quality marine-grade connector. Note that the unit must be hard wired without the use of any friction type connections (no spring-loaded alligator clamps, for example).

5. Neaten up and support the wires as required.

6. Test the AC power system for proper operation, while verifying that the galvanic isolator is functioning properly with no faults shown.

Remote Monitor

In addition to any indicators on the unit itself, your galvanic isolator should also have a remote monitor panel if the unit is installed in some out-of-the-way location, which most typically are. This allows you to monitor the operational status of the unit easily and often. This is crucial, as the unit can stop working for several reasons, including major electrical faults and nearby lightning strikes.

Maintenance

Follow all maintenance procedures for your galvanic isolator as required by the manufacturer. In most cases, this will be minimal, with recommendations to check the unit and connections for corrosion or physical damage on a regular basis (annually at a minimum). It’s also good practice to check the remote monitor for faults any time you plug into shore power or after any major electrical event (e.g., nearby lightning strikes).

It’s not often that a boat owner can install a piece of equipment that for around $200 protects both boat and crew. Galvanic isolators offer peace on both fronts at a reasonable price, a win for everyone.