A yacht or commercial vessel is an important asset worthy of the best corrosion and safety grounding protection available. While protective coatings and anodes can help protect your boat, a more reliable method is required to deal with the galvanic corrosion circuit created by the shore power connection.

The Galvanic Corrosion Problem:

Whenever a boat is connected to shore power, the hull and drive system are connected to the shore grounding system and to other adjacent vessels (also connected to shore power) via the grounding conductor in the shore power cable. This connection, while required for safety, creates a galvanic corrosion cell involving the dissimilar metals between boats and between a boat and the shore grounding system.

 

1. Anode – a metal surface which gives up metal ions (corrodes)
2. Electrolyte – a medium which conducts ionic current between the anode and cathode
3. Cathode – a metal surface that picks up metal ions
4. Metallic Bond – a continuous metallic path that allows current to flow from cathode to anode

 

Dairyland Galvanic Isolators Address Galvanic Corrosion

A simple method which essentially eliminates this galvanic corrosion problem is to insert an appropriately rated and certified Galvanic Isolator in series with the grounding conductor of the ship-to-shore power cable. The function of the Galvanic Isolator is to provide AC continuity of the grounding conductor (required for safety in the event of an AC fault) and to block the flow of corrosion-causing galvanic current. This effectively removes the galvanic corrosion circuit while retaining safety grounding.

 

Product Selection:

Dairyland Galvanic Isolators are sized to match the steady-state AC rating of the shore power service, which also provides an appropriate associated AC fault rating. If an alternate location is planned or higher fault current is expected, there are other methods available for rating the products. This can include sizing the products based on known fault currents, coordinating with upstream circuit breakers, or coordinating with grounding conductor size and ampacity. Contact us if additional information is needed.

Shore Power Size	30A	Dual 30A	50A	Dual 50A	100A	200A
Recommended Dairyland Galvanic Isolator	GI-50/60A-FSP	GI-50/60A-FSP	GI-50/60A-FSP	GI-100A-FSM	GI-100A-FSM	GI-200A-FSM

 

Alternate Product Selection Method – AC Fault Current

Dairyland Galvanic Isolators are typically sized to match the steady-state AC rating of the shore power service, which typically also provides an appropriate associated AC fault rating. If an alternate location is planned or higher fault current is expected, there are other methods available for rating the products. This can include sizing the products based on known fault currents, coordinating with upstream circuit breakers, or coordinating with grounding conductor size and ampacity. Contact us if additional information is needed.

Whenever a marine vessel is connected to AC shore power, a galvanic circuit is established, which can cause the vessel and its drive system components to corrode. When there is a direct bond between dissimilar metals in an electrolyte, a galvanic corrosion cell is established. In this application, the direct bond is the grounding conductor in the ship-to shore power cable, the electrolyte is earth and water, and the metals in the marina and marina electrical grounding system are normally different than the metals in a boat.

A simple way to eliminate this corrosion-causing circuit is to install a decoupling device, such as a Dairyland Galvanic Isolator, in series with the safety grounding conductor in the ship-to-shore power circuit. This can be done one of two ways. A decoupling device can be installed by the marina or port operator on the shore side of the circuit, or it can be installed on the vessel by the owner. Either installation location will prevent the flow of corrosion-causing galvanic current while retaining the safety features of the grounding conductor, provided an appropriately rated and certified decoupling device is used. The following application guide deals specifically with preventive measures that can be taken by the marina or port operator.

 

Effective Grounding Path

When a decoupler is used in the safety grounding conductor of electrical equipment, it must meet certain code requirements. In the U.S., the applicable code is the National Electrical Code (NFPA 70). The most important code sections are 250.4(A)(5) which defines the requirement for “an effective AC grounding path,” and 250.6(E), which allows the use of a certified decoupling device for “blocking the flow of objectionable DC current”. Other countries have similar codes. A key requirement of Section 250.4(A)(5) is that if a decoupler fails, it must fail as a “short circuit.” In the short circuit mode, it must still be capable of meeting all published current ratings. If this requirement is not met, it is not “an effective grounding path”. Dairyland products recommended for this application meet and exceed this criterion. That is, when the Dairyland decoupler recommended for this application is tested to current levels that will cause failure, the product will fail in the short circuit mode and be capable of carrying current well in excess of every current rating (i.e., steady-state current, fault current, and lightning current).

On a related note, decouplers that are installed on a vessel must meet requirements which are defined in American Boat and Yacht Council (ABYC) Standard A-28. The ABYC standard does not require or reference the sections of NFPA 70. However, ABYC A-28 does define a class of galvanic isolators that are considered “fail-safe,” and when tested to assure fail-shorted operation would provide the needed effective grounding. Use on a shore-based electrical system would depend on the manufacturer’s device certification, and whether it allows such use. Dairyland specifically has certification for product use on shore electrical systems covered by the NEC. Other devices defined within A-28 that are not considered fail-safe must have a monitoring system to indicate if the unit is functional, and these types of devices would not meet the shore-based grounding codes and may not be used.

 

Typical Application

A typical installation of a decoupling device on the shore side of a ship-to-shore AC power circuit is schematically illustrated for a single-phase service in Figure 1 below. Similar criteria apply to other types of single-phase service or three-phase services that utilize a safety grounding conductor. The typical device used is one of the Dairyland Galvanic Isolator models chosen for the appropriate shore cord current rating, which also indirectly selects the AC fault current rating. When placed in series in the grounding conductor that feeds the shore cord receptacle, the Dairyland Galvanic Isolator will block corrosion current, allow any AC to flow, and will switch to the shorted mode as needed to address AC faults or conduct lightning current.

 

AC Fault Current Rating

The key rating of a galvanic isolator used for this application is the AC fault current. The ABYC standard relates steady-state AC current ratings to AC fault ratings, however it is the fault rating that is most important. If a Dairyland Galvanic Isolator is selected based on matching its steady-state current rating to the shore cord rating, and a user feels that there is a higher AC fault current available at this point in the system, a larger fault-rated Galvanic Isolator may be needed. The Dairyland product fault rating should exceed the available system fault current, considering the current magnitude and the time duration of the fault. There are three typical methods for determining fault current if this value has not been provided.

1. AC fault current may be controlled by the current-time clearing characteristic of the circuit breaker for the circuit of interest and selecting a device with ratings that exceed the breaker curve settings. If chosen correctly, the breaker will trip before the device ratings are exceeded.

2. The fault current may indirectly be determined by examining the current-time capability of the grounding conductor size into which the Dairyland device will be inserted and selecting a device that has ratings that exceed the wire capability ratings. Such data can be determined from cable short circuit rating charts, such as by ICEA in standard P-32-382-2007.

3. The most conservative approach is to determine the maximum fault current available at the upstream transformer terminals and rate the Dairyland device to meet that value. Transformer nameplate data contains the appropriate information to calculate this value.

If assistance is needed with any of these methods, or with other rating and application aspects regarding Dairyland Galvanic Isolators or decouplers, please contact us.

 

 

Notes:

1. Decoupler: A DC isolation/AC coupling device. Recommended: Galvanic Isolator models by Dairyland. Listed per NFPA 70, Article 250.4(A)(5), 250.6(E).
2. Do not install the decoupler in series with the secondary neutral.
3. Observe polarity marks when installing the decoupler.
4. There cannot be any bond between the neutral conductor and the grounding conductor downstream of the decoupling device because this would bypass the decoupler.
5. One decoupling device per vessel is recommended, (to prevent bonding the vessels to each other through the grounding conductors, which would establish a vessel-to-vessel galvanic corrosion cell.) However, this will depend on whether this feeder panel serves multiple vessels and if the decoupler can be located at a pedestal or junction box so that the vessel of interest is isolated from all other grounds/vessels.
6. Similar installation procedures apply to other single-phase and three-phase services that utilize a safety grounding conductor.
7. If there is any question as to proper installation procedures, contact Dairyland for assistance.

Onshore safety grounding systems are designed to protect your vessel and others when they are connected to shore power. However, this safety measure can affect boat hulls and metal components due to the presence of dissimilar metals in the grounding system and your boat. Aided by the water’s electrolytic properties, this galvanic corrosion will ultimately damage your yacht’s drive system and hull.

Protective coatings and sacrificial anodes can help prevent some corrosion from occurring, but the most reliable method of eliminating corrosion on your boat is through installing a Galvanic Isolation device. Placing a Dairyland GI in series with the grounding conductor for the shore power cable will:

• Block the flow of DC (galvanic) current that can corrode your boat
• Ensure AC current can flow to ground during a lightning or AC fault event
• Provide peace of mind with a fail-safe design and a virtually unlimited lifespan

Based on our experience with similar products in the energy industry, Dairyland manufactures Galvanic Isolator models to a higher level of performance because we recognize the safety and liability implications due to inadequate grounding. Dairyland brings this experience to the marine industry by offering a much more complete and conservative approach to boat grounding and galvanic isolation.

 

Reliable, Fail-Safe Design

The American Boat and Yacht Council wrote standard A-28, which includes criteria for galvanic isolators and the NEC requirement for safety grounding under all conditions. Through Dairyland’s efforts and example, the standard includes provisions for fail-safe design, ensuring an effective grounding path even when tested to the limits of the conductor’s current rating. This assured safety grounding is a critical safety feature. All of Dairyland’s devices are rated as fail-safe and come with high levels of AC fault capability, which exceed the A-28 required ratings.

Additionally, each device is rated with high levels of AC fault protection as well as a significant lightning surge current rating. During an AC fault or lightning event, the isolator will instantly switch to conduction, passing all current from the boat to ground. These important product ratings will ensure that the isolator withstands these conditions and minimizes the chance of a failure.

 

Rugged Marine Construction

Other isolators on the market generate large amounts of heat, which is not ideal for a marine setting. Compensation is designed into these products through heat sinks or cooling fins, open vents, or even small fans–all potential points of failure.

Designed for a marine environment with harsh conditions, the Dairyland Galvanic Isolator is hermetically sealed against moisture and has no known limit to its lifespan to date. Its design removes the need for heat sinks or fins, and each model comes with a high steady-state AC current rating, to match or exceed the rating of the ship-to-shore cable. This rugged design makes the Dairyland GI completely maintenance-free. Historically across product lines, in the marine world and in the energy industry, Dairyland’s failure rate is lower than 0.001%, so you can rest assured knowing your purchase will outlast any expectations.

 

Third Party Verified

At the core of Dairyland’s design philosophy is the desire to offer rugged products that last. Our customers can look to our extensive third-party certifications as an indicator of our commitment to product performance. Across the board, Dairyland products undergo independent laboratory testing and our galvanic isolators meet and exceed marine industry standard ABYC A-28 and are also UL certified to meet national electric code requirements for “effective grounding path”. Our products are used throughout the world in a variety of harsh environments, from marine settings to desert pipelines and are trusted to perform.

Captain David Rifkin, an expert in safety grounding issues on boats details how to guard against galvanic corrosion between bonded components of dissimilar metals on the same hull.  This article was published in the February/March 2010 issue of Professional Boatbuilder.

In this paper, previously published in Technical Exchange December 2006, Capt. David Rifkin, an expert in safety grounding issues on boats discusses the ins and outs of transformer installation on a boat’s electrical system.  This paper covers topics of both polarization and isolation methods including an emphasis on safety of personnel for each.