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The Complexity of Plugging in Theory, Standards, and Best Practices Behind Installing and Maintaining Shore-Power Transformers

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The Complexity of Plugging in Theory, Standards, and Best Practices Behind Installing and Maintaining Shore-Power Transformers BEST PRACTICES The Complexity of PLUGGING IN Theory, standards, and best practices behind installing and maintaining shore-power transformers. Text and photographs by Steve D’Antonio ot long ago, I sat with a client Many professionals in the industry 105 and 106). In either case, their great- Nhunched in the lazarette of his stumble on the transformer question. est attribute is saving lives by reducing newly acquired boat. Looking over These units are indeed beneficial in the risk of shock, electrocution, and the humming shore-power trans- many ways, and those who decide electric shock drowning, or ESD (see former, he said, “I’ve heard these are whether to use, install, advocate for, or sidebar on page 112). When wired in useful, but I have no idea why. Can sell them should thoroughly under- isolation mode, a transformer is also a you explain it?” The boat’s builder, stand why. deterrent against galvanic corrosion, struggling to articulate a response, Shore-power transformers can be which results when nearby submerged, finally said, “Trust me, they are good wired in one of two configurations—for grounded metallic objects interact via to have.” isolation or for polarization (see pages the shore-cord-grounding conductor. Above—Looking inside a new Hubbell auto-boosting shore-power transformer, note that the winding is potted in a sand/resin mix- ture, increasing its durability, moisture resistance, and weight. 100 PROFESSIONAL BOATBUILDER Installation Left—Some transformers are located near an electrical panel to minimize While the operating principle of a cable runs, but they all must have plenty shore-power transformer is simple, of ventilation to remove the heat they there are peculiarities to be aware of to generate, and away from bunks, where ensure a safe, reliable, and effective the hum of operation would disturb installation. sleeping crew. In many ways a transformer behaves Below left—A true shore-power trans- much like a power source, a generator, former—the one at the head of the an inverter, or the utility company’s dock—is the source that shore power will transformer at the head of the dock. strive to return to. Sometimes its path runs dangerously through the water. The most important safety character- istic is the path electricity takes after it leaves the transformer. Like all elec- They also generate and radiate con- tricity, it seeks a return to its origin siderable heat and need appropriate and not necessarily to ground. Elec- ventilation and air circulation. In addi- tricity “returns” to earth ground only tion, they produce a noticeable hum, when the power source is referenced which means you wouldn’t want one to earth ground. In the case of the under a berth or opposite a headboard. transformer at the head of the dock or If installed in accommodation spaces, in a marina parking lot, this is accom- under a helm or behind an electrical plished with a ground rod bonded panel, for instance, they should rest on to the transformer’s neutral output and flexible mounts. Some transformers are the green grounding conductor for the mounted amidships, near an electrical system (known as a grounded neu- panel and between bow and stern tral). The importance of this safety should never swim around vessels or inlets, to minimize wire runs. Ideally, feature cannot be overemphasized. docks energized with power originat- shore-power transformers should carry Shore-power electricity, once it passes ing ashore. a marine UL listing (while most indus- through the vessel’s transformer, will As well as protecting swimmers trial transformers are UL listed, few “return” only to that transformer, from electrocution, a transformer carry the “Marine” prefix), and adhere either through the white neutral con- eliminates the possibility of reverse to ABYC’s standards. ductor or the green safety-grounding polarity. That’s why boats with 120V conductor. (These two are typically onboard transformers are exempt bonded or connected either at the from ABYC’s reverse polarity indica- transformer output or at the electrical tor and double-pole branch circuit panel, but not both.) On transformer- breaker requirements. equipped vessels, it’s important that The benefits of transformers are there be only one connection location. tempered by their size, weight, and (For more on neutral-to-ground con- cost: The average 30-amp unit may nections, see the ProBoat online measure a little less than a cubic foot resource at https://www.proboat. (0.03m3) and weigh 60 lbs (27.2 kg), com/2011/10/demystifying-the-neu- while a 50-amp, 240V unit measures tral-to-ground-connection/.) Put sim- half again as large and more than 200 ply, if the grounding/bonding system lbs (90.7 kg). Transformers, especially is intact, electricity that originates at ABYC/UL-compliant boosting ver- an onboard transformer will not travel sions, can cost in excess of $5,000 through seawater to reach a utility (4,442€). company transformer at the head of the dock or elsewhere ashore. Instead, it will return to the vessel’s onboard Be aware that not all transformers are transformer, thereby protecting per- created equal. While they all have the sons in the water. Note that even if potential to be a fire hazard, the better operators know their boats are appro- options for onboard installation are UL priately grounded/bonded, they listed, and the best are UL Marine listed. OCTOBER/NOVEMBER 2019 101 BEST PRACTICES: Shore-Power Transformers Inner Workings The specification tag for a transformer Through the principle of inductance, will tell you a lot, including primary and a transformer can achieve either polar- secondary voltage and whether it’s rated ization or isolation in the case of an to operate at 50 Hz, 60 Hz, or both. isolation transformer. In brief, shore The latter is particularly important for power travels from the dock’s wiring, vessels likely to cross oceans on their own bottoms or to be shipped overseas, through the shore-power cable(s) to where standard electrical service varies. the boat’s power inlet. (Note: One onboard transformer is required for each shore-power inlet that may be in One aspect of UL Marine compli- use.) Instead of going from there to the ance for shore-power transformers is vessel’s circuit breaker panel, the isola- the electrostatic shield between the tion transformer is inserted into the primary and secondary windings, circuit first. The incoming AC power which prevents arcing between the travels through the primary (or input) two. The shield (though not rated for winding of the transformer and back to lightning strikes) must be capable of shore. That’s as close as the actual dock- enduring surges of 4,000V for only side current ever gets to the boat’s elec- one minute to comply with ABYC trical system. Because electricity is elec- E-11. Unlike ordinary transformers, referenced to the shoreside ground; tromagnetically induced on the trans- the windings for marine-rated units however, that measure is required only former’s secondary (output, or boat- are typically potted in a mixture of when wiring in the isolation mode. side) winding from the nearby primary sand and resin, a process that improves No connections should be made to winding, no direct connection between shock resistance and prevents water the AC safety ground wire located the two is required. absorption. The shield is usually between the transformer and the inlet. 102 PROFESSIONAL BOATBUILDER The inner workings and wire counts, and cost is not a factor, they windings of an unpotted trans- may make sense. Otherwise, the anvil- former are more likely to suffer like ruggedness of traditional trans- moisture issues over their life- formers would have greater appeal. time compared to a potted transformer. ABYC Transformer Guidelines hundreds of feet of heavy wire ABYC guidelines for transformer and iron cores, high-frequency installations are predictably detailed switching transformers use and specific, including sections on electronic circuitry, in much overcurrent protection. The relevant the same way inverters do, to sections are available as part of com- achieve isolation. They may plete ABYC standards or, for the pur- (depending on the specifica- poses of this article, on ProBoat’s web- tions), therefore, be smaller site at www.proboat.com. Follow them! and lighter than traditional A few details are worth more expan- transformers of equal capacity. sive consideration. For instance, if a The units I’ve seen rely on fan transformer is located within 10'/3m cooling, while most conven- (a measurement of the wire, not as the tional transformers do not. crow flies) or less from a shore inlet, No mention of transformer func- That means if the fan fails, the unit although a conventional circuit tion would be complete without may shut down, and the vessel may, as breaker is required, an ELCI (equip- noting there is a high-frequency a result, be without shore power. For ment leakage circuit interrupter) switching version. Instead of using applications where every pound device is not. While that satisfies the OCTOBER/NOVEMBER 2019 103 BEST PRACTICES: Shore-Power Transformers Overcurrent protection for trans- Polarization vs. Isolation formers can be tricky. Read and There are few but important distinc- fully understand the requirements tions between wiring a transformer for spelled out in E-11. Err on the side polarization or for isolation. of caution, and if in doubt, remem- The primary difference is in its ber that it’s safest to install a breaker, including an ELCI. means of grounding. The polarization transformer’s primary and secondary ground are connected, i.e., the vessel’s as close as possible to the shore onboard AC safety ground, bonding, inlet.
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