Isolation Transformers

Clean, safe shorepower can be yours.

By Charles Fort

If you could put a single device on your boat that would make your boat shockproof for swimmers, prevent galvanic corrosion, stop the worry about reversed shorepower polarity, and give you clean AC power for sensitive electronics, would you want one? If you answered yes, read on.

Isolation transformers are a way to achieve all of those goals. Without getting too technical, think about an isolation transformer as your own private onboard power source that uses your boat's shorepower connection. Confusing? Not really. An isolation transformer takes your marina's often wild and unpredictable 120VAC shorepower and converts it to pure clean power. And by creating an onboard power source, it greatly enhances the safety of those on your boat or swimming nearby.

The Green Ground Safety Wire

Most of us know how important the green ground shorepower wire is. It carries fault current (electricity that's going somewhere it's not supposed to, like when shorepower shorts against a metal case onboard) back to shore where it can't hurt anyone.

But marina shorepower systems may be less than reliable. Due to long-term corrosion or improper installation, the ground wires are sometimes not properly connected, meaning you (and nearby swimmers) are not protected from a fault if the AC shorepower shorts into the DC system. This could happen because of a problem in any AC/DC appliance, such as a battery charger. If that happens, any fault current is going to follow a path all through the boat's DC ground and bonding system, which is connected to the engine and underwater fittings, such as thru-hulls and prop shafts. Because leaking current always searches for a way back to its source (in this case, the marina's shorepower system ashore), leaking current will exit the boat and head toward shore. If a swimmer passes through the current, they will be electrocuted and may be killed. This is called Electric Shock Drowning (ESD), and every year several people are killed this way.

The beauty of an isolation transformer is that because it's taken over duties as the boat's power source, any leaking current will simply return to the transformer on the boat, protecting everyone in the water. A great side benefit is that the transformer automatically corrects polarity problems from the shorepower. Reversed polarity can be dangerous because AC appliances that should be off when their power switch is turned off will still have current flowing into them. Even worse, when polarity is reversed on some household appliances, such as refrigerators, the metal case may be energized with 120VAC. Anyone who comes into contact with that refrigerator and a ground could be electrocuted. Need more convincing? Isolation transformers also prevent galvanic corrosion that can occur between boats in a marina that share a common ground through the AC shorepower. This connection can cause neighboring boats to damage or destroy each other's less noble underwater fittings, like aluminum outdrives. And finally, isolation transformers supply clean power to such sensitive AC electronics as computers and plasma TVs.

So all of this goodness must come with a price, right? Sure, you don't get something for nothing, and in this case you pay two ways: Cost and weight. Transformers are filled with copper wire and by their nature are heavy. A typical transformer for a 30-amp shorepower system weighs about 60 to 70 pounds and must be installed within 10 feet of the shorepower inlet. Small boats may not tolerate the extra weight well. Costs range from about $600 on up, plus installation, though the peace of mind may be well worth the price. 

— Published: July 2015

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Polarity Transformers

Polarity transformers are a slightly different type of device. Some experts feel these are even safer than isolation transformers because they retain the green shore power ground wire. They do everything a regular isolation transformer does but require the addition of a galvanic isolator to prevent boat-to-boat galvanic corrosion, increasing the cost somewhat. Either of these systems will vastly increase electrical safety of those onboard and in the water, as well as provide clean proper-polarity power.