How to prevent an earth loop on your boat

Your connected shore power may be creating unwelcome stray current corrosion, warns Andrew Simpson

It’s impossible to get to grips with electrically promoted corrosion – as distinct from that caused by oxidation, such as rusting – without appreciating the extent, vigour and unpredictability of the electrical frenzy that surrounds us.

Our boats are assembled from materials of varying potential. We then launch them into a soup, also of varying potential, and – as if that wasn’t enough – teeming with stray leakage current that we can neither see nor easily measure.

The figure above shows two yachts, both properly connected to the shore power. In compliance with recommended practice, each has its AC and DC grounds bonded together. Neither is drawing power from the pontoon but this doesn’t matter. Even with their systems dormant, the earth wires continue to link them, binding them into a galvanic relationship in which the most anodic boat will suffer damage. Leakage currents will make the situation even worse.

The temptation for boat owners is to break the loop by simply disconnecting the shore AC earth wire. But this is not without its risks even if the supply is protected by an onboard RCD (residual current device) – an absolutely essential safety measure.

A much safer strategy is to block the path for tiny DC currents to escape while still allowing full AC fault currents to run safely to earth. There are two ways of achieving this.

Isolation transformers

Although heavy and relatively expensive, these should be seriously considered for larger vessels with elaborate AC systems and those with conductive hulls – i.e. aluminium, steel or carbon fibre composite. Here there’s no direct connection with the shore supply.

The power is transferred by induction from one coil to another. A boat ground plate is necessary to maintain the neutral at earth potential but at least there are no polarity problems since isolation transformers don’t care which way round the shore power arrives.

Galvanic isolator

Lighter and less costly, these are a good choice for simpler systems.

A pair of diodes are arranged in parallel on the earth. An AC current will pass straight through – in or out – while a low voltage DC current will find one diode barring its way entirely, while the voltage drop (typically about 0.7V) across the other diode will negate its effect if coming in the opposite direction.

If that isn’t enough, the diodes can be wired in series to double their effect.

About the Author

RYA Electrics Handbook author Andrew Simpson is a yacht designer, boatbuilder and surveyor, who has written several other books on boating subjects, including the RYA Boat Maintenance Handbook

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