Earthing, Bonding, and Galvanic Isolation Explained

Few yacht electrical topics create more confusion than earthing, bonding and galvanic isolation. They are often discussed as if they are one problem. They are not.

Earthing is about electrical safety and fault paths. Bonding is about connecting conductive parts so dangerous voltage differences are controlled and corrosion systems behave as intended. Galvanic isolation is about reducing unwanted DC galvanic current paths, especially through shore-power connections and marina environments.

When these ideas are misunderstood, the symptoms can appear everywhere: accelerated anode loss, underwater metal damage, nuisance trips, shock risk, noisy audio, SatCom or radio interference, strange continuity readings, or engineers inheriting a refit where nobody is sure what was changed.

Applicability depends on vessel size, voltage, shore-power arrangement, hull material, class/flag requirements, whether the yacht follows large-yacht or small-vessel standards, marina supply conditions, transformer arrangement, galvanic isolator design, cathodic protection system and local electrical rules.

This article explains concepts and evidence. It is not a wiring design instruction.

Use this separation onboard:

• Earthing: how fault current returns so protective devices operate and exposed conductive parts do not remain dangerous.
• Bonding: how metal parts are connected together for safety, reference, corrosion control or lightning strategy.
• Galvanic isolation: how the yacht avoids becoming part of a marina-wide corrosion circuit through the shore-earth path.

The same conductor or connection may affect more than one issue, but the design intent matters.

At anchor, the yacht is mostly its own electrical environment. Alongside, it becomes connected to shore infrastructure and often to other vessels through marina earth paths, water, shore cables and immersed metals.

That connection can create a low-voltage DC path that has nothing to do with running the yacht's AC loads. The shore-power protective earth conductor may link the yacht's bonding system to the marina earth system and, indirectly, to other boats connected to the same infrastructure. At the same time, the yacht's underwater metals sit in seawater, which acts as an electrolyte. If there is a voltage difference between bonded metals, shore earth, neighbouring vessels or marina structures, a small current can flow through the water and the bonding path.

That small current is enough to move metal ions. In simple terms, the less noble metal gives itself up faster. Sacrificial anodes are fitted so they are the preferred part to waste away, but if the shore-power connection changes the current path, the anodes may disappear unusually quickly or another underwater fitting may become part of the sacrifice path.

Common yacht examples include:

• zinc or aluminium anodes losing material much faster when the yacht is connected to shore power than when it is at anchor,
• bronze through-hulls, propeller components, rudder stock fittings, stabiliser fins or trim tabs showing corrosion that does not match normal service history,
• a tender-lift, swim platform, passerelle or underwater-light installation introducing a new bonded metal path after refit,
• shore-power trips, audio hum, corrosion complaints and strange continuity readings appearing together after a yard period,
• one marina berth causing repeated corrosion symptoms while another berth does not.

This is why "bad anodes" is not always the answer. The real question is whether the yacht has become part of a wider marina corrosion circuit.

Isolation transformers and galvanic isolators are common tools, but they are not magic boxes. They must be correctly specified, installed, tested, labelled and maintained. Technical crew should know which one is fitted, where it is, what it protects, and what failure indicators mean.

An isolation transformer separates the shore supply from the yacht's onboard AC system through magnetic coupling rather than a direct conductive connection. Correctly designed and installed, it allows the yacht to establish its own onboard neutral/earth reference on the secondary side and reduces the direct galvanic path through the shore earth connection. On larger yachts or more complex shore-power arrangements, isolation transformers may be part of the shore-power converter, transformer room or main switchboard architecture. The key point for crew is not just that a transformer exists, but whether the shore connection in use actually passes through it.

A galvanic isolator is different. It is normally installed in the shore protective-earth path and is intended to block low-voltage DC galvanic current while still allowing dangerous AC fault current to pass so protective devices can operate. In practice, this means it is a corrosion-control device that must not compromise shock protection. Technical teams should know whether it is fail-safe, monitored, alarmed, correctly rated and included in periodic testing.

The bonding system is another part of the protection picture. Bonding connects selected metal parts so dangerous voltage differences are reduced and cathodic protection behaves predictably. It may include engines, shafts, struts, rudders, through-hulls, tanks, fuel fittings, stabilisers, underwater lights, metal swim-platform parts and other conductive items depending on the vessel design. Bonding is not simply "more wires are better." The right question is whether the actual bonding network matches the approved or intended design.

Sacrificial anodes and, on some vessels, impressed-current cathodic protection systems help control which metal gives itself up. Anode condition is therefore evidence, not just maintenance consumable status. A sudden change in anode life after a marina stay, shore-power modification, underwater-light installation, charger change or bonding repair should trigger investigation.

Corrosion protection can be defeated by innocent-looking changes:

• a shore-power earth bypass added during fault-finding,
• a galvanic isolator bypassed temporarily and never restored,
• a second shore-power inlet or converter path that does not pass through the same protection,
• a battery charger, inverter, UPS, AV rack, SatCom rack or IT cabinet introducing an unintended earth or bonding connection,
• underwater lights, stabilisers, thruster tunnels, passerelles or swim platforms added without updating bonding records,
• a marina cable, adaptor or pedestal fault creating abnormal earth or leakage conditions,
• contractor test leads, temporary supplies or yard wiring left in place.

For example, a yacht may have an isolation transformer on its main shore-power inlet but later add a separate temporary shore feed for chargers or yard loads. If that secondary feed bypasses the intended isolation strategy, corrosion symptoms may appear even though everyone believes the yacht is "transformer protected." Another common pattern is an AV/IT or charger installation that creates a new connection between equipment racks, DC negative, bonding conductors and shore earth. The symptom may be audio noise or network rack hum, but the underlying issue may be electrical reference and corrosion control.

Good protection therefore depends on architecture, not brand labels. The evidence should show shore-power path, transformer or isolator status, bonding layout, anode history, earth-fault records, recent modifications and any temporary supply arrangements.

During refits, bonding conductors are sometimes moved, extended, disconnected or reconnected casually. That is dangerous. A bonding change can affect shock protection, corrosion behaviour, lightning strategy, fuel-system safety, navigation electronics and survey evidence.

If a contractor says "we just need an earth," the technical team should ask:

• Is this protective earth, bonding, functional earth or shield/drain reference?
• What standard or drawing supports the connection?
• Where does it terminate?
• Is it part of the cathodic protection system?
• Is it temporary or permanent?
• Has continuity been tested?
• Are drawings updated?

Common mistakes include:

• assuming bonding and earthing are identical,
• disconnecting bonding to "fix corrosion" without understanding safety impact,
• adding random earth straps to solve noise problems,
• bypassing a galvanic isolator during troubleshooting and forgetting to restore it,
• not knowing whether an isolation transformer is actually in circuit,
• assuming one protected shore inlet means every shore-power path is protected,
• ignoring temporary yard supplies, chargers, UPS feeds and secondary shore feeds,
• treating anode loss as a diver-only issue,
• not recording shore-power corrosion complaints by marina, berth and supply.

Electrical safety and corrosion control need joined-up thinking between engineering, electricians, divers, class/survey where relevant, and sometimes corrosion specialists.

A yacht arrives in a marina and starts reporting nuisance trips, fast anode loss and intermittent audio hum. The AV team blames the amplifiers. The engineer suspects bad shore power. The diver reports unusual anode consumption. A contractor says the bonding looks "modified."

This is not one department's problem. The correct response is to review shore-power arrangement, isolation equipment, protective-earth continuity, bonding records, anode history, recent refit changes, and whether new equipment introduced an unintended connection.

Maintain an earthing, bonding and isolation evidence pack:

1. Identify shore-power arrangement.
2. Confirm isolation transformer or galvanic isolator status where fitted.
3. Confirm whether all shore inlets, temporary supplies and charger feeds pass through the intended protection.
4. Review bonding drawings against actual installation.
5. Check recent refit changes involving metalwork, AV/IT racks, chargers, shore-power gear, underwater lights, stabilisers, passerelles or swim platforms.
6. Review anode replacement intervals and compare alongside periods with time at anchor.
7. Record marina-side trips or corrosion complaints by berth, pedestal and shore cable.
8. Escalate unexplained anode loss, shock reports, repeated trips or undocumented bonding changes.