As soon as 230 volts come on board — via shore power, generator, or inverter — different rules apply than on the DC side. The reason is not only the voltage but also the environment: conductive structures, humidity, and a person who is often barefoot and grounded.
The Protective Earth Conductor
It is the central element of the entire system. Its task: to connect touchable metal parts to the earth potential in such a way that in the event of a fault, a current flows that triggers the protective device — before someone finds the fault with their hand.
From this follow the requirements: The protective earth conductor is continuously routed, never switched, never fused, and never interrupted. It is green-yellow, and this color is not used for anything else. All touchable metal parts of the AC system are connected to it.
WarningA broken protective earth conductor is not detectable from the outside. The system functions completely normally — until the first fault, and then precisely the device that is supposed to switch it off is missing.
Residual Current Protection
The second pillar is the residual current device (RCD). It compares the incoming and outgoing current and disconnects as soon as the difference exceeds a threshold — for personal protection, this is 30 milliamperes. A current flowing through a person triggers it long before a circuit breaker would react.
Typically, combined devices that unite residual current and circuit protection in one switch are installed: as main protection at the feed-in with 16 or 25 amperes and 30 milliamperes, followed by two-pole thermal circuit breakers for each consumer circuit, typically with 10 amperes.
Two-pole is not a detail: On board, it cannot be assumed which of the two conductors is the live conductor — at a reverse-polarity shore power connection, the assumption is incorrect. Therefore, both conductors are switched and disconnected.
The Three-Meter Rule
NoteIf the feed-in point is more than three meters away from the shore power connection unit, the supply line must also be protected near the feed-in with a two-pole circuit breaker according to EN ISO 13297.
The reason is the same as for the main fuse on the DC side: The section of cable between the feed-in and distribution is unprotected. For short distances, the risk is acceptable; for longer distances, it is not.
Multiple Sources — But Never Simultaneously
Shore power, generator, and inverter must never overlap. Two sources simultaneously on the same grid mean, at best, destroyed equipment, and at worst, backfeeding into the shore grid — meaning voltage on a cable that someone considers dead.
This is ensured by a transfer unit that always switches through only one source and keeps the others de-energized. With two sources — shore and generator — a changeover switch in the panel is sufficient; with three sources, a dedicated transfer unit handles this and additionally indicates which source is currently active.
The display is essential: Mains indicator lights and a voltmeter make it visible whether and with what voltage power is supplied. At a weak shore power connection, the voltage noticeably drops under load — without an indicator, this is only noticed by the behavior of the consumers.
Separation from the DC Side
Both systems are routed separately: separate cable ducts, separate terminal areas, clear labeling. Where paths must cross, this is done with distance and mechanical protection.
The fault case explains the rule: If both sides touch, 230 volts are applied to a DC circuit whose equipment, terminals, and insulation are not designed for it. Displays, sensors, and bus participants are then immediately destroyed — and the system is under a voltage that no one expects.
Cables and Connectors
- Three-core. Live conductor, neutral conductor, protective earth conductor — with the designated colors.
- Fine-stranded and flexible. As on the DC side, movement and vibration do not tolerate rigid cores.
- Outdoor connectors. The shore power connection is located where splashing water can reach.
- Strain relief at every connection. At the shore power connection, someone regularly pulls on the cable.
- No laying in the bilge. Neither for AC nor for DC.
Inverter as a Source
An inverter generates a voltage on board that is just as dangerous as that from the shore. Therefore, it belongs to the same system: its own protection, integration into the switching, protective earth concept according to manufacturer's specifications.
The last point is often overlooked: How the inverter's neutral conductor is connected to the protective earth conductor differs depending on the device and operating mode. This information is in the manual and is not a matter of discretion — it determines whether the residual current protection works at all in island operation.
The Connection with Corrosion
The protective earth conductor of the shore power connection connects the metal parts on board with those of all other connected boats. Electrically, this is correct and necessary; electrochemically, this creates a galvanic cell across the entire jetty.
The contradiction can be resolved with a galvanic isolator in the protective earth conductor — it blocks small DC voltages but allows fault currents to pass — or with an isolation transformer, which completely separates the system from the shore grid. The protective earth conductor itself is never simply omitted.
Typical Mistakes
- Protective earth conductor switched or fused. It is continuously routed and never interrupted.
- Single-pole circuit breakers used. At a reverse-polarity socket, the wrong conductor is then switched.
- Three-meter rule overlooked. The section to the feed-in remains unprotected otherwise.
- Sources not interlocked. Backfeeding into the shore grid is the most dangerous variant.
- AC and DC in the same channel. In the event of a fault, 230 volts are present on the DC side.
- Protective earth conductor disconnected due to corrosion. An isolator and isolation transformer are available for this.
Work on the AC side belongs in expert hands. This article explains the connections and replaces neither the standard text nor inspection by a qualified person.
