The shore power connection is the only energy source on board where a fault immediately endangers people. Accordingly, the requirements are different: residual current protection, a changeover switch that positively breaks before it makes, and a handling of the protective conductor that does not worsen corrosion.
What a shore power unit does
Between the socket on the jetty or at the pitch and the consumers, there are several interdependent functions:
- Residual current protection. Disconnects in case of an insulation fault before a dangerous current can flow through a person.
- Circuit protection. Protects the supply line against overload and short circuit.
- Polarity and mains control. Indicates whether the mains voltage is correctly applied — by no means a given with foreign installations.
- Distribution. Individually fused AC circuits instead of a multi-socket outlet.
Compact units combine these functions in one housing. A common combination is a residual current circuit breaker and a miniature circuit breaker in one device with 16 A and 30 mA tripping current, plus a control indicator.
The protective conductor and corrosion
This is where it differs from a house installation. The protective conductor connects the on-board electrical system to the earthing system of the shore installation — and thus indirectly to all other connected boats in the same harbor basin. This creates a closed circuit via the water, in which a current can flow that corrodes metal.
The result is galvanic corrosion, and it does not necessarily affect the boat that has the fault. It becomes visible on anodes that are consumed unusually quickly, and on underwater fittings.
Two components interrupt this path without eliminating the protective function:
- A galvanic isolator sits in the protective conductor and blocks small DC voltages, but allows a fault current to pass in an emergency.
- A isolation transformer completely isolates the system and creates its own on-board network. More complex and heavier, but a more thorough solution.
WarningInterrupting or omitting the protective conductor also solves the corrosion problem — and at the same time eliminates protection against an insulation fault. This is not a compromise, but a fault that endangers people. The isolator is designed to conduct in case of a fault precisely for this reason.
Switching between sources
As soon as a generator or inverter supplies AC voltage in addition to shore power, switching between the sources is necessary. The basic rule: No more than one source may be connected at a time, and the changeover must be positive — i.e., mechanically break before it makes.
Automatic changeover units take care of this and prioritize according to specification: If shore power is available, it is used; if it fails, the inverter takes over. Manual solutions are permissible, but must also be positively isolated — two separate switches that can be switched on simultaneously do not meet this requirement.
Supply line and plug
The connection cable is part of the installation and not an extension cable. Three points are crucial: a cross-section that matches the fusing and length, a design for damp rooms or outdoor areas, and a strain relief at the feed-in point so that tension on the cable does not affect the contacts.
A common mistake is the rolled-up cable reel: A coiled cable under load heats up significantly more than an uncoiled one. What seems tolerable on land is a fire hazard in a storage compartment.
Charger at the shore power connection
The charger is a consumer like any other on the AC side and belongs to its own, fused circuit. On the DC side, it is a charging source — with everything that entails: its own cable to the battery, fusing near the battery, charging curve matching the battery type.
With multiple charging sources, the same principle applies as everywhere: The charging end voltages must match each other. A charger with a fixed lead-acid curve next to a modern solar controller in a lithium system not only slows down charging, it prevents full charging.
Check before the season
- Triggering the residual current circuit breaker with the test button — the only test that everyone can and should do themselves.
- Observe the mains control indicator when plugging in: if it shows a deviating state, do not proceed.
- Check the connection cable and plug for discoloration at the contacts — this indicates contact resistance and thus heating.
- Inspect anodes. Unusual wear is an indication of a corrosion path, not undersized anodes.
Typical faults
- Protective conductor interrupted to stop corrosion. Eliminates corrosion and personal protection.
- Two sources connectable simultaneously. Without positive disconnection, a serious fault.
- Cable left on the reel. Heating under load, a fire hazard in the storage compartment.
- Residual current circuit breaker never tripped. A device whose function no one checks is not protection.
- Charger without its own fusing at the battery. The DC line is unprotected over its full length.
