Marinas and berths (§709) — individual RCD protection and galvanic corrosion

The 702-swimming pools guide and the 708-camping guide cover other wet and/or temporary environments. §709 (based on IEC 60364-7-709) regulates its own category: the shore-power supply in marinas for pleasure craft at a fixed berth — an environment combining increased shock risk (water, damp hands), mechanical loading (structure movement, impact damage), and a risk found in none of the other chapters: galvanic corrosion.

One socket-outlet, one vessel, its own RCD

The core of §709: every socket-outlet supplying shore power to a vessel must be individually protected by an RCD with an operating current of at most 30 mA, disconnecting all live conductors including the neutral. To prevent a long connection lead between the distribution board and the vessel from creating an unmanageable situation, there is also a practical upper limit of at most four socket-outlets per distribution enclosure, and each socket-outlet supplies only one vessel.

Note: this is stricter than an ordinary outdoor socket-outlet circuit in a domestic installation, where a shared circuit RCD for multiple outlets is common. A marina has no such option: every socket-outlet has its own RCD.

Why galvanic corrosion is a separate risk

A vessel at a berth typically has underwater metal parts (propeller, shaft, rudder, through-hull fittings) that are electrically connected, via the hull and the earth conductor of the shore-power cable, to the marina's earthing system. Once several vessels — often with slightly different metals or sacrificial anodes — are connected to each other through that same shared earth conductor and the seawater, a small galvanic DC voltage difference between the vessels can drive a DC current path through the water. That accelerates corrosion of the sacrificial anodes and underwater parts of one vessel or another, independently of the normal AC safety function of that same earth conductor.

Two common solutions to break this galvanic path without losing the safety function of the earthing:

Galvanic isolator: a component in the earth conductor that blocks the small galvanic DC voltage, while still passing the fault current needed by the RCD/protection during a genuine earth fault.
Onboard isolation transformer: complete galvanic separation between the shore supply and the installation on board via magnetic energy transfer — more thorough, but heavier and more expensive than a galvanic isolator.

Mechanical and environmental protection

Shore-power distribution enclosures and socket-outlets in a marina are exposed to spray, rain, sunlight and mechanical impact (mooring, wave action) — considerably harsher than an ordinary outdoor installation. Materials and IP rating must be matched to that, and the mounting height must account for the highest expected water level at that location (tide, wave action).

Note: this article gives the principle. The exact minimum IP rating, mounting height above the highest water level, and other construction requirements per berth type follow from the full standard and the layout of the specific marina.

Practical relevance

When installing or renovating shore-power facilities in a marina, each distribution enclosure must be assessed for: the number of socket-outlets (≤ 4), the individual RCD per outlet, the IP rating and material given exposure to salt water, and whether a galvanic isolator or isolation transformer is needed to limit corrosion damage to vessels in that harbour.

Common mistakes

Using one shared circuit RCD for multiple shore-power socket-outlets — §709 requires an individual RCD per outlet, not a shared circuit protection.
Feeding multiple vessels from one socket-outlet via a branch connector — contrary to the one-outlet-per-vessel principle.
Ignoring galvanic corrosion "because the RCD works fine" — the RCD protects against electric shock, not against the slow galvanic corrosion of underwater parts; that requires a separate measure (isolator or transformer).