The socket on the jetty is the only part of the system that you didn't build yourself, and yet you use it. Reversed conductors, a broken protective conductor, a socket without residual current protection — all of this happens, and none of it is visible when plugging in.
What can be wrong with a jetty socket
| Error | Effect on board |
|---|---|
| Live and neutral conductors reversed | single-pole switches disconnect the wrong conductor; devices remain live |
| Protective conductor broken | no leakage path in case of fault, no disconnection |
| Protective conductor connected to neutral conductor | operating current on the protective conductor, residual current protection disturbed |
| No residual current protection on the jetty | on-board protection must act alone |
| Voltage too low under load | chargers throttle back, motors run hot |
| Socket damaged or damp | contact resistance, heating, failure |
The first three points are safety-relevant, the others concern function and service life. Only the last one is visible.
Why polarity reversal matters on board
In a house installation, the position of the conductors is defined. On a jetty socket, it is not — and with some designs, the plug can be inserted in two directions anyway. A single-pole switch designed for the live conductor then disconnects the neutral conductor: The consumer is off, but remains live.
This is why switching and disconnection are consistently two-pole on board. It is also why a polarity indicator makes sense: it tells you before switching on whether the assumption about the conductor assignment is correct.
The display on board
A mains control panel in the switch cabinet automatically checks with each connection: Green confirms a correct connection, red indicates reversed live and neutral conductors. The measuring current is in the milliampere range and well below the tripping threshold of the residual current protection.
WarningSuch an indicator requires an existing protective conductor. In networks without a protective conductor, no fault detection is possible — there, a green indicator is not a confirmation, because it cannot measure at all.
In larger shore power units, mains indicator lights and an AC voltmeter are already included. The voltmeter is the second useful piece of information: If the voltage visibly drops when the charger is switched on, the supply line on the jetty is too weak or a connection is poor.
The routine when mooring
- Look at the socket. Damage, moisture, burnt spots, wobbly fit — visible defects are a reason to take another socket.
- Switch off all consumers on board. Plug in without load.
- Connect cable, on-board first, then at the jetty.
- Read display. Mains control green, voltage in expected range.
- Check residual current protection. Press the test button of the circuit breaker on board — it must trip. This is the only functional test possible at all, and it takes seconds.
- Switch on consumers individually and keep an eye on the voltage.
When leaving, reverse the order: first disconnect at the jetty, then on board. This way, no live plug is ever exposed.
The connection cable
It is part of the system, even if it is lying loose. An excessively long cable is often coiled up — under higher load, the coil heats up significantly because the windings heat each other up. Unrolling is not a precautionary measure, but necessary.
The second point is the cross-section: a thin extension cable over twenty meters makes 230 volts at the jetty significantly less on board. And the third is strain relief — someone regularly pulls on the cable at the shore connection, and the plug connection should withstand this.
If the display is red
Disconnect, try another socket, inform the harbor master. Do not: ignore the display because "the devices are working anyway". They are indeed working — the function is not affected by the polarity reversal, but safety is.
Anyone who frequently moors in different places and wants to solve the problem permanently will turn to an isolation transformer: It completely separates the on-board system from the shore power grid and gives it its own, defined potential — regardless of how the socket on the jetty is wired.
Typical errors
- Plugged in without display. Polarity reversal and missing protective conductor are not visible.
- Ignored red display. The devices still work — that's not a counter-argument.
- Test button never pressed. Residual current protection is the only testable safety component on board.
- Plugged in under load. Unnecessarily stresses contacts and creates arcing.
- Cable operated coiled up. The coil heats up significantly.
- Too thin extension. Significantly less voltage arrives at the end.
