Earth fault on a ship's network: how it is actually located

Most low-voltage networks on ships are unearthed, or earthed through a high resistance. The choice is deliberate: an insulation fault on one phase produces no fault current and does not trip the breaker. The vessel keeps running.

That is both the advantage and the trap. The system tolerates the first fault. It does not tolerate the second: a second fault on a different phase creates a full short circuit through the two points, often in cables not protected for that current.

What insulation monitoring shows, and what it does not

The insulation monitoring device measures the resistance of the whole network to earth. It tells you a fault exists and how severe it is. It does not tell you where. Identifying the outgoing feeder needs either a locating system with an injected signal, or a methodical procedure.

Before disconnecting anything: what the fault itself tells you

The most useful information is collected before anyone touches a breaker. Three observations narrow the search sharply:

• Does it come and go? Then it is tied to something that starts, heats or gets wet. Record what was running each time.
• Which phase? Most devices indicate the affected phase. A single-phase load immediately shortens the candidate list.
• How low is the resistance? A few kΩ suggests moisture or contamination over a large surface; close to zero suggests a conductor touching metalwork at a point.

The methodical procedure

1. Record the initial insulation value and the vessel's operating condition.
2. Split the networks: disconnect lighting transformers and secondary networks one at a time. The fault is often found at this step.
3. Isolate outgoing feeders on the main switchboard in sequence, watching the insulation value — starting with non-essential loads and with every interruption agreed in advance.
4. Once the feeder is identified, measure the circuit in sections: board, cable, load. The fault sits in the terminal box or the gland far more often than in the machine itself.
5. Visual inspection at the suspect points: moisture, salt deposits, sheath damage at a penetration, a loose terminal, condensation in an enclosure.
6. After repair, measure and record again — and measure every remaining circuit, because a second, smaller fault often hides behind the first.

The usual culprits

• Motors in wet spaces, particularly after a long period out of service.
• Heating elements and pipe trace heating.
• Deck cables with sheath damage at supports or penetrations.
• Light fittings in exposed and wash-down spaces.
• Generator and motor space heaters — frequently forgotten entirely.
• Old cable whose insulation has become hygroscopic over the years.

The point that matters

An earth fault is not urgent in the sense of requiring an immediate shutdown, but it is urgent in the sense that the vessel is operating without the safety margin it believes it has. Deferring the search because 'it is not affecting anything' is the most common cause of the next, far more expensive incident.