Shipboard Earth Fault Monitoring: First and Second Faults, Detection and Location

“Earth fault” is the common onboard term. In an unearthed IT system, insulation fault is usually more precise: insulation resistance has fallen enough to create an unintended conductive path from a live conductor to protective earth, the vessel’s earth reference, or the hull where the approved earthing arrangement uses it.

Many shipboard systems use an IT arrangement to preserve continuity of supply. Live conductors have no intentional low-impedance connection to earth, while exposed conductive parts remain connected to the protective-earthing system. A first fault can therefore be alarmed without immediately disconnecting the supply. Not all ship systems are IT systems; earthed and impedance-earthed systems require different monitoring and protection. [2][6]

In a healthy IT system, live conductors have no intentional low-impedance connection to earth, although the system may include a high-impedance connection for monitoring or functional purposes. Finite insulation resistance and system capacitance also create small leakage or charging currents. Cables, EMC filters, connected equipment and some surge-protection arrangements can all contribute. [6][9]

A first insulation fault creates a conductive path between a live conductor and protective earth, the vessel’s earth reference or—on many metal vessels—the hull. The fault may be resistive or nearly solid.

In an AC IT system, the resulting current is normally limited by the system’s insulation resistance, leakage capacitance, fault impedance and any intentional earthing impedance. It is therefore often too small to operate an overcurrent protective device. Provided the applicable protective-earthing and touch-voltage conditions remain satisfied, the insulation monitoring device alarms and the system may continue operating. [6][11]

The first fault nevertheless changes the system:

• The faulted conductor moves toward earth potential, depending on the fault resistance.
• The healthy conductors’ voltages relative to earth can increase.
• Cables, filters and connected equipment may experience greater earth-referenced voltage stress.
• A second fault on another live conductor can produce a high-current fault through the protective-earth network and cause protective devices to disconnect the affected circuits.

Continued operation after the first fault is a designed continuity response, not an acceptable normal condition. The fault should be located and cleared as soon as reasonably practicable. [6][11]

The critical case is a second insulation fault on a different live conductor while the first fault remains.

Where equipment frames share a common protective-earthing network, faults on different AC phases can create a phase-to-phase fault path through that network. On an insulated DC system, faults from opposite poles can similarly create a pole-to-pole fault path. Protection may then trip, equipment may be damaged, arcing may occur, and more than one service can be lost. [6][11]

If exposed conductive parts are earthed individually or in separate groups, second-fault protection follows TT-type principles and the required disconnection method differs. A second fault on the same phase or DC pole does not normally create a line-to-line short circuit, but it can worsen leakage, heating, tracking, and local touch-voltage conditions. [11]

1. Identify the affected network. Confirm whether the alarm relates to the main bus, emergency bus, transformer secondary, UPS output, DC system, or shore-derived section.
2. Check recent activity. Consider washdown, rain, shore-power changes, new equipment, contractor work, heaters, pumps, galley or laundry loads, and recent AV/IT work.
3. Use location equipment first. Read feeder, panel, or locating-current indications before opening circuits.
4. Narrow the fault progressively. Move from bus to feeder, subdistribution panel, final circuit, and load without randomly interrupting essential services.
5. Test safely. Insulation-resistance testing must be performed de-energized, with sensitive electronics, SPDs, drives, UPS equipment, and control modules isolated as required.
6. Repair and verify. Confirm the insulation value returns to an acceptable level, the alarm clears, and the cause, test results, and corrective action are recorded.

There is no universal insulation-alarm threshold suitable for every vessel. Thresholds and delays must suit the system voltage, leakage capacitance, connected equipment, class requirements, and monitor manufacturer’s instructions. [8][9]

Intermittent faults may appear only with moisture, vibration, temperature, load, equipment movement, or a particular operating mode.

IEC 61557-8 covers IMDs for unearthed systems up to 1,000 V AC and 1,500 V DC. IEC 61557-9 covers IFLS equipment over the same general voltage ranges. Equipment must be selected for the actual AC/DC topology, leakage capacitance, converters, filters, and expected electrical disturbances. [6][7]

For many yachts, locating the fault to the affected subdistribution panel gives the best balance between cost and operational benefit. Final-circuit tracing can then be completed locally without opening unrelated feeders at the main switchboard.

Where normally separate buses can be connected by a bus tie, the monitoring arrangement must respond to the change in network topology. Modern coordinated systems may automatically disable one monitor or use communicating devices so that the coupled network is monitored without conflicting measuring signals. [10]

• An IMD intended for an energized IT network is not a substitute for earth-leakage protection on an energized TN or TT system. IEC 61557-8 also permits IMDs to monitor de-energized TN, TT, or IT equipment, which is a separate application. [6]
• Independent IMDs can interfere when connected to the same galvanically connected network. Bus ties and shore/ship configurations require coordinated monitoring logic. [10]
• Neutral-to-earth links in generators, transformers, shore systems, UPS bypass paths, or connected equipment can change the effective earthing arrangement.
• VFDs, rectifiers, EMC filters, SPDs, and distributed cable capacitance affect measurement behaviour and must be considered during selection and commissioning. [6][9]
• An IFLS identifies the monitored branch carrying the locating signal; inspection and dead testing are still required to find the defective component. [7]
• Earth lamps or other earth-indicating systems may be accepted under particular class or flag rules, but should not be assumed equivalent to an active IMD that measures insulation resistance and can detect symmetrical deterioration. [8]
• IEC 61557-8 is a low-voltage product standard. High-voltage monitoring and neutral grounding require dedicated equipment and class-approved protection design. [4][6][8]

SOLAS

SOLAS Chapter II-1, Regulation 45 is the main statutory reference for precautions against shock, fire, and other hazards of electrical origin. The requirement carried into this regulation states that where a primary or secondary distribution system for power, heating, or lighting has no connection to earth, continuous insulation-level monitoring must be provided with an audible or visual indication of abnormally low insulation values. [1][2]

IEC Standards

The main standards relevant to system selection and protection include:

• IEC 60092-202:2016 — shipboard electrical protective systems. [3]
• IEC 61557-8:2014 — insulation monitoring devices for IT systems up to 1,000 V AC and 1,500 V DC. [6]
• IEC 61557-9:2023 — equipment for insulation fault location in IT systems. [7]
• IEC 60092-503:2021 — shipboard AC systems above 1 kV up to and including 36 kV. [4]
• IEC 60092-507:2014 — electrical installations in small vessels up to its stated size and tonnage limits. [5]

Flag and Classification Rules

Flag and class requirements determine the final arrangement for a particular vessel.

As one current example, ClassNK’s June 2026 rules require continuous insulation monitoring on unearthed power, heating, and lighting distribution. Within a defined restricted-service or small-ship application, the rules permit other earth-indicating systems for certain ships below 1,600 GT, excluding tankers, liquefied-gas carriers, and dangerous-chemical carriers. This is a class-specific allowance, not a general exemption for every vessel below 1,600 GT. [8]

ClassNK’s high-voltage rules separately recognize insulated, high-impedance, low-impedance, and directly earthed systems. Low-impedance and directly earthed systems require automatic disconnection of faulty circuits, while high-impedance systems may be designed to continue under a first fault subject to overvoltage withstand and protection requirements. The HV distribution system must have visual and audible earth-fault indication. [8]

This article applies to yachts and ships selecting, reviewing, or operating insulation and earth-fault monitoring systems. Applicability depends on flag state, class society, vessel service, voltage, system-earthing philosophy, construction material, hazardous areas, and whether the system supplies essential, emergency, propulsion, navigation, AV/IT, or operational-technology loads.

It is not a substitute for approved single-line diagrams, the vessel’s earthing and protection philosophy, class or flag requirements, manufacturer instructions, or a vessel-specific protection study.