Why earthing on ships is designed for continuity — and why it becomes deadly when misunderstood
Introduction — ships don’t earth systems the way shore plants do
A lot of shore electricians arrive onboard expecting one comforting rule: earth faults trip the breaker. On many ships that’s not what you want — and not what you have.
Ship power systems are often designed to keep running after a first earth fault. That’s not because the industry is casual about safety; it’s because losing power at the wrong moment (steering, propulsion control, fire pumps, navigation) can be more dangerous than a controlled first fault.
So shipboard earthing is a compromise between:
- continuity of supply
- shock protection
- fire risk control
- fault detection and location speed
If you don’t understand which earthing regime your ship is using, you can make the “right” move and create the worst possible outcome.
The three earthing philosophies you’ll meet
1) IT system (unearthed / isolated neutral, monitored)
This is the classic “ship style” arrangement: the system has no solid connection to hull/earth, so a single earth fault does not immediately produce a high fault current. The system stays online — but only if you detect and clear the first fault fast.
That “detect” part is not optional. Guidance and class practice expect insulation monitoring on IT systems, and marine vendors explicitly tie this to IEC ship standards.
Operational reality:
- First earth fault = alarm + hunt it down
- Second earth fault (on another phase) = you’ve just created a phase-to-phase short through hull structure. That’s where fires and arc events happen.
2) Earthed neutral systems (TN-style / direct or impedance-earthed)
Some vessels/sections use neutral earthing (direct or via an impedance/resistor). In these systems, earth fault currents are intentionally high enough to operate protection quickly — but still controlled.
Class rules (via IACS) are very explicit about what earth fault current must and must not be. In earthed neutral systems, IACS requires the earth-fault current to be no greater than the full-load current of the largest generator on that switchboard section, while also being not less than three times the minimum current needed to operate the earth-fault device.
That’s not academic. It’s designed to avoid two extremes:
- too little fault current → protective devices won’t clear reliably
- too much fault current → catastrophic damage/arcing
3) Hybrid and split systems (reality onboard)
In practice, ships often have mixed earthing:
- one philosophy on the main 440 V system
- another on emergency switchboard feeds
- DC and UPS systems with their own bonding approach
- HV systems with dedicated earthing switches and procedures
The danger is thinking “the ship is IT” when only part of it is.
What the regulations and enforcement actually want
SOLAS: prevent shock and fire, not “make it work”
SOLAS requires electrical installations to be arranged to avoid injury and prevent hazards of electrical origin (shock and fire). That’s the legal intent behind the earthing regime you select and maintain.
Flag/authority guidance: earth-fault indication is expected
UK MCA guidance explicitly expects means of monitoring via earth lamps or meter fitted to the switchboard.
That’s a practical inspection point: PSC/Surveyors will look for it, and they will test whether it functions in a meaningful way.
Class/IACS: earth faults must alarm, and the system must be designed for the chosen regime
IACS also expects visual and audible earth-fault indication, and in low impedance/direct earthed systems it expects arrangements to automatically disconnect faulty circuits.
How earth faults really develop on ships
Earth faults are often not “electrical failures” first — they’re contamination failures:
- salt spray ingress
- condensation tracking in terminals
- oil mist and dust deposits
- galley contamination and poor cleaning
- vibration fretting on cable terminations
- damaged gland seals letting moisture wick into insulation
A simple example: IMCA highlighted incidents where contamination caused an earth fault with clear fire potential (fat residue ingress into sockets in a galley environment).
That’s the pattern: housekeeping and sealing failures become electrical faults.
Real-world case: “Massive earth fault” after insulation breakdown
A classic escalation is: insulation degrades quietly, then a reconnection or load transfer triggers a major event. An Australian investigation report describes a situation where, after a fire, an attempt to connect the ship’s supply to equipment resulted in a massive earth fault following failure of insulation.
That’s exactly why ETOs treat insulation condition as a live safety boundary, not a paperwork metric.
Practical onboard method: how an ETO should manage IT systems
If your ship runs an IT system, your job isn’t “avoid trips”. Your job is “make first faults non-negotiable”.
A disciplined routine looks like this:
- Treat first earth fault alarm as an operational defect, not background noise.
- Localize fast: isolate by feeder groups, one at a time, with comms to operations.
- Prioritize high-risk areas first: galleys, deck wet zones, HVAC fan rooms, exposed junction boxes.
- Fix the cause, not just the symptom (drying a panel without restoring seals just buys you hours).
Knowledge to Carry Forward
Shipboard earthing is designed around consequences: continuity, shock risk, and fire escalation. IT systems tolerate a first fault only if you detect and clear it quickly. Earthed neutral systems must produce fault current that is high enough to trip protection, but limited enough to prevent catastrophic damage — and class rules explicitly constrain that window.
Tags
ETO, Earthing, Bonding, IT System, Insulation Monitoring, Earth Fault, Switchboard, IEC 60092, SOLAS II-1, IACS E11