Why DC quietly causes the most persistent faults onboard
Introduction — DC doesn’t trip the way AC does
DC systems are often described as “simple” because there is no frequency or phase. In practice, DC is harder to interrupt, harder to detect faults in, and easier to misunderstand.
On ships, DC systems power:
- control circuits
- protection relays
- emergency lighting
- alarms
- UPS systems
- battery-backed safety equipment
When DC fails, systems don’t stop cleanly — they behave erratically.
Typical shipboard DC systems
- 24 V DC — control and automation
- 48 V DC — telecoms and navigation
- 110 V DC — legacy emergency lighting and control
- Battery banks — lead-acid or Li-ion
- Rectifier-fed DC buses
These systems often appear isolated — but are tightly interconnected.
🔧 Regulatory anchors (explicit)
IEC 60092-101 / 201
Defines:
- DC voltage limits onboard ships
- insulation requirements
- segregation from AC systems
SOLAS Chapter II-1
Requires:
- reliable power for safety systems
- independence of emergency supplies
- predictable failure behaviour
DC instability violates SOLAS intent.
Why DC faults are dangerous
- DC arcs do not self-extinguish like AC
- Fault currents can persist
- Protective devices are slower and less intuitive
- Voltage drop causes unpredictable behaviour
This is why DC fires often smoulder instead of tripping.
Common DC failure modes onboard ships
- Ground faults masked by IT systems
- Battery degradation causing brownouts
- Rectifier failures introducing ripple
- Incorrect polarity during maintenance
- Poor bonding causing floating voltages
Each produces symptoms that confuse non-specialists.
Real-world failure: DC control loss causing blackout cascade
A container vessel suffered repeated blackouts traced not to AC generation, but to 24 V DC control instability. Battery capacity had degraded, causing:
- PMS logic resets
- breaker mis-operations
- generator trips
The AC system was healthy.
The DC system undermined it.
Protection and monitoring challenges
DC systems require:
- insulation monitoring
- voltage trend monitoring
- battery capacity testing
- alarm prioritisation
Ignoring DC health is equivalent to flying blind.
ETO judgement: DC is the nervous system
AC is muscle.
DC is nerves.
You can have strong generators and still lose the ship if DC collapses.
Knowledge to Carry Forward
DC systems fail quietly, persistently, and misleadingly.
They rarely produce dramatic trips — instead they destabilise everything they support.
A professional ETO treats DC integrity as mission-critical, not auxiliary.
Tags
ETO, DC Systems, AC vs DC, Marine UPS, Control Power, IEC 60092, Electrical Failure Modes, Ship Automation Power