Electrical, Control and Automation Systems: Sources of Power, Protection and Fail-Safe Design
Electrical questions test the architecture rather than component theory: which source supplies which service, where the emergency source may be located, how faults are detected, and how automation is designed to fail. Control and automation questions ask about alarms, remote control, and the human interface in an unattended machinery space.
Main and emergency sources
SOLAS II-1 Part D requires a main source of electrical power able to supply all services necessary for propulsion and safety. The emergency source, usually a generator with a transitional battery supply, must be self-contained, located above the uppermost continuous deck, readily accessible from open deck and not forward of the collision bulkhead. It must supply emergency lighting, fire detection and alarm, navigation lights, radio communications, internal communication, and the services needed for the survival and evacuation of the ship for 36 hours, with the emergency switchboard energised within 45 seconds of a main power failure.
Protection and distribution
- Insulated distribution systems need earth fault detection and alarm; a single earth fault must not cause loss of an essential service.
- Protection coordination must ensure that a fault in one final circuit is cleared by its own device and does not black out the switchboard.
- Cables, glands and enclosures must match the environment: IP rating, fire-resistant cable for essential circuits, and certified equipment in hazardous areas.
- Insulation resistance readings are trended rather than judged in isolation; a falling trend across several circuits signals contamination or cable damage.
Automation and control
Remote control of propulsion from the bridge, with an engineer's local position, is normal. Alarm and safety systems are designed fail-safe where a failure could be hazardous, and redundant where a single failure would remove an essential function. The IMO cyber risk requirement, in force through ISM since 1 January 2021, obliges the company to address unauthorised access to OT systems.
Worked example
An emergency switchboard must carry 60 kW for 36 hours. At a specific fuel consumption of 0.21 kg/kWh the requirement is 60 x 36 x 0.21 = 453.6 kg of fuel, so a tank holding roughly 500 kg with a workable suction head satisfies the rule; a 300 kg tank does not, however the generator is rated.
Traps
Placing the emergency generator inside the machinery space of a cargo ship; confusing essential with non-essential services on the emergency switchboard; assuming an earth return distribution is acceptable; treating alarms as a substitute for a safety action.