What to check on a Power Management System (PMS).
A PMS sits above the individual generator control units and the switchboard, watching total electrical load, bus frequency and available generating capacity, and making the decisions a duty engineer would otherwise have to make by eye: start the next standby generator before load exceeds running capacity, synchronise and close it onto the bus, and shed pre-selected non-essential load if generation capacity drops faster than a new set can come online. This distinguishes it from a simple generator auto-start relay, which only reacts to one signal such as low frequency, without…
What a power management system actually does
A PMS sits above the individual generator control units and the switchboard, watching total electrical load, bus frequency and available generating capacity, and making the decisions a duty engineer would otherwise have to make by eye: start the next standby generator before load exceeds running capacity, synchronise and close it onto the bus, and shed pre-selected non-essential load if generation capacity drops faster than a new set can come online. This distinguishes it from a simple generator auto-start relay, which only reacts to one signal such as low frequency, without the layered logic of load-dependent start, priority-based load shedding and blackout recovery sequencing a full PMS provides. On vessels running unmanned machinery spaces, the PMS is what makes multi-generator operation with minimal watchkeeping possible at all.
Main components
Load-dependent start and stop logic
Continuously compares total bus load against the capacity of running generators, factoring in a configurable spinning reserve margin, and issues a start command to the next available standby set with enough lead time to synchronise before the margin is used up.
Load-sharing control
Once two or more generators run in parallel, the PMS, or the individual governor and AVR controllers under its direction, divides real and reactive load between them proportionally, keeping each set within its rated output rather than letting one carry a disproportionate share.
Blackout prevention and load shedding
A pre-configured priority table ranks consumers from essential (steering gear, navigation, fire pumps) down to non-essential (accommodation air conditioning, workshop supplies). If load suddenly exceeds available capacity, a generator trip, for example, the PMS sheds load from the bottom of that table in a fixed sequence, fast enough to prevent a frequency collapse and total blackout.
Preferential trip and blackout recovery
Defines which breakers open first if a fault forces an emergency shutdown, and automates the sequence for restoring power afterward: starting the emergency generator, re-energising the bus, and reconnecting consumers in a controlled order rather than all at once.
Selection and sizing
- Number of generators and their individual capacity, since load-sharing accuracy and shedding granularity both depend on how many sets and how many load steps are available.
- Integration depth with the alarm and monitoring system a PMS that only talks to the switchboard cannot factor in, for instance, a bow thruster about to be engaged.
- Redundancy of the PMS controller itself; a single point of failure in the control system defeats the purpose of load-shedding logic built to prevent a blackout.
- Manual override capability, so an engineer can force a generator online or block a shed sequence during maintenance or drills.
Regulations and class
Class notations for unmanned machinery space operation require automatic standby generator start and documented blackout recovery capability as a condition of reduced manning in the engine room. IACS requirements for electrical installations expect load shedding to protect essential services defined under SOLAS Chapter II-1, and periodic survey includes a functional test of automatic start, load-dependent start/stop and load shedding sequences, not just a visual inspection of the switchboard.
Typical faults
| Fault | Cause | Consequence |
|---|---|---|
| Standby generator fails to start on demand | Auto-start logic disabled after manual testing, not re-enabled | No standby capacity when load rises, risk of blackout |
| Load sharing unbalanced between generators | Governor droop settings not matched after an engine overhaul | One generator overloaded while another runs light |
| Load shedding trips essential consumers | Priority table misconfigured after equipment changes | Steering or navigation load dropped instead of accommodation load |
| Nuisance generator starts | Load sensor drift or noisy signal from current transformers | Unnecessary running hours, fuel and maintenance cost |
What to look for in a supplier
- A documented, editable priority table for load shedding that the crew can actually verify against the vessel's real consumer list.
- Compatibility with the existing generator governors and AVRs, or a clear retrofit path if they need replacing together.
- Local backup control at the switchboard that keeps generators running if the PMS controller itself fails.
- A commissioning test report showing an actual blackout recovery drill, not just a paper configuration.
Re-verify the load shedding priority table any time a new large consumer is added a bow thruster or reefer container bank wired in without updating the table can end up shed ahead of something that should never lose power.