A marine electrical refit is not simply a replacement of old equipment with newer equipment. Every change affects a connected system: batteries, charging sources, DC distribution, AC supply, protective devices, bonding, monitoring, control circuits and the vessel’s operating routine. A component can be correctly specified in isolation and still create an unsafe or unreliable installation when its fault current, cable route, configuration or interaction with the rest of the vessel has not been assessed.
This checklist is organised around the safety framework of ISO 13297:2020 and Amendment 1:2022. That edition covers extra-low-voltage DC systems at nominal voltages of 50 V or less and single-phase AC systems at nominal voltages of 250 V or less on small craft. It excludes electrical propulsion systems, which are addressed by ISO 16315, and three-phase AC installations, which are addressed by IEC 60092-507. The currently applicable edition and amendments, national requirements, class rules where relevant, and the latest instructions from every equipment manufacturer must always be checked for the actual vessel. A general guide cannot determine a cable size, protective-device rating, isolation method or commissioning limit without the installation data.
Define the refit before disconnecting anything
Start with the reason for the work. A repeated low-voltage alarm, overheated conductor, unreliable shore connection, lithium conversion, larger inverter or additional navigation equipment each creates a different scope. Record the symptoms, operating conditions and previous repairs. Separate confirmed defects from assumptions and from optional improvements.
Create a change list that states what will be retained, removed, relocated and added. Include future loads that are already planned, but do not design around undefined possibilities. Identify whether the vessel must remain operational during the work and which safety-critical circuits require a controlled temporary supply. A refit should have a defined boundary; otherwise undocumented work tends to expand while the system is already dismantled.
Before starting, photograph equipment, labels, cable entries, busbars and terminal positions. Produce or correct the single-line diagram and circuit schedule. Unlabelled conductors should be traced and identified rather than assigned a purpose from colour alone.
Establish safe isolation and working conditions
Isolation is a planned state, not merely an open switch. Identify every possible energy source: battery banks, shore power, generator, inverter output, solar controllers, alternators, DC-to-DC chargers and any separate backup battery. Energy can arrive from more than one direction, and some equipment retains stored energy after its input is removed.
Use an agreed lockout and verification process appropriate to the vessel and the work. Confirm absence of voltage with a suitable instrument and prove the instrument before and after the check. Protect disconnected cable ends against contact and movement. Work involving live systems, high fault-current battery banks, unfamiliar AC distribution or damaged conductors is not a reasonable place for trial and error. Stop when safe isolation cannot be demonstrated.
Ventilation, access, fire risk and bilge conditions matter as much as the schematic. A technically correct connection made in a wet, inaccessible or mechanically exposed location is not a satisfactory installation.
Audit the existing system as it is, not as the drawing suggests
Compare the drawing with the vessel. Trace the positive and negative paths, protective conductors, main disconnects and connections between banks. Inspect for heat discolouration, loose or stacked terminals, corrosion, unsupported conductors, damaged insulation, sharp bends, unsealed penetrations and joints hidden outside appropriate enclosures.
Check whether labels remain readable and whether the installed protective devices correspond to the circuits they protect. Identify direct-to-battery connections and loads bypassing the main distribution. Review negative returns, bonding arrangements and any connection between AC protective earth and the vessel’s DC or bonding system according to the applicable design. Do not “correct” grounding by habit; first establish the intended topology and the requirements of connected equipment.
Measure rather than guess. Relevant checks can include open-circuit and loaded voltage, voltage drop along a complete path, battery-bank behaviour, charging-source output, leakage indications, terminal temperature under controlled load and insulation testing where appropriate. The test method must be compatible with connected electronics; sensitive equipment may need to be isolated before certain tests.
Design protection, conductors and fault paths together
Conductor selection depends on permitted temperature, insulation, bundling, ambient conditions, route, length, continuous and intermittent load, voltage drop and prospective fault current. A generic ampacity chart is not enough. Protection must interrupt an abnormal current before the protected conductor or connection is damaged, while also tolerating legitimate operating and starting behaviour.
Review protection near each energy source and wherever conductor capacity changes. Confirm breaking capacity and suitability for the system type, not only the current printed on the device. Ensure that disconnects are accessible, clearly identified and appropriate for their duty. Parallel conductors, multi-bank systems and equipment with multiple inputs require particular care because a fault may be supplied through an unexpected path.
Mechanical installation is part of electrical safety. Select appropriate terminals and tools, follow the manufacturer’s preparation and torque instructions, provide strain relief and support, and keep joints inspectable. Do not solder, crimp or stack terminals as a universal solution; use the connection method approved for the equipment and conductor.
Reassess batteries and every charging source
A battery change alters more than stored capacity. Review chemistry, battery-management behaviour, fault current, disconnect strategy, charging profiles, temperature limits, alternator loading, shore charger, solar controllers, DC-to-DC devices, inverter/charger settings and system monitoring as one architecture.
Determine what happens when a battery-management system opens under charge or load. Confirm that charging sources and critical consumers have a controlled operating path and that the alternator is protected from an uncontrolled event. Settings must come from the current battery and equipment documentation, not from another vessel with a similar-looking installation.
For AC equipment, verify supply selection, neutral and protective-conductor arrangements, transfer behaviour and prevention of unintended backfeed. Shore, generator and inverter sources must never be assumed to be interchangeable simply because they share the same nominal voltage.
Control the installation and record every deviation
Mark both ends of every conductor before final connection. Keep AC, DC, signal and network wiring separated as required, while respecting manufacturer guidance for electromagnetic compatibility. Protect routes against heat, fuel, moving parts, chafe and water. Maintain service loops only where they do not create unsupported cable or interfere with ventilation.
Use a staged inspection. Check routing and protection before covers hide the work; check terminations before energising; then compare the finished installation with the drawing. If a route or component changes during installation, update the design record immediately rather than relying on memory at handover.
Commission in a controlled sequence
Commissioning should begin with power removed. Perform a visual and mechanical inspection, verify identifiers, polarity, continuity and the intended isolation state. Confirm that protective devices, conductor sizes and terminal locations match the approved design. Test insulation or protective-conductor continuity only with methods appropriate to the connected system.
Energise in stages: sources first under controlled conditions, then distribution sections, then individual loads. Observe unexpected current, alarms, smell, sound and temperature. Verify battery and charging configuration, source-transfer behaviour, monitoring accuracy and alarm logic. Operate representative loads individually and in realistic combinations, including the situations that originally caused the complaint.
A successful power-up is not the end of commissioning. After a controlled operating period, recheck accessible terminations and inspect for abnormal heating or voltage drop. Test normal shutdown, emergency isolation and recovery after a source change. Confirm that safety-critical equipment behaves predictably when a non-critical source or network connection is lost.
Handover is part of the safety system
Deliver an updated single-line diagram, circuit schedule, settings record and equipment list. Record which checks were completed, what remains outside scope and any temporary arrangement that must be removed. Provide the location and purpose of main disconnects and explain the correct sequence for shore, generator, inverter and battery operation.
Keep serial numbers, manuals and configuration backups where they can be recovered after a device failure. A clear baseline makes later troubleshooting safer and prevents a future technician from having to reconstruct the refit from cable colours and assumptions.
Final decision point
Do not energise the system if the source of a conductor is unknown, a protective path cannot be verified, a damaged connection remains in service, equipment instructions conflict, or measured behaviour does not match the design. The correct next step is to isolate, document the uncertainty and obtain a competent review.
The strongest refit is not the one with the most new equipment. It is the one in which every source, conductor, protective device, configuration and operating action has a defined role, can be tested, and is reflected in the final documentation.