On a boat, fire most often starts in the electrical installation. Second on the list is something nobody notices: stray current eating the metal. Both are silent, both build over years, and neither shows itself to the eye.
This page sets out how the electrical system is assessed during a survey: which readings are taken, why, and what the numbers mean.
Scope note: A survey documents the condition and ranks the risks. Rewiring, panel building and commissioning are separate work, coordinated with an independent marine electrician where needed.
1. DC System: From Battery to Load
The boat's backbone is 12 or 24 volts. Faults here arrive slowly: a connection warms, resistance rises, it warms further.
| Measured | What is looked at |
|---|---|
| Battery bank and internal resistance | Capacity test and internal resistance; how closely the cells in a bank match. One failing battery drags the bank down. |
| Alternator charging | Voltage and current under running conditions; whether the charge profile suits the battery type. |
| Charger and bank separation | Start and service banks kept apart, use of diodes or DC-DC. |
| Cable size and voltage drop | An undersized run drops voltage, raises current and heats the cable. ABYC E-11 takes 3% on critical circuits and 10% elsewhere. |
| Fusing and circuit protection | Every circuit protected, fuse rated to the cable, protecting the appliance instead of the cable is a common error. |
| Main battery fuse | Expected within 18 cm of the positive terminal; that short unprotected run is a known starting point for boat fires. |
| Connections and terminals | Oxidation, looseness, crimping versus solder, heat marks. The thermal camera shows this under load. |
2. AC System: Shore Connection
230 V behaves differently afloat: the structure conducts, the environment is wet, leakage goes into the water.
- Residual current device (RCD): present, working, correctly placed. The test button is not enough; trip time is measured.
- Earth continuity: the green conductor unbroken from panel to appliance.
- Galvanic isolator / isolation transformer: the component that blocks galvanic current arriving through the marina earth; presence and integrity checked.
- Polarity and neutral-earth separation: reversed polarity or a neutral-earth bond aboard creates lethal leakage.
- Leakage measurement: current on the earth conductor read with a true RMS clamp meter. It should be close to zero.
- Shore lead and sockets: the lead end, heat marks and burning, the most commonly skipped item.
3. Current Into the Water: The Silent Damage
| Reading | What it means |
|---|---|
| Silver/silver chloride reference electrode | Hull potential in the water. On steel, more negative than −850 mV is expected; on aluminium the band is −950 to −1100 mV. |
| Anode consumption rate | An anode gone in a single season is not normal protection, it signals stray current. |
| Shaft and propeller wear | Faster wear than expected is often the visible face of an electrical problem. |
| Bonding system | Underwater metals tied to a common conductor; a broken link sacrifices that fitting alone. |
See cathodic protection survey for detail.
4. Thermal Camera: Looking Under Load
- The panel is opened and scanned with circuits running.
- An unexpected temperature difference between neighbouring terminals points to a loose connection.
- Alternator, inverter and charger are imaged separately while working.
- Findings enter the report with a photograph and the temperature reading.
5. What Comes Up Most Often
- Unfused additions: equipment added later, tapped into an existing circuit without protection.
- Size mismatch: a large fuse on thin cable, the cable burns before the fuse opens.
- Tangled wiring: no colour code, no labels. When a fault occurs nobody knows what is what.
- Domestic components: household sockets and connectors, which fail quickly in salt air.
- Joints in damp places: splices at bilge level without heat shrink or insulation.
- No galvanic isolator: current from the marina earth consumes anodes quickly.
- Battery ventilation: lead-acid batteries in an unventilated space allow hydrogen to collect.
6. What Changes with Age
| Installation age | What comes to the front |
|---|---|
| 0-8 years | Installation omissions; how later additions were connected. |
| 8-15 years | Battery bank life, terminal oxidation, charger and inverter age. |
| 15-25 years | Cable insulation hardening, panel component life, accumulated added circuits. |
| 25+ years | Assessment of the whole installation becomes the question; piecemeal repair is rarely economic. |
7. What Goes in the Report
Electrical findings carry the same classes as the rest: A, affects safety at sea, not to be gone to sea with; B, planned repair; C, monitor. Each is written with a photograph, the reading taken and the relevant standard reference.
For scope and fee see the survey calculator; if the boat has to be moved, the delivery calculator.
Taken from a real client report (96 pages, issued in Turkish). Boat name, make/model, report number, hull and engine numbers, place, date and people are withheld. More pages from the report
Frequently Asked Questions
It is part of a pre-purchase survey. An extended inspection focused solely on the electrical system can also be arranged; that adds readings and time.
No. A survey documents condition and sets priorities.
With a true RMS clamp meter on the earth conductor, alongside hull potential in the water read with a silver/silver chloride reference electrode. The two are read together.
That is not normal protection. Usually it is galvanic current arriving through the marina earth, or leakage aboard. Measurement tells them apart.
Yes, but only under load. A cold panel always looks normal; a loose terminal heats only while current flows, and that is when it is imaged.
Charge profile, BMS integration, alternator protection and cable sizing are assessed separately. A direct swap in a system designed for lead-acid is rarely enough.
Yes. The lead end, socket contact and heat marks are checked, one of the most skipped and most easily burned items.
Scope: The survey is non-destructive and nothing is dismantled; details on the survey types page.
Findings we often see on site
- Overheating electrical connection: On the thermal camera, one pole of the main battery switch is markedly hotter.
- Unfused circuit added later: A circuit added for extra equipment in household cable with terminal blocks, and no fuse.
- Leakage current on shore power: A steady current measured on the earth conductor with shore power connected.
Continue with this topic
- AC and shore power: RCD, polarity and galvanic isolation.
- Electrical measurements: Charging, draw, leakage, batteries and hot connections.
Related Pages
Navigation ElectronicsPlotter, radar, AIS, autopilotCathodic Protection SurveyAnodes, potential and stray currentLithium Battery ConversionCharge profile and BMSCommon Boat DefectsWhat comes up most oftenSurvey TypesWhich scope you needOnboard EmergenciesFirst response in the fieldSurvey QuoteFee by length and scope
Concerned about the electrical system?
Send the boat details and what you have noticed; we will set the scope together.
💬 WhatsApp 📝 Written quote 📞 +90 501 5 SURVEY