One of the leading causes of fires on boats is a condition known as an ‘arcing fault’. An arcing fault occurs when there is a loose or resistive connection in an electrical circuit which is operating with its normal current (amperage) flow. In other words, there is not a short circuit which would cause high currents to flow, tripping a circuit breaker or blowing a fuse.
The loose connection results in sparks, or resistance causes a buildup of heat, which, in both cases can burn surrounding materials and eventually start a fire. The danger lies in the fact that these faults can occur without any visible sign of a problem, making them difficult to detect until it's too late, with conventional circuit breakers and fuses providing no protection.
Shore Power Cords
Shorepower cords are particularly susceptible to arcing faults. This is because of the plug-style connections at both ends (as opposed to bolted connections), combined with the potential for boat movement (loosening the connections) and the marine atmosphere (promoting corrosion). In North America the standard marina outlets are rated at 30 amps and 50 amps, while in Europe it's often 6 or 16 amps. These relatively high amperages combined with the potential for loose and corroded connections increases the risk of arcing faults. This is, in fact, one of the more common causes of boat fires.
To prevent arcing faults, it's essential to regularly check both ends of your shorepower cord and the power inlet on your boat for any signs of burning, scorch marks, or other damage. If you notice any of these, it's crucial to replace the cord and fittings immediately to avoid further risks. If the dockside outlet shows signs of scorching or burning, don’t plug into it! If you plug in, at best you are likely to damage the plug on your shorepower cord, and at worst you may melt down the dockside outlet and your plug, and even start a fire.
Locking Plugs for Secure Connections
When plugging in your shorepower cord, make sure to use a locking plug that screws or clips into place. Support the shorepower cord to prevent any stress being placed on the plug. This is especially important if your boat is moored in a marina where it may be subject to rocking and rolling. Loosened plugs result in arcing faults and a potential fire hazard.
It's important to note that insurance companies may deny claims related to arcing faults if negligence is deemed a factor. Failure to properly secure shore power connections and cords or neglecting to regularly check for signs of damage have been construed as negligence. The boat gets burned up, maybe burning up adjacent boats and the marina, and the insurance company refuses to pay out. Taking proactive steps to prevent arcing faults, such as using locking plugs and conducting regular inspections, is important!
Onboard Issues
The shorepower connection almost always feeds an onboard battery charger supplying energy to the boat’s DC systems. These are commonly 12 volts, with 24 volt systems on larger boats and nowadays an increasing number of 48 volt systems. The lower the DC voltage the higher the amperage for a given power level required to run a device., For example, a bilge pump of a given size that requires 10 amps to run at 12 volts will only require 5 amps at 24 volts and 2.5 amps at 48 volts. The higher the amps, the greater the likelihood of heat generation at loose or resistant connections. Connection faults in DC systems are another leading cause of fires on boats. As with shorepower arcing faults, circuit breakers and fuses provide no protection.
Conclusion
In modern house construction we have something known as ‘arc fault circuit breakers’ or ‘arc fault circuit interrupters’ (AFCI). These can detect the fluctuating currents you get with many arcing faults and shut down the circuit to prevent a fire (they can also nuisance trip).
We do not, at present, have similar technology on our boats. The only antidote to arcing faults is to ensure all circuits are well installed with tight, resistance-free, connections. In particular, pay special attention to both ends of a shorepower cord, and, if doing work on DC systems, don’t start a job on Friday and leave loose connections over the weekend which you may forget to tighten on Monday. A single loose connection can set your boat on fire!
If you're interested in learning more about AC systems on boats and how to prevent arcing faults, be sure to check out the modules in our Advanced Marine Electrics course! Our comprehensive course covers various topics related to marine electrics, including shore power connections, grounding, and electrical safety.
