Ain’t nothing much worse than fire on a boat. The problem is that there is nowhere to run so you end up jumping overboard. To make matters worse, you lose your boat in the process and nobody wants to lose their boat. And if someone gets killed, then the whole boating experience is spoiled. I must warn you blog buddies that I am just now, starting to design the wiring for Borderline. Being an electrical engineer, I am inclined to get a bit carried away with the whole wiring thing so some of you may wish to look away now!
Because I am making a solar electric boat, I am particularly interested in electrical fires on boats so I did a bit of research.
It turns out that about 1/3 of all boat fires are caused by the DC electrical system. If you include AC electrical fires, you are up to 41%. (*) Looking at the pie chart above, you can see that a considerable number of boat fires are caused by an “off-boat source” which you as a boatie, can’t do much about. So just thinking about the boat fires that you can do something about , electrical fires account for more than half of those fires.
BOAT FIRES EXCLUDING OFF-BOAT SOURCES
Remember that the pie chart above is not talking about electric boats but rather we are talking about normal, everyday boats. For electric boats, the statistics are probably worse thanks to lithium batteries. ESPcat is going to solve the lithium battery safety problem, the same way Boeing did with their lithium airplane batteries. I am putting Borderline’s lithium batteries in a fireproof box with a flame-arrestor vent. I have to say that I can’t understand why anyone would not put their lithium boat batteries in a fire proof box but sadly, most do not.
It’s not just little boats that are catching fire. Have a look at this article “Why Are All These Super Yachts Catching fire?”(**) Why are there so many of them? Why isn’t something being done about the problem. I did a bit more research and I think I know the answer.
The wiring regulations for boats are simply too vague. If you are wiring a house, the electrician knows exactly what size wire to use and what size breakers. Boat wiring is more complicated than wiring a house but the regulations are less precise. Designing the wiring for a boat is left up to an electrician who is usually unqualified to do electrical design. Electrical fires start because some fool has either failed to fuse the circuit at all or has chosen a fuse that has too high a rating to protect the wire being used. A properly protected circuit is not able to start a fire.
Here are some basics:
1 – Use marine wire (wire where each individual copper strand is tinned) so it does not corrode in salt air. I have seen boats wired up by electricians calling themselves “marine electrician” who didn’t even know enough to use marine wire!
2 – Every circuit including the engine starting circuit must be protected with a fuse or circuit breaker. I prefer breakers but marine rated circuit breakers are expensive.
3 – Fuses are to protect the wire so match the fuse/breaker of each circuit, to the size of the wire (see table below). If you use too large a fuse, the wire can overheat and start a fire.
4 – Design “discrimination” into your fuse/breaker system so that when you get a short circuit the whole system does not fail but rather just the part of the circuit closest to the fault. I explain how to get discrimination, below.
5 – Label all wires or use color coded wire.
6 – Make a circuit diagram so you can maintain your boat’s wiring system.
7 – Install protective diodes on inductive loads.
Here is a table from BD fuses. (***) The table shows, for example, you are using AWG-16 wire, you are not allowed to use a fuse larger than 7.5 amps. If you use too large a fuse, it is possible for a fault to overheat the wire and cause a fire. Stick to the goddammed table!
For discrimination, ensure that for each step down in wire size, you use a fuse with ½ or less rating than the fuse upstream from it. Follow this rule and only one fuse can blow when a fault occurs.
Here is part of the wiring diagram for Borderline. See where I have used size AWG-4 wire for the engine starting circuit. I used a 100A fuse but the table above says I could have gone as high as 125A while still protecting the wire. For the AWG8 wire to the rest of the boat, I have used a 50A fuse. I can’t use anything larger as the table shows.
Below is more of Borderline’s wiring. Note that I have dropped down from AWG-8 to AWG-10 and have protected the AWG-10 with multiple 25A fuses. I could have used 30A and still protected the AWG-10 wire but I used 25 because it is ½ of the upstream 50A fuse and gives me discrimination. If I get a short circuit in the toilet flush motor, only the toilet motor fuse will blow. Not being able to flush is bad enough without having all the remaining electronics fail also.
The AWG-10 wire feeds a switch panel. The switch panel wires are AWG-16 and are protected by 7.5 amp fuses.
In every case, the fuses are equal to or less than the maximum value from the table above. For each step down in fuse size, the downstream fuse is ½ or less than the rating of the upstream fuse so we get discrimination. When one of those 7.5A fuses blows, nothing else fails except the one circuit. No matter what kind of fault you get, this wiring will never overheat and start a boat fire.
I am certain that many these boat fires are caused by improperly designed wiring systems. If everyone followed these basic design rules, there would be no electrical boat fires. If everyone designed discrimination into their boat wiring systems, you would see fewer examples of boats being stranded because of electrical failures.
If, I were to add an anchor winch that draws 125A, I would have to run AWG-4 wire all the way back to the battery and have a separate 125A fuse or circuit breaker to protect the wire. If I had a large engine rather than my tiny 8hp, I might have to use a 400A fuse and AWG-00 wire for its starter circuit. If you have a big diesel engine, you will be amazed how much better it will start if you use decent sized cables in the starter circuit. Big fuses are not all that expensive. You can purchase a 500A fuse for less than $10.(****).
Match the fuse and wire to the load. Put the fuse at the beginning of the circuit, closest to the battery so that a fault anywhere in the circuit will blow the fuse (or trip the breaker). I actually prefer breakers on a boat but marine quality breakers are expensive.
Don’t forget protective diodes across inductive loads. Any electrical device with coils of wire in it such as a motor or a solenoid or a relay, has inductance. Circuits with inductance, cause an arc in any switch that tries to open such a circuit. I did an experiment once where I took a relay with a clear plastic cover and set it up to control a second relay. With the lights out, I performed the experiment with a protective diode on the second relay and without the protective diode. You would be amazed at the huge arc created by the circuit without diode protection. That arc destroys contacts. You can see where I used 1N4007 diodes to protect contacts that are switching inductive loads. Without diode protection, contacts in push buttons or relays have a much shorter life.