Let’s explore an important yet often overlooked issue for electrical installations on boats: the proper and improper use of stainless steel in electrical circuits.
Misplaced Stainless Steel
It is a little known fact that stainless steel is almost electrically non-conductive; it has approximately 8% the conductivity of copper. Because of this, if stainless steel becomes part of a conducting circuit it acts like a resistor. When you put current (amps) through a resistor, you generate heat. In high current circuits, if the high currents are sustained for any length of time, a single stainless steel washer in the wrong place can generate sufficient heat to cause nuisance blowing of fuses or, if the fuse does not blow, melt down fuse holders and even start a fire.
ANL Fuses and Stainless Steel
A common source of problems is the installation of ANL and other high current fuses with stainless steel mounting posts, nuts, and washers. The fuse holders are frequently mounted horizontally. The boat builder builds the boat without the fuses. At the end of the project, the fuses are installed. The conductors are already in place. The fuse goes in on top of the conductor terminals. To hold things together, there's typically a lock washer, a flat washer, and a nut.
The fuse holder may be in a difficult-to-access space with poor light. When the fuse is slid into place, the installer does not notice that one of the washers has dropped down and is now between the fuse and the conductor terminal. An unintended resistor has been put in the circuit!
Let’s say this is a circuit with a high output alternator. The alternator is charging a well discharged lithium-ion battery bank which can accept the full output of the alternator for an extended period of time. The alternator is driving 200 amps through the washer. The washer heats up and transmits the heat to the fuse. The fuse is a thermal device and as such melts, open circuiting the alternator. The alternator is destroyed. This is not a fictional event!
Main battery fuses
Now let’s move this ANL fuse to a position where it is the main fuse in a DC system – the first fuse on the positive side of the main conductor coming off the battery or battery bank. Once again, a stainless steel washer has found its way between the terminal coming off the battery and the fuse. The battery is well discharged, the engine is running, the alternator is pumping out 200 amps, and the washer is heating up. The heat generated blows the fuse, disconnecting the battery. Once again, the alternator is destroyed, but before this happens it puts a spike of hundreds of volts through the boat’s wiring and blows out all the electronics that are turned on at this time. This is not a fictional event!
But maybe the fuse doesn’t blow. Instead, the heat generated by the washer softens the fuse holder and loosens the connection. The loose connection results in an arcing fault that sets the boat on fire. This is not a fictional event!
Proper Use of Stainless Steel in Electrical Circuits
The use of stainless steel to hold things together in electrical circuits is not a problem. It is commonly done. The critical issue is to ensure the stainless steel is doing only that - holding things together - and that it does not become part of the conducting circuit. The electrical components being held together need to be in physical contact with one another.
Over the years I have inspected the electrical systems on hundreds of boats. Over and over again I see stainless steel fasteners and washers which have become part of the conducting circuits. It is a very easy mistake to make. So check your own boat, especially those ANL fuses, to make sure that wherever you have bolted connections the conducting surfaces (the conductor terminals and whatever they are attached to) are in physical contact with no intervening stainless steel.
If your want to learn more about what to keep in mind when checking or renewing installations on your boat, check out our courses on Marine Electrical Systems!
