April 2

Understanding Marine Electrics: The Dangers of Misplaced Stainless Steel in Electrical Circuits on Boats

Welcome to another blogpost! We are investigating 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

Stainless steel is almost electrically non-conductive; it has approximately 8% the conductivity of copper. Stainless steel that becomes part of a conducting circuit 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, and at the end of the project, the fuses are installed. The conductors are already in place. The fuse goes 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!

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. 

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!

About the author 

Nigel Calder

Nigel is often referred to as THE guru when it comes to technical systems on boats.

He is a long-time member of the American Boat and Yacht Council (ABYC) electrical Project Technical Committee (PTC) which writes the standards for recreational boat systems in the USA, and has also been involved in European standards development.

Nigel is best known for his Boatowner’s Mechanical and Electrical Manual (now in its 4th edition), and his Marine Diesel Engines (in its 3rd edition), both considered the definitive English-language works in their field.

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