ISO 13297: DC installations on board (formerly ISO 10133)

NoteUntil the end of 2020, the onboard DC side was regulated by ISO 10133. It was withdrawn at that time and incorporated into ISO 13297, which has since covered both DC and AC systems in a single standard. The following requirements remain consistent with the current status—only the reference source has changed.

ISO 13297 covers extra-low voltage systems for the DC side—that is, everything up to 50 volts, which is the standard on board. At first glance, the requirements may seem meticulous; however, they essentially address two risks: fire caused by overloaded cables and failure of safety-critical loads.

Why extra-low voltage is not harmless

The voltage is too low to endanger a human being. The current is not: in the event of a short circuit, a battery bank delivers currents in the four-digit range. A 25-square-millimeter cable that is connected to a positive terminal without a fuse and comes into contact with a ground will glow and burn out in seconds.

From this follows the hierarchy of requirements: first protection against short circuits and overload, then mechanical safety of the cable routing, and then the availability of loads that are critical in an emergency.

Circuit protection: the three rules

1. Every circuit must be protected. The only practical exceptions are the starter cable and a few special cases where the fuse itself would pose a risk.
2. The fuse belongs at the beginning. As close as possible to the source—the unprotected section before it is the most dangerous part of the entire system.
3. The rating depends on the cable. The fuse protects the wire. A device that requires more protection will have its own built-in fuse.

WarningA main fuse must be located at the battery in immediate proximity to the positive terminal. It is the only protection for the thick supply cable to the distribution board—and this cable carries the entire current of the system.

Regarding the fuse type, the interrupting capacity is the deciding factor: the rating must be higher than the short-circuit current the battery bank can deliver. For large lithium or AGM battery banks, standard blade fuses are not designed for this—fuses with a high interrupting capacity must be used at the battery there.

Cables and their installation

The cross-section is determined by two conditions: the permissible voltage drop and the current-carrying capacity for the chosen installation method. The more stringent of the two requirements is decisive.

Connections

The requirement can be summarized as: connections must be mechanically firm, electrically permanent, and accessible. In practice, this means crimped cable lugs using appropriate tools, strain relief at every connection, and no connection points in places that are inaccessible later on.

Two common practices fail here. Tinned wire ends in screw terminals loosen because the tin flows under pressure. And terminal blocks (chocolate blocks) used as a permanent connection on board can withstand neither the movement nor the moisture.

NoteConnections to battery terminals are limited in number—something many overlook. Instead of stacking five cable lugs on one terminal, a busbar should be placed in between. This is also the point where the system becomes organized.

Disconnecting devices

Every battery bank needs a disconnection capability that allows the system to be de-energized—for maintenance, during long periods of inactivity, and in an emergency. It belongs in a location that can be reached without tools and without climbing.

The positive side is disconnected. Where both poles are disconnected, care must be taken that the sequence is correct and that no devices lose their reference as a result. Loads that must remain permanently powered—bilge pump, alarm, gas detector—are deliberately connected ahead of the main switch, but then with their own protection.

Safety-critical loads

Bilge pump, navigation lights, radio, and alarm systems are loads whose failure is more than just annoying. For these, the rule is: separate circuit, separate protection, no shared routing with comfort loads, and no dependence on electronics that could fail themselves.

This is also the argument against fully digital distribution: a switching module may be reliable—but the bilge pump still needs a path that functions without a display and without a bus system.

Labeling and documentation

Circuits are to be labeled, at the distribution board and preferably on the wire as well. This includes a plan that shows what leads where and what its fuse rating is.

The effort takes about half a day and pays off with the first fault. Without a plan, every troubleshooting process starts by tracing cables—and often ends with a fuse being chosen larger "as a test."

What is checked during inspection

<

• Is every cable protected, and does the rating match the cross-section?
• Is the main fuse located near the battery?
• Are positive and negative cables of the same gauge, and is the return lead routed throughout?
• Are cables fastened, feed-throughs protected, and bilge areas free?
• Are AC and DC cables separated?
• Are connections crimped, strain-relieved, and accessible?
• Is there an accessible disconnection device for each battery bank?
• Do safety-critical loads have their own circuits?
• Is everything labeled and documented?

Typical mistakes

• Unprotected section between battery and the first fuse. The most dangerous part of the system.
• Fuse chosen based on the load. The fuse protects the cable.
• Interrupting capacity too low. Large battery banks deliver short-circuit currents that simple fuses cannot handle.
• Ground via hull or structure. Undefined resistance, risk of corrosion.
• Cable lugs stacked on the battery terminal. Install a busbar instead.
• Bilge pump on the comfort circuit. Safety-critical loads need their own paths.

This article explains the principles but does not replace the standard text. Anyone wanting to have a system inspected needs the original document—it is available from national standards institutes.