It's "Maintenance Monday", and with the alarming frequency of boat fires in our Southwest Florida region lately, we're talking about marine battery chargers. The size of battery-charging equipment has grown to keep pace with larger and more sophisticated batteries. As charger output current has risen, cable sizes have increased in order to limit voltage drops or overheated conductors. This has led to direct-current cabling, which carries the charging current, and is much larger than the alternating-current conductors that operate the charger. This mismatch could pose a fire hazard if the AC grounding wire on the charger case — the green one — had to carry DC fault current home.
Fault current can result if wire chafe or an internal DC connection occurs and causes a short circuit to the case. DC current in the charger’s case would want to “go home” any way it could. The pathway for the fault current is possible because American Boat & Yacht Council standards require a safety connection at the electrical panel between AC grounding and DC negative. This connection must be maintained for shock hazard safety reasons, in case of potentially lethal AC voltages appearing on the DC wiring.
Because the much smaller green grounding wire that runs with the hot and neutral supply wiring, potentially carrying large DC current, could easily overheat, there is a defined standard for installation. The installer must provide a DC grounding conductor from the charger’s case — this applies to inverters, as well — that directly runs to the DC negative bus. It has to be able to carry fault current from the largest supply conductor (DC cable to battery), so it is sized to safely match ampacity.
The positive, or hot, 12-volt conductors in a multiple-output charger may carry their rated currents, but remember that the ampacity of the negative conductor should be the sum of the combined output (charging) current. This negative conductor terminates internally at the charger and is isolated from the case.
If you have questions regarding the charging system for your vessel's batteries, contact us today for a no-obligation, no-cost consultation at #MarinerServicesInc. at (239)776-4253!