Soundings - - Contents - Paul Mirto is a dig­i­tal il­lus­tra­tor, long­time boater and for­mer Coast Guards­man. mir­ BY ROGER HELLYAR-BROOK PAUL MIRTO IL­LUS­TRA­TION

Mod­ern bat­tery charger in­stal­la­tions can cre­ate fire haz­ards when ground wires are ac­ci­den­tally mis­matched.

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. It car­ries the charg­ing cur­rent, which 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 could re­sult if wire chafe or an in­ter­nal DC con­nec­tion oc­curred and caused 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, we have 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.

Roger Hellyar-Brook runs a marine con­sult­ing busi­ness, re­pair­ing and up­grad­ing boats of all types. He has spent more than 40 years in the marine in­dus­try and is the for­mer man­ager of the sys­tems pro­gram at The Land­ing School in Arun­del, Maine.

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