6.6. Earthing and grounding
Earthing and grounding
Earthing and grounding on board are significantly different due to the specific marine conditions: damp environment or immersion into sea water can cause significant damage to the connections and cables and special means of protection are to be used to avoid the damage.
Two types of corrosion can be present especially in DC circuits (AC circuits suffer from it to a lesser extent). If cables are immersed into sea water, which contains salt and acts as an electrolyte. In case of the galvanic corrosion, different metals immersed in the same sea water have different potentials and therefore a weak current occurs between them, as they create potential difference. By getting current, the water particles get charged and start moving from the cathode to the anode, thus making the corrosion much faster. In case of stray-current corrosion, the current leaking from the circuit can try to find its way to the ground through bilge water, sea water, etc. and the presence of current in sea water have a similar effect on the submerged metal parts. In case of stronger leaks, the hardware can be destroyed in a matter of several hours, compared to natural corrosion lasting for years.
To prevent the corrosion, only marine type hardware shall be used. For example, there is a slight difference in stainless steel composition, and only austenitic steel with higher amount of nickel should be used for marine applications, as other stainless steel types are more vulnerable in these conditions. In addition, the DC equipment (batteries, battery chargers, DC motors) have to be connected to the main ground point to ensure that the negative of the DC circuit has the path to return to the equipment through the ground.
There are several means used to deal with current leaks, lightning protection, and grounding:
1) Radio grounding plate.
2) Single grounding point.
3) Bonding of all electrical equipment on board.
All DC electrical equipment on board shall be grounded to avoid current leaks and corrosion.
References:
1. Calder, N. (2005) Boatowner’s Mechanical and Electrical Manual. 3rd ed. Adlard Coles Nautical. Bloomsbury.