Earthing systems for sailboats | cruising world

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Chassis grounds, like the green wire (undersized) here, must safely carry fault current in the event of a short circuit.
Steve D’Antonio

Recently, I met with a client to review and critique their vessel’s systems. One item I saw was the bonding or grounding system. These systems have similar objectives: to return the parasitic, galvanic or fault current to its source.

Let me clarify two related issues. First, electricity does not “seek ground” as so many wise dockside claim. No, whether from a battery or a shore power, it seeks to return to its source. An example of the concept of returning to the source is all too often tragic; it is drowning by electric shock or electrocution in water. When the alternating current, which comes from the shore power, “leaks” into the water in which the ship floats, it attempts to return to its source, which in most cases is a transformer located on the quay or in the marina car park. Once power passes through a transformer, that transformer becomes a power source. So if a shore power transformer is installed on board a vessel, the fault current will seek to return to that transformer – rather than through the water – and to the one supplying the marina, making it makes a safer option.


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Second, although the terms are frequently and naturally used interchangeably, “bonding” is often used in conjunction with underwater metals and corrosion prevention, while “grounding” often refers to the connection of equipment chassis and hardware to DC negative terminal. However, the two systems are almost always connected (with AC security and lightning grounding systems), so for the purposes of this discussion, they are one.

Undersized jumper wires from engine block
Undersized engine block jumpers, also like the one pictured here, pose a risk of overheating if they carry inrush or fault current.
Steve D’Antonio

In the case of my client’s boat, I noticed a 14 gauge wire connected to the engine block. It seemed rather new, and when I inquired about it, the owner confirmed that an electrician had installed it in the not so distant past. Poor block soil can lead to oil pressure and coolant temperature gauge issues, which may have prompted its addition. While this “fix” may have fixed a problem, it created a fire hazard.

Another electrical myth is that “electricity takes the path of least resistance”. Indeed, electricity consumes everything paths, the current flow being proportional to the resistance. Thus, more current flows through larger, low-resistance cable runs; when both are present, larger wires carry more current than smaller wires. But what if the larger wire breaks, or is inadvertently disconnected, or the connection loosens or corrodes? In this case, a small gauge wire connected to an engine block will be called upon to carry a high current, like that of a starter or alternator.

A few years ago, I was inspecting the engine room on a 60 footer. A mechanic had recently replaced the batteries, then started the engine to test its work. However, when removing the old batteries, he had dropped a cable behind a battery box and then failed to reconnect it when installing the new batteries. When he turned the key, instead of running through a cigar-sized 2/0 cable, the current from the starter instead took an alternate path through a 12-gauge jumper wire connected to the engine block. A few feet away, I remember feeling the heat on my face as it almost instantly became white-hot, and the insulation melted and then burned. Fortunately, the wire melted before anything caught fire.

corrosion
Bonding metals underwater prevents or reduces galvanic or stray current corrosion.
Steve D’Antonio

It doesn’t take a run-out scenario for something like this to happen. If the starter or alternator positive cable is rubbing against the engine block (ABYC standards prohibit starter positive cables from touching the block in any way), current will attempt to return to the battery through all paths. , including small connecting wires.

Simple morality? Each ground/bond wire connected to a motor or generator block, or other DC equipment, must be capable of carrying full inrush or fault/short circuit current, meaning it cannot be less than one size smaller than the largest DC-positive cable. In addition, the jumper wires must not be connected to current-carrying parts. If the frame of the block, thruster, etc., is common with the DC negative, then it should do not be linked. Engine blocks and generator sets that use ground-insulated starters and alternators can, on the other hand, be bonded.

Steve D’Antonio provides services to boat owners and buyers through Steve D’Antonio Marine Consulting.

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