According to the ABYC: "The main AC system grounding bus shall be connected to: The engine negative terminal or the DC main negative bus on grounded DC systems, or the boat's DC grounding bus in installations using ungrounded DC electrical systems."

To us, this is your boat's "Common Grounding Point".

In the TechNote on Alternating Current (AC) Ground System, we made a statement that "Once you connect the AC safety grounding "green" wire to your boat’s Common Grounding Point, you have in effect electrically tied your boat’s Common Grounding Point to every boat at the dock." Hopefully, the above animation and the accompanying description will shed some light on that statement.

Figure Five illustrates the flow of phase current as it is applied to a typical electrical load. It also illustrates the flow of neutral current, as it leaves the electrical load and returns back to earth via the marina's ground rod.

Figure Six illustrates how the "green" grounding wire works in a not so perfect world. Our toaster oven has a direct short to ground. The one good thing is that our "green" safety wire does not have any corroded connectors, no broken wires and has good "green" wire continuity from the appliance to the marina's ground-rod.

Figure Seven illustrates what happens in an ABYC compliant installation when the world has really gone astray.  Our toaster oven has a short and there is a problem in our grounding system.

Let review what is going on in Figures Five and Six before moving onto Figure Seven.

In Figure Five, we illustrate how things should work. In our illustration a boat called "Problem" is wired in accordance to ABYC recommendation and every thing is in good working order. The captain of the "Problem" plugs in his 1000 watt (8.33 amp, I=P/E)) toaster oven. Phase current of about 8.33 amps at line voltage (120 volts) would flow from the marina's power source through the pedestal, shore power cable, distribution panel, 15 amp circuit breaker, to the branch circuit and grounding receptacle supporting the toaster oven. The toaster oven in this case, places a load or resistance of about 14 ohms (R=E/I) on the circuit. The line voltage would drop to a low voltage (near zero) as it pushes the current through the 14 ohms load. The 8.33 amps of current at a very low voltage would now travel the "neutral" wire back to its source - ground.

In Figure Six, we illustrate how things work in a not so perfect world. Our good ship "Problem" has a problem. The toaster oven has a direct short to its case. In this scenario, the captain of the "Problem" plugs in his shorted toaster oven. Phase current at whatever level required by the circuit resistance and at a line voltage of 120 volts would flow from the marina's power source through the pedestal, shore power cable, distribution panel, 15 amp circuit breaker, to the branch circuit and grounding receptacle supporting the toaster oven. Phase current would flow from the marina's power source through the pedestal, shore power cable, distribution panel, 15 amp circuit breaker, to the branch circuit supporting the toaster oven. Because the toast oven has a short to its case on the phase side of the load, it offers no resistance to the 120 volts circuit. In fact, the heating elements of the toaster oven and the "neutral" ground wire are not even part of the circuit anymore. The "new" circuit utilizes the "green" safety wire as its return path to ground. Current would bypass the 14 ohms load placed on the circuit by the toaster oven and head directly to ground via the grounding receptacle's "green" wire connection to ground. Hopefully, the circuit breaker would trip disabling the circuit.

We said hopefully, because it is dependent on the resistance the grounding connection. The National Electrical Code (NEC) requires a ground-rod resistance of 25 ohms or less. At 25 ohms we are only drawing about 4.8 amps (I=E/R). Not enough to trip the breaker. We will need a ground-rod with less than 12 ohms (R=E/I) of resistance to trip the breaker. Normally the breaker will trip and disable the circuit. If it does not you should still be safe - your body would offer a higher resistance path to ground, so you should not become part of the circuit. The breaker tripping or not tripping comes into play in a big way in the scenario illustrated in Figure Seven.

One more point about Figure Six before moving on. The installation shown is typical of a Two - 30 Amp or 50 Amp x 120 volt - 3 wire shore power installation. In this type of installation you have two "green" grounding wires (one from each cord set) connected to a common grounding buss bar aboard the boat. In this design, you have built in redundancy for a single "green" wire failure. At least a single wire failure on the boat side of the circuit. On the dock side of the circuit, the "green" wires are usually connected to a common bus bar which is tied to the "neutral" wire which is tied to the marina's ground-rod. On two cord sets installations, the ABYC common ground point adds a third level of safety.

On boats that utilize a single 30 or 50 Amp x 120 volts - 3 wire or a single 50 or 100 Amp x 120/240 volt 4 - wire shore power system. In these installation the "green" wire redundancy is not available. All you have for redundancy is the "green" wire connection to the common ground point.

Let take a look at the ABYC recommendation and Figure Seven.

Lets assume that the boat named "Problem" is docked a few slips away from your boat and your boat is also wired in accordance to ABYC recommendations, in that both boats are equipped with a common grounding point. Your electrical system is in good working order. You are just setting back enjoying the evening.

Things are not so good on the good ship "Problem". The boat has a couple of problems. The first one being the shorted toaster oven as illustrated in Figure Six. The second more serious problem is in the grounding system supporting the boat. We have lost continuity of both "green" grounding wires between the grounding buss bar and the marina's power pedestal.

The captain of the "Problem" is totally unaware of any problems aboard his boat until he plugs in his toaster oven with a phase side direct short to ground. Phase current would flow from the marina's power source through the pedestal, shore power cable, distribution panel, 15 amp circuit breaker, to the branch circuit supporting the toaster oven. Line voltage (120 volts) would not see the 14 ohms load from the toaster oven. It would pass at full line voltage onto the "green" grounding wire - it would be looking for ground. It could not use the shore power cable's "green" wire connection to ground because there is a problem with the connection. So it will take the next best route to ground via the "green" wire connection to the boat's "Common Grounding Point". The "common grounding point" has a connection to the negative battery terminal. The negative battery terminal has a connection to the propeller via the propeller shaft and engine block. The negative battery terminal also has a connection to the boat's bonding system, which is connected to the boat's underwater hardware and fittings. "Problem's" new return circuit to ground will include her propeller and underwater hardware and fittings..

Phase current, at about line voltage would enter the water through "Problem's" propeller shaft and underwater hardware - looking for ground,. And, this is where your boat's "common grounding point" comes into play. Your boat's grounding system would now become part of the "Problem's" extended grounding system. Current would flow back to the marina's ground-rod via the water surrounding your boat up through your propeller shaft and underwater hardware to your negative battery terminal. The negative battery terminal is connected to your common grounding point, which is connected to your grounding buss. Your grounding buss bar is connected to your shore power cables and the marina's pedestal. At the pedestal it would follow the neutral wire connection to the marina's ground-rod.