Changing Currents
The owner of a 37-foot tug tied up his boat and plugged into shore power. He was surprised to see that half the boat’s AC panel would not work.
His boat is set up with two 30-amp shore power cords. While the side of the panel powering the air conditioner worked fine, there was nothing happening on the other side. He tried swapping cords, and even pedestals, to no avail. Air conditioning on a hot summer day is certainly a priority, but he also needed his battery charger and water heater.
There was power at the dock pedestal, and at the end of the shore power cords. That meant the problem had to be between the boat’s AC inlet and the AC panel.
Unable to determine the specific cause, he called the boatyard.
AC and DC considerations
There were several pieces of equipment and wiring scenarios to consider. This particular boat was built in the mid-1990s, before the American Boat and Yacht Council recommended installing an Equipment Leakage Circuit Interrupter to protect the boat against ground faults. Early ELCIs were notorious for nuisance tripping, and retrofitting an ELCI to an older boat often illuminated ground leakage faults.
This boat was not fitted with an ELCI, so that couldn’t be the problem.
ABYC standards also specify that if the shore power inlet is not within 10 feet of the AC panel, a breaker must be installed within 10 feet of the inlet, measured along the conductors. This requirement often results in an awkwardly located breaker somewhere out of sight, even though the standard is for it to be accessible. If that breaker trips, and if its location is unknown, the result would be a mysterious loss of power.
On this boat, the inlet was within 10 feet of the AC panel, so no additional breaker was required. The owner was aware that the boat had a galvanic isolator, but it was installed in the AC ground wire and therefore couldn’t affect the hot and neutral wires coming into the boat. We also determined that the boat was not fitted with an isolation transformer.
The boat was, however, fitted with an inverter/charger. Such combination units have become popular aboard boats needing AC power without running a generator; there can be advantages to a combination unit versus a stand-alone inverter. A device that converts battery-fed direct current into alternating current can, with electrical wizardry, also turn AC into DC for battery charging.
DC flows from one post on the battery through a wire to a load, and then back to the other battery post. It is always a circuit, and the electrons flow in one direction. AC, on the other hand, is switching flow back and forth (60 times a second in the United States). It is this rapid shuffling of electrons that powers useful equipment. So, if we want to convert 12-volt DC into 120-volt AC, we need to increase the voltage and make the DC rapidly change directions by switching it on and off—and at each switch, we need to flop the polarity.
Enter rectifiers, diodes and silicon chips. These technologies slice the current into extremely small segments that can simulate a true sine wave (the visual representation of the AC waveform, measured with an oscilloscope). Early inverters produced an AC sine wave with a sharp, blocklike wave, similar to a castle crenellation. Modern inverters can replicate the smooth sine wave that the power company produces. Today’s inverters are considerably more efficient, quieter and electronics-friendly.
Each inverter/charger installation must provide a way to power the battery charger, and a way to supply AC output to the boat.
Power to the battery charger can be supplied by bringing shore power directly into the inverter/charger (with proper overcurrent protection). This is called a pass-through installation, since the incoming shore power has to pass through the inverter on its way to the panel. Alternatively, the battery charger can be supplied from the vessel’s AC breaker panel, either all or a subsection of it. Supplying a section of the panel can exclude high-wattage loads such as a water heater (forgetting to turn off a water heater powered by an inverter would quickly deplete most battery banks).
We suspected and confirmed that the boat’s inverter/charger was wired in line with one of the shore power inlets between the inlet and the AC panel supplying all the power to that leg. When the boat was plugged into shore power, the AC flowed through the inverter/charger via a transfer switch to power the AC panel, or a section of it.
The output is typically not inverted power in this mode. The battery charger also uses some of this power to charge the batteries.
Key connections
In our home electrical panel, the neutral and the safety ground are connected. On board, things are different: It is not safe to have AC neutral-to-ground connections except at sources of power such as generators or inverters. Any shore power AC fault, where some amount of amperage escapes the wires or equipment, will always try to find its way back to the power company’s grounding rod on shore. These faults mean that the water or equipment can become electrified, causing shock or death.
We want our safety ground to have an uninterrupted path back to that ground rod. So, we connect the safety ground to the boat’s underwater hardware. If the shore power ground wire fails, then the current will find its way back to the ground rod through the water.
This is where the neutral-to-ground connection becomes so important: If they are connected aboard the boat, and if polarity becomes reversed so that the neutral becomes hot, then all of the underwater hardware and the bonding system become hot and potentially lethal. For this reason, on a boat, they can only be tied together at the source of power, which means on the dock, on the generator or in the inverter.
Since an inverter/charger can be both a load (when it is charging batteries) and a source of power (when it is inverting), it needs a way to connect and disconnect its neutral-to-ground connection, depending on its mode. An internal transfer switch meets this requirement by switching sources of power and the neutral-to-ground connection.
On many units, you can hear an audible click as the relay inside makes the connection. Because many of these switches are electromechanical, they have contacts that can experience arc damage, or the switch itself can fail or become sticky.
There can never be more than one source of AC connected together, unless the waves and frequency have been matched by sophisticated electronics. Some new devices (including inverter/chargers) can waveform and frequency match, and they offer interesting capabilities for supplementing weak shore or generator power with inverted power from the batteries. This technology can help deal with spiking start-up loads from AC motors. It can also allow multiple inverters to be linked together (sometimes called “stacked”) to provide more wattage.
Aside from these devices, every other power source needs electrical separation.
What can go wrong?
Inverters/chargers are one of the most commonly misinstalled components aboard a boat.
Most units are not ignition-protected, so they can’t be installed in the same space with gasoline engines or their fuel lines and valves. Because batteries can emit hydrogen gas (which is lighter than air) when they are charging, the units cannot be installed above a battery bank. Another consideration is the weight of the unit; many are quite heavy. Whenever possible, the units should be through-bolted to a strong bulkhead or shelf. No one wants a 50-pound lump of metal flying across the engine room.
Also, the inverter/charger’s AC output wiring can get tricky. As mentioned earlier, the AC input wiring will be powered from the AC panel on a dedicated breaker or directly from the shore power inlet. If we go for a dedicated breaker panel and a separate neutral bus for all the loads (as was the case in our example), that is straightforward. But if we segregate out breakers on an existing panel, great attention will need to be paid to move all the neutral loads to a separate bus. Any connecting bars supplying the hot breakers will need to be cut so they can be segregated. Any breaker indicator lights (including circuit board-based lights) also will need their supplies segregated.
Better inverter/chargers can be equipped with remote control displays and temperature compensation for the battery charger. Follow the directions for the installation of the temperature probe on the battery, and check that it remains in place periodically.
If the charger is counting on a probe glued to the side of a battery for its calibration, and if that probe falls off and hangs in the air, the difference in temperature can be enough to trick the charger into over- or undercharging the batteries.
Bypass surgery
This particular boat owner noticed that his inverter/charger remote display panel wasn’t lighting up. When he inspected the inverter/charger itself, the display was likewise dead. With a multimeter, he checked that AC was going into the unit, but saw that nothing was coming out. Some inverters have “pop out” breakers to protect against an output overload, but these had not tripped.
All of these observations pointed to an internal failure. Because the boat was wired with the failed AC leg solely running through the inverter, there was no way to get power through it.
Now that the owner knew the culprit was the inverter/charger itself, a workaround became possible. With the help of a cellphone, a multimeter and the boatyard, he disconnected the AC input and output wiring and butt-connected the black wire to black, white to white, and green to green, effectively taking the inverter out of line.
This effort provided power back to both sides of his panel, although he could only charge his batteries from the alternator on his engine because the temporary bypass removed the inverter/charger’s battery-charging function from the system.
Additional tips
It is absolutely essential, when working on shore power or with inverters, to disconnect the shore power cord and turn off the inverter battery switch. If you unplug the shore cord and leave the inverter active, you can electrocute yourself with AC power supplied from the inverter.
Also critical, if you have a pass-through inverter installation: To prevent a loss of power, install a bypass switch between the input and output AC wiring to the inverter. A roll switch or a set of breakers with a mechanical interlock (only one breaker can be activated at once) should be installed in a plastic junction box on the bulkhead beside the inverter.
The breakers would allow a choice between normal inverter/charger operation or complete inverter/charger bypass. This bypass to route power around a failed inverter/charger can be accomplished with a dedicated roll switch as well.
Having this setup makes troubleshooting easier, and provides a simple way to take a malfunctioning inverter/charger out of series.
