Text AC Ground Systems

Alternating Current (AC) Ground System

Introduction:

The term "ground" is probably one of the most misunderstood terms in electricity today. It is also one of the most important terms to understand when it comes to marine pleasure craft wiring. Hopefully, the following TechNote will shed light on the subject of Alternating Current ground practices in the marine environment. However, it is not our intent to make you a marine electrician. Nor is it our desire to replace the need to seek the services of a qualified professional. It is our desire to give you the appropriate level of competency to adequately survey your boat's Alternating Current AC Ground System and to consider the recommendations of a qualified marine electrician. We will focus on building your knowledge - based on our interpretation and "real world" implementation of the American Boat and Yacht Counsels (ABYC) Standards and Recommendation.

For the "do-it-yourself" weekend electrician a special warning:

WARNING: Working on your boat’s AC electrical system is extremely dangerous!, And unless you thoroughly understand the characteristics of alternating current electrical systems, you should seek the assistance of a professional marine electrician!

Why is it important?

Basically for two reasons – First, a good understanding of AC ground systems at best may prevent you from electrocuting yourself or at least keep you from getting a nasty electrical shock. Second, it may just help keep your boat afloat by reducing the potential of stray current corrosion.

Electric current will take the path of least resistance. If something is wrong in your electrical system, a properly installed ground system will carry any potentially lethal current directly to Earth, reducing the real risk of electrocution or electrical shock.

Also, a properly installed ground system can help reduce but not totally eliminate stray current corrosion of your boat’s below-the-waterline metal fittings. Serious ground problems can eat away metal fittings in days.

Understanding basic grounding practices can save lives. That’s why it is important.

Probably the best source for marine pleasure craft wiring advice is the American Boat and Yacht Council (ABYC) standards and recommended practices. You would be best advised to review the ABYC standards or seek a marine electrician that understands and follows ABYC recommendations before attempting to develop or alter your boat’s AC or DC electrical ground systems.

General Ground Systems

Lets take a look at some of the ground systems you may come in contact with everyday.

Your automobile - a non-insulated ground return system:

Your automobile’s electrical system can be called a grounded current-carrying system, but most often it is called a non-insulated ground return system. And, if your automobile's electrical system is like mine, you probable have a couple of other choice names to call it. For our purpose, we are going to call it simply a "ground return system". Under this system, the positive terminal of the battery is connected with insulated conductors directly to the engine starter. Other electrical device loads such as lights, instruments, and accessories are also connected to the positive termial of the battery with insulated conductors. However, they also utilize switches and fused-links or circuit fuses as part of thier circuit. Each positive current-carrying conductor is insulated from other conductors, the chassis, the engine and the Earth. The negative battery terminal on the other hand, is directly connected to the engine block and chassis. In other words, the negative battery terminal is grounded to your automobile’s engine block and chassis. Your automobile's Earth ground or zero voltage reference point for all current-carrying circuits is the engine block and chassis via their connection to the negative battery terminal. The grounded automobile chassis acts as a very large negative current-carrying conductor.

By definition, your automobile's chassis is a grounded conductor that is intentionally grounded to Earth or to some conducting body that serves in place of the Earth - the automobile chassis via the negative battery terminal. It is the reservoir of "neutral charge" and the zero voltage reference pointfor the electricity utilized by your automobile.

True, your automobile is not an Earth-grounded system because it is insulated from the Earth by rubber tires, but it holds true to the definition of a grounded system. The automobile frame is acting as "some conducting body that serves in place of the Earth" via its connection to the negative battery terminal. Like the Earth’s difference in electrical potential, the entire automobile frame has a difference in electrical potential from the positive battery terminal due to its connection to the negative battery terminal.

Your home – an insulated ground return system:

Your home’s electrical system can be called a "current-carrying insulated ground return system". This means that all the current-carrying conductors are insulated from other conductors, other structures and Earth. Each electrical device load has a complete circuit (phase and neutral) from the distribution panel to itself and back to the distribution panel through the use of these insulated wires. Both the phase and neutral wires are current carrying wires under normal operating conditions. Also, the phase wire has an over-current protection device and the neutral current-carrying conductor normally does not.

The electrical utility company supplies power to your home via a two or three wire system. A two-wire system consists of a single-phase wire and a single-neutral wire. A three-wire system consists of two-phase wires and a single-neutral wire. In either case, the utility supplied phase wire(s) from their transformer are connected at your home to a power meter and the main circuit breaker(s) at the distribution panel. The utility supplied neutral wire (which is most likely grounded to Earth at its transformer - utility ground) is connected to the same power meter and to a neutral buss bar inside the distribution panel. Also, the neutral wire at the power meter or most likely at the neutral buss bar is tied to a buried metal rod driven into the ground just outside your home. This grounded connection is called the "house ground". See Figure One.

The main circuit breaker feeds phase-current to the individual branch circuit breakers. Each branch circuit breaker supplies phase current over an insulated wire to each electrical device load. Unlike your automobile, which uses it’s chassis (ground) as the electrical device's negative return path, your home uses a separate insulated (neutral) wire from each electrical device. This separate neutral wire is connected to the neutral buss bar at the distribution panel. The neutral buss bar provides a single grounded connection to Earth. This connection keeps your home’s electrical system tied to the Earth’s ground potential. Both the phase and neutral conductors are current carriers under normal conditions. The phase conductor in never connected to ground, but your neutral conductor must be connected to ground. By definition, your home’s neutral wire is a grounded connector that is intentionally connected to the Earth.

Inside your home’s distribution panel there is another buss bar. It is called the grounding buss bar. The grounding buss bar is tied to the neutral buss bar. Remember, it is the neutral buss bar that supports a direct connection to Earth via the "house ground". (NOTE: On your boat, the grounding buss bar and the neutral buss bar must not be tied together.)

The grounding buss bar provides a safety ground connection for each branch circuit. Typically, each branch circuit in your home consists of a black phase wire, a white neutral wire, and a "green" or bare safety grounding wire. The green wire connects to the grounding bus at the distribution panel and the electrical device loads metallic enclosure at the other end of the branch circuit. Unlike the current carrying phase and neutral wires, the "green" ground wire is never a current-carrying conductor under normal operation. See Figure One.

The primary purpose of the grounding wire is to keep you alive. For example: If a phase wire inside of your toaster comes in contact with its metallic case, the case will becomes electrically "hot". Without the grounding wire there is no circuit to ground. No circuit breaker would trip - there is no current flow. At least no current will flow until you complete the path to ground by touching the case. At best, you could get a nasty shock - at worst you could die. The grounding wire is designed specifically to carry current to ground during an abnormal condition to protect you from an electrical shook. Hopefully, the current draw to ground from the faulty phase wire will be enough to trip the breaker – thus giving you some warning about an electrical problem. If not, at least when you touch the toaster the grounding wire should provide a lower-resistance path to ground thus keeping the current away from you. See Figure Two.

By definition, your home’s "green or bare" wire is a grounding connector used to connect the non-current carrying metal parts of equipment raceways, and other enclosures to the system grounded conductor and Earth.

For more detailed information and an animated illustration of a typical Ground Fault scenario.
>>Click Here<<

Your boat – a hybrid-ground system:

Your boat could be considered a hybrid-ground system because it incorporates several different ground systems. Each one has its own characteristics - its own electrical potential from Earth’s ground and its own potential for disaster. For example, on most boats you will find a DC non-insulated ground return system, a DC insulated ground return system, an AC insulated ground return system, an AC safety grounding system and the boat's bonding system. On some boat’s, you can find separate lightning and HF radio grounds. Also, your boat should have a Common Grounding Point. Following the ABYC recommendation - all of the grounding system on your boat except the AC insulated ground return (neutral) wire, will have a connection to your boat’s Common Grounding Point. The purpose of the common grounding point is to keep all your boat’s grounding systems at the same ground potential.

Note: 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. This may or may not be a problem. (more on this later)

Our focus in this TechNote is Alternating Current (AC) Ground Systems, but we will briefly review each ground system that you may find on your boat.

DC non-insulated ground return and insulated ground return systems.

Your boat’s DC electrical systems most likely consist of two ground systems: a non-insulated ground return system and an insulated ground return system. The non-insulated ground return system is like the one found in your automobile. The negative battery terminal is connected to your boat’s engine block. All electrical equipment on the engine, such as the electronic sending units (temperature, oil pressure, etc), the alternator, and the starter use the engine block as a large negative wire with a common negative battery connection forming the zero voltage reference point for the DC non-insulated ground return system. The non-insulated ground return system is the most common system used on boats but not the most desirable.

All other DC electrical equipment (bilge pumps, lighting, radios, heads, etc) found on your boat should use an insulated ground return system. This system utilizes a distribution panel typically equipped with a main circuit breaker, several branch circuit breakers and a common negative buss bar. Each electrical device has a positive insulated wire connection to one of the branch circuit breakers. The branch circuit breaker is connected to the main circuit breaker. The main circuit breaker is connected to the positive terminal of the battery. The insulated negative wire from the electrical device is connected to the common negative buss bar. The negative buss bar is connected to the negative terminal of the battery forming the zero voltage reference point for the DC insulated ground return system.

Bonding system

Your boat’s bonding system (if equipped with one) ties together all major metal objects (rigging, engine and propeller shaft, metal water tanks, through-hull fittings, etc) on the boat to equalize differences in potential of the various types of metal. Its primary purpose is to help slow down galvanic (electrolytic) corrosion.

Your boat’s bonding system is not a current-carrying system. Nothing should be electrically grounded to your boat’s bonding system. Although the need to "bond" all the boat’s metal objects is a debatable subject, the need to keep it separate from your boats AC or DC ground systems is not. Each bonded device on your boat has a single connection to the bonding system. The bonding system has a single connection to your boat's Common Grounding Point which is the negative battery terminal.

Your boat's Alternating Current (AC) Ground and Grounding System

The alternating current (AC) insulated ground current-carrying system utilized on your's boat is often refered to as a "free-floating" system. But this is only true for the neutral and the phase current-carrying conductors. They are not connected to any other alternating current (AC) ground or phase source aboard your boat. The alternating current (AC) safety "green" grounding conductor is not "free-floating". It is connected, by ABYC standards, to the boat's common grounding point. The common grounding point ties all the grounding systems aboard your boat together. This is an important point to understand from a safety standpoint and from a stray current corrosion standpoint.

Although, there seems to be some room for debate about the green wire (grounding conductor) connection to the boat’s Common Grounding Point, there is no debate concerning the neutral wire. It is never grounded to anything on the boat - shore power grounded at the marina - AC generator grounded at the AC generator - inverter grounded at the inverter. Of course there are some exceptions to that. Hopefully, the following will explain:

The ABYC puts it this way, "A grounded neutral system is required. The neutral for AC power sources shall be grounded only at the following points:"

"The shore power neutral is grounded through the shore power cable and shall not be grounded on board the boat."

"The secondary neutral of an isolation transformer or polarization transformer shall be grounded at the secondary of an isolation or polarization transformer."

"The generator neutral shall be grounded at the generator."

"The inverter output neutral shall be grounded at the inverter. The inverter output neutral shall be disconnected from ground when the inverter is operating in the charger or the feed-through mode(s)."

"When more than one shore power inlet is used, the shore power neutrals shall not be connected together on board the boat."

Exception to the above: "For systems using an isolation transformer or polarization transformer, both the generator or inverter neutral and the transformer secondary neutrals may be grounded at the AC main grounding bus instead of at the generator, inverter, or transformer secondary."

Pretty straightforward: Each electrical device load on your boat has a separate neutral wire connection from the load to a neutral buss bar at the boat's distribution panel. The neutral buss bar has a single feeder wire connection to a simultaneous trip (hot and neutral power feeder - more on this later) main circuit breaker.

If your boat's only source for AC power is the marina's shore power, then the rest of the neutral wiring is simple. You will have a single feeder wire from the neutral buss bar to the load-side of the main neutral circuit breaker. There will be a single feeder wire from the source-side of the main neutral circuit breaker to your boat's shore power inlet connector. The shore power cable supplies the neutral wire from the boat's inlet to the docks shore power outlet connector. From there, the neutral wire should be connected to an Earth ground via the marina's ground-rod and power distribution system. And hopefully, the marina has done a good job designing and maintaining its Earth ground system because this is your zero voltage reference point. See Figure Four.

If you utilize more than one shore power connection, the neutral wires from each connection must not be tied together on board the boat. They will have their own circuit breaker, feeder wire, and neutral buss bar. The reason for this is very simple. Remember the neutral wire is a current carrying wire. If you tie them together aboard your boat and then disconnect one of your shore power cables, current will still be on your boat's inlet side of the shore power connection. This is a shock hazard. See Photo One

If your boat has multiple sources for AC power - shore power and/or an AC generator and/or a DC/AC inverter - things get a little more complex. For right now, let's just say that each source has its own neutral zero voltage reference point. And, never will one source's neutral wire be tied to another source's neutral wire. Nor will one source's phase wire be tied to another source' phase wire. They are separate systems and must be kept electrically separate.

How do you keep everything separated?

According to the ABYC: "Individual circuits shall not be capable of being energized by more than one source of electrical power at a time. Each shore power inlet, generator, or inverter is a separate source of electrical power."

"The transfer from one power source circuit to another shall be made by a means that opens all current-carrying conductors, including neutrals, before closing the alternate source circuit, and prevents arc-over between sources."

"A means for disconnecting all power sources from the load shall be provided at the same location."

Exception to the above: "The grounded neutral from a polarization transformer, isolation transformer, generator or inverter may be permanently connected to the same main AC grounding bus and is not required to be switched."

Again, everything is pretty straightforward: But it is important, and so we will go over it again. The neutral grounds from the AC shore power system, the AC generator system, and the AC inverter system are not connected together. They are separate ground systems. They are kept apart by a correctly installed AC Source Selector switch.

Remember an AC electrical load device cannot have more then one AC power source attached at any given time. An AC power source selector not only switches the phase source(s), it also switches the neutral source. However, it does not switch the green safety-grounding conductor. All green safety wires are connected to a grounding buss bar at the distribution panel. The grounding buss bar is then tied to your boat’s Common Grounding Point.

Your Boat’s - Common Grounding Point

Remember, because of all the various electrical systems on your boat, including the non-electrical bonding system, each with its own ground potential, it is desirable to establish a common grounding point for all systems. The purpose of the common grounding point is to keep all of your boat’s ground systems at the same ground potential.

Your boat’s AC system falls into this category, with its neutral ground current-carrying conductor and its green non current-carrying conductor. However, the neutral conductor should never be connected to your boat’s common grounding point. Remember, the neutral wire is a current carrying wire and if you connect it to your boats' common grounding point, current could flow though underwater fittings to ground inviting stray current-corrosion. Your boat’s common grounding point is for the green non-current carrying grounding conductor.

Again 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."

That is their recommended Common Grounding Point

Pretty straightforward: You connect your incoming grounding "green" safety wire to the negative terminal of the battery on DC grounded return systems or the main DC negative bus bar on insulated ground return systems. However since the majority of boats utilize both systems and the negative battery terminal is usually connected to the negative buss bar at the DC distribution panel, it might be the best place to connect the AC green wire.

One other point the ABYC makes about the green safety wire - which is very important.

There shall be no switch or over current protection device in the AC grounding (green) conductor.

Pretty straightforward: Do not connect a switch or circuit breaker to the grounding conductor.

load time 82 sec @ 28.8kbs	load time 74 sec @ 28.8kbs	load time 74 sec @ 28.8kbs
An animation illustrating the phase to ground "neutral" wire flow of current in a typical marine shore power system See Figure Five	An animation illustrating the phase to grounding "green" wire flow of current in a typical marine shore power system with a ground fault. See Figure Six	An animation illustrating the phase to common grounding point flow of current in a shore power system with a faulty grounding conection. See Figure Seven

We are going to end this TechNote on Alternating Current (AC) Ground Systems, but be sure that we still have more to say on the subject of ground, grounded, and grounding. Future TechNotes will be discussing, the need to polarize your AC system, the use of ground-fault circuit interrupter (GFCI) circuit breakers and outlets, polarization transformers, isolation transformers, generators, and inverter wiring. So come back.

End of Alternating Current (AC) Ground Systems

After reading the above, if you still need some help or have a direct question about Alternating Current (AC) Ground Systems, please feel free to e-mail TechNotes@onemanonline.com.

Related Articles:
"Safe Shorepower", by Don Casey
http://www.boatus.com/boattech/Casey/36.htm
This is a very good overall article on AC wiring in the marine environment, and for the most part it agrees with what the ABYC is recommending.

"BUYING, OWNING AND MAINTAINING BOATS AND YACHTS", by David H. Pascoe, Marine surveyor
D. H. Pascoe & Co., Inc. Marine Surveyors, Destin, Florida
The complete article:
http://marinesurvey.com/yacht/ElectricalSystems.htm
The part about grounds:
http://marinesurvey.com/yacht/ElectricalSystems.htm#Grounds
This is also a very good overall article on AC wiring in the marine environment. However, we do not agree on what the ABYC is recommending.

"Galvanic Isolators: Don't Plug-in Without One",
From Seaworthy, BoatUS Marine Insurance Report
http://www.boatus.com/seaworthy/galvanic/default.asp