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The Connection Between Bonding, Grounding And Lightning

Basic electricity for boat builders, boat repairers and owners. How are all these systems connected?  The connection between AC, DC, Bonding, Grounding, and Lightning Protection systems.

I have talked to many boat owners and found that the electrical connection between AC and DC is very misunderstood by most of them.  Throw in Bonding, Grounding and Lightning protection and people get so confused it becomes a real muddle.

To separate the issues and understand what the connection is, where it is, and why it is, you first need to start with some definitions. You also need to think about these as separate systems that just happen to be connected at one, and only one, point on the boat.

Also: ABYC and other authorities such as ISO, make a distinction between the term Ground and the term Grounding.  In Europe and other parts of the world this may be called Earth and Earthing.  Ground in AC systems usually refers to the white neutral wire and it's connection to the earth (or ground) and Grounding refers to the third, green wire.

DC:  Direct Current. A low voltage system is usually 12 Volts, but on large vessels can be 24, or 48 volts, to power basic equipment using batteries.  Anything over 50 volts, AC or DC, is considered high voltage.  DC is a two wire system, positive (red) and negative (black or yellow), but there can be a third wire, a green grounding wire or a grounding buss connected to the boat's ground. This is not normally a current carrying conductor.  See DC grounding conductor below. See DC systems

AC: Alternating Current. Basically the same 60 cycle, 110 Volt system (nominally 120 Volt)  you have at home, with some very important differences. It is a three wire system with a hot wire (black), a neutral (white and also very hot) and a green wire. The green wire (the safety wire) is an alternative path for current back to earth and is not normally a current carrying conductor. See AC grounding conductor below.   220 Volt AC systems are rarely seen on boats in the USA but are common in Europe.  These systems will have a fourth red wire and are often seen in many homes in North America powering electric clothes dryers See AC systems

Bonding: A separate system electrically connecting all metal fittings on the boat that are in contact with the water, including the sacrificial anode(s), and connecting them to the earth ground, so they are all at the same voltage potential, zero.  There should never be any current in the bonding system. The resistance of this circuit should be less than one (1) ohm. 
See Bonding

Lightning Protection: A separate system to provide a direct, low resistance  path for lightning to earth ground while protecting the boat and it's occupants. ( a very simplified definition)  See Ike's Electrical Links for links to lightning protection systems information.

Grounding System: See the ABYC definition of Ground below.

The following are taken directly from ABYC Standard E-11 AC and DC electrical Systems on Boats.

11.4.1 AC grounded conductor

A current carrying conductor that is intentionally maintained at ground potential.

NOTE: This may be referred to as the neutral (white) conductor in AC electrical systems.

11.4.2 AC Grounding Conductor (green or green with a yellow stripe) -a conductor, not normally carrying current, used to connect the metallic non-current carrying parts of AC electrical equipment to the AC grounding bus, engine negative terminal or its bus, and to the source ground.
1. This may be referred to as the ground (green, or green with yellow stripe) conductor in AC electrical systems.
2. The source of electrical power may be the shore AC power, an inverter, an isolation transformer, or a generator.

11.4.9 DC Grounded Conductor - a current carrying conductor connected to the side of the power source that is intentionally maintained at boat ground potential.

11.4.10 DC Grounding Conductor - a normally non current carrying conductor used to connect metallic non current carrying parts of direct current devices to the engine negative terminal, or its bus.

11.4.11 Engine Negative Terminal - the point on the engine at which the negative battery cable is connected.

11.4.16 Ground

The potential of the earth's surface. The boat's ground is established by a conducting connection (intentional or accidental) with the earth, including any conductive part of the wetted surface of a hull.

11.4.26 Polarized System (AC) - a system in which the grounded and ungrounded conductors are connected in the same relation to terminals or leads on devices in the circuit.
11.4.27 Polarized System (DC) - a system in which the grounded (negative) and ungrounded (positive) conductors are connected in the same relation to terminals or leads on devices in the circuit.

Grounding is making a connection between systems or equipment on a boat to bring that system or equipment to zero voltage potential, or earth ground.  The grounding system may be connected to the engine negative terminal, or connected to a grounding buss that is connected to the engine negative terminal.  This is not the same as the Bonding system above.  This is normally used for DC electrical equipment. Some equipment on boats, that are not part of the electrical system, need to be at zero potential and provide a path to ground for any static electricity that may occur.  In particular the fuel system, fuel tank, fuel fill, and fuel tank fittings need to be connected to ground to dissipate any static electricity that may occur during fueling.  The wires for the fuel system ground go to the engine negative terminal or the grounding buss.



Ground Fault:  A ground fault is an AC hot wire, either black or white, inadvertently touching something it isn't supposed to be touching such as a metal case.  It is not the same as two hot wires shorting to each other. It is wires shorting to ground.  This can occur due to a breach in the insulation, a broken wire, or a loose connection. Chafing on a sharp edge or rough surface can abrade the wire and cause a break in the insulation.

Where are all of these systems normally connected?

The DC System is connected by the DC grounding conductor to the engine negative or it's Buss. This is usually the wire from the battery negative terminal to the engine cranking motor, but it can be a separate wire from the battery negative to the engine block.

The AC system is connected to earth ground in two places, one on the boat and one not on the boat. On the boat the AC grounding conductor (the green wire) is connected to the earth ground at the engine negative terminal.  The other connection to earth ground is always at the source of power, the power generating station, where the AC Grounded conductor (the white wire, neutral) and the AC Grounding conductor (the green wire) are connected to each other and connected to earth ground by a rod driven into the earth.

 Normally the green grounding wire and the white neutral wire are not connected anywhere on the boat.  The exception to this is, if you have an isolation transformer, then there is no physical connection from the boat to the earth ground at the power station, only on the boat.  The AC Grounded conductor (the white wire, neutral) and the AC Grounding conductor (the green wire) are connected to each other at the transformer, the source of power for the boat. 

If you have an on board generator, or an inverter then the AC Grounding conductor is connected to earth ground internally inside the generator or inverter, the source of power.  But the only connection on the boat that matters to you the owner/installer/technician,  is the connection to the engine negative terminal or the grounding buss.

Additionally, the green Grounding conductor is connected to the metal case of AC appliances, such as electric ranges, refrigerators, washer/driers, microwaves, etc. This provides a path to ground if there is an accidental short (a ground fault) in the  equipment to the case.  If you touch the metal case the current goes through the wire instead of through you.  It is extremely important to check any AC household appliances to see if the green wire is connected internally to the white wire.  This is commonly done on household appliances. It needs to be disconnected.  If you leave this connected, then the green Grounding wire and the white Neutral wire are connected on the boat and a shock hazard exists because current is now flowing in the green wire.  The green Grounding wire should only be connected to the metal case.

Bonding wire: The bonding wire, also a green wire, is connected to earth ground at the engine negative terminal or the grounding buss.

Lightning protection system: The lightning protection system is also connected to the engine negative terminal. But when making connections for the Lightning Protection system care must be taken to follow the instructions in ABYC TE-4 Lightning Protection.  ABYC

ABYC has anticipated combining DC Grounding systems and Bonding systems.  In E-11 it says:

11.17.4 Combined Grounding and Bonding Systems
The DC grounding conductors may be combined with the following systems providing all the requirements with respect to conductor size are met for each system (see FIGURE 5 and FIGURE 11). You can purchase a current copy from ABYC at Lightning Protection - See ABYC TE-4, Lightning Protection. Cathodic Bonding - See ABYC  E-2 Cathodic Protection. (This is an old copy from 2013) Static Electricity Grounding - See E-, ABYC H-24,Gasoline Fuel Systems, and ABYC H-33, Diesel Fuel Systems. 

The reason behind all of these systems being connected at the engine negative terminal is the same for all of the systems.  This puts the system at zero voltage potential during normal operation, and provides a zero voltage reference point from which to measure voltage potential on the positive and hot side.   The Grounding wire acts as an alternative path for current back to earth ground, but only when there is a ground fault in the system. This prevents shock hazards and prevents you from being the path for current back to ground. It also prevents stray current corrosion and galvanic corrosion.  This is why it is very important to insure that the green Grounding conductor is properly connected and does not have any breaks in it, in order to conduct current back to the source of power.

See the diagram below:  Click on it to Enlarge.

Grounding Buss Connections

Earth Ground:  Some people do not agree with the premise that connecting the grounding wire to the engine block provides a path back to earth.  However, water, whether fresh or salt, is an electrolyte, and current will travel through it.  Fresh water is not as good a conductor as salt water and is a high resistance path to ground. If you were measuring leakage current around a boat in fresh water, and then did the same measurement in salt water the current would be much higher in fresh.  This is why AC leakage is so dangerous to swimmers in fresh water.   See Hot Docks  Electroshock Drowning  There is still a danger in salt water although there is not as large a current field around the boat.  But the current will definitely travel through the water back to earth.  This has been establish by research and experimentation.

However, this should never happen.  There should never be current from the grounding point into the water.  If this occurs then there is a ground fault in either the Black Hot wire or White Neutral wire and a  fault in the green Grounding wire.  It is broken somewhere. It is vitally important to maintain the integrity of the green Grounding wire.

For DC systems some people prefer to not have a connection from the battery negative to the engine block.  This is what is commonly called a floating, or isolated system.  They believe this prevents stray current from entering the water.  Their belief is simply that if you do not have an electrical connection to any metal in touch with the water, then you can't have any stray current.  There is some merit to this argument.  It will prevent stray current corrosion, but it will not prevent galvanic corrosion. See Corrosion On Boats

This is difficult to achieve in practice because the starter (cranking motor) is grounded to the engine block on most engines, and by connecting the negative wire to the battery negative, you are connecting the engine block as well.  This is the same as using the engine block as a grounding point.

© 2007 All rights reserved. revised 04/27/2021

Basic Electricity
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