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Old 02-09-2004
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Kevin Hughes is on a distinguished road
Understanding Grounding and Bonding


Onboard electrical systems can either be a blessing or a beast, and often that distinction is rooted in what the boat owner knows about them, so read on.
Many boat owners install and maintain their own electrical circuit wiring to provide power for light fixtures, pump motors, and electronic navigation equipment. Although many such systems are not perfectly in compliance with electrical codes, many are patterned after what most experts would agree are safe and adequate systems. The connection of conducting materials that aren't part of the electrical utilization system is another matter entirely. Because all the "experts" cannot agree upon a standard method for this procedure it seems deserving of additional attention. So, this series of articles concerns this very special portion of a vessel's electrical system—that of the current that flows outside the designated wiring.

Among the DIY crowd, the topic of grounding generates more opinions than any other scientific discussion I can think of; although I believe more of a consensus has been forthcoming in recent years. I will tell you right now that the statements I make herein are my own opinion. It may make a difference to you that I am a Certified Master Electrician and Electrical Engineer who has worked in the field for over 25 years on most every type of electrical and electronic equipment in a vast array of installations. And I have sailed for 20 years. My wife and I recently moved aboard a sailboat after completely refitting all the shipboard systems to the highest degree possible. I will try to present the theory of bonding and grounding, as I understand it, and as it applies to seagoing vessels, particularly recreational ones.

Bonding and Grounding Objectives    The bonding (the intentional interconnection of separate metallic components) and grounding (the intentional connection of a metal or system of metal components to a specific reference point) of the non-current carrying metal parts of a boat may accomplish three specific and separate objectives.

  1. Reduce the corrosive effects of dissimilar metal galvanic corrosion on expensive and critical boat parts.
  2. Eliminate electrolysis because of being the "ground" for another boat or an entire marina because of wiring defects.
  3. Protect persons and equipment from damage due to lightning.

Let's begin by briefly discussing what occurs when the objectives above are ignored and destructive or dangerous conditions exist.


Mast bases are notorious locations for galvanic corrosion due to the propensity for differing metals to inhabit these spaces.
Galvanic Corrosion  
When two different type metals are in contact and subject to a corrosive environment, the least noble metal will be sacrificed. (John Vigor's Practical Mariner's Book of Knowledge is an excellent source to identify the galvanic series of metals in seawater and most other essential sailing data.) A corrosive environment would include submersion in salt water or even freshwater, or when subjected to seawater spray.

The process of accelerated corrosion begins because of an exchange of ions, electrons, and other atomic and subatomic particles at the point where these metals touch. This exchange of particles at the junction of these metals causes an electrical difference of potential between the metals. This technical term sits at the root of all the evils discussed in this entire article, so it needs to be understood. One way to understand "difference of potential" is to know that it can be measured in volts. In the case of dissimilar metals that sit in contact with one another, it is generated chemically. So what we are dealing with is a battery of sorts. Electrical current will flow if a connection or circuit is complete between the metals having a difference of potential.

A common lead-acid battery ceases having a difference of potential across its terminals, or is "dead" when the lead oxide plates are chemically converted to lead sulfate through use and discharge of the battery. A brass plumbing fitting threaded into a Monel bilge tank will cease to have a difference of potential when all the zinc in the brass corrodes away and the remainder of the fitting crumples into dust.

In the case of submerged dissimilar metals, the more anodic (or less noble) metal corrodes and gives up part of its structure (in the form of metal ions conducted through the seawater) to the more cathodic (or more noble) metal. The anodic metal is consumed. Go ahead, read this section again; it is the condensation of three or four chapters of really technical stuff so don't feel bad if you didn't get it right away.


Electrolytic corrosion is the beast that lurks beneath the surface. No matter the size of the marina, if boats sit in close enough proximity to one another, ions will be changing locations.
Electrolytic Corrosion  
Electricity sent out on a wire from its source to utilization equipment to do some work must have a return path to the power source. In a house, there is a ‘hot' wire and a ‘neutral' wire. In a boat, there should be red positive wires and black negative wires. In an automobile, one wire feeds a device and the frame of the vehicle is sometimes used to complete the electrical path back to the ‘grounded' battery wire.

Comparatively speaking, seawater is a pretty good conductor of electricity. So all the underwater exposed metal parts of your boat are electrically connected to all the underwater exposed metal parts of any other boats within a reasonable distance. (A conductor's ability to pass electricity is greatly reduced by distance). Think of these parts and fittings as being ‘in contact,' as in the example given above of dissimilar metals.

A mistake or fault in the wiring supplying power to your boat, the marina docks or any of the other boats nearby could cause a difference of potential between the submerged metal parts in your boat and the submerged metal parts connected to the faulty wiring. With good ol' seawater completing the circuit between these metal parts, accelerated corrosion will occur at one or both of these metals parts because of the forced migration of electrons or metal ions. (If the metals are different, it's galvanic; the "forced" difference of potential makes this situation defined as electrolysis.)

Lightning    Lightning is certainly the most striking of the topics involved in grounding and bonding. It occurs millions of times during the tens of thousands of thunderstorms each year in the US alone. Although facts and figures are collected every day by satellites and research laboratories such as the one I frequented in Tucson, AZ, thorough knowledge of lightning has not yet been attained. It assumes several forms—most everyone has seen and heard bolt lightning and sheet lightning; but other effects of lightning such as ball lightning, St. Elmo's Fire, and other atypical static discharges have been described for centuries and still are not fully understood.


Though not much of the information available about  lightning is definitive, there are some reasonably good approaches to grounding and bonding boats and their metal parts and electrical systems.
Basically, lightning is caused by a difference of potential created during atmospheric disturbances; most commonly in and around thunderstorms. The difference of potential causing lighting is measured in millions of volts. The lightning occurs when a current can be conducted between bodies of potential difference. In the case of cloud-to-ground flashes of lighting, which are the most devastating to boaters and our immediate concern here, the difference of potential is so great that it creates a condition called ionization right in the air, which allows the normally insulating air to conduct electricity. Ionized air is created in steps by the great difference of potential.

The idea of a lightning strike being a bolt of electricity coming out of the clouds and zapping the earth or object on the earth is incomplete. In most cases, the creation of ionized air begins at both ends of the path the lightning will travel. A lightning bolt does not usually consist of a single zap to equalize the difference of potential; sometimes several exchanges in both directions occur to dissipate the vast electrical energy involved.

It is very difficult to study lightning for many reasons. It occurs very randomly even within ideal conditions and varies greatly in its attributes; a bit akin to snowflakes, no two lightning bolts are alike. And lightning is an extremely brief event of an immense magnitude. For our purposes, we only need to study the results of lightning, what effects it has when occurring in the vicinity of sailboats.

Because lightning is the most allusive problem considered here, with the most controversial solutions, we will save it for last. To solve the first two problems with an ideal bonding and grounding system is really a piece of cake. Next month we'll see what can be done about all this corrosion.

The Following User Says Thank You to Kevin Hughes For This Useful Post:
gieholzer.60@gmail.com (08-28-2013)
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