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.
- Reduce the corrosive effects of dissimilar metal galvanic corrosion on expensive and critical boat parts.
- Eliminate electrolysis because of being the "ground" for another boat or an entire marina because of wiring defects.
- 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.
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.
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.
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.
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