2/9/2020: Significant edits to better connect concepts and include technique descriptions.
My previous post (Corrosion Article) discussed corrosion of underwater metals caused by various stray electric currents in the water. In that post, I made passing reference to “bonding,” “bonding conductors,” and to underwater metals “being bonded together.” This article looks specifically at the bonding system of a boat. The objective is to provide a basic understanding of why bonding is installed, what it does, and consider the maintenance needs of the bonding system.
As boaters, we are constantly involved in discussions of the design, equipment, materials, techniques and components of the AC and DC divisions of a boat’s electrical system. When those systems fail, there are usually symptoms, anxieties and inconveniences that boaters notice. Although Internet boating discussion lists are filled with electrical topics, only rarely does one see discussion related to a boat’s “bonding system.”
As in other electrical technical areas, “bonding” is an area where there is a body of common concepts and terminology that apply across a wide range of AC and DC situations. Just as in the “Corrosion” topic, the concepts of bonding are consistently the same, but an understanding of context is essential to avoiding confusion. Experienced electrical practitioners often take shortcuts with context. For the layman, the only way to get past that is to invest some time in understanding the concepts. After that, understanding context gets easier very quickly.
The terms “grounding” and “bonding” are often used interchangeably, but in fact, they are different. Following are definitions with which most experts would agree:
“Ground” is the single-point of electrical connection between an electrical sub-system (like a boat) and the physical earth. This connection is made for the purposes of:
- providing a lightning discharge path,
- providing a path to bleed off static charge,
- sub-system voltage stabilization, and
- reducing RF interference.
“Bonding” is an electrical connection (usually a network of electrical connections) which electrically interconnect metallic housings and device enclosure components. Bonding:
- provides a low-resistance path for ground-fault currents to ensure circuit protection devices (circuit breakers) trip,
- prevents dangerous “touch-voltages” from appearing on exposed metal surfaces, and
- provides a path for galvanic currents and AC and DC stray currents.
NOTE: there are two important reasons to have a bonding system on boats; 1) mitigation of galvanic currents and 2) AC electrical safety. Sometimes, we hear and read that a bonding system is not needed or desirable, because it actually provides one of the conditions that are NECESSARY in order for corrosion to happen. But, that is a very narrowly-framed point-of-view that ignores the importance of the bonding system to the safety of the AC Electrical System aboard a boat.
Figure 1 is a simplified topology overview of the three major divisions (AC division, DC division and Bonding division) of the electrical system of a typical cruising boat, whether sail or power, whether slow displacement hull or go-fast sport fish. It is representative of the great majority of US-manufactured boats. This topology view is consistent with the “model” electrical system upon which the principal ABYC Electrical Standard, E-11, is based (“AC and DC Electrical Systems on Boats,” July, 2003 – 2018, Figure 10).
The ABYC E-11 standard treats a boat’s DC System as the “central-most” division of the electrical system of the boat, to which all other divisions are attached in a peer-to-peer relationship. This seems a reasonable assumption, since AC systems and bonding systems are neither required nor essential on a boat, but the DC system is always needed for engine starting and the operation of bilge pumps, navigation lighting and (usually) sound-signaling device requirements.
All of the conductors shown in green in Figure 1 are part of the boat’s “bonding system,” or “bonding network.” That entire network of conductors works together. In typical dockside conversation, the “bonding system” is often thought of as limited to the wiring shown on the right-had side of Figure 1. The usual term applied to the AC portion of the bonding system is the “AC safety ground.” Note, however, that the AC safety ground is an integral part of the overall bonding network of the boat.
In normal operation, all bonding systems are “silent” and “invisible.” When “everything is right,” the bonding system does nothing, and “everything works fine.” Bonding networks are so quiet and invisible that a boat owner might never know if a fault had appeared.
In fact, the primary purpose of the bonding system is to spring into action to protect us when an electrical fault does occur in either the AC or DC system. The only “normally active” purpose of the bonding system is to control corrosion due to DC galvanic currents.
Today we are very fortunate that the reliability of electrical components is very good. The mathematical probability, confirmed by life experience, is that electrical faults are relatively infrequent. Given that the bonding system comes into play only when there is a fault, it probably won’t actually be needed very often. If the bonding system does have a defect, unless there is another, second fault, there will be no failure symptom or danger to people or pets. Yes, there may be an increased rate of corrosion, often interpreted as “electrical issues in the basin and nothing to worry about.” These are “handled” as a routine maintenance item, but the underlying cause is often not diagnosed or corrected. The bonding system adds complexity to the boat, but can save many headaches, much expense and even heartache for the boat owner if it is intact when needed. Some bonding system faults can create dangerous situations leading to fire, electric shock, loss of property and in the ectreme, loss of life.
The heart of the DC division of the boat electrical system is the battery/battery bank, including all B+ and B- wiring and all subordinate DC device attachment wiring. “B+” is the term for the DC positive feed (+12V, +24V) that originates at the positive post of the boat’s battery. “B-” is the term for the DC negative conductor that returns DC power to the negative post of the battery. In the common lexicon of conversation, the DC return circuit is often referenced as its “ground” conductor. However, the B- conductor in the DC system carries DC current back to the battery, so it is more properly analogous to the “neutral” conductor of the AC division.
Bonding circuits are intended to carry only galvanic and fault currents; never currents that power equipment or attachments. To avoid undesirable voltage drops in the bonding system, and problems with accelerated electrolytic corrosion, no B- connections should ever be made to any part of the bonding system. Such connections are analogous to a “code violation.”
ABYC E-11, Figure 10, shows the “DC Main Negative Buss” as the central collection point for all DC B- return circuits, as well as for the “AC Safety Ground” and the bonding network connections. The boat’s AC Safety Ground and the various branches of the DC bonding system are all connected together at one place, and at one place ONLY: the “DC Main Negative Buss.”
Neither ABYC nor NMMA “require” the installation of DC bonding systems. Bonding systems are “optional.” However, ABYC E-11 does specify requirements for the bonding system if one is installed. Among US boat manufacturers, bonding systems are the “normal” manufacturing practice.
The primary purposes of the bonding system are to:
- hold exposed metal parts at to “touch potential” that is safe for people and pets;
- provide a low resistance path for fault currents to trip “circuit breakers;”
- provide a single point-of-access to protect multiple structural metals of the boat from corrosion, via a sacrificial anode (zinc, aluminum or magnesium, depending on composition of minerals in local waters);
- provide a path for certain DC stray currents to safely exit the boat via the AC shore power safety ground;
- disperse static electricity formed in high winds and from nearby electrical storms, and
- reduce (attenuate) spurious RF electrical “noise” created by on-board equipment (battery chargers, inverters).
Many of the conductors of a “bonding system” are installed in the very hostile environment of the boat’s bilge. The various metal objects tied to the bonding system include:
- thruhulls, seachests, sea strainers and packing glands,
- rudder “stem iron,” rudders, rudder “shoes” (skegs), tillers and miscellaneous metal support structures of the steering system,
- various steering system components (quadrant, cables, metallic hydraulic lines, hydraulic pumps),
- trim tabs and thruster systems,
- anchoring system components, including all-chain rodes,
- exhaust system fittings and ports,
- radio counterpoise and static dissipation “ground plates,”
- fuel tanks, fuel filling ports and tank vents,
- potable water and black water tank access and vent ports,
- generator, battery charger and inverter chassis frames,
- solar panel and wind generator frames,
- handrail and bridge enclosure frames,
- heat pump and circulator pump frames,
- stove and water heater frames,
- refrigeration (compressor) frames,
- etc, etc, etc…
In short, lots ‘o stuff.
Figure 2 shows the hull penetrations on a typical trawler (Sanctuary) built with individual thruhulls (without a seachest).
The complete collection of all of these metal components are “bonded” – connected together into a single electrical network – as shown in Figure 3.
Figure 3 is only one example of the construction of a bonding system. Other configurations are acceptable. Take particular note of the large gauge conductor shown in orange. That conductor is the “backbone” of the DC portion of the bonding system. That backbone conductor runs the length of the hull. To the backbone are attached all of the green stranded wire pigtails connecting the metal structures of the boat to the backbone. Also note the transom anode (zinc, aluminum or magnesium), which provides primary galvanic protection to all of the metals connected to the bonding system. When the boat is at anchor, away from shore power, it is the transom zinc that is the “ground” connection point. That is, the single point of electrical attachment to the earth, the primary dispersal point for static electricity and lightening and the electrical connection that establishes the “touch potential” for people and pets for the entire electrical system of the boat.
It would not be unusual if a boat’s owner did not know when the bonding network was last tested. It may have been quite some time; perhaps, never, even on older boats. It is possible that weakness(es) are present in the bonding system. I suggest full integrity testing of the bonding system should be done every three to five years.
Most if us have measured the terminal voltage of flashlight batteries many times. We have probably all measured our boat’s 12V (or 24V) lead/acid batteries at some time. Figure 4 reminds us of the very simple task of measuring the terminal voltage of a “AA” battery:
This “typical” battery is a classic galvanic cell consisting of two “half-cells” (copper and zinc) located in an electrolyte. Since the battery is always seen as a packaged unit, the term “half-cell” is not commonly used except by engineers, battery manufacturers and technicians specializing in corrosion mitigation. The terminal voltage of a “AA” battery is measured with a digital voltmeter. When a load is connected across the battery terminals, current flows to illuminate a flashlight, for example, or power a radio or GPS.
Key concept: batteries are used to provide the voltage needed for circuits. With batteries, their intended use means there should be a voltage between the positive and negative terminals. A direct short circuit across a battery is never desirable, as it will dramatically accelerate the rate at which the battery becomes exhausted. Inside a short circuited battery, the halfcells will become wasted (a form of corrosion) at an extremely fast rate, accompanied by the generation of heat and gasses. However, in the case of the “accidental” battery created by the electrochemistry of dissimilar metals in seawater, the whole point of the bonding system is to create an electrical short circuit across the various exposed terminals of that “battery.” Bonding creates a path for electrochemical galvanic currents to circulate. Bonding holds all of the metal surfaces at the same, safe touch voltage, but in so doing, bonding also ensures the presence of the conditions needed for corrosion to occur. That is the reason for the presence of the transom anode (zinc, aluminum or magnesium) in the bonding network. The transom anode serves as a sacrificial metal (sacrificial anode) that protects all of the important, expensive, valuable more noble metals attached to the bonding backbone from corrosion.
For measuring and troubleshooting the bonding system of a boat, a reference “half-cell” is used. The reference cell is external to the bonding system. The reference cell behaves in a known and predictable way when submerged in sea water. The reference cell becomes one of the halves of a “battery.” The metals attached to the bonding network of the boat become the other half-cell. In use, the reference half-cell is immersed in seawater outside the hull of the boat, and that seawater is the electrolyte of the “battery.” Note here that “sea water” is a collective term. The water in which a boat lots can be “fresh,” “brackish” or “ocean” in mineral concentrations. It is minerals in the water (primarily salt) that affect conductivity of the water. Since the water is the electrolyte of our corrosive galvanic cell, the voltage measured across the half-cell terminal will vary in different bodies of water and vary in different places on large bodies of water.
A Silver/Silver Chloride half-cell is the best reference cell with sea water (chemical symbol: Ag/AgCl) because it has known and stabile behavior characteristics in that application. That is, the voltage that other metals will produce against a silver/silver chloride half cell are very consistent across a wide range of temperature and electrolyte salinity.
Conceptually, measuring between the Ag/AgCl half-cell and the bonding network of the boat is the same as measuring between the terminals of a conventional “AA” battery. The bonding system and the Ag/AgCL half-cell immersed in sea water, become the “battery” being tested. The DVM measures the “terminal voltage” of that “battery.”
Figure 5 shows the measurement configuration described above:
As a boat owner, there are two ways to proceed with the testing of the bonding system. One is to hire an ABYC-Certified Corrosion Specialist. This analysis is a form of “boat survey,” although is is highly specialized and not all surveyors offer it as a service. Two is for owners to “do it themselves.” In the DIY case, one must obtain an Ag/AgCl half-cell, available from http://www.boatzincs.com and other Internet sources at a cost in the range of $140 – $150. Or, a “Corrosion Meter,” such as is offered by Promariner.
DIYers will begin their testing by connecting the Ag/AgCl half-cell to the negative terminal of the DVM. Then lower the Ag/AgCl half-cell over the side into the water near the hull, to about the level of the boat’s running gear. The half-cell should not rest on the sea bed. The guiding principle here is, if the bonding system is fully intact and functional, all metals connected to the bonding system are expected to be at the same voltage. Probing any of the bonded metals with the DVM should produce the same voltage reading. If different voltages are noted, something is not right, and corrective action is advised.
The bonding system of a boat – whether connected to shore power or not – should produce a reading on the DVM of between -400mV and -1000mV, depending on the mineral composition of the water. Knowing that the bonding system has all of its metal structures tied together, we therefore know all of the readings must be found at the same voltage if the bonding system is intact.
So how does one figure out what a “nominal reading” for any particular locality should be? There are two ways to get a good approximation of nominal “hull potential:”
- A friend who is an ABYC-Certified Corrosion Specialist offers this simple suggestion: “When starting a corrosion survey, I use a pencil zinc and check the potential between it and the Ag/AgCl reference cell. This gives me an indication of the best reading that I will be able to get for any protected metals on the boat and it also tells me that the DMM is working and that the Ag/AgCl reference cell is behaving.” This is completely independent of marina power distributions system, fast, easy and safe.
- Place the Ag/AgCl reference cell into the water of the boat basin, and probe the ground terminal (make sure to probe the “ground” terminal!) of any nearby pedestal 120V/240V power connector. This works because the AC Safety Ground on the pedestal is electrically connected to the earth (grounded, earthed) at the service entrance panel of the dock. Boats floating in the water of any boat basin are referenced to ground through the basin’s conductive water. (A Galvanic Isolator interrupts this circuit.) The basin water is the electrolyte through which galvanic currents flow. Connected in this way, the Ag/AgCl reference cell is one half-cell of a “battery” and the earth connection acts as the other half-cell. And when the boat’s shore power cord is connected to the pedestal, the bonding system on the boat is held at the level established by the shore power service ground. This method works well on docks with good electrical systems, but can be fooled by non-compliant boats. If using this system, and reading look “suspicious,” revert to method one, above.
Once a good baseline voltage is established, start to evaluate the integrity of the bonding system and any point along the bonding network that is convenient. Proceed to probe each of the various metal objects found all over the boat; that is, all the stuff previously mentioned [thruhulls, packing glands, sea chests, rudder posts and rudders, steering system components, exhaust fittings, main engine/transmission, Generator frame(s), battery charger/inverter chassis frames, solar panel and wind generator frames, handrail and enclosure frames, heat pump unit chassis frames, fuel tanks, fuel filling ports and tank vents, potable water tanks, thruster systems, black water tank, etc, etc, etc]. The voltage measured by the DVM should be the same as seen at the shore power connection everywhere. If it is not, something is “wrong!” Note: from a purely “corrosion” point-of-view, only those components immersed in water are affected. Other bonded metal equipment/components are checked for the purpose of verifying frame-to-frame touch potential safety and ability to trip the disconnect circuit breaker in an electrical fault event.
The last two steps in this analysis are to discover the cause of any inconsistent voltage reading, and then to make appropriate corrections. Some symptoms one might encounter include:
|Wide variation of voltages between different metal objects.||
|Most metal objects have consistent voltages except for one or two isolated objects, “here and there.”||Loose, corroded, broken or missing bonding connections to the affected metal object(s).|
|A collection of several metal objects measure one voltage, but that entire collection is different from the baseline voltage.||Broken bonding buss somewhere along the length of the backbone.|
|The baseline voltage is grossly different than expected (-400mV to -1000mV).||
|No reading occurs when the metal object is probed.||Bonding connections absent.
(Note: this will only happen with metal objects above the waterline and not in contact with the water.)