It’s a phenomenon I’ve encountered on a number of occasions in my marine industry career. The causes are simple and easily understood, while the solutions can be maddeningly elusive. The side effects can be everything from static or distortion on an otherwise high-quality television screen to humming or buzzing on a VHF radio, and in some cases, malfunctions in autopilot control systems, some of which may be dramatic. I’ve troubleshot more client complaints that start with the observation, “when I key the microphone the boat makes a hard turn to port/starboard,” than I can recall.

At the recent annual International Boat Builders’ Exhibition held in Miami, I attended a seminar on the subject of advanced electrical and electronic troubleshooting, one of the subtopics of which was indentifying and resolving electromagnetic and radio frequency interference, otherwise known as EMI and RFI respectively. The fact that this subject was a major portion of the presentation and generated the most questions from the audience is telling indeed. Distilled into a single phrase, cruising vessels have become significantly more complex, especially electrically and electronically, in the two decades I’ve worked in the industry. Unfortunately, the industry itself, while becoming adept at selling and installing the gear, hasn’t kept pace with prevention, mitigation, or resolution of EM and RF interference issues. I recently received an email from a reader, referencing a recent column on the subject of high-output charging systems and their associated wiring, which exemplified the interference issue. In part, it reads:

  1. Many high-output alternators use an isolated ground terminal and specify that this ground be brought back to the battery bank. If this is done, the charging meter does not work correctly. The ground needs to go to the shunt on the side away from the battery.
  2. We had a problem with the Cruise Command (ZF Marine) on our Nordic Tug 37 recently. Both ZF Marine and Cummins pointed to the Balmar alternator as being a source of electromagnetic interference and the source of the problem. It was determined that there was a problem with the Cruise Command unit and a replacement board was exchanged. However, we did find that the case of the Cruise Command was not bonded. Balmar also suggested adding “drains” to the positive alternator output as well as spiraling the ground wire for the MC-612 harness to effect a drain for it as well. I point this out, because this may be a concern for others as well.

My response:

Thanks for your note and comments regarding high-output charging systems. Indeed, you are correct. If a shunt-style ampere or amp-hour meter is used, all negative cables, both house supply and charging, regardless of charge source, must be attached to the side of the shunt that is not connected to the battery. This is a common protocol and one that’s well understood by experienced marine electricians.

I’m a strong advocate of including negative output cables in high-output alternator installations as well as wiring both positive and negative output cables directly to the house bank to minimize loss as much as possible. Engine blocks and attached components are typically made of iron and steel, which are poor conductors when compared to properly-sized copper cable. Thus, sending alternator output current via this path is undesirable. It’s important to remember that positive and negative cabling run from an alternator or battery charger (or any high-current consumer or producer) to a battery bank must be routed in such a way that they remain very close to each other, preferably touching, to minimize the production of electromagnetic interference. Doing so cancels the emission of electromagnetic interference; interestingly, separating the cables often by just a few inches while maintaining their parallel paths can actually amplify the production of EMI.

As far as the alternator acting as a source of electromagnetic interference, this is true to an extent, however, virtually all electrical gear, especially electric motors, generators, and transformers, as well as cables carrying high current, has the potential to create EM and/or RF interference including starters, inverters, chargers, radar, single sideband and VHF radios, etc. Provided a few anti-EM and RF interference protocols are followed, this won’t present a problem. In my experience it’s rare in modern high-output alternators, Balmars included. For example, the alternator output cables should not be run parallel and adjacent to control cabling for any sensitive gear such as electronic shift and throttle controls, engine instruments, autopilots, thrusters, and windshield wiper circuits, to name a few, as they can induce interference-inducing current on these cables.

Occasionally an alternator’s regulator can suffer from interference caused by nearby equipment (regulators should not be installed adjacent to inverters, for instance).

In these cases the regulator control harness negative wire can be wrapped around the other regulator control wires, however, these are small gauge wires, typically 16 gauge. The large-gauge alternator output cable typically requires no such treatment other than keeping positive and negative cables from a producer or consumer as close as possible to each other. If high-current consumer or producer cabling are run parallel to other wiring this can lead to inductive interference, which, as mentioned previously, is best avoided by separating these cables and wires.

Finally, as you mentioned, bonding all electrical equipment, particularly equipment whose manufacturers specifically call for this protocol, is a prerequisite in the battle against EM and RF interference. Because I’ve never encountered an instance where an alternator’s suspected production of EMI caused damage to an engine’s electronic control computer or related circuitry, it is likely that, in your case, the problem was caused by the manner in which the gear was wired. Many new, electronically controlled diesel engines are equipped with high-output alternators, often as an option from the engine manufacturer, with no noticeable or undesirable side effects.

Another especially common and insidious cause of interference for electronic engine control circuits involves the connection of ground and bonding wires to multiple locations on an engine block. This practice should be strictly avoided for at least two reasons. First, doing so can induce current to flow through the block, which in turn could create problems like the one you experienced with your computerized engine management system. Second, if the engine’s own large-gauge ground cable becomes disconnected, loose, corroded, or otherwise compromised, starter cranking and alternator output current may flow through other small-gauge negative/bonding wiring that is connected to the block. It’s important to remember that current flows through these wires under normal circumstances as electricity will take all paths back to its source, not just those of least resistance. However, provided large-gauge, low-resistance cables are connected, they will safely carry the bulk of the current. If called upon to carry very high current, a starter commonly draws 300-500 amps, for instance, the smaller gauge wires will quickly overheat and burn, possibly igniting nearby flammable materials. To avoid both of these scenarios all bonding and ground cables should be connected to an engine block at a single point or via a single cable and then to a common ground bus bar. If it is somehow disconnected or experiences high resistance it’s likely the engine won’t start, which will alert the user to a problem and prevent additional damage from occurring.

Thus, it’s clear to see that this often-vexing problem can lead to a variety of cascading and frequently difficult-to-resolve problems. The discussion of EM and RF interference and prevention could easily fill an entire article and then some. In short, following relatively simple EM and RF interference prevention protocols (in some cases this involves no more than physical separation of equipment producing interference from that which is being interfered with) during installation or modification of a vessel’s electrical and electronic equipment often goes a long way toward preventing the occurrence of these phenomena from the outset.