What is an Isolated Ground Engine?

And Why You Should Care.

This question came up recently while I was working on a client’s boat. This boat had a NEW generator and a NEW drive engine installed. Both these–very expensive–pieces of equipment were ordered with isolated grounds as an extra cost option. There were obvious grounding issues on the boat that we needed to sort out, and the answers were frustrating and disappointing.

As we traced the issues through the system it eventually became obvious that the people who built and installed these expensive pieces of equipment seemed to have no idea what “isolated ground” actually means.

Terminology Confusion

One of the issues here is that English vocabulary is really bad at describing the different things that are sometimes called “ground”. This gets really bad on a boat where there is no “ground” involved! There are basically three things that might be called “ground”.

First, is the AC Safety “ground”, the green wire in the AC circuit. This normally has no current flow, but is there to divert dangerous high voltage AC current away from people when there is a short from the hot wire to the case of a piece of equipment. The idea here is that the green wire diverts the AC current back to its source and trips a breaker in the process preventing use of a circuit with a dangerous fault.

Second, is the battery negative. This is frequently called the “ground”. On an Amel there are two separate battery grounds, one for the 12V starting battery, and one for the 24V house battery. They are, by design, not connected together. Because they are not connected together, and not connected to any other part of the boat, they are sometimes described as “floating grounds”. This seems to confuse people a lot, but it is actually very much the standard approach in boats with metal hulls, so really not unique, just a standard marine wiring system used in a slightly different application.

Finally, we have the bonding system. This is used to connect all of the underwater metal parts of the boat together to prevent certain types of corrosion. On an Amel as delivered from the factory, neither of the battery negatives are connected to the bonding system in any way. The AC safety ground is connected to metal bodied AC powered pumps, which are also connected to the bonding system. If these pumps are replaced with plastic bodied pumps, this connection between the bonding system and the AC safety ground might not be present. An AC generator will also have a direct connection to the bonding system, and a connection to the AC safety ground at the generator.

Also, note that in several places through this article I describe properties of automotive electrical systems. Those systems have become exponentially more complex in the last decade, and the newest car models might not look like my simplistic description.

So What IS an “Isolated Ground”?

Almost all boat engines are derived from engines originally designed for use in road vehicles or construction equipment. There is no need or benefit for an isolated ground in these applications, so most engines and engine accessories are designed and built without even considering it. The Negative cable from the battery is bolted to the engine block, and everything that needs electrical power uses the engine block or vehicle frame as the “ground” to complete the electric circuit back to the battery. This is an un-isolated ground. On a car, truck or tractor it is an easy, cheap, and efficient way to assemble an engine. With no downsides.

In an ideal world, you would design a boat engine with an isolated ground from scratch. You would not connect the battery negative to the engine block. Rather each piece of gear on the engine that needed electricity would have a direct wired connection to the battery negative of its own. Since finding an engine built completely to this standard is virtually impossible, some workarounds need to be invented.

Why do we want our ground isolated?

Because they float in electrically conductive salt water boats have corrosion and wiring issues that road vehicles do not. Connecting the battery negative to various metal parts around the boat can set up stray currents that flow through parts of the boat and the surrounding water, instead of inside wires. These stray currents can cause corrosion issues ranging from irritating to catastrophic. An isolated ground engine is one of the ways we can control where electricity goes in an effort to keep it always inside the wires where it belongs.

How do we isolate an engine?

In a “regular” engine, the sensors for the water temperature, oil pressure, etc. all use the engine block as their path back to battery negative. We need to install instruments that have TWO wires, one of which goes to battery negative, so the device is electrically isolated from the engine block. These are readily available–for a price. The same is true for alternators. Any alternator designed for the marine market is made with the battery negative isolated from the engine. Alternators designed for cars, trucks and other vehicles typically are not isolated; not surprisingly they are cheaper.

Now things get more complicated. It is very difficult to find a starter motor that is electrically isolated from the engine block. They DO exist, but they are rare, and not available for every engine. Diesel engines sometimes use preheaters to help starting in cold weather. These are almost always grounded to the block. Finally, many engines use an internal solenoid to shut off fuel flow and stop the engine. Also grounded to the block. What to do?

Each of these things, a starter motor, the preheaters, and the stop solenoid, we only need for a few seconds at a time. We set up the engine with the battery negative connected to the engine block but only through a solenoid. When we push the button to preheat the engine, the grounding solenoid closes, and we connect the engine block to the battery negative for the few seconds we preheat. Same thing when it is time to run the starter, or stop the engine. In each case the grounding solenoid closes just for the few seconds we need those things and then it opens and the engine is isolated again.

Why Don’t All Boats Have Isolated Ground Engines?

Because it costs more to build a boat that way, and few customers know enough to ask for it. That is the only reason.

Boats Not Built By Amel

The ABYC actually requires a connection between battery negative and the bonding circuit. However, the requirements for that connection are very specific. In actual practice very few boats comply with that standard. The requirement is that there be ONLY ONE place the battery negative and the bonding circuit be connected together. This is usually the main negative bus bar. However, most boats are made with unisolated ground engines, so that automatically makes TWO connections. If there is a generator with an unisolated ground, that makes THREE. The more different connections between the bonding circuit and battery negative the more likely stray current corrosion issues become.

The ABYC (contrary to what you might read on internet forums) is not a bunch of idiots. There are good reasons for the recommendations they make, but they are not necessarily the ONLY way to build a reliable, safe boat. Amel’s approach is different, and it works. In both cases, people get in trouble when they mix and match different systems without understanding all of the consequences of making changes.

The Issues We Found.

On this client’s boat both the engine and the generator were ordered (as they should have been) as isolated ground units. The drive engine from Beta Marine was ordered with a second, optional, 24V alternator in addition to the standard 12V one. When the engine manufacturer assembled this unit they installed all the right bits including an isolated 12V alternator. EXCEPT… they installed an UN-isolated 24V alternator?!??! This is a total screw up, because it destroys the isolation of the ground from engine block! It also makes a connection between the 12V and 24V battery negatives that should not exist.

Either they really do not understand the whole point of an isolated ground engine, or maybe they just installed the wrong part? We could swap out the alternator for a proper isolated model, or we can fix this by installing some plastic washers and bushings, but we shouldn’t have to do either of these things on a brand new engine just to bring it up to the specification that was supposed to have been delivered.

The Onan generator was a simpler mistake by the manufacturer, but no less serious. They just shipped an un-isolated unit. No technical mistakes, just the wrong generator. So battery negative is ALWAYS connected to the block and frame. The guys installing the unit, I suspect, recognized the issue, but didn’t say anything. Their “solution” was to NOT connect the bonding system to the frame of the generator. Maybe they thought that that made the ground “isolated”? In any event, the AC safety ground is connected to generator frame, and to the bonding system, hence destroying the isolation that was ordered and paid for.

The quick and easy solution to this problem is to install a switch in the battery negative wire to the generator so we can disconnect the battery negative from the generator frame and block when we are not using it. It doesn’t get used all that many hours in a week, so corrosion potential will be minimized. In the long run, Onan should be able to supply the solenoid and wiring needed to isolated the engine.

I can not even begin to describe the frustration and struggle involved in figuring out how and why there were multiple unwanted connections to BOTH the 12V and the 24V electrical systems.

Why Do We Care?

If the bonding circuit is connected (somehow) to battery negative, that gives a path back to the battery for electric current. If nothing else is wrong, that’s fine, and no damage is done. In fact, most boats actually have a deliberate connection between these two systems. BUT… if there is ANY place on the boat in contact with salt water that is accidentally connected to battery positive, now we have a potentially very bad situation. Current starts to flow.

We again get into a very messy terminology issue here. In normal electrical terminology we talk about current flow going from positive toward negative, and that almost always is good enough. But in this case we have to be very careful, because we need to think about where the electrons are going. And they move the OTHER way, leaving the negative side and entering the positive side.

Where the electrons enter the circuit from the surrounding salt water, they convert the metal into negatively charged ions that dissolve in the water. It makes little difference what kind of metal this is, the voltage of the battery is sufficient to dissolve pretty much any metal off the positive side of this circuit. This dissolving of metal can happen VERY quickly. destroying pumps, thru-hulls, valves, drivetrain components, propellers, and so on in weeks, or even days..

This problem can appear in other, more subtle, ways. If there are two places where the bonding system is connected to battery negative, that can lead to a difference in voltage between those two points, and current flow between them. This voltage will be smaller than a direct connection to battery positive, and might vary depending on how much current is flow through the various parts of the DC electrical system. Current will be smaller, and intermittent, but still able to cause significant corrosion.

More Terminology

The kind of corrosion we are talking about here is NOT galvanic corrosion. That requires no outside battery to drive it.

It is absolutely NOT “electrolysis”. That is how your spa removes excess hair from your body.

It IS “stray current corrosion” or in a more technical term, “electrolytic corrosion.”

These terms are frequently used interchangeably by both human writers and AI Bots. If someone doesn’t understand the difference between these words, they very likely do not have a good understanding of the concepts behind them.

What Can Happen:

Here is a scenario where an engine without an isolated ground can be a problem:

The setup: Both the drive engine and the generator are NOT isolated ground systems, so both are connected to battery negative AND the bonding system.

When the drive engine is running, the alternator can be pushing a lot of power back into the batteries. If there is any kind of connection issue in the “normal” circuit path through the engine block back to the battery negative, there will be voltage drop and the block of the engine will be at a higher potential than the battery negative itself. Some current will find another way back to the battery. That flow might be down the C-drive, out the propeller, and then back through the cooling circuit into the generator. This puts the prop and C-drive at risk for rapid electrolytic corrosion because they become the positive side of the circuit out into the water.

A common work-around to the installation of a unisolated ground engine, is to install a switch in the battery negative wire to the engine. This isolates the engine when it is not running, but necessarily UNisolates it when it is running. Such a work-around, while better than nothing, will not solve the problem as described in the above paragraph.