• Bill Kinney

Check Your Bonding System

Updated: Aug 19

The subject of a boat's bonding system is a complex one and surrounded by a lot of mis-information and confusion. In this post, I make no effort to clear all that up, but will present a tool to help diagnose problems with ONE particular type of bonding system.

This is usually referred to as a "floating ground" or "floating negative" system. In this type of bonding system all of a boat's underwater metals are connected together, and they are NOT connected to the DC battery system at all. This system is not compliant with ABYC rules, and is more common on metal hulled boats (especially aluminum), but there are GRP hulls that are wired this way, most notably boats made by Amel.

What can go wrong?

These systems work well. The only catch is that successful function is predicated on them ACTUALLY staying isolated from the battery systems. Accidental connections between the battery positive or negative wiring and the bonding system can cause problems, some of them serious. How might this happen? I have two examples.

In one case the metal housing of a toilet macerator pump was connected to the bonding system, as it should have been. After years of operation conductive carbon dust built up in the motor housing and created a low level short circuit between the negative wire inside the motor and the motor housing, which was connected to the pump housing, which was connected to the bonding system... you get the idea.

In another case, wires supplying power to the navigation lights passed through the bow pulpit. Chafed insulation allowed a connection between the battery and the metal tubing of the pulpit. The pulpit was connected electrically to the bonding system... and you see where this is going.

How Do You Know There is a Problem?

In concept, it is pretty simple. To test for a negative connection to the bonding system, you intentionally connect the battery positive to the system, and look for current flow. If there is current flow, there must be a round trip circuit from battery positive through the bonding system, and back to the battery negative. Reverse the connections to look for an unwanted connection to battery positive.

Why not just test voltage? It would seem reasonable, yet voltage is MUCH too sensitive a test. It you look at voltages between the bonding system and the battery you can find yourself chasing ghosts of connections that are not really there, or have current flows too small to matter.

The Concept

Rather than set up an amp meter each time we want to do this test, it makes sense to install a quick, easy test that tells us instantly if we have a problem or not, and we can then dive deeper in troubleshooting if we need to. The easiest way to do this is with lights. Simple incandescent indicator lights rated for the voltage of the system. LED lights do NOT work for this application.

What we want is a simple way to connect battery negative to the bonding system and look for current flow, and then do the same with battery positive. In later models of the Super Maramu as well as more recent designs, Amel incorporated the "Masse Light" to do this very thing. I have reverse engineered this simple circuit, and made an improvement.

The Details.

Here is the basic schematic for the lights to test your bonding circuits integrity:

In addition to the two 24 Volt incandescent lamps, you need a simple momentary-on push button switch (for the lamp test) and a single-pole/double-throw momentary on switch to connect the battery to the bonding system. A "momentary" switch returns to the off position when it is released. This prevents accidentally leaving the battery connected to the bonding system, which is exactly what we are trying to prevent!

I have been on several Amel boats that had the "Masse Light" installed. The question frequently came up, "The lights do not come on, but how do I know if they actually still work?" So I stole a page from the aviation industry, and added a switch to force the lamps to light and check their functionality.

The Build

I assembled the parts in a small box that I could install pretty much anywhere on the boat that has access to battery wires and the bonding circuit. Certainly, if you have the available panel space, you could mount the lights and switches into an existing panel.

In Use

When the toggle switch is pushed to the left, if the light comes on it indicates the battery positive is connected to the bonding system. If pushed to the right, and the light comes on, it indicates that the battery negative is connected to the bonding system. Pushing the "Lamp Test" button lights both lamps to be sure they are working.

A brightly lit test lamp indicates a serious problem, a dim, flickering lamp is a problem that is less serious, but should still be tracked down. In general, connections that supply too little current to light a small test lamp like this can be ignored.

Test regularly, and often. It is not unusual that a fault shows up only when a particular piece of equipment is turned on. If you want to find that fault, you have to test when it is turned on.

More common are faults where the battery negative shows a connection to the bonding system. They are important to track down and solve. A connection to battery positive is less common, but should be more urgent to repair. If a positive current can find its way back to the batteries by flowing out of any part of the bonding system corrosion can be EXTREMELY rapid.

How to track down a bonding isolation failure, sometimes called a "ground fault", will be the subject of another post here...



SPDT Toggle Switch This link is to a Honeywell MilSpec S&C 1TL1-5 sealed toggle switch. These are what I use on Harmonie after being disappointed many times with cheap switches that corrode and fail in the marine environment. They are expensive, but worth it, in my opinion. Depending on your location and access, there usually are places a little cheaper than Amazon for this specific switch. There are a lot of cheaper options if cost is a serious concern.

24 Volt Incandescent Test Lamps Available in a variety of colors, and sizes. Just be SURE to use incandescent lamps NOT LEDs. Connecting this lights does require soldering. With a bit of searching you might find some that have ready terminals.

Push button lamp test switch Nothing special about these, just a standard, waterproof, momentary-on, switch.


For products we give links to we might get a commission if you buy through our link. This commission never effects the price you pay, and we never get paid up front for such a link. All links are products we either use ourselves (and pay full price for) or would recommend without reservation.

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