three hulls, two people, one trip around the world…

Protecting Boats Against Impacts of Lightning Strikes

For the past few years, insurance companies have been reporting an ever-increasing amount of incidents in the maritime world caused by lightning strikes. Here, we focus on how to protect the NMEA 2000 bus on a vessel against voltage surges caused by nearby lightning.

When reading up on how to protect against lightning strikes on a boat (e.g., Palstek, Blitzschutz auf Yachten by Michael Herrmann, VDE Merkblatt Blitzschutz auf Yachten), one generally finds that a lot of attention is paid to reducing the probability of a direct hit. And if hit, how to make sure the lightning strike has a perfect path to ground, avoiding human injuries and reducing damage to the vessel’s structure. Far less attention, though, is paid to protecting the electronic equipment on board, sometimes arguing this is only 3rd prio. However, at least in our experience, it is not very likely to have a direct lightning hit, but rather much more likely is a strike close by, perhaps within a perimeter of 50 metres. In 4 years we have had 5 such events. All equipment that was not powered on survived, but for the rest, every time we had some casualties.

When the lightning is very close, but not a direct hit, electronic equipment can be damaged by the large voltages induced on the electrical cables. The longer the cable, the more voltage gets induced. This means that two types of cabling are particularly affected, as they tend to be long: The 12V / 24V + and – cables to supply all devices with power, and the NMEA 2000 bus connecting all the devices. For the supply lines, some surge protectors exist, e.g., as offered by Dehn (German, English), and also discussed elsewhere (German), but only very few of these protectors work for DC and if so, they are specified for higher voltages. As to protecting the NMEA 2000 bus, virtually nothing is offered.

However, in my 5 lighting events so far, I could see consistently that the bulk of the damage was done via the NMEA 2000 bus, but only for those devices that were also connected to the 12 V / 24 V supply lines. The only exception is for wind, but it can be argued that it also has two inputs – the NMEA bus and the wire coming from the wind sensor. Usually, the NMEA 2000 chipset in devices like autopilot, chart plotter, or VHF radio got fried, whilst the device itself would still work as a standalone. (Well, with the autopilot there was no telling… 😉 ) However, devices that were only connected to the NMEA bus usually survived (except for the wind sensor).

In part, this may be due to my having somewhat reduced the negative impacts on the 12V / 24V cabling. There, given that I was far away from Europe at the time, I had not installed these professional surge protectors as offered by Dehn, but instead, I had opted for an easier and less invasive approach: Wherever I could access a 12V / 24V cable, (preferably + and – together), I added ferrite clips around them. Lots of them! These are not as efficient as proper surge protectors, granted, far from it, but they do damp sudden voltage changes and thereby give the protection circuitry in the devices more time to react.

But this approach obviously does not work for the NMEA 2000 bus, since these ferrites would smooth the data signals on the bus, making them unreadable in the extreme.

So, what to do instead?

A good friend of mine reminded me that on the physical level the NMEA 2000 bus is identical to the CAN bus used for many decades in the automotive industry and, more recently, also in Smart Home. Searching the internet we found two candidates that were offered as surge protectors for the CAN bus as used in Smart Home:



In essence, these devices offer multiple-stage protection with gas discharge, diodes, and ferrites. In the end, I decided to go for the CAN-UES2, mostly because in its technical circuit drawing I could identify all CAN bus cables. Here is a link for the NMEA 2000 cabling signals and colour codes. It is very simple, really: CAN_L is bLue, CAN-H is wHite, +12V is red, and GND is black.

Where do I need to install these surge protectors? Ideally, every device on the NMEA bus gets a surge protector in its drop line.

Unfortunately, it is a bit tedious to install these surge protectors, as they do not come with NMEA 2000 cabling. So, what one has to do is cut a drop line into two pieces and insert the surge protector in the middle. One has to make sure that the correct side of the surge protector is facing toward the bus, as it is not symmetrical. Also, one will want to make sure that the shielding metal sleeve of the NMEA cable is routed from one side to the other. And since the gas discharge will induce a lot of voltage in its neighbourhood when triggered, one will also want to have the surge protector in a metal case, so that nearby devices are less affected.

I have finally found a neat metal case to put the surge protector in. The grey cable is my PE and connects to my PE backbone in our vessel, which I have just installed. For every device to be protected, make sure the NMEA surge protector is connected to the same point in the PE backbone as the surge protection for the power supply of that device is. It then does not matter if the PE backbone overall sees some voltage shift, as the reference point on it is the same for both protectors.

Additionally, I have split the NMEA bus into two segments, connected with a galvanically isolated active ‘NMEA to NMEA converter’ bridge, and each segment is powered separately with a DC/DC converter, also with input and output galvanically isolated. This will reduce the induced voltage within each segment, simply because it is shorter. One power supply is located underneath the nav table where the majority of all the electric stuff is, the other one is located at the helm station. Do not have one DC/DC converter feed both segments, as this will kill the galvanic isolation offered by the bridge. Also, do not be tempted and deploy instead of the galvanically isolated active bridge a simple T-piece which only cuts the power, but lets the bus signals through. This would create an imbalance between the signal lines and the power lines on the bus, allowing large voltages to be induced between these two, which likely would mean that all those devices on the bus that only connect to the bus, but not to a separate power supply — which so far had been fine in my installation — would now become vulnerable. So, there is a risk of making things worse this way.

And finally, when applying these changes to the installation on my boat, I noticed that the input and output GND lines of the 24V/12V converters for the bus were actually connected with each other. That does not seem to make any sense, given that those are special, pricy DC/DC converters with isolated input and output — and why then go to all the trouble and expense, as well as accepting a reduced efficiency of these converters, only to reconnect the input and output in the end again? Does not make any sense to me, so I cut that connection. And whilst I was on this, I did the same for a couple of other DC/DC converters, like for the Fusion radio. The protection against lighting offered by galvanically isolated input/output of DC/DC converters is gone when connecting their GNDs on the input and output side…

All this will not help, anyway, when hit directly by lightning. But as said, this seems a much less likely scenario.

Incidentally, if you are interested in testing methodologies for CAN bus interfaces, have a look at this TI document. It talks about essentially the same protection elements as used in the surge protectors above. These should be built into the NMEA ports of our devices, but this does not always seem to be the case. For instance, my B&G autopilot features a separate domain for the NMEA bus circuitry, talking to the main board via opto-couplers, which is great. It also features thyristors (but only two, not three), but there are no gas-discharge tubes seen anywhere.

Now, this will take care of the NMEA bus, but as mentioned, the main risk seems to be for devices that have an NMEA bus connection as well as a separate power supply. So, we need something similar for the power supply of those devices.

This is how I have protected the chart plotter and the network expansion port. I have also added a 24V/24V DC/DC converter before this, with galvanically isolated input/output. The Dehn Blitzconductor is placed in close proximity to the chart plotter(s) / network expansion port. So, the plotter at the helm station has its protector in the steering console, and the NEP has its protector in the technical room.
This is how I protect the autopilot (stage 1).
This is how I protect the autopilot (stage 2). I have added a 24V/24V DC/DC converter as stage 3, with galvanically isolated input/output.
And this is how it finally looks like for my two autopilots. You can see the Dehn protectors, the DC/DC converter, and the PE base. The lids have been taken off for better viewing… 😉

It goes without saying that all these protections should be placed as close as possible to the device to be protected. Also, very important is that any surge-protection circuitry for the power supply of a device uses the same reference point for PE as the surge protector for its NMEA bus does. After all, it is relative voltages that matter.

The VHF radio as well as the stereo radio are both connected to 24 V / 12 V DC/DC converters with galvanically isolated input/output and thus may already be OK as far as protecting their power supply is concerned. They do need bus protection, though.

Finally, I have placed a Dehn surge protection unit identical to the one for the autopilot next to my main electrical distribution board where the master on/off switch is, and all the massive 100+ A fuses are. A much smaller variant of that is placed underneath the nav table where the distribution hub for all the electronics is and where the majority of all the DC/DC converters are placed.

2nd stage protection of the central electrical hub under the nav table.



This Post Has 4 Comments

  1. Manfred Lugert

    I would also protect all coaxial cables going into devices, i.e. from the VHF antenna(s), radio/tv/sat. Furthermore all control/signal cables not on NMEA 2000 bus – there will be a few.
    Finally, I would consider to try to optimize the „common“ potential (grounding?) for all devices and the surge protectors – possibly there are lower impedance (-) minus lines in the neighborhood of the installation places, compared with the NMEA 2000 bus. It might be useful to bring a low impedance (-) bar to some strategic places where the protectors can be connected with short thicker wires, i.e. in the navigation station, the pilot location, beside the autopilot or at the bottom of the mast, where all cables could be „grounded“.
    A final thought: How sensitive are the new LiFePo batteries with build-in DC boosters and BMS? There could be even a fire risk in some cases…

    1. trimaran-san

      Yes, indeed, there is much more that one can do! What I did is only a start, aiming at the dominant failure pattern that I have seen so far.

  2. Josh

    Is it possible to replace the nmea2000 chipsets within individual devices? I had a direct hit to my B&G MHU this past summer. It fried all electronics on the NMEA2000 network. Possible to revive these instruments?

    1. trimaran-san

      Hmm, good question! I had opened my fried B&G autopilot and one could clearly see the electrically isolated domain on the PCB housing the NMEA2000 circuitry. They communicated via two opto couplers. So, I suppose, if you find replacement parts for those and can unsolder the old ones and replace them, you might be good, but it is not a trivial task at all.

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