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 (connected to PE), so that nearby devices are less affected. In my first attempt I used aluminium foil for this, but more professional would be to put all this into an aluminium casing and connect to that.
Above you can see my first attempt at this. I have tested the compatibility of these CAN bus surge protectors with the NMEA 2000 bus, and they are not affecting the network at all. So far tested (B&G): Zeus3 chart plotter, Triton2 monitor, VHF radio, high-precision compass, depth sounder and autopilot.
In the above pictures, I have connected the PE contact (Functional Earth) to the NMEA bus cable shield, which is not optimal and likely means that the gas discharge tubes will never trigger. If you have a good PE point nearby, then it is a much better idea to connect the PE with that, rather than the NMEA cable shield, and keep these two disconnected. This is also what the manufacturer recommends. For devices, which are only connected to the NMEA bus, the version shown above should be OK, but for devices that have supply as well as NMEA attached, it is much better to connect the PE contact to the same PE point the device is connected to.
Additionally, I want to split the NMEA bus into two segments, connected with a galvanically isolated bridge, and each segment powered separately. This will reduce the induced voltage within each segment, simply because it is shorter.
Most likely, 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 rest 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. This is work in progress.
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 and thus may be OK as far as protecting their power supply is concerned. They do need a bus protection, though.