November 7, 2014

Lightning Protection Aboard

We're sailing into rainy season--which means thunder and lightning

Do you diligently put your portable electronics in your oven or microwave oven during a thunderstorm?  You might be fooling yourself about the lightning protection you have.  Here’s a reasonable test – put a mobile phone in the oven and dial it from another phone.  Do you hear ringing?  If so, radio frequencies are penetrating the box and your oven is NOT an effective Faraday Cage (FC).  We were happily putting our electronics in the oven until I had a ‘oh duh’ moment – the oven has a glass door.  This is a big hole that prevents it acting as an effective FC.  Lightning acts on a variety of frequencies, but much of the high energy radiation is the high frequency radio spectrum (which means short wavelengths – more about this shortly)

A Faraday Cage is named after Michael Faraday, the famous British scientist. He found that an empty metal container will only allow external electrical charges on the surface of the container.  Properly done, they shield the interior from external electromagnetic radiation if the surface is thick enough, and any holes are significantly smaller than the wavelength of the radiation.  Microwave ovens are allowed to leak some amounts of radiation, so depending on the particular oven, they might not be as good as you hope to protect against lightning.  Normal ovens, with their large gaps (or glass doors) are likely very poor FC.  

Construction details:

Start with a metal box, big enough for all your portable electronics.  This might be larger than you think because the modern cruising sailboat is laden with electronics that should go in the box when lightning threatens.  We bought an aluminum tool box and modified it.  Other suitable boxes could be army surplus steel ammunition boxes (though may be hard to explain to customs when they search your boat), cookie tins with tight fitting lids, galvanized garbage pails with lids, etc.  It has to be metal (no, it doesn’t have to be iron or steel, just conductive). 

Seal up all the small holes – this is harder to do than you might think.  We cut off some front facing latches because they prevented our box from fitting into a particular tight spot. We taped over the resulting holes with mylar foil tape, inside and out.  You have to really pay attention to very small gaps for best performance.
conductive foam gasket

The lid was not a tight fit, just a typical shoebox type toolbox construction.  So we fitted a conductive foam gasket to the top of the box.  This is not something you can buy at your local hardware store (check online electronics stores like or – it’s a compressible foam gasket with a very fine metal mesh cover (nickel plated copper in our case) that provides a tight seal that our box lacked.  We added some overcenter clamps to hold the lid down firmly onto the gasket.  These gaskets are specifically designed for FC’s to protect sensitive electronics.

Finally we lined the box with some corrugated plastic sheeting to prevent the electronics from touching the walls of the box – this might be not necessary, but the electrical charge on the outside of the box can travel through the walls of the box.

Do you need to ground your FC?  Nope, not really.  A FC will work if it is grounded or not.  However lightning is attracted to large metallic objects and it is a good idea to ground your box if your regular lightning grounding system conductors (cables from mast or chainplates) pass relatively close to the box to prevent side-flashes.

What lives in our box:

  • 2 laptop computers
  • 3 spare autopilots
  • 240V battery charger (because we are seldom at a dock we use a portable battery charger for our batteries)
  • Spare VHF
  • Spare fishfinder
  • Handheld VHF
  • 4 ebooks
  • Spare GPS
  • 4 x portable hard drives
  • 2 laptops
  • Desktop mini computer
  • 2 phones
  • iPod
  • 2 DSLR cameras
  • About 5 lenses
  • Camera flash
  • Several point and shoot cameras
  • Chargers for cameras
  • Video camera
  • Label maker
  • Pactor Modem

What doesn’t fit:

  • Radar
  • SSB/ham radio
  • Big LCD computer monitor
  • Hard wired autopilot
  • VHF
  • Depthsounder

What about stuff that is hard wired or is too big to go in the box?  Unplug as many antennas, power cables, etc. and hope for the best.  It’s all you can do.

Disclaimer: Evan is a lowly mechanical engineer, not an electrical engineer, so all this advice is at your own risk.


Behan said...

This is really interesting- very cool. Hope I can work out how to sneaky-make and surprise Jamie with one of these. But WHERE will it fit?! Hmm...

Unknown said...

Great post very interesting! Also, lightning protection is an invaluable thing to have. Many people think they may not need because their home will not get struck by lightning. Regardless of the probability or chances that it can happen it is still best to protect your home. It has changed drastically since Benjamin Franklin first invented the lightning rod. They have changed with the times and have been updated to fit the modern world.