Letter Re: EMP Grounding Straps and Cables (Updated)

Thursday, Feb 25, 2010

Good Morning Sir,
My question pertains to a February 24, 2010 blog post, where there was mentioned an EMP ground for one’s vehicle. This is the first I have heard of a ground wire for today’s vehicles that would prevent electronics from being damaged. Is this true sir? Thanks for providing us all the education to survive. - Tim S.

JWR Replies: A grounding strap offers only marginal EMP protection for a vehicle. The type that were mentioned are the sort that you can see used on many trucks, especially fuel delivery trucks, where the concern is a buildup of static electricity.

With a quick web search, I found one vendor on the Internet with straps at reasonable prices. They do eventually wear out, so you should probably buy several. But again, they are more for static electricity discharge protection than EMP protection. Sadly, the only way to make your vehicle truly safe from close proximity EMP is to convert it to a traditional ignition system. Alternatively, if you leave the electronic ignition system installed, you'd have to carry spare ignition components in a couple of layers of Faraday protection. ( Alternating layers of aluminum foil and ziploc bags should work fine.

I should also mention that once parked, while preparing to unload fuel, gas tanker trucks use a separate grounding cable, for even greater protection, from a static discharge kablooey.

It is important to note the EMP is a different animal than lightning, so the grounding rules are not quite the same. For example, a ground connection can actually be counterproductive to EMP shielding if you use a lengthy linear object underground, such as a water or sewer pipe. For anyone with a basic understanding of lightning protection, it may sound hard to believe, but EMP can actually couple with underground linear metal objects! So if you do decide to use a ground for any of your electronic gear, then don't use anything longer that a six foot long ground rod.

A SurvivalBlog reader who is an Electromagnetic Compatibility (EMC) engineer added these comments:
"Many people are under the false impression that a ground connection is some sort of magic sump into which they can dump electric current and electromagnetic fields they don't want. To some degree this impression comes from the fact that power lines and other electrical wires entering our buildings are grounded at the point of entry. The reason for this ground is to give a path for lightning strikes to wires external to the building a lower resistance path back to the source of the electric current (in this case the Earth) than though something inside the building. The service entrance ground rod does not play a part in electrical safety insidethe building provided by the ground wires run with the hot and neutral power wires. The key issue here is that the ground wires are connected to the neutral wire at the service entrance bond point. That same bond point is where the ground rod is connected, but the physical path to earth ground is not why the ground wires in the house help safety.

By the way this is why portable generators do not need to be grounded per the National Electrical Code (NEC). All they need is the internal bond from neutral to the ground wire.

In a similar way, when it comes to electromagnetic energy (radio waves) the important issue is shielding rather than grounding. The most effective shielding is made of a continuous conductive surface that totally surrounds what we want to protect. This is why the advice to wrap equipment that we wish to protect from EMP in aluminum foil is excellent. The continuous conductive surface of the foil with joints that overlap each other provides extremely effective shielding from all types of electromagnetic waves including those from EMP. Grounding the foil to an earth ground makes no difference in its effectiveness.

Static electricity also is stopped by shielding, and discharges to a conductive shield flow around the outside surface of the shield and do not damage equipment inside the shield. Again a connection to earth ground will make zero difference in the protection provided by the shielding.

Ground straps on vehicles provide a path to equalize the local static electric potentials and reduce the chance of a static discharge that might cause fuel fumes or other explosive or flammable gasses or liquids to ignite. A separate ground wire as you mention is even more effective. In both cases they work because they reduce or eliminate static electric potential differences that could cause a spark, not because they are tied to the physical earth."

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