A young fellow I know, who is a professor of engineering at an American university, was discussing with some friends (the friends neither engineers nor professors) about how an EMP would affect things, asserted that most modern cars would NOT be affected because, being made of a metal shell, they are a Faraday Cage.
Is that true? Are the modern cars that are made of metal a Faraday Cage and thus protected from an EMP?
How does glass compare to metal regarding electromagnetic radiation? That question should give you a clue.
No they are not. If they were you're cell phone or GPS device wouldn't work while in it. However, because it is insulated from the ground by the rubber tyres it wouldn't ground a charge. However, the US government have conducted tests on how a big EMP would affect cars at that fancy test site they have where they create a major EMP, and they've found the electronic and electrical systems of modern cars will get fried by an EMP while many of the much older cars that have no electronic components that are turned off when the EMP occurs will operate afterwards. There used to be an Interesting news vid on that on the Internet years ago.
However, because it is insulated from the ground by the rubber tyres it wouldn't ground a charge.
Does that mean a Faraday Cage has to be grounded to be effective?
Does that mean a Faraday Cage has to be grounded to be effective?
No, but a Faraday cage will only stop EM radiation in wavelengths larger than the largest holes in it. A car has big holes in it called windows.
No, but a Faraday cage will only stop EM radiation in wavelengths larger than the largest holes in it. A car has big holes in it called windows.
In this context, that could be considered true. However the hole size EM wavelength can get through is considered half the wavelength in question.
Does that mean a Faraday Cage has to be grounded to be effective?
No. Planes are regularly struck by lightening every year while in flight. As with cars, they are a floating ground instead of an earth ground. Then there is the air friction static electricity for jets and helicopter blades. Aircraft of all sorts are designed with this in mind.
Obviously dragging a ground wire on the ground isn't going to work.
Obviously dragging a ground wire on the ground isn't going to work.
Especially when you consider NASA's rockets and satellites which have to fend off cosmic radiation. Running a long metal wire to the ground simply isn't feasible, yet they still operate quite efficiently, despite their also having 'windows' on board.
An interesting article regarding lightning strikes on aircraft.
http://www.boeing.com/commercial/aeromagazine/articles/2012_q4/4/
You may also find taking apart a surplus military equipment gage interesting. Especially from the Navy or Airforce. Starting in the late 60's, there was a shift in design criteria. At that time, anything considered critical to mission was required to be EMP proof, though they didn't use those words.
The gage shells were protected internally with mu metal. This prevents magnatization of internal ferrous components. Some information on that:
https://mumetal.com
Components shielded in that manner do not build up secondary electricity producing magnetic fields. Protection from induced magnetic fields is key to EMP protection and RF interference. I think a lot of people are under the mistaken impression that it's a direct hit of electricity that does the damage.
You may also find taking apart a surplus military equipment gage interesting. Especially from the Navy or Airforce. Starting in the late 60's, there was a shift in design criteria. At that time, anything considered critical to mission was required to be EMP proof, though they didn't use those words.
That can be seen even today.
When I was first trained to work with the PATRIOT missile system, I was fascinated by an odd quirk. There are 2 "electronics bays" on each side of the launcher, with big metal doors covering them. And a tongue and groove system that overlaps when they are closed to the launcher, with a gasket.
https://media.defense.gov/2019/Apr/27/2002122114/780/780/0/190424-F-QS178-1008.JPG
But the gasket is not rubber, or cork, or what you would expect to keep water out. It was a roll of metal mesh, that actually let water go through, and after a rain we often had to open the doors to let the water escape. For about 2 weeks this puzzled me, why put in a gasket that did nothing to keep water out?
Then I remember when it was actually made (JFK was the President when the program started, Ford was when it was first tested). I went up to our Chief Warrant Officer, and asked him if I was right, and I was. It was to create a Faraday Cage around the electronics, so they would be hardened against an EMP effect.
Between the tongue and groove and metal gasket, the sensitive electronics inside are protected. And all over inside of the various parts are layers of mesh to provide additional protection.
Next time you have an MRI, pay attention to the door frame as you enter the room. That same setup is used around the door frame.
yet they still operate quite efficiently, despite their also having 'windows' on board.
They must still be using Windows 3.1 ;-)
AJ
Especially when you consider NASA's rockets and satellites which have to fend off cosmic radiation. Running a long metal wire to the ground simply isn't feasible, yet they still operate quite efficiently, despite their also having 'windows' on board.
Unmanned rockets generally don't have windows, and the windows in satellites and space craft tend to be significantly smaller than the windows in a car.
Also "cosmic radiation" is charged particles, not EM so a very different kind of shielding is required.
I tend to think of it as a gaussian surface but it is effectively a Faraday cage. For example, if your car gets struck by lightning, you are safe. It has zero to do with the tires. Faraday cages are not impermeable to all fields.
EDITED TO ADD: Like others have said, the holes in the cage are much larger in a car than in traditional cages and would let much more through. Still safe from a current though.
Is that true? Are the modern cars that are made of metal a Faraday Cage and thus protected from an EMP?
No its not true.
most modern cars would NOT be affected because, being made of a metal shell, they are a Faraday Cage.
That's true AND false - they would not be much affected by EMP, but (mainly) not from the Faraday cage effect (more on this later). Modern cars would not be much affected because the automotive environment is one of the worst places for electronics: the power supply (a nominal 12V from the battery/alternator) has transient spikes into the 10's of kV, load dump surges of 100s of V for 10s of ms, and long term (10s of seconds and more) dips (to 6V or less, whilst starting) and surges (to near 30V if a 12V flat battery is mistakenly jumped from a 24V lorry with the engine running hard to 'put some charge in'), and a very wide temperature range too. By the time the designer has accounted for all that, and for the high levels of EM noise under the bonnet, it'll shrug off all but the highest levels of EMP anyway.
As far as EMP goes, there are three frequency ranges to consider; the lower frequencies where the wavelength is notably longer than the equipment (or holes in the Faraday cage), higher frequencies where the wavelength is notably shorter than the device or holes, and the transition region between them. We can ignore the lower frequencies, because the EM radiation effectively ignores the holes/equipment and won't transfer appreciable energy into the car. The higher frequencies would propagate through the window apertures quite well, but the energy in nEMP is tailing off quite rapidly at these frequencies. In between, we get increasing propagation into the car with increasing frequency, but decreasing available energy with increasing frequency, which balance out and will reach a maximum somewhere between - I'll estimate this as about 1/4 of the max energy outside the car (half for each effect), and this would be at about 250MHz (where the EMP is probably down to half-strength). The field strength from the EMP will be a maximum of 50kV/m (otherwise the air would ionize, absorbing the EMP energy) and only in the area which has the highest fields (see Wikipedia page for diagram) - most areas would be much lower. At this field strength, a typical simple wire (rather than a carefully crafted antenna) that happens to be a quater-wavelength long (the length for maximum pickup, about a foot at 250MHz) and oriented in the (in)correct direction would generate a pulse of up to 15kV (typically less than half this), which is within spitting distance of what the engineers should have designed it to cope with anyway.
Given that the pulse amplitude will likely be much lower because of chance orientation of any chance lump-of-wire antenna which is likely to be a non-optimum length an sited in a place which has reduced field strength due to metal in the way and being unlikely to be in the highest field strength areas anyway - the car will probably survive the EMP.
BTW, I also found this page which confirms the 'probably survive' opinion.
ETA: Sorry for the wall-of-text
ETA2: It was interesting to design a bit of automotive kit back in the 80's, but not as bad as doing Intrinsic Safety designs in the late 90's - kit that you could put into the fuel tank of a car and it wouldn't, couldn't cause problems, even if two separate failures occur or Joe Fumblefingers wires up the sensor to the mains.
A young fellow I know, who is a professor of engineering at an American university, was discussing with some friends (the friends neither engineers nor professors) about how an EMP would affect things, asserted that most modern cars would NOT be affected because, being made of a metal shell, they are a Faraday Cage.
Is that true? Are the modern cars that are made of metal a Faraday Cage and thus protected from an EMP?
No, they are not.
A Faraday Cage is more than just isolation from the "ground effect" of electricity. And it is the same with vehicles in the air, including aircraft and rockets. All are largely "safe" from electromagnetic effects like lightning, but that can cause issues with their own isolated internal circuits being overloaded.
Because they are not "grounded", people on them do not suffer the effects of being electrocuted, but the random current still causes havoc with the electronics on board unless they are shielded.
EMP and other more severe particle effects need their own shielding, not just being or not being grounded. And cars do not fit this, so are as susceptible to the effects of EMP as anything else. In fact, most into survival tend to prefer having at least one vehicle that is 1970's or older, without all the modern electronics that they have in them today. At most, replace the coil and you are good to go again. You may not have a working radio, but the rest of the car will function fine.
