EMP, mitigating and repair examples.

For most properly shielded equipment, the danger to equipment will not be directly. It will be the power conducted to the equipment from long wires connected to that piece of equipment.

This is my experience in electronics, and dealing with surge and lighting induced damage on electronic equipment..

Radio receivers. If the receiver has a metal chassis, then the threat vectors are the connections to the outside world. The antenna. The power cord. The speaker wire.

The first stage the antenna signal hits is the RF preamp stage. Older/cheaper units use a BJT. Newer/more expensive units use a MOSFET stage. That is where the brunt of the EMP will be felt. A BJT will fail from the base voltage going past it’s max rated peak reverse voltage. or exceeding it’s max instantaneous rated base current. One or the other depending on the pulse polarity. It will either fail in an open, or shorted state. MOSFETS will fail from excess gate voltage causing a breakdown in the gate isolating layer. It will fail in a gate shorted condition. Repair can be affected by replacement of the failed transistor. In an emergency situation, you can jumper the antenna lead to the output transformer of the first stage to bypass it if no replacement parts are on hand. sensitivity will go down, but the unit will remain mostly functional.

Mitigation can be obtained by using a set of diodes or zener diodes back to back on the input. They should be considered sacrificial because they will short when they take a solid hit. That will protect the input stage from the initial strike, and all subsequent strikes because the input will be directly shorted. When the threat passes, then you can replace the diodes and resume normal operation.

Power cord. Will either enter a transformer, or rectifier bridge for a switching supply. Common mode surges will cause arcing in the transformer, or arcing in the feedback circuits for a switching supply. Differential mode surges will cause excessive voltage across the rectifiers, excessive voltages across the filter capacitors, and excessive voltages across the regulating components.

Transformers supplies. For common mode voltages, the primary threat is winding arc over. If the voltages from primary to secondary, or transformer case becomes too great, then it will arc over, possibly damaging the transformer. For differential voltages. The transformer acts as a band pass filter. The high frequency leading edge of the EMP will be muted. The low frequency component will cause excessive core magnetization and it will cause the supply fuse to blow with no other damage to the supply. The part of the wave that falls between the two extremes will damage components after the transformer. If the voltage goes past the maximum PIV of the filter diodes, then they will short. If there is enough energy, then the filter caps may be stressed and fail. Or the regulators could be damaged from the spike in voltage.

Repair should be conducted by checking to see if the primary fuse is blown. If it is, put an incandescent light as a current limiter in line and see if the power supply powers up as normal. Replace any blown components as needed.

Switching supplies. Common mode voltages may cause arc over in the feedback components. But normally it will cause arc over in the spark gap that most switching supplies have in them for lighting protection. Differential mode voltage spikes will cause damage to the rectifier diode on the input, and damage to the primary filter capacitor from excess voltage. Or damage to the switching transistor. Those failures will cause the input fuse to blow. If the supply has a MOV, then it will most likely blow the input fuse with no other damage.

Use a light as a current limiter in place of the fuse and power up the circuit so you can isolate and repair the damage if possible.

For emergency use, you can isolate the internal supply and power the radio with an outside supply of suitable voltage and current capability. If the radio is designed to operate on AC and DC, you may be able to just hook it to a 12V dc supply and operate it with no repair needed for the time being.

Mitigation can be done by using MOVs across input lines, and a MOV from each input to ground. Fuses on each line before the MOVs. The surge will be caught by the mov’s and blow the fuses. Replace fuses and return to service.

Speaker wire. If you have an external speaker wire connected. The voltage surge will hit the final audio amp. Depending on the design of the audio amp, it will either suffer internal damage(integrated amp), shorted transistors (component amp), or no damage at all if it uses catch diodes that will sink the current to the supply rails.

If there is damage, then you can repair or replace the damaged components as needed.

For emergency service, you can isolate the AF power amplifier from the DC supply rail and use the headphone jack on the front of the radio with a set of headphones. In a lot of radios the headphone jack is before the AF power amp that drives the speakers. So they will still work even with the power amp disconnected.

Mitigation for radios with relatively isolated and short speaker wires can be done by hooking catch diodes between the output and the two power supply rails. That will stop the output from swinging beyond power supply voltages and causing voltage reversing of components and resulting damage. If it is a capacitor isolated output, and if the length of the speaker wire is longer, or hooked to other equipment, then you can use back to back zener diodes to catch any input surge.

On radios with a plastic case, there could be damage with internal circuits that are not directly connected to external components. Damage and repairs will depend on the type of radio, and it too vast to detail here.

Transmitters. Beyond the stated conditions listed for receivers. The only thing additional that can be damaged is the RF power amplifier stage that is hooked to the antenna. The final power amplifier transistor may me shorted. That will cause the DC supply rail to be shorted, and the supply fuse to blow. Test and repair as needed.

For emergency service you can bypass the final output transistor by hooking the driver stage transistor to the final output tank. Output power will be lower, but the radio will still be functional for the most part.

Mitigation can be had by putting back to back zeners on the output. Again, the zeners will probably short in a strike, but they can easily be replaced.

Computers.
They will be vulnerable though their connected devices. Power supply damage. Ethernet transceiver damage. USB port damage. PS2 mouse and keyboard port damage.

The damage can be remedied by using a plug in Ethernet card, and connecting working devices to undamaged ports that may remain. If no working USB ports remain, then use a plug in USB expansion card. For example, if you are using USB mouse and keyboard, and are not using your PS2 mouse, and keyboard ports. If it damages your USB ports, then just drag up a working traditional PS2 style mouse and keyboard to get the computer back up to operation.

If the power supply is damaged, then that is a simple swap out with a working one.

Video card damage can be remedied by using a plug in card, or replacing the existing plug in card.

Desktop computers almost always have a solid metal case to prevent RF interference. They basically are built inside a faraday cage. SO if you leave one setting with nothing connected, then it should be impervious to any EMP damage. So it is a perfectly viable backup plane to leave a spare computer setting in the closet with nothing plugged in. After all the fecal mater stops flying, you can drag out your spare from the closet and fire it up.

Same thing with radios that have a solid metal case. Just leave any spare radios setting with nothing plugged in, and they should be ready to go after a strike and your primary radio is damaged.

Ethernet hubs and routers will probably be damaged. Keep a spare hub set back ready to be put into service when needed.

DSL and cable modems will be damaged at the phone, cable or Ethernet port. Replace with a working spare as needed.

Their power supplies may be damaged internally. Repair or replace the supply as needed.

Mitigation on all that electronic equipment can be done with surge suppressors on input lines. And surge suppressing strips.

Generators usually have solid metal shell, so as long as there is nothing plugged into them, then they should be operational when needed. The most likely damage would be a little solid state unit that kills the motor if the oil level drops too low. If that is damaged, then disconnect it and return to normal operation. If you have something plugged in when the strike happens, then the voltage regulator in the generator head may be damaged if it has one. If damaged then replace regulator.

Cellular phones should not be damaged as long as they have nothing plugged into them. cell phone chargers can be damaged. The phone may be damaged if it is plugged into the charger when an attack happens.

Compact fluorescent lights can sustain damage to the electronics. Electronic fluorescent ballast can be damaged. Keep working spares on hand and replace as necessary. Some can be repaired, but a lot are potted in plastic or tar so it will be impossible to repair them.

Microwave ovens.
The electronics may be damaged, from a surge on the AC supply line, or the fuse may just be blown.
Replace the fuse if needed. For emergency use, if the electronics is damaged, you can disconnect the main power wires from the relay on the control board and hook them to a toggle switch that you will mount in the front panel. That will allow you to manually turn the microwave on and off. Timing of cook times will be done manually.

On an inverter microwave, the damage may be more extensive, and there will be no way to put it back in service even in an emergency situation without extensive repaires.

Refrigerators.
I don’t see them being damaged, but if they are, it will be a shorted winding in the compressor. If you know someone that does HVAC work, you can have them change the compressor, or you can just swap out the entire refrigerator.

Mitigation can be done by MOVs installed on the compressor terminals.

With all the above examples, mitigation will not be 100% effective. But it will reduce the odds of any one piece of equipment from being damaged to a great degree.

Vehicles.
They are not likely to be damaged, but if you want to be absolutely sure, then…..
There is a few main things that can be damaged. The alternator/voltage regulator, the brain, the power train control module, MAP sensor, mass airflow sensor, and the ignition module.
Keep spares on hand in case such damage happens.

The ignition coil may also be damaged by a failed ignition module so keep a spare one of those too.

The instrument panel, and the radio may be damaged, but they are not critical items for the operation of the vehicle. Radio can be protected by putting a protection circuit in the antenna lead.

Refrigerators.
I don’t see them being damaged, but if they are, it will be a shorted winding in the compressor. If you know someone that does HVAC work, you can have them change the compressor, or you can just swap out the entire refrigerator.

Mitigation can be done by MOVs installed on the compressor terminals.

With all the above examples, mitigation will not be 100% effective. But it will reduce the odds of any one piece of equipment from being damaged to a great degree.

Vehicles.
They are not likely to be damaged, but if you want to be absolutely sure, then…..
There is a few main things that can be damaged. The alternator/voltage regulator, the brain, the power train control module, MAP sensor, mass airflow sensor, and the ignition module.
Keep spares on hand in case such damage happens.

The ignition coil may also be damaged by a failed ignition module so keep a spare one of those too.

The instrument panel, and the radio may be damaged, but they are not critical items for the operation of the vehicle. Radio can be protected by putting a protection circuit in the antenna lead.

Home heating furnace.
The most likely damaged part will be the 24v transformer that supplies thermostat line voltage. If you have an electronic thermostat, then it may be damaged.

For emergency use, you can remove the electronic thermostat and jumper the heat line to manual control the furnace. If you have an old mercury thermostat, then you can install it.

Mitigation is by putting MOVs on the input to the 24V transformer.

With large electric motors and other residential equipment, the most likely effects of an EMP will be tripped breakers and blown fuses.


That is my perspective on the entire situation.

S$F

I just finished reading One Second After and it scared the crap out of me. I'll be honest, my grasp of things electrical sucks, but I'm going to make my husband read this. He'll understand.

Great job.

This is some helpful info. I've always really feared this scenario as I think as a society we are so dependent on electronics/electricity at this point it would be one of the most far reaching SHTF scenarios you can imagine. I mean can you imagine what this would look like? I can, and it totally scares the # out of me. I've been looking for some EMP protection myself, and setting up DIY farraday cages on the cheap, this guide was well reviewed and seems to have everything I need to get going.

reply to post by Mr Tranny


Question for you...would the grounded lightning rods on buildings provide any additional protection against an EMP, or would it simply not be a factor?

reply to post by Gazrok


It would simply not be a factor.

Grounding rods are there to give a path for DC currents to flow to ground. That is to prevent the voltage of the building from building to a high level and arcing over someplace that would be damaging to something.

With an EMP strike, there is no DC current buildup that needs to be bleed off to anything. It’s strictly an AC affair. Some parts of the EMP are very low frequency and approach quasi DC, but a metal building is too small to capture RF energy at that low of a frequency, so it is a non issue.

The shielding that a metal building provides in regard to an EMP is shielding from the traveling wave front. When it hits the surface of the building, it reflects off, and that leaves a shadow region behind that metal surface. At lower frequencies where the wavelength becomes appreciable in regard to the building structure, then the whole structure will resonate in response to the wave that impacts it. But since the stuff inside the building is fully surrounded by that resonating structure, then the stuff inside resonates with that structure, and no current flows in anything inside the building. That is because everything inside is in a uniform magnetic/electric field.

Remember, RF current flows at the surface of an object. That is why a tesla coil will not hurt a person. Because the RF current flows on the surface of the skin, and no current flows through the nervous system deep inside the body.

You don’t even need a fully enclosed structure to guard a piece of equipment from an EMP.
A metal roof will leave an RF shadow region under it. Anything under that roof will be shielded from the brunt of that EMP. The lower frequency component of the EMP will cause the roof to resonate from the energy that the EMP dumps into it, but that frequency will be dependent on the size of the roof. Unusually a few MHz or less. The small items under that roof will be to small to absorb that low frequency resonation, so they will not be bothered. The high UHF region of the EMP that can bother those smaller items under the roof will be reflected by the roof back into the sky.

When a wave hits a metal object it will behave differently deepening on it’s frequency, and the objects size. If the metal object is large in reference to the incoming frequency, then the power will be mostly reflected. If the frequency is close to the resonant frequency of the object, then the object will absorb the power and continue to resonate with that energy even after the wave passes. If the frequency is long in regard to the size of the object, then the object will have no affect on the traveling wave front and absorb no power from it.

So, when a wideband wave like an EMP hits a large metal object like a metal roof, then the higher frequency component bounces back, the mid frequency component couples to the object, and the lower frequency component passes through. Considering that everything under that roof is smaller than the roof that is over it…. That would indicate that any of the lower wavelength frequency components that pass through the roof and impact the objects below it, will be of no threat to those objects. That is because the objects under the roof are too small to absorb that longer wavelength energy to be damaged by it. All the higher frequency components of the EMP that could damage the smaller objects was reflected by the big metal roof above them, leaving an RF shadow below it.

reply to post by Mr Tranny


Interesting. Both stables and the storage building have metal roofs...so does the "ManCave" (back porch). Presumably, the wave would be coming from above.

Does this mean that a metal garage door over a car would likely shield it? Or that a metal roof over a generator would shield it from a wave coming down?

Just asking, and assuming I understood you correctly.

But then, why would a car engine be affected, if a hood is over it?

Originally posted by Gazrok
reply to post by Mr Tranny

 

Does this mean that a metal garage door over a car would likely shield it? Or that a metal roof over a generator would shield it from a wave coming down?

As long as the object that is doing the shielding is significantly larger than the device being shielded, then yes.

A garage door is not much bigger than the car, so that would be a no, not so much.
A roof is a lot bigger than a generator, so that would be a yes.

But then, why would a car engine be affected, if a hood is over it?

edit on 11-4-2013 by Gazrok because: (no reason given)

The EMP doesn’t really directly hit the engine. It gets into the wiring in the car. Once the EMP gets into the wiring then it travels to susceptible components and damages them. That can be via a couple different ways. Wires going between poorly connected body panels. Higher frequency components of the EMP go through the windows and impinge on interior wiring. Or exposed exterior wiring.

If all body panels are properly bonded, and the amount of unshielded wires in the interior, and exterior of the vehicle is kept to a minimum, then the vulnerability of the vehicle is pretty low.

When there is full electrical continuity from one end of the vehicle to the other, then it will resonate as one object. That will mean there is a uniform electric field inside that vehicle, and no current will flow through any interior wiring. Thus no damage.

If the individual parts of the vehicle resonate differently, then there will be voltage developed on anything that crosses between those different parts