Swelex solutions for EMC, RFI shielding and EMI shielding

EMC (Electromagnetic compatibility)

EMC is the ability of a device, equipment or system to function satisfactorily in its electromagnetic environment without unacceptably affecting anything in that environment.

EMI (Electromagnetic Interference)

Electromagnetic interference is the opposite of EMC. It is when a conflict is experienced between different pieces of equipment in a system. It means that one piece of equipment emits interference, conducted or radiated, to such an extent that another piece of equipment in the system experiences problems or malfunctions. All electrical and electronic devices emit a certain amount of interference, to a greater or lesser extent. It is when the emitted interference reaches a level where other equipment is disturbed that we get a conflict situation, EMI.

There are many different standards, both civilian and military, that prescribe how much an equipment can emit in terms of interference: for example, the generic standard EN61000-6-4:2007/A1:2011 for EMC emissions in industrial environments in Europe and FCC47 Part 15 Subpart B in the USA.

It is good that there are standards to rely on, but it is definitely no guarantee that conflicts will not occur, only a good start to achieving EMC. Of course, it also depends on how much interference the receiving equipment can withstand before it malfunctions.

The EMC/EMI phenomena can thus be divided into 4 parts:

• radiated energy emitted (emission)
• resistance to radiated energy (immunity)
• Conducted energy emitted (emission)
• resistance to conducted energy (immunity).

RFI (Radio Frequency Interference)

The term RFI, radio frequency interference, is not used very often in Europe or Sweden. In the United States, however, it is widely used. Radio frequency interference, as it is called in Swedish, is interference that has such a frequency content that it falls within the radio frequency range. This frequency range is between 0.1 MHz and 3 000 MHz, which is a considerable range, where a variety of interference phenomena can occur. RFI is equated in Europe with EMI and thus has a much broader scope.

EMP (Electro-Magnetic Pulse)

Electro-magnetic pulse is what we usually associate with the electromagnetic effect of an atomic bomb blast, whether detonated on the ground or at high altitude. The definition of an EMP is a very brief transient voltage event: for example, the spike or transient that forms when a relay is activated, when a traditional fluorescent lamp is switched on, or when a thermostat switches on or off. This produces a rapid spike or transient that we can reasonably call an EMP.

NEMP (Nuclear Electro-Magnetic Pulse)

Nuclear electro-magnetic pulse is what we really mean when we say EMP, but in Swedish we want to strengthen the expression by saying EMP-puls. It is the electromagnetic pulse that is formed in a nuclear explosion. It is only one - but very fast - pulse with a rise time of 3 ns (see HEMP below), so a very fast pulse.

Today, we are well aware of what happens if a nuclear device is detonated at ground level. This ground cannot be used in the foreseeable future. Therefore, it is a high-altitude nuclear explosion that is tactically useful nowadays.

HEMP (High Electro-Magnetic Pulse)

High-altitude EMP is the EMP pulse emitted by a nuclear explosion at high altitude. A nuclear charge is sent up with a missile to a height above the atmosphere, where it is detonated. All other effects, such as shock waves and gamma rays, can then be discounted. The result is an EMP pulse that illuminates virtually all of the Earth visible from the explosion point.

Note also that the figure 3 ns on the pulse rise time only applies to the E1 type threat pulse for a HEMP. It is this fast component E1 that is described in EMMA and which we in Sweden have used, and still use, as a demanding threat pulse on many of our military strategic systems. In addition to E1, a HEMP also produces E2 with effects similar to lightning pulses and E3 with late time components (of second magnitude) and with effects similar to the effects of solar storms. The generation of all HEMP components is thus a complex process, and yet a low-altitude explosion has not been considered here.

The pulse is formed by a Compton process, where the static geomagnetic field together with the gamma rays refract electrons to form a very fast electromagnetic pulse that illuminates the Earth's surface.

LEMP (Lightning Electro-Magnetic Pulse)

Lightning electromagnetic pulse is the electromagnetic pulse produced by a lightning strike. When the course of a lightning strike has reached the phase where current begins to flow in the current path, a magnetic field is formed around the lightning path.

We remember from physics lessons at school that if you draw a current in an electrical conductor, a magnetic field forms around the conductor; that's exactly what happens around the lightning rod.

In the case of a lightning strike, several effects are dominant; the magnetic field that spreads for kilometres in the surrounding area is one of them.

RÖS (Clearance Signals)

RÖS, revealing signals, is a military term used to describe:

Improperly emitted electromagnetic signals that are perceived by unauthorised persons, which can then contribute to the disclosure of classified information (Source: FMV).

The English word for RÖS is TEMPEST. However, there is no rational explanation for this English word. No one knows where it comes from, but we do know what it means, namely the same as in Swedish, röjande signaler (Eng. compromising emanation).

The electromagnetic energy that can be detected from a distance and that can be deciphered to give an interpretation of the "useful signal" contained in the electromagnetic energy is referred to as the ROM.

A descriptive example is the black van parked in the banking district of a large city, or parked on the quay next to the visiting foreign warship. It is very likely that the van contains powerful measuring antennae and sensitive receiving equipment that tries to pick up electromagnetic signals that are inadvertently propagated from the bank premises or the warship. Some advanced signal processing can then tell us what information has come out that could be used in a hostile way.

ESD (ElectroStatic Discharge)

Electrostatic discharge is the same as static electricity. If we call the lightning pulse a macro phenomenon, we can call static electricity a micro phenomenon.

ESD is very similar to lightning. The phenomenon is built up by separating strong charges, after which nature tries to harmonise them again, like communicating vessels. A charge accumulation builds up for different reasons and under different conditions. A charge separation is then created, and this is when things start to get dangerous.

The separated accumulation of charge, either positive charge or negative charge, tries to equalise itself according to the laws of nature by collecting the opposite charge in its vicinity. If the isolation between them eventually breaks down, a static discharge occurs.

It is precisely the discharge that is dangerous for electronic equipment, if it is not adequately protected against this threat.

If we go back to school physics lessons again to understand what ESD is, we can describe the example of a cat skin and an ebonite rod. You can rub them together and then separate them, thereby creating a strong charge separation. The cat skin or the ebonite rod attracts dust and hair, causing them to stick together if they have opposite charges, but if they have the same charge they will push apart.

There are usually very high voltages associated with electrostatic discharges. It is usually said that there is no spark below 3 kV, while our electronics are sometimes much more sensitive to voltage transients with lower amplitudes, perhaps down to 400-500 V.

For larger sparks than those produced at 3 kV, an extension of the spark by one centimetre is said to correspond to a voltage increase of 1 kV.

If a person is working at a table and gets up from the chair, they can be briefly charged up to 30 kV (depending on the material of the chair cushion and trousers).