When a current is interrupted by an unsuppressed electromechanical switch (either a manual switch, contactor or relay) the resultant voltage rise across the gap causes a repetitive ignition and extinction of an arc discharge – the so-called “showering arc” effect. The nature of the arc is determined by the magnitude of interrupted current, the circuit and load inductance and stray capacitance, and the rate of separation of the contacts. It generally results in a burst of fast, low energy current transients which couple along the supply circuit or are radiated from the conductors on either side of the switch. The burst repetition rate varies from 10kHz to 1MHz.
The duration and rise time of these transients is short compared to the travel time in building wiring systems. Typically the rise time of each strike is less than 10ns and the duration is tens to hundreds of ns; transit time is roughly 5ns/m. Thus the supply wiring appears as a transmission line, so that the line characteristic impedance determines the transient source impedance, and the transients themselves decay with distance. As a result, these fast switching transients are only a threat to local susceptible equipment. But since there are many sources of such transients in the typical electrical installation, and because fast-rise events have considerable upset potential for digital circuits (though little or no potential for damage), equipment reliability will be enhanced by taking them into account.
The test for EFT burst immunity appears in IEC 61000-4-4. A similar transient test, but using a damped sinusoid waveform, also appears in DEF STAN 59-411 DCS05.
The following video demonstration illustrates an EFT/Burst event by using a chattering relay. Both time domain and frequency domain measurements were conducted.
Surge
High energy transients are generally a result of lightning coupling to the supply network, or are due to major power system disturbances such as fault clearing or capacitor bank switching. Lightning can produce surges of several joules by the following mechanisms:
direct strike to primary or secondary circuits: the latter can be expected to destroy protective devices and connected equipment; the former will pass through the service transformers either by capacitive or transformer coupling
indirect cloud-to-ground or cloud-to-cloud strikes create fields which induce common-mode voltages in all conductors
ground current flow from nearby cloud-to-ground discharges couples into the supply grounding network via common impedance paths
primary surge arrestor operation or flashover in the internal building wiring causes dV/dt transients (for lightning protection of buildings, see BS 6651: 1999)
Fault clearance upstream in the distribution network produces transients with energy proportional to 0.5·L·I2 trapped in the power system inductance. The energy will depend on the let-through current of the clearance device (fuse or circuit breaker) which can be hundreds of amps in residential or commercial circuits, and higher for some industrial supplies. Power factor correction capacitor switching operations generate damped oscillations at very low frequency (typically kHz) lasting for several hundred microseconds.
The test for surge immunity appears in IEC 61000-4-5 whose voltage waveform is shown in the graph. A lightning surge transient test for aircraft systems also appears in DEF STAN 59-411 DCS09. A relatively lightweight surge transient is specified in MIL STD 461F CS106.
ESD
Electrostatic discharge is a source of transient upset which most typically occurs when a person or other body that has been charged to a high potential by movement across an insulating surface then touches an earthed piece of equipment, thereby discharging through the equipment. A current pulse of tens of amps can flow for a short period with a very fast (sub-nanosecond) rise-time - the idealised waveform for the ESD test is shown opposite. Even though it may have low energy and be conducted through the equipment case, such a current pulse couples very easily into the internal circuitry.
Other ESD scenarios are possible. The equipment does not have to be earthed for a damaging transient to occur, since the mechanism relies on charge equalization which can depend only on the equipment’s self-capacitance. While personnel are common sources of static potential, other ungrounded equipment and even the victim equipment itself can be the source. The discharge current need not actually flow in the victim equipment to cause upset; if the discharge is to another nearby conductive object, the transient generates high local E- and H-fields which then couple to the victim circuitry.
The effect of the high transient current is most severe for edge-triggered digital or microprocessor circuits. If the transient occurs during a critical logic switching period then the logic state of the circuit (e.g. the microprocessor’s program counter) will be corrupted, causing incorrect operation and a possible lock-up of the program. Less severe effects include data and memory content corruption. On the other hand, if high levels of energy are coupled directly into IC pins as may occur at external interfaces, the IC may be physically destroyed or degraded.
The commercial ESD test appears in IEC 61000-4-2, and a virtually identical test (using the same test generator) is specified in DEF STAN 59-411 DCS10.
It is worth mentioning that, apart from direct ESD damage, the electromagnetic field generated during an ESD event can also couple with nearby circuits, devices, or equipment through both near and far-field coupling. Consequently, tens to hundreds of volts can be induced on wires or traces on the PCB if precautions are not taken. This can pose additional problems, as demonstrated in the following video.
Power supply variations
Power supplies themselves can suffer dips and interruptions, and though these may be regarded as an issue for power supply design, EMC susceptibility generally is taken to include this class of disturbance. IEC 61000–4–11 defines immunity test methods for these phenomena.
It specifies tests for voltage dips and short interruptions (an interruption is a dip to 0% of the supply) and for short-period voltage variations. The preferred values for period and level of dips are listed in the standard. Tests of dips and interruptions are significant as these are referenced in the informative annexes of the generic immunity standards. The generic standard requirements are for a half-cycle dip to 70% of rated voltage, a 5-cycle dip to 40% of rated voltage and a 5 second interruption. The performance criterion which applies to the latter two tests is that temporary loss of function is allowed, provided that it is self recoverable or can be restored by operation of the controls; i.e. a latch-up or a blown fuse is unacceptable.
Testing can be done either using electronically-controlled switching between the outputs of two variacs (variable voltage transformers) or by controlling the output of a waveform generator fed through a power amplifier.
Automotive transients
The automotive sector has a set of requirements of its own, as the automotive EMC environment is different from other residential or industrial environments. The automotive ESD test appears in ISO 10605, and transient immunity tests conducted via supply and signal lines are referenced in ISO 7637. This includes several different waveforms, some of which cover similar phenomena to the EFT burst discussed above. Two waveforms which are specific to the automotive 12/24V DC supply are the supply voltage starter motor reduction, test pulse 4, which is related to the AC power supply dip test; and the load dump, test pulse 5. This occurs when the supply battery is disconnected (accidentally or intentionally) while the alternator is generating charging current and with other loads remaining on the alternator circuit. The alternator continues to provide battery charging current for a few hundred milliseconds, which has to be dissipated in the rest of the load and therefore causes a high voltage surge from a low source impedance.
The load dump is a particularly severe transient and requires high energy protection devices to clamp it properly. This is best done centrally, at the alternator, rather than by every individual item of equipment that is connected to the supply; one problem that distributed clamping raises is that the lion's share of the clamping is effected by the device with the lowest clamping voltage, and other devices on the same supply are rarely activated. Most modern vehicles are therefore designed with centralised system-level clamping.
With the automotive industry shifting towards electrification, an increasing number of electric vehicles (EVs) are replacing traditional internal combustion engine vehicles. Unlike their counterparts, EVs lack an alternator as the power source for the low-voltage battery. Instead, they rely on a DC-DC converter to step down high voltage to low voltage, effectively charging the low-voltage network. Consequently, conventional immunity tests, such as load dump tests, lose their relevance in this context. As a result, several manufacturers are beginning to replace these outdated testing methodologies with more appropriate ones tailored to the unique characteristics of electric vehicles. EMC test industry continues to advance in tandem with technological progress. It is important for engineers to stay informed about these developments and adapt their testing methodologies accordingly.
Ring wave
The ring wave represents a very typical oscillatory transient occurring frequently in power supply networks and control and signal lines, due to load switching, power faults and lightning. The propagation of the wave in the power and signal lines is always subject to reflections, due to the mismatched line impedance. These reflections create oscillations, whose frequency is related to the propagation speed, length of line and parasitic parameters such as stray capacitance. The rise time is slowed due to the low-pass characteristic of the relevant line. The resultant phenomenon at the equipment ports is an oscillatory transient, or ring wave, compared to the unidirectional surge which is the normal test waveform for high energy events.
The ring wave is defined in IEC 61000-4-12 with a voltage rise-time of 0.5µs and an oscillation frequency of 100kHz. Despite its prevalence, the ring wave is rarely called up in commercial standard requirements, which are mostly satisfied by the IEC 61000-4-5 surge. Oscillatory transients are though more commonly required in military standards, particularly DEF STAN 59-411 DCS06, DCS08 and DCS12, and MIL STD 461 CS116.
NEMP
One of the effects of a nuclear explosion is a pulse of EM energy. DEF STAN 08-4 discusses the NEMP environment and test methods / facilities. An unclassified description of the NEMP environment and its causes can be found in IEC 61000-2-9.
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