Electronics Guide

Lightning-Induced Surges

Lightning strikes produce extremely high currents (typically 20,000 amperes, sometimes exceeding 200,000 amperes) that induce surges in power and communication systems through multiple mechanisms. Direct strikes to power lines inject current directly into the distribution system. Nearby strikes induce voltages through electromagnetic coupling to overhead and underground cables. Ground potential rise during strikes creates voltage differences between equipment grounded at different points.

The energy in a lightning-induced surge depends on the strike characteristics and the coupling mechanism. Direct strikes to utility primary lines can propagate surges with crest voltages of 20 kV or more at service entrance points. Secondary effects from nearby strikes typically produce lower voltages but still significantly exceed equipment ratings. The surge waveform includes both fast-rising components (reflecting the initial strike) and slower decay components (reflecting the current flow through system impedances).

Geographic factors significantly affect lightning exposure. Some regions experience frequent thunderstorm activity while others see lightning rarely. Building characteristics including height, construction, and lightning protection systems influence the likelihood and severity of lightning-related surges. Risk assessment considering these factors guides the selection of appropriate protection levels.

Switching Transients

Power system switching operations generate transient overvoltages through the interruption and re-energization of electrical circuits. Capacitor bank switching, transformer energization, fault clearing, and load switching all produce characteristic transient signatures. While generally lower in amplitude than lightning-induced surges, switching transients occur more frequently and affect equipment connected to the switched circuit.

Capacitor bank switching for power factor correction produces transient oscillations that can significantly exceed normal line voltage. The switched capacitance forms a resonant circuit with system inductance, producing damped oscillations at frequencies typically between 300 Hz and 1000 Hz. Multiple capacitor banks switching in sequence can cause voltage magnification through resonance with existing oscillations.

Transformer energization produces inrush currents that can reach 10 times the rated current, causing voltage dips followed by recovery transients. The asymmetric saturation of transformer cores during energization produces complex waveforms with significant harmonic content. Large transformers in industrial facilities and utility substations create the most significant energization transients.

1.2/50 Microsecond Voltage Waveform

The open-circuit voltage waveform has a rise time of 1.2 microseconds and a decay time (to 50% of peak) of 50 microseconds. This relatively slow waveform represents the voltage developed across high-impedance loads or open circuit conditions. The long duration allows significant energy transfer even at moderate peak voltages.

Test levels range from 500 V (Level 1) through 4 kV (Level 4), with higher levels available for special applications. The appropriate level depends on the installation environment, with outdoor equipment and equipment near service entrance points requiring higher levels than equipment in protected indoor locations.

8/20 Microsecond Current Waveform

The short-circuit current waveform has a rise time of 8 microseconds and a decay time to 50% of peak of 20 microseconds. This waveform represents the current delivered into a low-impedance load and determines the energy that protection devices must absorb. The generator source impedance determines the relationship between open-circuit voltage and short-circuit current.

For power port testing, the generator impedance is typically 2 ohms, producing peak currents of 2000 amperes at 4 kV. For communication line testing, a higher impedance of 12 ohms limits peak current while maintaining appropriate voltage levels. Some applications, particularly telecommunications and utility installations, require testing with dedicated current generators delivering tens of thousands of amperes.

10/700 Microsecond Telecom Waveform

Long telecommunications lines act as transmission lines that slow the rise time and extend the duration of coupled lightning surges. The 10/700 microsecond waveform, defined in ITU-T recommendations and referenced by IEC 61000-4-5, represents this extended duration surge. The longer duration delivers significantly more energy than the 1.2/50 waveform at the same peak voltage.

Power Port Testing

Surges on AC power ports can appear as line-to-line (differential mode) or line-to-ground (common mode) transients. Both modes are tested because real surge events can produce either or both modes depending on the source and coupling mechanism. Common-mode surges are typically tested at higher levels than differential-mode surges, reflecting the generally higher voltages of lightning-induced common-mode transients.

The test setup uses coupling networks that inject the surge while isolating the surge generator from the mains supply and protecting other equipment on the same circuit. Decoupling is typically provided by inductors that present high impedance at surge frequencies while passing normal power frequency current. The coupling network also defines the effective source impedance seen by the equipment under test.

Signal and Telecom Port Testing

Signal and communication ports are tested with surges coupled between lines (line-to-line) and from lines to ground (line-to-ground). The appropriate test level depends on the expected exposure, with outdoor lines requiring higher protection levels than indoor wiring. Shielded cables tested with surges between shield and conductors evaluate the cable's ability to protect internal signals from surges on the shield.

The coupling impedance for signal port testing is typically higher than for power ports, reflecting the higher impedance of typical signal circuits. This limits the peak current while maintaining appropriate voltage stress. For very high-impedance circuits, gas discharge tubes or spark gaps in the coupling network may protect the generator while allowing full voltage to be applied.

Performance Criteria

Performance during and after surge testing is evaluated against defined criteria. Criterion A requires normal performance with no degradation. Criterion B allows temporary functional degradation with automatic recovery after the surge ends. Criterion C permits loss of function requiring operator intervention to restore normal operation. The applicable criterion depends on the safety implications of equipment malfunction and the requirements of the product standard.

Damage to protection components that does not affect normal operation may be acceptable under some criteria, though most products aim to survive the specified number of surges without component replacement. The test sequence typically includes positive and negative polarity surges at multiple phase angles to identify worst-case conditions.

Metal Oxide Varistors

Metal oxide varistors (MOVs) are the most widely used surge protection components due to their combination of fast response, high energy absorption, and low cost. MOVs exhibit a highly nonlinear voltage-current characteristic, presenting high impedance below the clamping voltage and low impedance above it. This allows the MOV to remain effectively invisible during normal operation while providing a low-impedance shunt path during surges.

MOV energy rating is specified in joules and indicates the single-surge absorption capability. Multiple surges degrade the MOV, eventually leading to failure through either open circuit (end of life) or short circuit (catastrophic failure). High-quality MOVs include thermal disconnection mechanisms that safely disconnect the device before thermal runaway can cause fire.

The clamping voltage of an MOV depends on the surge current level, with higher currents producing higher clamping voltages. Selection must consider both the maximum continuous operating voltage (MCOV) requirement and the desired clamping voltage at rated surge current. MOVs degrade faster when operated near their MCOV limit, trading protection effectiveness for component longevity.

Gas Discharge Tubes

Gas discharge tubes (GDTs) provide extremely high surge current capability in a compact package. When the voltage across the tube exceeds the DC sparkover voltage, the gas ionizes and the tube enters arc mode with a very low voltage drop (typically 10-25 V). This low arc voltage allows GDTs to divert very high surge currents without excessive power dissipation.

The primary limitation of GDTs is their response time. The sparkover process takes time, particularly for slowly-rising transients, allowing the voltage to significantly exceed the DC sparkover rating before the tube fires. This relatively slow response makes GDTs unsuitable as the sole protection for sensitive electronics, though they excel as the primary protection stage in multi-stage schemes.

GDTs are commonly used for telecommunications line protection, where their high current capability and ability to withstand repeated operations without degradation are particularly valuable. Modern GDTs achieve arc sustainment voltages below 20 V with surge current ratings exceeding 20,000 amperes.

TVS Diodes

Silicon avalanche diodes (TVS diodes) provide the fastest response of any surge protection technology, with clamping occurring in less than one nanosecond. This fast response provides precise voltage limiting that protects sensitive semiconductors from even the fastest surge wavefronts. TVS diodes are available in voltage ratings from 3.3 V to hundreds of volts.

The energy capability of TVS diodes is more limited than MOVs or GDTs, making them most suitable as secondary protection following an energy-absorbing first stage. The clamping voltage of TVS diodes is well-defined and varies only slightly with current, providing predictable protection levels. Unidirectional and bidirectional configurations address DC and AC applications respectively.

Thyristor Surge Protectors

Thyristor-based surge protectors (often called silicon controlled rectifiers or crowbar devices) provide a unique combination of fast switching and low on-state voltage. When triggered by an overvoltage, the thyristor latches into conduction, crowbarring the line to near-ground potential. The low on-state voltage allows sustained conduction of follow current without the thermal limitations of clamping devices.

The crowbarring action of thyristor protectors can be a disadvantage in some applications, as the device remains conducting until the current drops below the holding current. On AC power lines, this means the device will not reset until the next zero crossing. Special designs including gate-assisted turn-off address this limitation for applications requiring fast reset.