EMC & Standards

Combination Wave

Q: Why does the combination wave have two different waveform specifications?

The two waveforms represent the two extreme load conditions a surge generator encounters. When driving a high-impedance (open) load, the voltage waveform dominates and follows the 1.2/50 microsecond shape (1.2 microsecond rise time to peak, 50 microsecond half-value decay). When driving a low-impedance (short) load, the current waveform dominates and follows the 8/20 microsecond shape (8 microsecond rise to peak, 20 microsecond half-value decay). Real equipment presents impedance between these extremes, so both voltage stress and current delivery capability are tested simultaneously. The generator's effective source impedance is 2 ohms, which determines the relationship between open-circuit voltage and short-circuit current: a 2 kV open-circuit voltage produces 1 kA short-circuit current.

Q: What test levels are specified for RF equipment?

IEC 61000-4-5 defines four severity levels for AC power port testing. Level 1 is 0.5 kV (line-to-line) and 1 kV (line-to-ground) for light industrial equipment. Level 2 is 1 kV and 2 kV for general commercial equipment. Level 3 is 2 kV and 4 kV for heavy industrial environments. Level 4 is 4 kV and variable (typically 4 to 6 kV) for severe environments including outdoor installations and equipment near lightning-prone areas. For signal and telecom ports, levels are typically half the power port values. RF base station equipment, outdoor antennas with connected cables, and radar installations typically require Level 3 or 4 testing because they are directly exposed to lightning-induced surges through antenna feeds and power connections.

Q: How does the combination wave differ from other surge standards?

The combination wave (IEC 61000-4-5) differs from related standards in key ways. The 10/700 microsecond wave (ITU-T K.20/K.21) has a slower rise and much longer duration, representing telecom line surges from distant lightning strikes conducted over long cable runs. The ring wave (IEC 61000-4-12) produces a 100 kHz damped oscillation representing oscillatory surges from switching transients in power distribution networks. ESD testing (IEC 61000-4-2) uses a much faster nanosecond-rise discharge representing electrostatic discharge events. The combination wave specifically models the surge arriving at equipment from a nearby lightning strike coupled through the AC power distribution system, which is the most energetic and destructive common transient threat.

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A standardized EMC test waveform specified in IEC 61000-4-5 and IEEE C62.45 that simultaneously produces a 1.2/50 μs open-circuit voltage surge and an 8/20 μs short-circuit current surge from a single generator. The generator's 2 Ω source impedance means a 2 kV open-circuit voltage delivers 1 kA into a short circuit. This waveform models lightning-induced surges arriving at equipment through AC power distribution, telecom lines, and signal cables. Test severity levels range from 0.5 kV (Level 1, light commercial) to 4 kV or higher (Level 4, outdoor/industrial), with RF base stations and radar installations typically requiring Level 3 or 4 due to direct lightning exposure through antenna feeds.

Category: EMC & Standards

Voltage Shape: 1.2/50 μs

Current Shape: 8/20 μs

Understanding Combination Wave

Lightning protection design for RF systems begins with understanding the threat waveform. When lightning strikes a power line or nearby ground, the electromagnetic pulse couples into building wiring and signal cables as a fast-rising, high-energy transient. The IEC 61000-4-5 combination wave generator replicates this threat in a controlled laboratory setting. The "1.2/50" designation means the open-circuit voltage rises to peak in 1.2 μs and decays to 50% of peak in 50 μs. The "8/20" designation means the short-circuit current rises to peak in 8 μs and decays to 50% of peak in 20 μs.

The 2 Ω source impedance is critical: it represents the impedance of the AC power distribution network as seen by the surge at the equipment terminals. This means the surge energy delivered to the equipment under test (EUT) depends on the EUT's own impedance. A high-impedance EUT sees mostly voltage stress; a low-impedance EUT (such as one with a metal-oxide varistor protection device) sees mostly current stress. Surge protection devices (SPDs) must be rated to absorb the full 8/20 current at the test level without failure, while the protected equipment must withstand the let-through voltage without damage or malfunction.

Waveform Parameters

Open-Circuit Voltage:
V(t) = V_peak × k × [exp(−t/τ₁) − exp(−t/τ₂)]
Rise time: 1.2 μs ± 30% | Duration: 50 μs ± 20%

Short-Circuit Current:
I(t) = I_peak × k × [exp(−t/τ₃) − exp(−t/τ₄)]
Rise time: 8 μs ± 20% | Duration: 20 μs ± 20%

Source Impedance:
Z_s = V_oc / I_sc = 2 Ω

Example: 2 kV open-circuit → 1 kA short-circuit. Energy per pulse at 2 kV into matched 2 Ω load: ≈ 50 J. Repetition: 5 positive + 5 negative surges at 1-minute intervals per IEC 61000-4-5.

Surge Waveform Standard Comparison

Standard	Waveform	Rise / Duration	Source Z	Threat Modeled	Typical RF Application
IEC 61000-4-5	Combination wave	1.2/50 μs V, 8/20 μs I	2 Ω	Lightning on AC power	Base stations, radar
ITU-T K.20/K.21	10/700 μs	10/700 μs V	25 to 50 Ω	Telecom line lightning	Telecom RF interfaces
IEC 61000-4-12	Ring wave	0.5/100 kHz damped	12 to 30 Ω	Switching transients	Indoor power supplies
IEC 61000-4-2	ESD	<1 ns / 60 ns	330 Ω	Electrostatic discharge	Handheld RF equipment
MIL-STD-461 CS116	Damped sine	10 to 100 kHz	1 to 50 Ω	EMP/HEMP	Military RF systems

Common Questions

Frequently Asked Questions

Why does the combination wave have two different waveform specifications?

The two shapes represent extreme load conditions. Into high impedance (open), the 1.2/50 μs voltage dominates. Into low impedance (short), the 8/20 μs current dominates. Real equipment falls between these extremes. The 2 Ω source impedance links them: 2 kV open-circuit = 1 kA short-circuit. Both voltage stress and current delivery are tested simultaneously.

What test levels are specified for RF equipment?

IEC 61000-4-5 defines Level 1 (0.5/1 kV) for light commercial, Level 2 (1/2 kV) for general, Level 3 (2/4 kV) for heavy industrial, and Level 4 (4+ kV) for severe/outdoor. RF base stations, outdoor antennas, and radar require Level 3 or 4 due to direct lightning exposure through antenna feeds and power connections.

How does the combination wave differ from other surge standards?

The 10/700 μs telecom wave (ITU-T K.20) is slower and longer for distant lightning. The ring wave (IEC 61000-4-12) models switching transients. ESD (IEC 61000-4-2) has nanosecond rise times. The combination wave specifically models nearby lightning coupled through AC power distribution, the most energetic common transient threat.

Related Terms

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