Many engineers focus only on impulse voltage amplitude when performing Clause 14 transient over‑voltage tests for home appliance safety, while overlooking source impedance. Wrong source impedance changes the pulse energy completely, leading to misleading test results: your product may pass lab testing yet fail in real‑world conditions, or get falsely rejected. This blog explains impedance selection rules referencing the IEC 60255‑5 impedance quick‑reference table.
According to IEC 60335‑1 (GB 4706.1) Clause 14, household appliances shall withstand 1.2/50 μs lightning impulse voltage to verify insulation resistance against transient over‑voltages from mains supply. Note that identical impulse voltage with different source impedance delivers totally different energy and stress to DUT, which is critical for valid safety evaluation.
Clause 14 refers to the impedance system defined in IEC 60255‑5. Four typical source impedances (2 Ω, 12 Ω, 40 Ω(42 Ω), 500 Ω) are specified for different test points and must not be misused.
| Source Impedance | Application | Key Test Characteristics |
|---|---|---|
| 2 Ω | Relay assemblies, coils, contacts | High‑energy output, simulates direct mains surge impact, evaluates component resistance to high‑energy pulses |
| 12 Ω | Power terminals to earth safety test | Default impedance for IEC 60335‑1 Clause 14 main‑circuit test, simulates induced over‑voltage on power supply lines |
| 40 Ω (42 Ω) | Signal lines, control & communication terminals | High‑voltage with limited current, evaluates insulation of signal interfaces without destroying sensitive ICs |
| 500 Ω | Insulation material specimens (near‑zero DC resistance) | Ultra‑low energy, only detects material breakdown; avoids thermal damage to electronic components |
12 Ω ±10% is the standard source impedance for 1.2/50 μs impulse generator for household appliance type‑testing. It replicates real‑life induced lightning surges on mains lines.
Common mistake: Using 500 Ω for power‑terminal testing. Its low energy may hide insulation defects. Your unit passes lab test but fails under real‑world mains surges.
2 Ω for relay, coil and contact evaluation
Use 2 Ω when assessing on‑board relays and electromagnetic coils. This setting delivers maximum surge energy, simulating harsh direct mains stress for contact and coil endurance verification.
40 Ω (42 Ω) for control and communication ports
Do not apply 12 Ω high‑energy impulse to UART, keypad or communication interfaces. The 42 Ω impedance provides high test voltage while limiting surge current. It checks interface insulation without burning downstream sensitive chips, reproducing realistic transient interference on signal cables.
500 Ω for standalone insulation material testing
500 Ω is reserved for material‑level testing of plastic or film insulation samples only. Its minimal energy targets dielectric breakdown itself instead of damaging attached electronics. Do NOT use this setting for full‑product terminal testing.
Impulse voltage compliance is not merely about reaching required voltage levels. Source impedance defines pulse energy, and energy determines test authenticity.
For IEC 60335‑1 Clause 14 appliance testing: use 12 Ω for power‑to‑earth terminals; 2 Ω for relay coils & contacts; 40 Ω for signal / communication ports; 500 Ω only for insulation material specimens. Following the IEC 60255‑5 impedance guidance ensures repeatable and meaningful safety test results.
Tip: Confirm your 1.2/50 μs impulse generator supports configurable source impedance before running tests. Different impedance settings produce distinct output energy.
Many engineers focus only on impulse voltage amplitude when performing Clause 14 transient over‑voltage tests for home appliance safety, while overlooking source impedance. Wrong source impedance changes the pulse energy completely, leading to misleading test results: your product may pass lab testing yet fail in real‑world conditions, or get falsely rejected. This blog explains impedance selection rules referencing the IEC 60255‑5 impedance quick‑reference table.
According to IEC 60335‑1 (GB 4706.1) Clause 14, household appliances shall withstand 1.2/50 μs lightning impulse voltage to verify insulation resistance against transient over‑voltages from mains supply. Note that identical impulse voltage with different source impedance delivers totally different energy and stress to DUT, which is critical for valid safety evaluation.
Clause 14 refers to the impedance system defined in IEC 60255‑5. Four typical source impedances (2 Ω, 12 Ω, 40 Ω(42 Ω), 500 Ω) are specified for different test points and must not be misused.
| Source Impedance | Application | Key Test Characteristics |
|---|---|---|
| 2 Ω | Relay assemblies, coils, contacts | High‑energy output, simulates direct mains surge impact, evaluates component resistance to high‑energy pulses |
| 12 Ω | Power terminals to earth safety test | Default impedance for IEC 60335‑1 Clause 14 main‑circuit test, simulates induced over‑voltage on power supply lines |
| 40 Ω (42 Ω) | Signal lines, control & communication terminals | High‑voltage with limited current, evaluates insulation of signal interfaces without destroying sensitive ICs |
| 500 Ω | Insulation material specimens (near‑zero DC resistance) | Ultra‑low energy, only detects material breakdown; avoids thermal damage to electronic components |
12 Ω ±10% is the standard source impedance for 1.2/50 μs impulse generator for household appliance type‑testing. It replicates real‑life induced lightning surges on mains lines.
Common mistake: Using 500 Ω for power‑terminal testing. Its low energy may hide insulation defects. Your unit passes lab test but fails under real‑world mains surges.
2 Ω for relay, coil and contact evaluation
Use 2 Ω when assessing on‑board relays and electromagnetic coils. This setting delivers maximum surge energy, simulating harsh direct mains stress for contact and coil endurance verification.
40 Ω (42 Ω) for control and communication ports
Do not apply 12 Ω high‑energy impulse to UART, keypad or communication interfaces. The 42 Ω impedance provides high test voltage while limiting surge current. It checks interface insulation without burning downstream sensitive chips, reproducing realistic transient interference on signal cables.
500 Ω for standalone insulation material testing
500 Ω is reserved for material‑level testing of plastic or film insulation samples only. Its minimal energy targets dielectric breakdown itself instead of damaging attached electronics. Do NOT use this setting for full‑product terminal testing.
Impulse voltage compliance is not merely about reaching required voltage levels. Source impedance defines pulse energy, and energy determines test authenticity.
For IEC 60335‑1 Clause 14 appliance testing: use 12 Ω for power‑to‑earth terminals; 2 Ω for relay coils & contacts; 40 Ω for signal / communication ports; 500 Ω only for insulation material specimens. Following the IEC 60255‑5 impedance guidance ensures repeatable and meaningful safety test results.
Tip: Confirm your 1.2/50 μs impulse generator supports configurable source impedance before running tests. Different impedance settings produce distinct output energy.
