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CISPR 28: Information Technology Equipment – Telecommunications Network Immunity Requirements
Enhanced EMC immunity requirements for ITE used in telecommunications networks covering surge, EFT, and RF immunity at higher test levels
Introduction to CISPR 28
CISPR 28 specifies immunity requirements for Information Technology Equipment (ITE) used in telecommunications networks and similar environments. While closely related to CISPR 24, CISPR 28 focuses specifically on ITE that is part of or connected to telecommunications infrastructure, including switching equipment, transmission equipment, base stations, and network termination units. The standard addresses the higher immunity levels needed in telecommunications environments where equipment is exposed to more severe electromagnetic disturbances from nearby transmitters, power line faults, and lightning-induced surges. CISPR 28 ensures that telecommunications network equipment maintains operational integrity under these challenging conditions, preventing service disruption.
Telecommunications equipment installed in central offices, base stations, and outdoor cabinets faces significantly higher EM stress levels than typical office ITE. CISPR 28 test levels are generally 6-10 dB higher than CISPR 24 for surge and EFT phenomena, reflecting the more demanding installation environment.
Immunity Test Requirements
CISPR 28 specifies enhanced immunity test levels for telecommunications ITE. For surge immunity, the standard requires ±4 kV line-to-earth (compared to ±2 kV in CISPR 24) and ±2 kV line-to-line. EFT testing requires ±2 kV on power ports and ±1 kV on telecommunications ports. Radiated RF immunity is tested at 10 V/m (compared to 3 V/m in CISPR 24), recognizing that telecom equipment is often co-located with high-power radio transmitters. Conducted RF immunity uses 10 V (EMF) on power and telecom ports. The standard also includes specific test procedures for telecom-specific ports including DSL lines, E1/T1 interfaces, and fiber optic cable metallic members.
| Immunity Test | CISPR 28 Level (Telecom ITE) | CISPR 24 Level (General ITE) | Difference |
|---|---|---|---|
| Radiated RF (80-1000 MHz) | 10 V/m | 3 V/m | +10.5 dB |
| Conducted RF (0.15-80 MHz) | 10 V (EMF) | 3 V (EMF) | +10.5 dB |
| Surge — Line-to-earth | ±4 kV | ±2 kV | +6 dB |
| Surge — Line-to-line | ±2 kV | ±1 kV | +6 dB |
| EFT — Power ports | ±2 kV | ±1 kV | +6 dB |
| EFT — Signal/telecom ports | ±1 kV | ±0.5 kV | +6 dB |
The 10 V/m radiated RF test level in CISPR 28 corresponds to the field strength at approximately 100 m from a 100 W UHF transmitter. Telecom equipment installed on rooftops or towers may experience field strengths exceeding 50 V/m — requiring additional site-specific mitigation measures beyond standard compliance.
Engineering Design for Telecom ITE Immunity
Designing telecommunications equipment to meet CISPR 28 requires robust protection at multiple levels. Primary protection at the building entry point uses gas discharge tubes (GDTs) for lightning surge protection, with 10/700 µs surge waveform capabilities up to 10 kV. Secondary protection on circuit boards uses TVS diode arrays with 600 W-1500 W peak pulse power ratings for each telecom line. Isolation transformers with reinforced insulation (minimum 1500 VAC, 60 second hipot test) are essential for telecom port protection, providing both surge isolation and common-mode noise rejection.
Thermal management of protection components is critical — repeated surge events can heat TVS diodes beyond their rated junction temperature. Proper PCB layout with adequate copper area for heat dissipation, careful selection of clamping voltage to ensure the protected circuit’s breakdown voltage is never exceeded, and coordination between primary and secondary protection stages (using series resistors or PTCs for current limiting) are essential design practices. For outdoor telecom cabinets, additional environmental sealing (IP65 or better) and humidity control prevent moisture-related tracking and flashover on protection circuits.
A three-stage telecom port protection design — GDT (primary) + series resistor + TVS array (secondary) + common-mode choke — provides 20 kV surge withstand capability and passes CISPR 28 requirements with 10 dB margin, at a typical component cost of $2-5 per protected line.
Compliance Testing and Field Performance Correlation
While CISPR 28 laboratory testing provides a standardized immunity assessment, field conditions in telecommunications installations often exceed test levels. Factors include: proximity to high-power broadcast transmitters (field strengths exceeding 30 V/m), ground potential rise during power system faults (several kV), and lightning-induced surges on long cable runs (up to 10 kV). Engineers should design with additional margin — the standard EN 300 386 (European telecom EMC standard) often serves as a reference for enhanced requirements. Site-specific protection measures, including additional surge arrestors, enhanced grounding, and cable routing optimization, should be documented in the equipment installation manual.
Telecom equipment installed outdoors must withstand combined environmental and EM stress — including lightning, temperature extremes (-40°C to +70°C), and condensation. Protection circuit design must account for these environmental factors to ensure long-term reliability of the immunity protection components.
Frequently Asked Questions
Q: Is CISPR 28 still current or has it been superseded?
A: CISPR 28 has been largely superseded by CISPR 24 and later CISPR 35, but it remains referenced by some telecommunications equipment specifications and national regulations for network infrastructure EMC.
A: CISPR 28 has been largely superseded by CISPR 24 and later CISPR 35, but it remains referenced by some telecommunications equipment specifications and national regulations for network infrastructure EMC.
Q: How do I perform radiated RF testing at 10 V/m?
A> The test follows IEC 61000-4-3 methodology with higher amplifier power. A 10 V/m uniform field requires approximately 10-20 W amplifier power in a standard 3 m semi-anechoic chamber, compared to 1-3 W for 3 V/m.
A> The test follows IEC 61000-4-3 methodology with higher amplifier power. A 10 V/m uniform field requires approximately 10-20 W amplifier power in a standard 3 m semi-anechoic chamber, compared to 1-3 W for 3 V/m.
Q: Do fiber optic ports need immunity testing?
A> The optical fiber itself is immune to EM disturbances, but metallic members of hybrid fiber-copper cables, the equipment chassis, and any metallic cable glands require testing per the standard’s requirements for telecom ports.
A> The optical fiber itself is immune to EM disturbances, but metallic members of hybrid fiber-copper cables, the equipment chassis, and any metallic cable glands require testing per the standard’s requirements for telecom ports.
Q: What is the coordination strategy between primary and secondary protection?
A> Primary protection (GDT) handles high-energy surges but has slower response (>100 ns). Secondary protection (TVS) responds in <1 ns but handles less energy. Series impedance (resistor or inductor) between them ensures the TVS conducts first, then the GDT fires to divert the remaining energy. This ensures coordinated protection without either device being destroyed.
A> Primary protection (GDT) handles high-energy surges but has slower response (>100 ns). Secondary protection (TVS) responds in <1 ns but handles less energy. Series impedance (resistor or inductor) between them ensures the TVS conducts first, then the GDT fires to divert the remaining energy. This ensures coordinated protection without either device being destroyed.
