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PD IEC TR 61156 — Multicore and Symmetrical Pair/Quad Cables for Digital Communications Technical Report Series
Multicore and Symmetrical Pair/Quad Cables for Digital Communications
🔌 PD IEC TR 61156 is the British Standards Institution (BSI) Published Document implementing the International Electrotechnical Commission (IEC) technical report IEC/TR 61156, addressing multicore and symmetrical pair/quad cables for digital communications. Prepared by the UK technical committee EPL/46 “Cables, wires and waveguides, radio frequency connectors and accessories for communication and signalling” and classified under ICS 33.120.20 (wiring accessories), the series provides technical guidance on the electrical transmission characteristics, test methods, and measurement-correction of symmetrical pair/quad cables — an important reference for characterising the transmission performance of LAN, structured cabling, and digital communication cables.
💡 The performance of symmetrical pair/quad cables (e.g., Cat 5e/6/6A twisted pair) is governed by electrical transmission parameters. The technical reports unify transmission-line equations, measurement corrections, and nominal values so results from different test environments are comparable and judgeable.
1. Series Structure and Overall Framework 📌
This batch of the PD IEC TR 61156 series comprises 3 technical report parts, covering electrical transmission characteristics and test methods, return-loss/input-impedance measurement correction, and nominal DC resistance values.
| Part | Year | Title | Core Content |
|---|---|---|---|
| PD IEC/TR 61156-1-2 (+A1) | 2009 (+2014) | Electrical transmission characteristics and test methods of symmetrical pair/quad cables | Provides basic transmission-line equations, primary/secondary parameters, and frequency characteristics |
| PD IEC/TR 61156-1-5 | 2013 | Correction procedures for the measurement results of return loss and input impedance | Specifies RL/OSRL/SRL measurement and PRL/GRL/FRL correction methods |
| PD IEC/TR 61156-1-6 | 2016 | Nominal DC-resistance values of floor-wiring and work-area cables | Gives typical conductor-diameter and DC-resistance values and resistance unbalance |
⚠️ This series is an IEC/TR technical report of reference nature. Always reference the current BSI version (including amendments), with current IEC 61156 product specifications taking precedence.
2. Electrical Transmission Characteristics: Part 1-2 Fundamentals 📐
Part 1-2 starts from the basic transmission-line equations and systematically explains the electrical transmission characteristics of symmetrical pair/quad cables:
- Characteristic impedance and propagation coefficient: gives the equations and physical meaning of characteristic impedance and propagation coefficient.
- Phase and group velocity: analyses phase and group velocity and their effect on digital signal transmission.
- Primary parameters: conductor resistance (R), inductance (L), capacitance (C), and conductance (G) with their frequency dependence.
- Secondary parameters: attenuation, phase shift, impedance, and return characteristics derived from primary parameters.
Part 1-2 also suggests test methods for the parameters, providing a theoretical and methodological basis for cable performance testing.
3. Return-Loss Measurement Correction: Part 1-5 Procedures 📏
Part 1-5 specifies correction procedures for return loss and input-impedance measurement results. Periodic structural defects in the cable cause structural return loss, while parasitic effects from connectors and test fixtures affect measurement accuracy:
| Measured/Corrected Quantity | Description |
|---|---|
| Return loss (RL) | The ratio of reflected to injected signal at the cable input, indicating impedance matching |
| Open/short return loss (OSRL) | Return loss measured under open/short conditions for fixture correction |
| Structural return loss (SRL) | The return-loss component caused by periodic structural defects in the cable |
| Parasitic-inductance-corrected return loss (PRL) | Return loss corrected for test-fixture parasitic inductance |
| Gated (GRL) / fitted (FRL) return loss | Time-domain gating or frequency-domain fitting to remove connector-reflection effects |
The Part 1-5 correction procedures make return-loss results consistent across test environments and are key for arbitration measurements of cable performance.
4. Nominal DC Resistance Values: Part 1-6 Data 🔍
Part 1-6 gives the nominal DC-resistance values of horizontal floor-wiring and work-area wiring cables for digital communications, including:
- Nominal conductor diameter and typical DC resistance: gives typical DC resistance for horizontal floor-wiring cables by conductor-diameter range.
- Work-area cables: gives DC resistance values for work-area wiring cables.
- Resistance unbalance: analyses resistance unbalance between pairs and its evaluation methods.
- Stranding diameter and lay length: evaluates the effect of stranding parameters on resistance unbalance.
The Part 1-6 nominal values provide reference for cable design, selection, and supply calculations (e.g., PoE).
✅ The three parts go from theory to measurement to data: Part 1-2 on “how to calculate transmission characteristics,” Part 1-5 on “how to correct measurement results,” and Part 1-6 on “what the nominal DC resistance values are,” together supporting symmetrical cable performance characterisation.
5. Engineering Practice and Application Notes 🛠️
- 🔍 Cable selection: choose the appropriate category (Cat) and conductor size per transmission characteristics and nominal DC resistance.
- 🔧 Performance testing: correct return-loss measurements per Part 1-5 to ensure comparable results.
- 📊 Power-supply calculation: use nominal DC resistance to calculate PoE voltage drop and power loss.
- 📋 Alignment with current standards: use with current IEC 61156 product specifications and structured cabling standards (e.g., ISO/IEC 11801).
6. Frequently Asked Questions ❓
❓ Q: What is return loss and why does it matter?
A: Return loss is the ratio of reflected to injected signal at the cable input, indicating impedance matching; excessive reflection causes signal reflection and errors, degrading high-speed transmission.
A: Return loss is the ratio of reflected to injected signal at the cable input, indicating impedance matching; excessive reflection causes signal reflection and errors, degrading high-speed transmission.
❓ Q: Why is correction needed for return-loss measurements?
A: Test-fixture parasitic inductance and connector reflections contaminate results; PRL/GRL/FRL corrections remove instrumentation effects to obtain the cable’s true performance.
A: Test-fixture parasitic inductance and connector reflections contaminate results; PRL/GRL/FRL corrections remove instrumentation effects to obtain the cable’s true performance.
❓ Q: What are nominal DC resistance values used for?
A: Cable-size selection, link-attenuation calculation, and power-over-Ethernet (PoE) voltage-drop and power-budget estimation.
A: Cable-size selection, link-attenuation calculation, and power-over-Ethernet (PoE) voltage-drop and power-budget estimation.
🔍 Q: Why is this series published as a Technical Report (TR)?
A: Because it is an IEC technical report providing theory, methods, and reference values rather than mandatory limits, for reference in cable performance analysis, testing, and design.
A: Because it is an IEC technical report providing theory, methods, and reference values rather than mandatory limits, for reference in cable performance analysis, testing, and design.