Level 2 specification
NR/L2/TEL/31107
Limits and Test Method of Induced Voltages on Telecommunications Cables due to Electrification Systems
Issue 2, 06 March 2010 · Copper design & immunisation
The induced-voltage limits (longitudinal VL and transverse VT) on copper telecoms cables in AC electrified areas, and the test process used to prove them before, during and after electrification changes
Clause map — what each clause deals with
| 1 | Purpose | What the standard is for: defining test limits and methods for voltages induced on copper telecoms cables by AC electrification, in normal and credible failure modes, to protect staff, equipment and service quality |
| 2 | Scope | Applies to lineside telecoms cables in AC electrified areas when electrification, traffic or the telecoms cables themselves change; sits under the GE/RT8016 interactions plan |
| 3.1 | Designated Project Engineer (DPE) | What the DPE does before a change: risk assessment to set the level of modelling, test plan, tests, reports and the commissioning certificate |
| 4.1 | Abbreviations | Abbreviations used (BT, RC, OHLE, FTE, VL, VT and so on) |
| 4.2 | Definitions | Definitions including accessible voltage, booster transformer, longitudinal and transverse voltage, hot wire working |
| 5 | Introduction | How AC traction induces a longitudinal voltage along lineside conductors and the risks that follow (shock, equipment damage, noise) |
| 6.1 | Process | The overall compliance process (Figure 1): define route, model, test plan, pre-change tests, upgrade, commissioning tests, post-commissioning tests, report |
| 6.2 | Definition of Route | Defining the telecoms cables on the route and the maximum circuit length of each section |
| 6.3 | Modelling | Assessing expected VL by modelling or calculation before the change, scaled to the risk |
| 6.4 | Test Plan & Migration Plan | What the test plan must contain — tests, locations, pairs used, electrification and traffic conditions |
| 6.5 | VL and VT In-Traffic Tests | Pre-change in-traffic VL/VT tests, minimum 2 hours at peak traffic (longer for big changes) |
| 6.6 | Report | Results go to the DPE; changes may start once acceptance criteria are met |
| 6.8 | Commissioning Tests | Tests when commissioning AC electrification: load bank (only for new system types), short circuit at the worst-case location, degraded-condition tests not normally required |
| 6.9 | Post-Commissioning Tests | In-traffic VL/VT tests after the change (2 hours minimum at peak); hot wire working if VL limits are not met |
| 6.10 | Reports & Certificate | The DPE's modelling report, test report and the Telecommunications Commissioning Certificate (Appendix A) |
| 7.1 | Normal Conditions | The longitudinal voltage limit under normal conditions — Table 1, time-dependent: 60 V rms steady state (>300 s), 65 V up to 300 s, rising to 370 V for 20 ms events |
| 7.2 | Degraded Conditions | Hot wire working is invoked under degraded conditions (booster/autotransformer outage, missing screening conductor) unless the risk has been shown tolerable |
| 7.3 | Fault Conditions | Fault-condition VL limits (Table 2, time dependent) and the 645 V rms absolute maximum to protect lineside equipment |
| 8 | Transverse Voltage Limits | Transverse (pair-to-pair differential) voltage: 1 mV rms psophometrically weighted under normal conditions, with permitted excursions |
| 9.1 | Test Process | The step-by-step VL/VT set-up and test process (Table 3) |
| 9.2 | Selecting Test Pairs | Choosing the cable and pairs that give the least protection (longest cable, worst screening) |
| 9.3 | Tests Prior to Terminating Cable | Insulation resistance and conductor resistance/balance tests on the test pairs before termination, recorded on the Cable Test Sheet |
| 10.1 | Terminating Test Pairs for Longitudinal Voltage | How the VL pair is earthed at the remote end (30 ohm or better earth, remote from the railway) |
| 10.2 | Tests on Terminated Longitudinal Voltage Pairs | Resistance to return rail (>1 ohm) and VL loop resistance checks |
| 10.3 | Test Equipment Specification for Longitudinal Voltage - Continuous | Minimum spec for the recorder used for continuous VL measurement |
| 10.4 | Test Equipment Specification for Longitudinal Voltage - Short Circuit | Minimum spec for recording VL during short-circuit tests |
| 11 | Transverse Voltage Test Method | Why VT is measured (speech clarity, digital interference) and the 300–3000 Hz weighting |
| 11.1 | Tests on terminated transverse voltage pairs | Resistance check on the 600 ohm terminated VT pair before measuring |
| 11.2 | Psophometric transverse voltage measurements | ITU-T O.41 psophometric weighting and how to apply it |
| 11.3 | Unweighted transverse voltage measurements | When an unweighted VT measurement is needed (trackside system compatibility evidence) |
| 11.4 | Test Equipment Specification for Transverse Voltage | Minimum spec for VT measurement equipment |
| 11.5 | Using Digital Post-Processing | Applying the weighting filter by post-processing a digitised recording |
| Appendix A | Telecommunications Commissioning Certificate | The certificate the DPE signs off after successful verification |
| Appendix B | Cable Test Sheet | The spreadsheet used to record pair tests and calculate derived values |