FTN design & development specification (D&D)
SP FTN (D&D) 006
Joint, Termination and Test Aluminium Screening Conductors
Issue 1.1, November 2006 · FTN D&D specs & typical drawings
How new aluminium screening conductors on the FTN are jointed, terminated, earthed and tested. Designers use it for earth-tap spacing, earth farm siting exclusions, the resistance-to-earth target and the continuity test method.
Clause map — what each clause deals with
| 1 | Introduction | Why screening conductors are needed in ac areas, when a new one is installed, assessing existing ones, and dc/non-electrified cases. |
| 2 | Scope | Jointing, terminating and testing of new screening conductors across the FTN. |
| 3 | Reference Documentation | Related FTN specs and BR documents on closures and immunisation. |
| 4 | Abbreviations | Short list of acronyms. |
| 5 | Glossary | Meaning of earth electrode, earth farm, earth tap, immunisation, screening conductor. |
| 6 | Jointing | Joints live in the route, supplied lengths and earth interval, straight-through vs earth-tap vs branch joints, shear-bolt connector kits and heatshrink. |
| 7 | Terminating | Terminators are shorter shear-bolt units that always carry an earth tap. |
| 8 | Earthing | Earth connection interval and at ends; earth wire type. |
| 8.1 | Existing Earth Farms | When an existing earth farm may be reused: proximity, exclusion distances, resistance target, 10 km averaging method (figure), adding electrodes. |
| 8.2 | New Earth Farms | Siting exclusions for new earth farms, no building earths, soil resistivity survey, expected electrode and rod counts, escalation. |
| 9 | Identification and Labelling | Labelling follows the FTN labelling spec. |
| 10 | Testing | Tests to declare a new screening conductor fit for immunisation. |
| 10.1 | Testing of New Screening Conductors | Basis of acceptance, avoiding noise or touch potential during tests, and test method approval. |
| 10.1.1 | Continuity Test | Step-by-step dc continuity test between earth taps using the tester in Appendix B, with the ac current check before disconnecting earths. |
| 10.1.2 | Earthing Resistance Test | Mean earthing resistance target over any 10 km of new conductor. |
| 11 | Submission of Test Results | Results submitted as for copper and fibre cable test results. |
| App A | Jointing Kits | BR catalogue numbers for through-joint, earth-tap and branching kits. |
| App B | Configuration of DC Continuity Tester | Circuit diagram of the battery, diode, resistor and contactor tester with measuring points. |
| App C | Screening Conductor Earthing Flowchart | Decision flowchart for reusing an existing earth farm, adding electrodes or installing a new one. |