A corporate campus operating two administratively interconnected buildings experienced a complete loss of network connectivity between the sites. The inter-building link, carried over an underground OS2 single-mode fibre cross-connection, had gone dark with no prior warning - taking down all data, voice, and management traffic between the two buildings. One building was left effectively isolated from the core network infrastructure. The incident was classified as a Priority 1 outage. An experienced fibre and infrastructure engineer was engaged to diagnose the fault, locate the break, and restore the link with minimum possible downtime.
Full link outage - one building had completely lost connectivity, affecting all users and systems at that site. Business operations were severely impacted from the moment the fault occurred.
Unknown fault location - the fibre route ran through underground ducting between the two buildings, with the physical break location unknown at the point of engagement. The route passed through a shared duct with other low-voltage services, and recent groundworks in the area were identified as a likely cause.
Single-ended fault - with one termination point unreachable from a network perspective, remote diagnostic options were limited, placing full dependency on physical OTDR testing from the available end.
Armoured external cable - the fibre cable was an armoured, multi-core external-grade installation, requiring appropriate tooling and materials for field repair.
Time pressure - the client required restoration of at least a minimum number of serviceable fibre cores as quickly as possible to restore critical connectivity, with full restoration to follow.
Attended site and conducted an immediate review of available information: last-known working state of the link, any recent site activity (groundworks, construction, deliveries) near the cable route, and alarm data from the network monitoring platform.
Confirmed with the network team that the fault was physical - optical transceivers at the live end were reporting no light received on all affected cores, ruling out a configuration or equipment issue.
Retrieved the original as-built documentation for the fibre route, including the route drawing, cable schedule, splice records, and previous OTDR baseline traces taken at installation.
Identified that civil groundworks had been carried out in the area between the two buildings approximately 48 hours prior to the fault occurring - establishing a high-probability fault zone on the route.
Connected a calibrated OTDR (Optical Time Domain Reflectometer) to the fibre patch panel at the available (live) building end.
Set up the OTDR with the correct parameters for the installed OS2 single-mode cable - refractive index, pulse width, and range, and ran bidirectional traces on all affected fibre cores from the live end.
The OTDR traces returned a clear hard fault event on all cores, presenting as a high-loss reflective event (consistent with a physical break or crush) followed by a flat noise floor - confirming a complete cable break rather than a high-loss splice or bend issue.
Calculated the fault distance from the OTDR event cursor: 120m from the termination point at the live building end.
Cross-referenced the fault distance against the route drawing to identify the precise physical location on the surface above the cable route -pinpointing the fault to a section of the route that fell directly within the area of recent groundworks.
Documented the OTDR traces and fault distance calculation and shared findings with the client and civil team to confirm the dig location.
Coordinated with the client's facilities team to arrange safe excavation at the identified fault location, ensuring utility strike prevention procedures (CAT scan / hand dig protocol) were followed before any mechanical excavation commenced.
Upon excavation, the damaged cable was exposed - the armoured fibre cable had been mechanically crushed and severed by groundworks equipment, with visible armour deformation and sheath damage at the break point.
Photographed the damage in situ for inclusion in the incident report and client records.
Assessed the extent of the damage: the break was clean and localised to a short section of cable, confirming that a field splice repair using a re-entry splice kit and underground-rated fibre splice enclosure was the appropriate restoration method.
Cut back the damaged section of cable on both sides of the break point, exposing clean, undamaged fibre beyond the crush zone on each side.
Prepared both cable ends - stripping armour, outer sheath, and buffer tubes, cleaning and organising individual fibre cores by colour code, and installing the cable into the underground-rated re-entry splice enclosure (gel-filled or heat-shrink type rated for direct burial / underground installation).
Performed fusion splicing on all fibre cores using a core-alignment fusion splicer, verifying each splice loss on the splicer display in real time - re-splicing any core exceeding the 0.1 dB threshold before proceeding.
Protected all splices with heat-shrink splice protection sleeves and organised splice trays within the enclosure.
Sealed and closed the underground splice enclosure in accordance with the manufacturer's specification, ensuring full environmental protection against moisture ingress.
With the field splice complete and the enclosure secured, performed OTDR testing from both ends of the repaired link to verify:
The splice event was visible and within acceptable loss limits
No additional fault events were present on the link
End-to-end continuity was confirmed on all repaired cores
Performed bidirectional insertion loss (IL) measurement using an OLTS across all repaired cores, confirming the total link loss remained within the original certified link budget.
Confirmed with the network team at both ends that optical transceivers were receiving light and that the network link had restored successfully - connectivity between the two buildings was confirmed restored.
With the field splice complete and the enclosure secured, performed OTDR testing from both ends of the repaired link to verify:
The splice event was visible and within acceptable loss limits
No additional fault events were present on the link
End-to-end continuity was confirmed on all repaired cores
Performed bidirectional insertion loss (IL) measurement using an OLTS across all repaired cores, confirming the total link loss remained within the original certified link budget.
Confirmed with the network team at both ends that optical transceivers were receiving light and that the network link had restored successfully - connectivity between the two buildings was confirmed restored.
The inter-building fibre cross-connection was fully restored within 2 hours of the engineer attending site, returning all data, voice, and management traffic between the two buildings. Key results included:
Fault located precisely using OTDR distance measurement, enabling targeted excavation and avoiding unnecessary disruption to the wider cable route.
All fibre cores successfully repaired via field fusion splicing, with post-repair insertion loss measurements confirming full compliance with the original link budget.
Network connectivity fully restored - confirmed by the client's network team before site was left.
Root cause identified and documented - mechanical damage from third-party groundworks, providing the client with clear evidence for any cost recovery or contractor liability process.
Updated as-built records and OTDR baselines delivered, ensuring the permanent record accurately reflected the repaired link state.
Recommendations accepted by the client, including the introduction of a mandatory cable route notification procedure for all future groundworks on the campus.
External armoured OS2 single-mode loose-tube fibre optic cable
Bidirectional OTDR trace analysis and baseline comparison
Optical Loss Test Set (OLTS) - insertion loss measurement
Core-alignment fusion splicing - field repair conditions
Underground-rated re-entry fibre splice enclosure installation
Armoured external fibre cable handling and preparation
Fibre end-face inspection and cleaning
able route drawing interpretation and as-built update
Incident and repair report authoring
Civil coordination - safe excavation and cable reinstatement
Live outage fault management and client communication
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