Cable Manufacturer Case Study: Fire-Performance Cable for a Metro

A metro extension needed cable for tunnels where the evacuation route is the tunnel itself. That changes the specification: the cable has to resist flame spread in a bundle, it has to avoid releasing corrosive halogens, it has to keep visibility when it burns, and the circuits that feed emergency systems have to keep working. We produced the batch in three calls against the tunnel programme, with each drum marked to its pulling section. No drum had to be re-cut on site.

Project Snapshot

The Problem: Four Different Tests and One Cable Route

A tunnel cable specification is not one requirement, it is four, and they pull in different directions.

Flame spread is the first. In a tunnel, cables are installed in bunched trays rather than as single runs, so the test that matters is not how one cable behaves but how a group behaves when the bottom of the bundle is ignited. A sheath that resists a single flame can still carry fire along a full tray.

Halogen content is the second. When halogenated insulation burns it releases acidic gas, and in a confined tunnel that gas attacks equipment and makes the space harder to enter. A low smoke zero halogen construction answers this, and it usually means changing the compound rather than the copper.

Smoke density is the third. Escape in a tunnel depends on visibility, and a cable that meets the flame test while producing dense smoke fails the actual purpose of the specification.

Circuit integrity is the fourth, and it is the one contractors underestimate. Emergency lighting, ventilation and communication circuits have to keep working for a defined period while a fire is in progress, which is a separate test from any of the three above. Cable that has passed the flame test will still go open circuit when the insulation fails.

Why We Chose a Certified Cable Manufacturer Over Regional Assemblers and Split Supply

Regional assemblers. Excluded. They can supply a cable that carries the right current, but the evidence pack for the fire performance is where these projects are won or lost, and an assembler cannot produce a test report for a construction it does not control.

Split supply across several factories. Excluded. Flame performance is a property of the whole construction, and mixing sheaths from different lines makes the batch impossible to defend at inspection.

One certified manufacturer with an in-house laboratory. Excluded the three options above. The construction is controlled end to end, every batch is tested and can be documented, and drum lengths can be set to the contractor's schedule.

The Cable We Supplied

The power circuits are XLPE insulated and steel tape armoured, armoured electrical cable built for pulling and for the mechanical abuse a tunnel site delivers before the cable is ever energised. The 12/20 kV sections follow the same construction logic as our higher voltage XLPE power cable, with the insulation thickness and screen set by the voltage class.

The control circuits use a control cable construction with the same halogen-free sheath family, so the whole installation speaks one language at inspection.

The sheath is low smoke zero halogen across both types. That single decision sets the compound, the extrusion process and the flame performance together, which is why it has to be made before production rather than after.

Installation: How the Work Ran

Delivery was staged to the tunnel programme in three calls, because storage at the portal is limited and cable that sits in the open is cable that gets damaged.

Each call matched an excavation section, so the drums arrived as the tunnel was ready for them and not before. Drum markings carried the section number, the length and the drum identity, which let the pulling crew confirm at a glance that the right drum had reached the right shaft.

The same staging approach was used on a 33 kV medium voltage delivery we supplied for a substation project.

Testing and Quality Control

Because the fire performance is a construction property, the testing starts before extrusion. Incoming compounds are checked, and the sheath compound is verified against the specification for each production run.

The client's own record, as published, is that every batch is inspected rather than sampled, and the group holds a CNAS-accredited laboratory on site, so batch documentation is produced in-house. For this project the contractor received batch certificates with each call, and the drums were inspected against the pulling schedule on arrival.

Results: What the Numbers Show

The full batch was delivered across three calls inside the tunnel programme, with no drum rejected on arrival and no length re-cut on site. The contractor's pulling sections matched the drum schedule, which removed a joint per section from the original plan.

Documentation was the difference. Each call came with batch certificates and drum identities that matched the schedule, so the contractor closed out the inspection file without chasing the factory. The published commercial terms behind the order are a 300 m minimum for power cable, an 18 working day lead time, and FOB or CIF shipment, which is what makes staged delivery practical for a project of this size.

What This Case Means for Your Project

On a tunnel or metro job, the cable is chosen for how it behaves in a fire and how well its performance can be evidenced. Current-carrying capacity is the easy part of the specification, and documentation is what separates a supplier from a risk.

If your project specifies fire performance, low smoke halogen-free construction or circuit integrity, send us the route, the voltage class, the pulling sections and the test regime. Our team will propose the construction, confirm the batch testing that supports it, and quote with the delivery staged to your programme. You can also discuss the specification with our engineering team before the cable schedule is issued.