Copper Trunks
The multi-core copper cables that carry analogue CT/VT secondary signals and DC trip/close commands from the switchyard to the control building — the legacy wiring that the IEC 61850 process bus replaces.
Also: copper trunking, control cables, secondary wiring
Copper trunks are the bundles of multi-core copper cables that connect the field equipment in a substation switchyard — instrument transformers, circuit breakers, disconnectors — to the protection and control panels in the central control building.
What they carry
Copper trunks carry analogue signals, not digital protocols:
- CT secondary currents (1 A or 5 A AC) — scaled replicas of the primary power-system current, one pair per phase per CT.
- VT secondary voltages (typically 110 V AC) — scaled replicas of the primary voltage.
- DC trip and close commands (48 V, 110 V, or 125 V DC) — discrete wired circuits that fire the circuit breaker’s operating mechanism.
- Alarm and status contacts — binary signals indicating breaker position, gas pressure, temperature limits.
Each bay can have dozens of individual copper pairs. A large transmission substation with forty bays may have thousands of individual conductors running between the yard and the control room, often through underground cable trenches.
Why they matter
Copper trunks are the reason substation secondary-system upgrades have historically been expensive and disruptive. Replacing relays means re-terminating the copper at both ends. Extending or reconfiguring a bay means pulling new cable through congested trenches. The physical wiring is often the constraint on project timelines, not the relay hardware.
What replaces them
The IEC 61850 process bus replaces copper trunks with a fibre-optic Ethernet network. A stand-alone merging unit (SAMU) mounts at the existing CT and VT terminals in the yard, digitises the analogue signals at 4 kHz, and publishes them as Sampled Values over fibre. Trip and close commands travel as GOOSE messages over the same fibre network.
The key advantage of the merging-unit retrofit is that the copper trunks do not need to be ripped out in one operation. The SAMU connects to the existing CT/VT secondary terminals — the same terminals the copper was connected to — and the fibre runs alongside or replaces the copper incrementally. The primary plant and the instrument transformers stay untouched.