A short overview of the fibre optic cables used in wind farm SCADA networks: why they are dielectric, how they are built, and what to look for in a specification.
If you have worked on a wind farm, you know that alongside the medium voltage power cables running from each turbine to the substation there is always a smaller, lighter cable carrying the SCADA signals — the optical fibre. It is easy to overlook compared to the 30 kV cables, but without it the wind farm has no communication backbone, no remote monitoring, and no centralised control.
In this short post I want to go through the key characteristics of the optical fibre cables typically specified for wind farms, based on a standard BoP specification I worked with.
Why Dielectric?
The first thing you notice when you read an optical fibre specification for a wind farm is the insistence on having no metal parts in the cable. The cable uses a PE sheath with rodent protection made from glass fibre yarns — not the steel wire armour you might see in other industries.
The reason is lightning. Wind turbines are tall structures in open terrain, and they get struck by lightning regularly. If the communication cable between turbines contained any metallic component — a steel armour, a copper strength member, even a metallic moisture barrier — it could conduct lightning-induced currents along the entire cable route, potentially damaging the SCADA equipment in multiple turbines and in the substation. By keeping the cable entirely dielectric (non-metallic), you break that conduction path. The fibreglass braid provides the mechanical protection that steel armour would normally give, without the electrical conductivity.
This is one of those design choices that seems obvious once you know the reasoning, but I have seen specifications from other infrastructure sectors where metallic-armoured fibre cables are standard. In wind farms, dielectric is not optional — it is a fundamental requirement.
Two Cable Types: Unitube and Multitube
A typical wind farm specification defines two cable constructions depending on the fibre count needed.
The unitube cable is used for runs requiring up to 12 fibres. All the fibre strands sit inside a single central tube filled with jelly (a thixotropic gel that blocks water ingress). The construction is straightforward: fibre optic core, central jelly-filled tube, fibreglass reinforcements with waterblocking, a ripcord, an inner jacket of linear low density polyethylene (LLDPE), the dielectric armour of fibreglass braid, another ripcord, and the outer LLDPE jacket. It is a compact, easy-to-handle cable.
The multitube cable handles up to 48 fibres. The construction is similar in principle but adds a GRP (glass reinforced plastic) central strength member and distributes the fibres across multiple loose tubes rather than a single central tube. The loose tubes are also jelly-filled. The additional strength member is needed because the cable is physically larger and heavier, and it needs to maintain the same handling characteristics during installation.
Both cable types share the same mechanical requirements for installation: a tensile strength of 2,150 N during pulling (dropping to 1,200 N as the permanent allowable load once installed), a temperature operating range of −40°C to +60°C, and a kink radius limit of 20 times the cable diameter. The multitube cable has a slightly higher crush resistance (3,000 N versus 2,000 N for the unitube), reflecting its larger cross section.
One detail worth noting is the attenuation behaviour across the temperature range. The cables are expected to operate without any measurable attenuation variation (≤0.05 dB) between −30°C and +60°C. Outside that range, down to −40°C, a maximum variation of 0.1 dB/km is allowed. In practice this means the cable should perform identically in a Spanish summer and a Scandinavian winter, which is exactly what you need for a standardised specification that will be used across different markets.
The Fibre Itself: Single Mode G.652
The fibres inside the cable are single mode, matched cladding, optimised for transmission at 1310 nm with good performance at 1550 nm as well. They comply with ITU-T G.652.B — which is essentially the standard single mode fibre used in telecommunications worldwide. There is nothing exotic about it.
The key optical parameters are an attenuation of ≤0.4 dB/km at 1310 nm and ≤0.25 dB/km at 1550 nm. For a wind farm where the longest cable run might be 15–20 km from the furthest turbine to the substation (through the daisy-chain or ring topology), these attenuation values give enormous margin. The cladding diameter is the standard 125 µm, and the mode field diameter is 9.2 µm at 1310 nm.
A parameter that matters for longer links and higher bandwidth applications is the polarisation mode dispersion (PMD), specified at ≤0.5 ps/√km when cabled and ≤0.2 ps/√km as a link design value. For a SCADA network running at relatively modest data rates this is more than sufficient, but it also means the infrastructure could support higher-bandwidth applications in the future if needed.
Standards and Marking
The cables are tested according to EN 187000 and CEI 60794 (the European and IEC standards for optical fibre cables), and the fibres themselves meet IEC 60793-2-50 Category B.1.1.
The colour coding follows the standard 12-colour sequence: red, green, yellow, blue, white, violet, orange, black, grey, brown, pink, and turquoise, with a second set using striped variants of the same colours for cables with more than 12 fibres. The cable marking includes the manufacturer, year, fibre count, fibre type, a traceability code, and metre markers — all the information you need to identify a cable in the field years after installation.
How Many Fibres?
A question that comes up in every project is how many fibres to specify. The SCADA system itself typically needs only 2 fibres per link (one transmit, one receive). But standard practice is to install more — a 12-fibre unitube between turbines in a string, and a 24 or 48-fibre multitube on the backbone runs back to the substation. The extra fibres cost very little compared to the trenching and are there for redundancy, future SCADA upgrades, condition monitoring systems, or even leasing to third parties for telecommunications.
In my experience, you never regret having spare fibres. You frequently regret not having them.
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