Single-mode fiber has a very thin core, around 9 µm, and carries light in a single mode, so it reaches tens of kilometers; it is used between substations and in large switchyards. Multimode fiber has a 50 or 62.5 µm core and is more tolerant of low-cost connectors and transceivers, but its reach at high speeds is hundreds of meters. In a substation the choice depends on distance, speed, the device ports and expected growth.
Why fiber in a substation
A substation is one of the most hostile electromagnetic environments for a data network: transients from disconnector switching, fault currents, ground potential rise and strong fields near the power equipment. Optical fiber does not conduct current or pick up those disturbances, and it galvanically isolates the devices it connects.
That is why links between the switchyard and the control house, between panels and between substations are made in fiber. Copper is kept for short connections inside a single panel. The substation network engineering guideline IEC TR 61850-90-4 starts from that basis when it covers the physical medium. The practical question is which type of fiber to use.
The physical difference
A fiber has a core, where the light travels, surrounded by a cladding 125 µm in diameter.
- Single-mode. Very thin core, around 9 µm. Light travels in a single mode, almost in a straight line, with no dispersion between paths. It works in the 1310 and 1550 nm windows. The reference standard for standard single-mode fiber is ITU-T G.652.
- Multimode. Core of 50 µm or 62.5 µm. Light travels along many paths (modes) of different lengths, which arrive at different times and spread the pulse. That modal dispersion limits reach at high speeds. It works mainly at 850 nm and 1300 nm. It is classified in OM categories: OM1 is 62.5 µm and the later ones are 50 µm, with more bandwidth.
Telecommunication fibers and cables are specified in the IEC 60793 (fibers) and IEC 60794 (cables) series.
Distances and speeds
Reach depends on the combination of fiber and transceiver. For 1 Gbit/s Ethernet, IEEE 802.3 specifies:
| Interface | Fiber | Wavelength | Specified reach |
|---|---|---|---|
| 1000BASE-SX | Multimode 62.5 µm | 850 nm | About 275 m |
| 1000BASE-SX | Multimode 50 µm | 850 nm | About 550 m |
| 1000BASE-LX | Single-mode | 1310 nm | 5 km |
| 1000BASE-LX | Multimode 50 or 62.5 µm | 1310 nm | 550 m |
Manufacturers offer longer-reach single-mode transceivers, beyond the basic specification, which are chosen from the optical power budget of the link. At 100 Mbit/s, the speed of many IEDs, multimode reach is longer than at 1 Gbit/s; the exact figure comes from the port data sheet.
For teleprotection, the IEEE C37.94 interface used multimode fiber in its first edition for runs of about 2 km, and the 2017 edition added single-mode fiber for longer distances.
Connectors and optical power budget
In substations you mostly see LC connectors on switches and SFP transceivers, ST (BFOC/2.5) on older protection and teleprotection equipment, and SC on some fiber patch panels. Conversions between types are made with the proper patch cords, never by forcing adapters.
Each link has an optical power budget: the power delivered by the transmitter minus the receiver sensitivity. Broadly, that margin has to cover:
- Fiber attenuation per kilometer, higher in multimode than in single-mode.
- The loss of each connector and each splice.
- A margin for aging, dirty connectors and future repairs.
A link that works with a bare margin starts failing when a connector gets dirty or a splice is added. Measurement with an OTDR and with a light source and power meter is part of commissioning the communication network.
Which one goes on each run
- Does the link leave the substation or exceed a few hundred meters?
- YesSingle-mode, sized with the optical power budget of the link
- NoDo the devices at both ends have multimode ports, and does the planned speed allow it?
- YesMultimode is acceptable; leave spare single-mode fibers if growth is expected
- NoSingle-mode, or the type the device ports require
In practice:
- Between substations, over OPGW or ADSS, always single-mode. The multiplexer and fiber optic network is designed on it.
- In large high and extra-high voltage switchyards, where the distances between relay kiosks and the control house grow, and on the process bus with 1 Gbit/s links, single-mode avoids falling short.
- Inside the control house and in compact substations, multimode is still common, especially when the IEDs only have multimode ports.
Many projects choose single-mode across the whole switchyard so as not to mix types or limit future speeds, and use multimode only where the device requires it. What is always avoided is mixing types on the same link.
Common mistakes
- A multimode patch cord on a single-mode link, or the other way round: the link does not come up or has intermittent errors.
- Dirty connectors, which add loss and reflection.
- Excessive bend radii in trays and panels.
- Poor records of which fiber in the cable goes to which port, which turns every maintenance job into a search.
- Transceivers of different reach at each end, which saturate a receiver on short links.
- Poor fusion splices in the entry box, which only show up when the full link is measured with an OTDR.
We help with the design
Tiza Group does the communications engineering of medium, high and extra-high voltage substations and industrial plants: network architecture, fiber type per run, optical power budget and specification of switches and transceivers. Contact us.
Related guides
- Multiplexer and fiber optic network: OPGW, SDH and MPLS-TP
- Substation Ethernet switch
- IEEE C37.94: optical interface for teleprotection and 87L
- Media converters and serial device servers
- Substation communication network
- Direct or multiplexed fiber for line differential protection
- Substation communications by voltage level
- IEC 61850-3 and IEEE 1613: environmental and EMC requirements
References
- ITU-T G.652, Characteristics of a single-mode optical fibre and cable.
- IEC 60793 (optical fibers) and IEC 60794 (optical fiber cables).
- IEEE 802.3, Standard for Ethernet (1000BASE-SX and 1000BASE-LX interfaces).
- IEEE C37.94-2017, Standard for N Times 64 kbps Optical Fiber Interfaces between Teleprotection and Multiplexer Equipment.
- IEC TR 61850-90-4:2020, Network engineering guidelines.
Frequently asked questions
Can a single-mode port be connected to a multimode port?
Not directly. Core size, wavelength and transceiver power are different, and the link either does not work or works with unpredictable losses. Both ends and the cable must be of the same type, or a media converter is used.
What is the maximum distance of multimode fiber?
It depends on the speed and the fiber type. At 1 Gbit/s with 1000BASE-SX, IEEE 802.3 specifies about 275 m on 62.5 µm fiber and 550 m on 50 µm fiber. At lower speeds the reach is longer, and at higher speeds shorter.
Why use fiber rather than copper cable inside the substation?
Because fiber does not conduct current or pick up electromagnetic interference. The switchyard has switching transients, fault currents and ground potential differences that can damage copper ports or corrupt data. Copper is limited to short runs inside a single panel.