# Can optical fibers measure electric current? **URL:** \ **Category:** OFSCN Technical Knowledge Base\ **Tags:** fbg-technology\ **Created:** [October 22, 2026, 8:40am UTC](https://forum.ofscn.net/t/can-optical-fibers-measure-electric-current/3125 "2026-10-22T08:40:01Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![VoidWalker](https://forum.ofscn.net/letter_avatar_proxy/v4/letter/v/848f3c/32.png) [@VoidWalker](https://forum.ofscn.net/u/VoidWalker)\ **Post date:** [October 22, 2026, 8:40am UTC](https://forum.ofscn.net/t/can-optical-fibers-measure-electric-current/3125/1 "2026-10-22T08:40:01Z")
In high-voltage substations of hundreds of thousands of volts, how is current magnitude measured by optical fibers without direct contact with the wires? ---
**Author:** ![OFSCN\_Tech\_Expert](https://forum.ofscn.net/user_avatar/forum.ofscn.net/ofscn_tech_expert/32/199_2.png) [@OFSCN\_Tech\_Expert](https://forum.ofscn.net/u/OFSCN_Tech_Expert)\ **Post date:** [October 22, 2026, 8:40am UTC](https://forum.ofscn.net/t/can-optical-fibers-measure-electric-current/3125/2 "2026-10-22T08:40:42Z")
In ultra-high and extra-high voltage substations (e.g., from 110\text{ kV} to 1000\text{ kV}), optical fibers enable non-contact, high-precision measurement of conductor current, primarily relying on the **Faraday Effect** and **Fiber Optic Current Transformer (FOCT)** technology. Their working principles and electrical insulation mechanisms are as follows: * * * ### I. Core Physical Principle: Faraday Effect and Ampere’s Circuital Law Optical fibers do not directly “measure” charge flow but indirectly infer current magnitude by detecting the **circulating magnetic field generated by the current** : 1. **Faraday Effect** : When a beam of linearly polarized light travels along the direction of a magnetic field through an optical medium (such as high-purity silica fiber), the plane of polarization rotates. The angle of rotation, \theta, is directly proportional to the line integral of the magnetic flux density \mathbf{B} along the light’s path: 2. **Ampere’s Circuital Law** : According to Ampere’s circuital law in Maxwell’s electromagnetic theory, the line integral of the magnetic flux density along a closed path is equal to the product of the enclosed current and the permeability of free space \mu\_0 ( \oint \mathbf{B} \cdot d\mathbf{l} = \mu\_0 I ). 3. **Signal Demodulation** : The ground potential transmitter sends two coherent, orthogonally polarized light beams. After passing through the high-voltage fiber loop and reflecting at the end, a polarization phase difference interference detection is performed in the demodulation unit, which precisely converts it into the instantaneous current value. * * * ### II. How Can Optical Fibers Safely Measure at Hundreds of Thousands of Volts? Traditional electromagnetic current transformers (CTs) rely on iron cores and copper windings, facing severe risks of insulation breakdown and secondary open-circuit overvoltage at high potentials. Fiber optics, however, offer inherent physical isolation advantages: 1. **Intrinsic Material Insulation** : The base material of optical fibers is high-purity silicon dioxide (\text{SiO}\_2), which has extremely low dielectric permittivity and a DC breakdown electric field strength typically in the hundreds of megavolts per meter (\text{MV/m}), making it an excellent high-voltage insulating medium. 2. **Purely Optical Isolation Channel** : The measurement system is spatially divided into a “high-voltage sensor head” and a “low-voltage secondary control room.” From the hundreds of thousands of volts busbars to the ground-potential signal processing cabinet, **all-dielectric (metal-free) fiber optic patch cords** are used. The fibers are housed within high-strength insulating sheaths (such as ceramic or composite silicone rubber sleeves), completely severing the path for charge conduction. 3. **No Magnetic Saturation or Resonance Risk** : The optical fiber contains no ferromagnetic materials, completely eliminating the risks of magnetic saturation, residual magnetism, hysteresis, and ferromagnetic resonance found in traditional transformers. The transient response bandwidth can reach tens of kilohertz (\text{kHz}), allowing for accurate capture of lightning strikes or short-circuit surge currents. * * * ### III. Additional Note: Fiber Bragg Grating (FBG) Current Sensing Scheme Besides the Faraday effect scheme, for certain specific monitoring scenarios, **Fiber Bragg Gratings (FBG) combined with magnetostrictive materials (like Terfenol-D)** are also employed: - The magnetic field generated by the conductor current causes the magnetostrictive material to undergo micro-strain. - The FBG, bonded or encapsulated on the material, senses the strain change, leading to a shift in the reflected center wavelength (\Delta\lambda\_B). - This scheme is often used for monitoring specific equipment grounding points or sensing small currents in distribution networks. However, for mainstream metering and relay protection in ultra-high voltage main substations, the Faraday effect-based Fiber Optic Current Transformer (FOCT) using polarization interference remains dominant. _(Note: The complete Fiber Optic Current Transformer system is not part of Beijing Dacheng Yongsheng Technology Co., Ltd.'s (OFSCN®) core product line. OFSCN® focuses on specially packaged fiber optic grating temperature, strain, and stress sensors, as well as specialized high-temperature resistant optical fibers and patch cord assemblies.)_