How can the optical cable splice box of the power system ensure communication security through electromagnetic shielding design?

Challenges of the electromagnetic environment of the power system to the optical cable splice closure
The electromagnetic interference sources in the power system are mainly divided into two categories:
Strong electric field interference
The power frequency electric field (50Hz) and transient overvoltage (kV level) generated by high-voltage transmission lines and substation equipment may enter the optical fiber communication system through capacitive coupling.
Example: In a 500kV substation, the unshielded optical cable caused communication interruption when struck by lightning. Fault location showed that the interference source was the arc discharge generated by the circuit breaker operation.
Strong magnetic field interference
The transient magnetic field (T level) caused by short-circuit current, geomagnetic storm, etc. generates induced current in the optical fiber through magnetic coupling, causing the bit error rate to increase.

Attenuation principle of metal braided mesh shielding layer
1. Multi-layer shielding mechanism
The metal braided mesh built into the junction box adopts a double-layer structure:
Outer high-density copper mesh: pore size ≤ 0.2mm, braiding density ≥ 95%, providing primary electromagnetic shielding;
Inner aluminum foil layer: thickness ≥ 0.1mm, reflecting residual electromagnetic waves and preventing static electricity accumulation.

2. Electromagnetic field attenuation path
When external electromagnetic waves (such as industrial frequency electric fields) penetrate the outer shell of the junction box, the metal braided mesh achieves attenuation through the following mechanisms:
Reflection loss: high-frequency electromagnetic waves are reflected on the metal surface, and the attenuation coefficient is positively correlated with the conductivity of the material;
Absorption loss: the porous structure of the braided mesh causes electromagnetic waves to reflect multiple times between the pores, and the energy is gradually dissipated;
Eddy current loss: the magnetic field generates induced current in the metal layer, consuming electromagnetic energy through Joule heat.

3. Verification of attenuation performance
Laboratory tests show that the design can attenuate the 50Hz electric field strength from 10⁰ V/m to below 10⁻³ V/m, meeting the Class A standard of GB/T 17626.3-2016 "Electromagnetic compatibility test and measurement technology radio frequency electromagnetic field radiation immunity test".

Innovative design of modular grounding system
1. Independent grounding architecture
The shielding layer of the traditional junction box and the metal reinforcement core of the optical cable share the grounding terminal, which is easy to cause:
Ground potential counterattack: the potential difference between different grounding points generates a loop in the shielding layer;
Grounding resistance superposition: multiple grounding loops increase the system impedance.
The modular design solves this problem in the following ways:
Independent grounding of the shielding layer: using a dedicated grounding terminal, which is physically isolated from the metal layer of the optical cable;
Grounding resistance optimization: built-in low-impedance grounding module to ensure that the grounding resistance is ≤1Ω.

2. Quick connection technology
To simplify on-site installation, the junction box adopts the following innovations:
Pre-crimped terminals: the shielding layer and the grounding module are connected through spring contacts, without welding;
Visual grounding indication: the grounding status is displayed through LED lights to reduce the risk of human error.

3. Compatibility design
The modular system needs to be compatible with the grounding requirements of different types of optical cables (such as OPGW, ADSS) through:
Replaceable grounding module: adapt to metal reinforcement cores of different wire diameters;
Grounding path selection: supports direct grounding and grounding through lightning arresters.

Technical advantages and application scenarios
1. Performance advantages
High shielding effectiveness: metal braided mesh + aluminum foil double-layer shielding, 20dB higher than the traditional galvanized steel plate shielding effectiveness;
Low maintenance cost: modular design reduces the fault repair time of the grounding system from 4 hours to 30 minutes;
Environmental adaptability: the combination of non-metallic shell and shielding layer avoids secondary electromagnetic interference caused by the metal shell.

2. Typical applications
UHV transmission lines: In ±800kV DC lines, the shielding effectiveness of the junction box has passed the lightning impulse test (1.2/50μs waveform, 100kA peak current);
Smart substation: Meet the requirements of IEC 61850 standard for communication link immunity and ensure the reliability of GOOSE message transmission;
New energy grid connection: In wind farms and photovoltaic power stations, effectively isolate the harmonic interference generated by the inverter.

Technology evolution trend
1. Intelligent monitoring
Online monitoring of grounding resistance: Real-time feedback of grounding status through voltage divider resistors;
Electromagnetic field strength warning: Built-in Hall sensor, triggering an alarm when the external field strength exceeds the limit.

2. Miniaturization and integration
Volume reduction: The foldable shielding layer design is adopted to reduce the volume of the junction box to 1/2 of the traditional model;
Functional integration: Integrate the shielding layer with the optical splitter to reduce the device nodes.

3. Application of environmentally friendly materials
Recyclable metal mesh: woven with tinned copper wire, which is convenient for material recycling after retirement;
Lead-free grounding module: complies with RoHS directive requirements and reduces environmental impact.