Guía de puesta a tierra para dispositivos de protección contra sobretensiones: Cómo garantizar una disipación eficaz de la energía de las sobretensiones

Guía de puesta a tierra para dispositivos de protección contra sobretensiones: Cómo garantizar una disipación eficaz de la energía de las sobretensiones

 

Los dispositivos de protección contra sobretensiones (SPD) son componentes esenciales para proteger los sistemas eléctricos y electrónicos de sobretensiones transitorias. Sin embargo, la eficacia de un SPD depende fundamentalmente de un sistema de conexión a tierra y unión bien diseñado e implementado. Esta guía describe los principios, requisitos y prácticas recomendadas esenciales para la conexión a tierra, específicamente en relación con la instalación de SPD, centrándose en crear una ruta de baja impedancia para la disipación de la corriente de sobretensión, minimizar las diferencias de tensión y garantizar la seguridad del sistema.

Introducción

Las sobretensiones transitorias, originadas por rayos (directos o indirectos) u operaciones de conmutación en la red eléctrica, pueden inducir corrientes destructivas en las instalaciones eléctricas. Los DPS actúan como protectores de seguridad, desviando estas sobretensiones de los equipos sensibles. El factor crítico para el éxito de un DPS es la conexión a tierra. Sin una conexión a tierra de baja impedancia, la energía desviada no puede disiparse de forma segura, lo que podría provocar fallos en el DPS, daños en los equipos o tensiones de contacto peligrosas. Esta guía detalla los requisitos de conexión a tierra esenciales para el funcionamiento eficaz de los DPS.

 

El papel de la conexión a tierra en la funcionalidad del SPD 

Un SPD funciona cambiando rápidamente su impedancia en respuesta a una sobretensión:

1. Limitación de voltaje: limita el voltaje a un nivel de protección (arriba) aceptable para el equipo aguas abajo.

2. Desviación de corriente: proporciona una ruta de baja impedancia para que la corriente de sobretensión masiva (Iimp, In) fluya lejos del circuito protegido.

3. Disipación de energía: La corriente de sobretensión debe conducirse de forma segura hacia la masa de tierra a través del sistema de puesta a tierra.

 

Las funciones principales del sistema de puesta a tierra para los SPD son:

Proporcionar una ruta de baja impedancia: minimice la resistencia y la inductancia en la ruta desde el SPD hasta el electrodo de tierra para permitir un flujo de corriente de sobretensión rápido con un aumento de voltaje mínimo.

Establecer un potencial de referencia: cree un punto de referencia estable (potencial de tierra) para que el SPD controle los voltajes de manera efectiva.

Evite diferencias de potencial peligrosas: asegúrese de que todas las partes conductoras (gabinetes de equipos, carcasas de SPD, estructuras metálicas) estén conectadas entre sí y a tierra, minimizando el riesgo de voltajes de contacto peligrosos durante un evento de sobretensión.

Disipar energía de forma segura: Permita que la energía de la sobrecarga se disperse sin causar daño en el suelo.

Principios fundamentales de puesta a tierra para dispositivos de protección contra sobretensiones (SPD)

1. La baja impedancia es fundamental: La combinación de resistencia e inductancia (impedancia) de la conexión a tierra del DPS debe ser lo más baja posible. Una impedancia alta provoca una caída de tensión significativa (V = I *Z) durante una sobretensión, lo que eleva la tensión detectada por el equipo protegido y puede superar el nivel de protección del DPS (Up).

Short and Direct Paths: Earth conductors connecting the SPD must be as short and straight as possible. Avoid sharp bends and loops, as inductance increases with conductor length and loop area. Inductance is the dominant factor for fast-rising surge currents.

2.Adequate Conductor Size:Conductors must have sufficient cross-sectional area (CSA) to handle the maximum prospective surge current (Iimp or In as per the SPD classification and risk assessment) without fusing or causing excessive heating. Minimum requirements are typically defined in standards (e.g., IEC 62305, IEC 60364-4-44, NFPA 70/NEC) but often start at 6mm² (10 AWG) copper for Type 2 SPDs, increasing significantly for Type 1 SPDs (16mm² / 6 AWG or larger). Always consult SPD manufacturer specifications and relevant standards.

Minimizing Earth Loop Area:Connecting SPDs at different points (e.g., service entrance, sub-distribution) to different earth electrodes or long, separate earth paths can create large earth loops. Magnetic fields induced by surge currents flowing in these loops can couple voltages into sensitive data or control cables. Equipotential Bonding is the solution (see below).

3.Bonding is Integral: Earthing SPDs effectively requires Equipotential Bonding. This means connecting:

The SPD's earth terminal directly to the main earthing terminal (MET) or earthing busbar.

The MET to the earth electrode system.

All extraneous conductive parts entering the building (metal pipes, structural steel, cable sheaths, etc.) to the MET.

All exposed conductive parts of electrical equipment (enclosures) to the Protective Earth (PE) conductor, which is also connected to the MET.

The goal is to keep all metalwork at, or very near, the same potential during a surge event, preventing dangerous sparks or voltages between them.

 

Earthing System Components & SPD Connections

Earth Electrode System:The interface with the ground mass (earth rods, plates, meshes, foundations). Must have low earth resistance (target values depend on local regulations and risk, but often <10 Ohms is desirable, though lower impedance is more critical than ultra-low resistance alone for fast transients). Regular testing is essential.

Main Earthing Terminal (MET):The central point where the earth electrode conductor, protective conductors (PE), bonding conductors, and SPD earth conductors converge. This is the crucial hub for SPD earthing.

SPD Earth Conductor:

Must be a dedicated, insulated copper conductor (usually green/yellow).

Connected directly from the SPD's designated earth terminal to the MET or a dedicated SPD earthing busbar directly connected to the MET with a very short link.

Avoid: Daisy-chaining SPD earth connections or connecting them only to local equipment earth points that may have a longer, higher impedance path back to the MET.

Connection Method: Use reliable, corrosion-resistant methods (e.g., compression lugs, exothermic welding, approved clamping connectors). Ensure clean, tight connections with good metal-to-metal contact. Paint or corrosion must be removed at connection points.

 

Specific Considerations for Different SPD Types/Locations

Type 1 (Coordinated) SPDs (Service Entrance):Handle the largest surge currents (direct lightning partial currents). Require the most robust earthing:

Largest conductor size (often ≥16mm² / 6 AWG, potentially much larger).

Shortest possible path to MET/earth electrode (<0.5m ideal, <1m strongly recommended).

Direct connection to MET is mandatory. Bonding of incoming metallic services (power, telecom, water, gas) at this point is critical for equipotentiality.

Type 2 (Distributed) SPDs (Sub-distribution Boards): Handle induced surges. Earthing remains critical:

Conductor size typically 6-10mm² (10-8 AWG) minimum.

Connection directly to the local distribution board's earth bar, which must have a low-impedance connection back to the MET via the main PE conductor.

Ensure the PE conductor between boards is adequately sized.

Type 3 (Point of Use) SPDs: Mounted close to equipment. Usually plug-in or socket types. Rely on the fixed wiring's PE conductor. Emphasizes the importance of correct overall installation grounding/bonding.

Data/Communication Line SPDs:Require careful earthing relative to the power system SPDs to avoid creating ground loops. Best practice is to earth them to the same reference point (e.g., MET or local bonded earth bar) as the power SPD protecting the equipment they serve. Use shielded data cables with shields bonded at both ends where possible.

 

Common Earthing Pitfalls & Solutions

1.Long Earth Leads:

Problem:High inductance causes high voltage drop.

Solution: Mount SPD close to MET/earth bar; use shortest possible straight conductor.

2.Daisy-Chaining Earths:

Problem:Shared path increases impedance for downstream SPDs; failure of one connection compromises others.

Solution:Use "star" earthing – individual SPD earth leads to a common point (MET or SPD busbar).

3.Inadequate Conductor Size:

Problem:Conductor can fuse open or act as a fuse itself during large surge, rendering SPD useless.

Solution:Calculate/select based on SPD rating and standards; always err on larger size.

4.Poor Connections (Loose, Corroded):

Problem:High resistance/contact failure.

Solution:Use proper connectors, clean contact surfaces, tighten securely, protect from corrosion.

5.Separate/Isolated Earth Electrodes for SPDs:

Problem:Creates large potential differences between power system earth and SPD earth during a surge.

Solution:All SPDs must connect to the single, common earthing system (MET) of the structure.

6.Ignoring Bonding:

Problem:Dangerous touch voltages between earthed SPD and nearby bonded metalwork during a surge.

Solution:Implement comprehensive equipotential bonding as per standards.

 

Verification and Maintenance

Initial Verification:

Measure earth electrode resistance (Fall-of-Potential method).

Measure continuity of all earth and bonding conductors (low-resistance ohmmeter).

Visually inspect conductor sizes, routing (short/straight), connection quality, and labeling.

Periodic Maintenance:

Regular visual inspection of SPD status indicators and connections.

Retest earth electrode resistance periodically (especially in dry/corrosive soils) and after major modifications.

Check continuity of critical bonding connections.

Follow SPD manufacturer's recommended replacement schedule (SPDs degrade with use).

 

Standards and References  

Compliance with relevant national and international standards is essential. Key standards include:

IEC 62305 (Protection against Lightning - Parts 1-4): Comprehensive lightning protection, including SPD earthing/bonding (LPZ concepts).

IEC 60364 (Electrical Installations of Buildings): Especially Part 4-44 (Protection against voltage disturbances and electromagnetic disturbances) and Part 5-54 (Earthing arrangements and protective conductors).

IEC 61643 (Low-voltage surge protective devices): Series covering SPD requirements and application.

IEEE 142 (Recommended Practice for Grounding of Industrial and Commercial Power Systems) - "Green Book".

IEEE 1100 (Recommended Practice for Powering and Grounding Electronic Equipment) - "Emerald Book".

National Electrical Codes (e.g., NFPA 70/NEC Article 250, 285, 800, 810; BS 7671 Section 443, 534).