Electrical systems are constantly exposed to transient overvoltages caused by lightning strikes, switching operations, and grid disturbances. Without proper protection, these surges can damage sensitive equipment, reduce system reliability, and increase maintenance costs. To address this, Surge Protective Devices (SPDs) are widely used in power systems.
However, the selection and installation of an SPD depend on the earthing (grounding) system—most commonly TNS, TN-C, and TT. Each earthing system requires a different SPD configuration to ensure maximum protection. This article explains the working principle of SPDs, their applications in TNS, TN-C, and TT systems, and provides a selection guide for choosing the right SPD.
A Surge Protective Device (SPD) is a device designed to protect electrical systems from transient overvoltages by limiting surge voltage and diverting surge currents safely to earth.
SPDs operate by creating a low-resistance path to ground when voltage exceeds a certain threshold. This prevents damaging surges from reaching sensitive loads, such as lighting systems, control circuits, and electronic devices.
Type 1 SPD: Installed at the origin of the installation, capable of handling direct lightning currents.
Type 2 SPD: Installed in sub-distribution boards, provides protection against switching surges and indirect lightning.
Type 3 SPD: Point-of-use protection for sensitive equipment.
TNS system: Neutral and protective earth are separate throughout the system.
TN-C system: Neutral and protective functions are combined in a single conductor (PEN).
TT system: Protective earth is provided by a local ground electrode at the consumer’s installation.
In TNS systems, SPDs connect between phase, neutral, and earth conductors.
In TN-C systems, special care is needed as neutral and earth are combined.
In TT systems, SPDs must be paired with proper earthing and residual current devices (RCDs) to ensure safety.
Different grounding arrangements influence how surge currents are discharged and determine the SPD connection scheme, voltage protection level, and grounding method.
Widely used in residential, commercial, and industrial networks due to its safety and reliability.
SPD provides protection by clamping surges between phase-to-neutral and phase-to-earth.
Connect SPD between each phase and neutral.
Connect SPD between neutral and earth.
Ensure short cable lengths for minimal voltage drop.
Dedicated grounding conductor ensures safe diversion of surge currents into the earth.
Since neutral and earth are combined (PEN), installation requires special care to avoid safety issues.
SPD is installed between phase and PEN conductor.
No separate neutral-to-earth protection path.
Use Type 1 SPDs at the main distribution board.
Ensure PEN continuity is reliable.
Higher risk of neutral faults.
Additional protective devices (e.g., RCDs) may be required downstream.
Common in rural areas and installations with independent earth electrodes.
SPD discharges surges to the local earth electrode. Residual current devices are essential for fault clearance.
Install SPD between phase and neutral.
Install SPD between neutral and earth.
Ensure earth resistance is within standard limits (usually <10 Ω).
Because of separate earth electrodes, proper coordination between SPD and RCDs is critical.
Identify the earthing system before selecting SPD.
Choose SPD type (Type 1, 2, or 3) based on installation point.
Ensure compliance with IEC 61643-11 standard.
| Earthing System | SPD Connection Scheme | Grounding Considerations |
| TNS | Phase → N, Phase → PE, N → PE | Reliable grounding conductor |
| TN-C | Phase → PEN | Ensure PEN integrity |
| TT | Phase → N, N → PE | Local earth electrode + RCD |
This helps electricians quickly match SPD models with earthing systems to avoid misinstallation.
TNS: Dedicated PE conductor.
TN-C: Shared PEN conductor.
TT: Local earth electrode with low resistance.
Keep SPD connecting leads short (<0.5m ideally).
Verify earthing resistance before commissioning.
Use appropriate SPD type according to system exposure.
Incorrect SPD wiring between conductors.
Poor grounding leading to ineffective surge protection.
Using the wrong SPD type for the earthing system.
Q1: Why is SPD selection different for TNS, TN-C, and TT systems?
Because each earthing system has unique grounding arrangements, requiring different SPD configurations.
Q2: Can the same SPD be used in all systems?
No, SPD wiring and grounding must match the specific earthing system.
Q3: What type of SPD should be used for residential buildings?
Type 2 SPDs are most common, but the choice also depends on whether the system is TNS, TN-C, or TT.
Q4: What happens if SPD is installed incorrectly?
Incorrect installation may result in failed surge protection or safety hazards.
Q5: How often should SPDs be inspected?
Regular inspection during scheduled maintenance is recommended, especially after severe storms.
Surge Protective Devices are vital for safeguarding electrical systems against transient overvoltages. But their effectiveness depends heavily on correct selection and installation according to the earthing system (TNS, TN-C, TT).
In TNS systems, SPDs provide straightforward protection with dedicated grounding.
In TN-C systems, SPD installation is more complex due to the shared PEN conductor.
In TT systems, SPDs require proper earth resistance and coordination with RCDs.
By following best practices in SPD installation and grounding, electrical systems can achieve reliable surge protection, ensuring safety, reducing equipment damage, and extending system lifespan.
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