Surge Protective Devices (SPDs) are critical for safeguarding electrical systems against transient overvoltages. Proper SPD selection ensures system reliability and equipment longevity.
This article explores SPD selection criteria based on IEC and IEEE standards, including formulas, tables, and real-world applications. Learn how to optimize SPD choice for various electrical environments.
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- Calculate SPD nominal discharge current for a 400 V system with 20 kA surge current.
- Determine maximum continuous operating voltage (Uc) for a 230 V AC supply.
- Estimate protection level (Up) for a Type 2 SPD in a commercial building.
- Find required SPD class for a data center with 10 kA surge current rating.
Comprehensive Tables for SPD Selection Parameters According to IEC and IEEE
| Parameter | IEC Standard Reference | Typical Values | Description |
|---|---|---|---|
| Maximum Continuous Operating Voltage (Uc) | IEC 61643-11 | 275 V, 320 V, 385 V, 440 V, 510 V | Maximum voltage SPD can continuously withstand without degradation |
| Nominal Discharge Current (In) | IEC 61643-11, IEEE C62.41 | 5 kA, 10 kA, 20 kA, 40 kA, 65 kA | Current level SPD can discharge repeatedly without failure |
| Maximum Discharge Current (Imax) | IEC 61643-11 | 10 kA, 20 kA, 40 kA, 100 kA, 125 kA | Maximum surge current SPD can withstand once |
| Voltage Protection Level (Up) | IEC 61643-11 | 600 V, 900 V, 1200 V, 1500 V | Voltage at SPD terminals during surge, must be below equipment withstand voltage |
| Response Time (tA) | IEC 61643-11 | < 25 ns | Time taken by SPD to respond to surge event |
| SPD Classes (Type 1, 2, 3) | IEC 61643-11, IEEE C62.41 | Type 1, Type 2, Type 3 | Classification based on location and surge current capability |
| SPD Class | IEC 61643-11 Definition | Typical Application | Nominal Discharge Current (In) |
|---|---|---|---|
| Type 1 | For direct lightning current (10/350 µs waveform) | Main distribution boards, outdoor installations | 10 kA to 100 kA |
| Type 2 | For switching and indirect lightning currents (8/20 µs waveform) | Sub-distribution boards, industrial and commercial buildings | 5 kA to 40 kA |
| Type 3 | For sensitive electronic equipment protection | Final distribution, near sensitive loads | 0.5 kA to 5 kA |
Essential Formulas for SPD Selection and Performance Evaluation
Understanding the key formulas involved in SPD selection is crucial for accurate design and protection. Below are the fundamental equations with detailed explanations.
1. Maximum Continuous Operating Voltage (Uc)
The Uc is the maximum RMS voltage the SPD can continuously withstand without degradation.
- Uc: Maximum continuous operating voltage (Volts RMS)
- Vrms_system: Nominal system voltage (Volts RMS)
Typically, Uc is selected to be equal or slightly higher than the system nominal voltage to avoid SPD damage during normal operation.
2. Nominal Discharge Current (In) and Maximum Discharge Current (Imax)
These parameters define the SPD’s ability to handle surge currents repeatedly (In) and withstand a single high-energy surge (Imax).
- In: Nominal discharge current (kA), typically 8/20 µs waveform
- Imax: Maximum discharge current (kA), single surge withstand capability
SPD selection requires In to be greater than the expected surge current in the installation environment, with Imax providing a safety margin.
3. Voltage Protection Level (Up)
Up is the voltage at the SPD terminals during a surge event and must be lower than the equipment’s withstand voltage.
- Up: Voltage protection level of SPD (Volts peak)
- Uw: Equipment withstand voltage (Volts peak)
Ensuring Up is less than Uw prevents damage to sensitive equipment during transient surges.
4. Coordination Between SPD and Equipment
To ensure proper coordination, the following inequality must hold:
- ΔV: Voltage drop in the wiring and system impedance (Volts)
This accounts for additional voltage stresses due to wiring inductance and system characteristics.
5. Surge Current Calculation Based on Lightning Protection Zone (LPZ)
According to IEC 62305, the expected surge current (I) can be estimated by:
- I: Expected surge current (kA)
- k: Coefficient depending on installation type (typically 0.6 to 1.0)
- A: Lightning current amplitude (kA), e.g., 10 kA to 200 kA
- C: Coupling factor based on distance and shielding
This formula helps estimate the surge current magnitude for SPD rating.
Real-World Application Examples of SPD Selection
Example 1: SPD Selection for a Commercial Building Distribution Board
A commercial building has a 400 V three-phase supply. The main distribution board requires SPD protection against indirect lightning surges. The expected surge current is 20 kA (8/20 µs waveform). The equipment withstand voltage is 1200 V peak.
- Step 1: Determine system nominal voltage: 400 V line-to-line RMS.
- Step 2: Select Uc ≥ system voltage × √2 (peak voltage). For 400 V RMS, peak voltage = 400 × 1.414 = 565 V. Choose Uc = 640 V (standard value).
- Step 3: Nominal discharge current In should be ≥ 20 kA. Select SPD with In = 20 kA.
- Step 4: Check voltage protection level Up. SPD Up = 900 V (typical for Type 2 SPD).
- Step 5: Verify Up ≤ Uw (1200 V). Since 900 V < 1200 V, protection is adequate.
- Step 6: Confirm SPD class: Type 2 SPD suitable for sub-distribution boards.
Result: A Type 2 SPD with Uc = 640 V, In = 20 kA, and Up = 900 V is selected for the main distribution board.
Example 2: SPD Selection for a Data Center Sensitive Equipment Panel
A data center operates on a 230 V single-phase supply. Sensitive IT equipment requires protection from surges with a withstand voltage of 800 V peak. The expected surge current is 5 kA.
- Step 1: System nominal voltage: 230 V RMS.
- Step 2: Calculate peak voltage: 230 × 1.414 = 325 V. Select Uc = 320 V (standard value).
- Step 3: Nominal discharge current In ≥ 5 kA. Choose SPD with In = 5 kA.
- Step 4: Voltage protection level Up should be less than 800 V. Select SPD with Up = 600 V.
- Step 5: SPD class: Type 3 SPD for final distribution near sensitive loads.
Result: A Type 3 SPD with Uc = 320 V, In = 5 kA, and Up = 600 V is ideal for protecting sensitive data center equipment.
Additional Technical Considerations for SPD Selection
- Coordination with Lightning Protection Zones (LPZ): IEC 62305 defines LPZs to guide SPD placement and class selection.
- SPD Location: Type 1 SPDs are installed at service entrance; Type 2 at distribution boards; Type 3 near sensitive equipment.
- SPD Energy Handling: Consider energy let-through ratings and thermal stability for high-energy surges.
- Response Time: Faster response SPDs reduce transient voltage stress on equipment.
- SPD Monitoring and Maintenance: Use SPDs with status indicators and plan periodic inspections per IEC 62305-4.
- Coordination with Other Protective Devices: Ensure SPDs do not interfere with upstream circuit breakers or fuses.
Authoritative References and Standards
- IEC 61643-11: Surge Protective Devices Connected to Low-Voltage Power Systems
- IEEE C62.41.2-2011: Recommended Practice on Surge Voltages in Low-Voltage AC Power Circuits
- IEC 62305: Protection Against Lightning
- NEMA: IEEE C62.41 Surge Environment Classification
By adhering to these standards and applying the formulas and selection criteria outlined, engineers can ensure optimal SPD performance and electrical system protection.