Surge Protector (TVSS) for Equipment Protection Calculator – IEEE, IEC

Surge protectors, also known as Transient Voltage Surge Suppressors (TVSS), safeguard sensitive equipment from voltage spikes. Calculating the correct surge protector rating is critical for effective equipment protection.

This article explores IEEE and IEC standards for surge protector calculations, providing formulas, tables, and real-world examples. Learn how to select and size TVSS devices accurately for your electrical systems.

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• Calculate required surge current rating for a 3-phase 400V system with 100kA prospective short circuit current.
• Determine maximum let-through voltage for a TVSS device protecting a 230V single-phase load.
• Estimate energy absorption capacity for a surge protector in a commercial building with 50kA surge current.
• Calculate coordination voltage for a multi-stage surge protection system per IEC 61643-11.

Common Values for Surge Protector (TVSS) Parameters According to IEEE and IEC Standards

Key Formulas for Surge Protector (TVSS) Calculations According to IEEE and IEC

Understanding and applying the correct formulas is essential for selecting and sizing surge protectors. Below are the fundamental equations used in TVSS calculations, with detailed explanations of each variable.

1. Maximum Continuous Operating Voltage (MCOV)

The MCOV is typically selected based on the nominal system voltage (Un) and is the maximum voltage the surge protector can continuously withstand without degradation.

No direct formula; MCOV is selected as:

• MCOV ≈ 1.1 × Un (for AC systems)

Where:

• Un = Nominal system voltage (Volts)
• MCOV = Maximum continuous operating voltage (Volts)

2. Nominal Discharge Current (In) and Maximum Discharge Current (Imax)

These values are usually specified by the manufacturer and relate to the surge current the device can handle repeatedly (In) and once (Imax).

For calculation purposes, the required discharge current rating can be estimated from the prospective surge current (Isc) at the installation point:

Required In ≥ 0.5 × Isc

Where:

• In = Nominal discharge current rating (kA)
• Isc = Prospective surge current at installation point (kA)

3. Voltage Protection Level (Up)

The voltage protection level is the maximum voltage the TVSS allows to pass through to the protected equipment during a surge event.

It is related to the MCOV and the device’s clamping characteristics:

Up ≤ 1.5 × MCOV

Where:

• Up = Voltage protection level (Volts)
• MCOV = Maximum continuous operating voltage (Volts)

4. Energy Absorption Capacity (W/R)

The energy absorption rating indicates how much energy the device can absorb without failure, often expressed in Joules.

It can be calculated from the surge current and voltage as:

W/R = 0.5 × C × (Up)^2

Where:

• W/R = Energy absorption capacity (Joules)
• C = Capacitance or equivalent energy storage parameter (Farads)
• Up = Voltage protection level (Volts)

Note: In practical TVSS devices, W/R is provided by manufacturers based on standardized test waveforms (e.g., 8/20 µs).

5. Coordination Voltage for Multi-Stage Protection

When using multiple surge protection devices in series (e.g., primary and secondary protectors), coordination voltage ensures proper clamping without device damage.

Uc = Up1 + Up2 + Margin

Where:

• Uc = Coordination voltage (Volts)
• Up1 = Voltage protection level of first stage (Volts)
• Up2 = Voltage protection level of second stage (Volts)
• Margin = Safety margin (typically 10-20% of Up values)

6. Let-Through Voltage Calculation

The let-through voltage is the voltage that appears across the protected equipment during a surge event and is critical for equipment safety.

Vlt = Up + (I × Zs)

Where:

• Vlt = Let-through voltage (Volts)
• Up = Voltage protection level of TVSS (Volts)
• I = Surge current (Amperes)
• Zs = Source impedance (Ohms)

Real-World Application Examples of Surge Protector (TVSS) Calculations

Example 1: Selecting a Surge Protector for a 230V Single-Phase Residential System

A residential building has a nominal voltage of 230V single-phase. The prospective surge current at the service entrance is 20kA (8/20 µs waveform). The goal is to select a TVSS device that protects sensitive electronics.

• Step 1: Determine MCOV

MCOV ≈ 1.1 × Un = 1.1 × 230V = 253V

• Step 2: Select Nominal Discharge Current (In)

Required In ≥ 0.5 × Isc = 0.5 × 20kA = 10kA

Choose a TVSS with In = 10kA or higher.

• Step 3: Determine Voltage Protection Level (Up)

Up ≤ 1.5 × MCOV = 1.5 × 253V = 379.5V

Select a device with Up ≤ 380V.

• Step 4: Check Maximum Discharge Current (Imax)

Choose Imax ≥ prospective surge current, so Imax ≥ 20kA.

• Step 5: Verify Energy Absorption

For residential applications, a W/R of 100-200 Joules is typical.

Final Selection: A TVSS device rated for MCOV 275V, In 10kA, Imax 20kA, Up 350V, and W/R 200J is appropriate.

Example 2: Multi-Stage Surge Protection for a 400V Three-Phase Industrial System

An industrial facility operates a 400V three-phase system with a prospective surge current of 100kA. The design requires primary and secondary surge protectors coordinated per IEC 61643-11.

• Step 1: Calculate MCOV

MCOV ≈ 1.1 × 400V = 440V

• Step 2: Select Primary Stage TVSS

Primary device In ≥ 0.5 × 100kA = 50kA

Choose In = 50kA, Imax = 100kA, Up1 = 900V (typical for primary devices)

• Step 3: Select Secondary Stage TVSS

Secondary device In ≥ 0.5 × 10kA (assumed residual surge current) = 5kA

Choose In = 10kA, Up2 = 600V

• Step 4: Calculate Coordination Voltage (Uc)

Assuming 15% margin:

Margin = 0.15 × (Up1 + Up2) = 0.15 × (900 + 600) = 225V

Uc = Up1 + Up2 + Margin = 900 + 600 + 225 = 1725V

• Step 5: Verify Let-Through Voltage

Assuming source impedance Zs = 0.01Ω and surge current I = 100kA:

Vlt = Up1 + (I × Zs) = 900V + (100,000A × 0.01Ω) = 900V + 1000V = 1900V

The coordination voltage (1725V) is less than the let-through voltage (1900V), indicating the secondary device will clamp residual surges effectively.

Final Selection: Primary TVSS: In 50kA, Imax 100kA, Up 900V; Secondary TVSS: In 10kA, Up 600V; coordinated for optimal protection.

Additional Technical Considerations for Surge Protector Calculations

• Waveform Standards: IEEE C62.41 and IEC 61643-11 define standard surge waveforms (e.g., 8/20 µs) used for testing TVSS devices.
• Short Circuit Current Rating (SCCR): Ensure the TVSS device’s SCCR matches or exceeds the available fault current at the installation point.
• Response Time: Faster response times (<1 ns) provide better protection for sensitive electronics.
• Installation Location: Primary protectors are installed at service entrances; secondary protectors near sensitive equipment.
• Environmental Factors: Temperature, humidity, and pollution levels affect TVSS device performance and lifespan.
• Maintenance and Monitoring: Some TVSS devices include status indicators or remote monitoring for end-of-life detection.

Authoritative References and Standards

• IEEE C62.41.2-2002: Guide on Surge Voltages in Low-Voltage AC Power Circuits
• IEC 61643-11: Low-voltage surge protective devices – Part 11: Surge protective devices connected to low-voltage power systems – Requirements and test methods
• NEMA Standards for Surge Protective Devices

By applying these calculations and standards, engineers can ensure reliable and effective surge protection tailored to specific electrical systems, minimizing downtime and equipment damage.