Varistors and surge suppressors are critical components in protecting electrical systems from transient voltage spikes. Selecting the right device requires precise calculations based on IEEE and IEC standards.
This article explores comprehensive methods for varistor and surge suppressor selection, including formulas, tables, and real-world examples. Engineers will gain expert insights into device ratings and application criteria.
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- Calculate varistor energy rating for a 230 V AC system with 10 kA surge current.
- Determine maximum continuous operating voltage (MCOV) for a 120 V DC application.
- Find appropriate surge suppressor clamping voltage for a 400 V transient event.
- Estimate varistor response time and energy absorption for a 20 kA lightning surge.
Comprehensive Tables for Varistor and Surge Suppressor Selection
Table 1: Common Varistor Voltage Ratings and Corresponding Maximum Continuous Operating Voltage (MCOV)
| Varistor Voltage (VV) | Maximum Continuous Operating Voltage (VMCOV) | Typical Application Voltage | Energy Rating (Joules) | Surge Current Rating (Imax, kA) |
|---|---|---|---|---|
| 130 | 110 | 100 V AC | 50 | 10 |
| 180 | 150 | 120 V AC | 70 | 12 |
| 275 | 240 | 230 V AC | 100 | 15 |
| 385 | 320 | 277 V AC | 150 | 20 |
| 510 | 440 | 400 V AC | 200 | 25 |
| 680 | 600 | 480 V AC | 300 | 30 |
Table 2: Surge Suppressor Clamping Voltage and Energy Absorption Ratings (IEEE C62.11 / IEC 61643-11)
| Nominal System Voltage (Vn) | Clamping Voltage (Vc) | Energy Absorption (Joules) | Surge Current Capability (Imax, kA) | Response Time (ns) |
|---|---|---|---|---|
| 120 V AC | 350 | 100 | 10 | 25 |
| 230 V AC | 600 | 150 | 15 | 20 |
| 400 V AC | 900 | 200 | 20 | 15 |
| 480 V AC | 1100 | 300 | 30 | 10 |
Table 3: Varistor Energy and Surge Current Ratings per IEC 61051-1
| Varistor Diameter (mm) | Energy Rating (J) | Surge Current (Imax, kA) | Typical Application |
|---|---|---|---|
| 7 | 20 | 5 | Low power electronics |
| 14 | 50 | 10 | Consumer appliances |
| 20 | 100 | 15 | Industrial equipment |
| 30 | 200 | 30 | Power distribution |
Essential Formulas for Varistor and Surge Suppressor Selection
1. Maximum Continuous Operating Voltage (MCOV)
The MCOV is the highest RMS voltage that the varistor can continuously withstand without degradation.
- VV: Varistor voltage rating (peak voltage at 1 mA current)
- MCOV is typically 10-20% below VV to ensure longevity
2. Energy Absorption (E)
Energy absorbed by the varistor during a surge event is critical for device sizing.
- E: Energy absorbed (Joules)
- C: Varistor capacitance (Farads)
- Vc: Clamping voltage (Volts)
3. Clamping Voltage (Vc)
The voltage at which the varistor starts conducting heavily to clamp the surge.
- Vc: Clamping voltage at surge current I
- VV: Varistor voltage at reference current Iref (usually 1 mA)
- I: Surge current (Amperes)
- Iref: Reference current (Amperes)
- α: Non-linearity coefficient (typically 0.3 to 0.5)
4. Surge Current Rating (Imax)
The maximum surge current the varistor can safely absorb without damage.
- Imax: Maximum surge current (Amperes)
- E: Energy rating (Joules)
- t: Surge duration (seconds)
5. Response Time (tr)
Varistor response time is the delay between surge onset and clamping action.
- Faster response times reduce transient damage risk
- IEC 61051 and IEEE C62.11 specify maximum allowable response times
Real-World Application Examples
Example 1: Selecting a Varistor for a 230 V AC Power Supply
A 230 V AC industrial power line requires surge protection against lightning-induced surges up to 10 kA. The varistor must have an MCOV above the RMS voltage and an energy rating sufficient for repeated surges.
- Step 1: Calculate MCOV
Choose a varistor with VV slightly above 325 V, typically 385 V.
- Step 2: Verify energy rating
For 10 kA surge current and typical surge duration of 8/20 µs, select a varistor with at least 150 J energy rating (see Table 1).
- Step 3: Confirm clamping voltage
Using the clamping voltage formula with α = 0.4 and Iref = 1 mA:
Datasheet clamping voltage is approximately 900 V at 10 kA, suitable for protecting downstream equipment.
Example 2: Surge Suppressor Selection for a 120 V DC Control Circuit
A 120 V DC control circuit is exposed to transient surges from switching inductive loads. The surge suppressor must clamp voltage spikes and absorb energy without failure.
- Step 1: Determine MCOV
Select a varistor with VV around 150 V to ensure margin.
- Step 2: Calculate energy absorption
Assuming surge current of 5 kA and surge duration of 10 µs:
Given varistor capacitance C = 2000 pF (2 × 10-9 F) and clamping voltage Vc = 350 V:
This energy is minimal; therefore, the varistor must be rated for repetitive surges and have a surge current rating above 5 kA.
- Step 3: Verify response time
Ensure the varistor response time is less than 25 ns to protect sensitive electronics.
Additional Technical Considerations
- Thermal Management: Varistors dissipate energy as heat; proper heat sinking or spacing is essential to prevent thermal runaway.
- Voltage Derating: Operating varistors below their MCOV extends lifespan and reduces failure risk.
- Surge Waveform: IEEE C62.41 defines standard surge waveforms (e.g., 8/20 µs, 10/1000 µs) used for testing and selection.
- Standards Compliance: IEC 61643-11 and IEEE C62.11 provide guidelines for performance, testing, and marking of surge protective devices.
- Coordination with Other Devices: Surge suppressors should be coordinated with upstream and downstream protective devices for optimal system protection.
Authoritative References and Further Reading
- IEEE Standard C62.11-2012 – Surge Protective Devices
- IEC 61643-11 – Low-voltage surge protective devices
- Eaton Surge Protection Solutions
- Littelfuse Varistor Technical Resources
By integrating IEEE and IEC standards with practical calculations and device data, engineers can confidently select varistors and surge suppressors tailored to their system requirements. This ensures robust protection against transient overvoltages, enhancing system reliability and safety.