What is an SPD
A Surge Protection Device (SPD) is an electrical device designed to limit transient overvoltages and divert surge currents to ground, protecting downstream equipment from damage. SPDs are essential for solar installations in lightning-prone areas, which includes most of India.
SPDs use voltage-limiting components, most commonly metal oxide varistors (MOVs), gas discharge tubes (GDTs) or silicon avalanche diodes. Under normal voltage, the SPD presents very high impedance and draws negligible current. When voltage exceeds the clamping threshold, impedance drops dramatically, allowing surge current to flow to earth instead of damaging equipment.
For Indian solar installations, CEA Measures Relating to Safety and the inverter safety standards (IEC 62109-1/-2) point to SPD on both the DC side (between PV modules and inverter) and the AC side (between inverter and grid). Indian thunderstorm activity makes SPD non-optional for plant longevity, a risk our surge protection buyer’s guide covers in more depth.
Why SPDs matter
Equipment protection. Inverters, panels, batteries and connected appliances cost lakhs to crore. SPDs cost a few thousand and prevent catastrophic loss.
Insurance compliance. Insurers require SPD-protected installations for claim coverage.
Regulatory compliance. CEA Measures Relating to Safety and the inverter safety standards (IEC 62109-1/-2) point to SPDs on both sides.
System availability. Surge damage causes long downtime for repairs. SPDs prevent the damage.
Investment protection. Solar is a 25-year asset. SPD is the cheapest way to protect that investment.
How an SPD works
Under normal voltage conditions, the MOV (most common SPD technology) presents very high impedance (megaohms). Negligible current flows through the SPD; downstream equipment receives clean power.
When a voltage surge exceeds the MOV’s clamping threshold (typically 600 V to 1.5 kV for low-voltage applications):
- MOV impedance drops to milliohms within nanoseconds.
- Surge current flows through the SPD to earth.
- Voltage across the protected equipment is clamped at the residual voltage of the SPD.
- Energy dissipated in the SPD reduces its remaining life slightly.
A status indicator (window or LED) shows the SPD’s health. Red indicates end of life; replace immediately.
Technical deep dive
SPD types
| Type | Surge waveform | Use location |
|---|---|---|
| Type 1 | 10/350 µs lightning current | Service entry, near LPS down-conductors |
| Type 2 | 8/20 µs surge current | Sub-distribution boards |
| Type 3 | Combination waveform | Appliance-level, sensitive electronics |
| Type 1+2 | Combined | Compact installations |
Most solar installations use Type 2. Service entry to lightning-protected buildings adds Type 1.
SPD ratings
| Parameter | Description |
|---|---|
| Uc | Maximum continuous operating voltage |
| Up | Voltage protection level (residual voltage) |
| In | Nominal discharge current (8/20 µs) |
| Imax | Maximum discharge current (8/20 µs) |
| Iimp | Impulse discharge current (10/350 µs, Type 1) |
Typical Indian residential SPD selection
| Location | SPD type | Uc | In |
|---|---|---|---|
| DC side, combiner box | Type 2 | 1000 V DC | 20 kA |
| AC side, inverter output | Type 2 | 275 V AC | 20 kA |
| Service entry (with LPS) | Type 1+2 | 275 V AC | 25 kA (Imax) + 12.5 kA (Iimp) |
MOV vs GDT
| Technology | Pros | Cons |
|---|---|---|
| MOV | Fast response, low cost | Wears out with each surge |
| GDT | Long life, high discharge capacity | Slow response, voltage follow-through |
| Hybrid | Combines both | Higher cost |
Real-world applications
Solar inverter DC input protection.
Solar inverter AC output protection.
Combiner box on utility plants.
Distribution panel at building entry.
Telecom tower equipment protection.
EV charger protection.
Common SPD problems
End of life not noticed. Indicator window goes red but no one looks at it.
Inadequate earthing nullifying SPD function.
Wrong type (Type 3 where Type 2 is needed).
SPD installed but not connected to earth.
Insufficient discharge capacity for site lightning exposure.
Cascading failures when one stage of SPD fails.
Best practices
Install Type 2 SPD on both DC and AC sides.
Verify earthing resistance below 5 ohms.
Place SPD as close to protected equipment as possible.
Use SPDs with status indicators.
Annual visual inspection of indicators.
Replace SPDs immediately when end-of-life indicated.
Coordinate Type 1 with Type 2 if both used.
Use a back-up fuse to disconnect a failed SPD.
Comparison: SPD types
| Type | Test current | Application | Solar use |
|---|---|---|---|
| Type 1 | 10/350 µs, kA-scale | Service entry, direct lightning current | Buildings with LPS |
| Type 2 | 8/20 µs, kA-scale | Sub-distribution | Standard solar |
| Type 3 | 8/20 µs and 1.2/50 µs combo | Appliance-level | Sensitive electronics |
Indian market context
Major Indian SPD brands include Phoenix Contact, ABB, Schneider Electric, Siemens, Havells, OBO, Citel, DEHN and Mersen, most of which are stocked through EPC channels like Heaven Green Energy’s equipment shop.
ALMM does not list SPDs. BIS certification under IS 16480 applies.
CEA Measures Relating to Safety 2023 references SPD installation.
Indian thunderstorm density (especially north-east and central India) drives SPD demand.
PM Surya Ghar residential installations include SPD on both DC and AC sides in most installer specifications.
Standards and certifications
| Standard | Scope |
|---|---|
| IS 16480 | Indian SPD standard |
| IEC 61643 series | International SPD standard |
| IEC 60364-4-44 | Protection against voltage disturbances |
| IEC 62109-1/-2 | Inverter safety |
| CEA Measures Relating to Safety 2023 | Indian safety regulations |
Common mistakes
Skipping DC SPD to save cost.
Installing SPD without proper earthing.
Wrong Uc value (clamping voltage too high for the system).
Inadequate discharge capacity for site exposure.
Not replacing end-of-life SPDs.
Mixing brands or generations without coordination.
Treating SPD as a one-time install instead of consumable.
Key takeaways
SPDs are mandatory protection devices for Indian solar installations. Type 2 SPDs on both DC and AC sides protect against indirect lightning and switching surges. They wear out and must be replaced when end-of-life is indicated. Proper earthing is essential for SPD function. The cost of SPDs is negligible compared to the inverter and module investment they protect.
Further reading
For how SPD plays out in real projects, these guides go deeper:
- How to Choose Solar Inverters with Surge Protection
- Solar Inverter Components: 7 Parts to Inspect
- Monsoon Solar Prep: Complete Checklist for India
Sources
- IS 16480, Indian Standard for surge protection devices.
- IEC 61643 series, International SPD standard.
- IEC 60364-4-44, Protection against voltage disturbances.
- CEA (Technical Standards for Connectivity of the Distributed Generation Resources) Regulations, 2013 (as amended 2019), Central Electricity Authority.
- CEA Measures Relating to Safety and Electric Supply Regulations 2023.
- IEC 62305 series, Lightning protection.
- Manufacturer technical documentation from Phoenix Contact, ABB, Schneider, Siemens, Havells, OBO, Citel, DEHN.
- IEEE C62 series on surge protection.
Frequently Asked Questions
What is SPD in simple words?
What does SPD stand for?
Are SPDs required for solar?
What are SPD types?
Do I need both DC and AC SPDs?
Where do SPDs go in a solar system?
Can SPD prevent direct lightning strikes?
How does an SPD work?
Does SPD wear out?
How often should SPD be replaced?
Are SPDs BIS-certified?
What is SPD discharge capacity?
Do off-grid systems need SPDs?
What is the typical cost of SPDs?
Does SPD need earthing?
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Co-founder & CEO driving Qbits Energy's product strategy and growth across India.