After a heavy storm, a contractor discovers that all the street lights on a road have stopped working overnight. The installer says the lightning was too strong. The fixture manufacturer notes that no surge protective device was specified. The project owner questions the product quality. Putting aside who is responsible, the main question is whether the right level of surge protection was chosen for this installation from the start.
This article explains how surge protective devices are classified, what the main parameters mean, and how to choose the right specification for your project.
Surge Protective Devices Classifications Explained: T1, T2, and T3
Under IEC 61643-11 and equivalent national standards, surge protective devices (SPDs) fall into three protection levels. These levels are based on where the device sits in the electrical system and how well it protects.
Level | Installation Point | Test Waveform | Key Parameter | Protection Target |
T1 (Class I) | Main distribution board, building entry point | 10/350μs | Iimp ≥ 25kA | Dissipate direct lightning strike energy |
T2 (Class II) | Sub-distribution boards, lighting control panels | 8/20μs | In ≥ 20kA, Imax ≥ 40kA | Clamp residual surge voltage |
T3 (Class III) | Terminal equipment, sensitive devices | 8/20μs | In ≤ 10kA, Up ≤ 1.0kV | Fine-level protection at the device |
The three levels work together in sequence. T1 absorbs most of the energy from a direct strike. T2 reduces the voltage that remains after T1, and T3 adds the final layer of protection at the fixture or device. If any layer is missing, the risk of failure goes up. T2 or T3 alone cannot handle the energy from a direct strike, and T1 alone does not provide enough protection at the fixture.
Note: IEC Class I/II/III and the commonly used field designations B/C/D grade are broadly equivalent. When evaluating suppliers, check which labelling system they use. Then match the terms between systems.
Key SPD Parameters: Iimp, In, and Voltage Protection Level
It is easy to claim surge protection, but hard to prove without checking the actual numbers. Here are the key parameters you should ask about.
Maximum impulse current (Iimp) — T1 only
Iimp measures the peak current the device can handle during the 10/350μs waveform. This simulates a direct lightning strike. T1 SPDs must handle at least 25kA, and higher-end units can reach 50kA. This parameter shows if a T1 device can survive a direct strike without failing. When choosing T1 products, always check the Iimp value first.
Nominal discharge current (In) and maximum discharge current (Imax) — T2 and T3
In represents the surge current the device can handle repeatedly — the standard operating condition. A rating of 10–20kA is typical for commercial outdoor applications. Imax is the single-event maximum the device can survive without damage, generally specified at 40kA or above for T2 devices. Both should be confirmed in the datasheet, not just claimed on the product label.
Voltage protection level (Up)
Up is the residual voltage at the device’s output terminals during a surge event — the voltage that gets through to the protected equipment. Lower is better. For T2 devices protecting distribution panels, Up ≤ 1.5kV is the accepted benchmark. For T3 devices protecting terminal equipment directly, Up ≤ 1.0kV is the appropriate target.
How to Select the Right SPD Level for Street Lighting
Three questions determine the appropriate specification for any outdoor lighting installation.
1.Where are the fixtures installed?
- Rooftop and elevated installations — Fixtures mounted on rooftops or elevated structures face a higher risk of direct lightning strikes. T1 protection is typically required at the supply entry point.
- Standard outdoor street lighting — Residential roads, commercial areas, and industrial perimeter lighting generally require T2 protection at the distribution panel. T3 protection can also be added at the fixture when the unit value justifies the extra protection.
- Indoor applications — Tunnel lighting and underground car parks can typically be protected with T2 or T3 alone.
2.What is the local lightning exposure level?
High-frequency lightning zones — coastal regions, mountainous areas, tropical climates — warrant stepping up protection levels relative to standard specifications. In environments with overhead cable runs or extensive exposed wiring, transient surge levels can reach 4kV/2kA or higher during indirect strike events. Local lightning density maps (annual lightning ground flash density, Ng) provide the data needed to make this assessment rigorously.
3.What is the replacement value of the equipment being protected?
Surge protection makes clear economic sense when equipment is expensive. Solar street lights can cost several hundred to over a thousand dollars each. Smart poles with sensors, communication hardware, and control systems cost even more per unit. As a rule of thumb, spending 5–10% of the equipment’s value on SPDs is reasonable. For very valuable installations, spending more is often worth it.
SPD Specification by Application Type
Application | Recommended Level | Key Parameters | Notes |
Main distribution board | T1 | Iimp ≥ 25kA | Direct strike protection |
Street lighting distribution panel | T2 | In ≥ 20kA, Up ≤ 1.5kV | Standard outdoor lighting baseline |
LED fixture internal protection | T3 | In ≤ 10kA, Up ≤ 1.0kV | Terminal device protection |
Smart poles / connected infrastructure | T1 + T2 combined | Cascaded protection | High equipment value; multi-level approach |
Indoor general lighting | T2 | In ≥ 10kA | Sub-distribution system protection |
Street Light Surge Protection: Procurement Tips
1.Don’t accept “lightning protection included” as a specification. Ask for the SPD classification (T1/T2/T3), the Iimp or In rating, and the Up value. A product that claims lightning protection but cannot provide these parameters typically contains nothing more than a basic metal oxide varistor — which offers limited and unreliable protection.
2.Check response time. An SPD must respond in ≤ 25 nanoseconds to be effective. Slower devices allow damaging voltage peaks to pass through before clamping kicks in.
Plan for replacement. SPDs are consumable components. Each surge event degrades them incrementally. Include SPDs in annual maintenance inspections — look for signs of overheating, discolouration, or physical damage, and replace units that have experienced significant events.
3.Verify the earthing system before assuming the SPD is working. Surge protection effectiveness is entirely dependent on a properly designed and maintained earthing installation. A correctly specified SPD connected to an inadequate earth will provide little or no protection in practice.
In Conclude
Specifying surge protection for street lighting comes down to matching the protection level to the installation’s actual risk profile:
• T1 at the main distribution board to handle direct lightning strike energy — minimum Iimp of 25kA
• T2 at the street lighting distribution panel for indirect and induced surge protection — minimum In of 20kA, Up ≤ 1.5kV
• T3 at terminal fixtures where equipment value justifies the additional layer — Up ≤ 1.0kV
When specifying or sourcing fixtures and distribution equipment, confirm the SPD classification and parameters explicitly. A claim of “surge protection included” without supporting specification data is not a specification — and in a post-storm blame conversation, it won’t be adequate defence either.
Yourlite has been manufacturing and exporting LED lighting products since 1996, supplying commercial, industrial, and outdoor lighting to more than 1,280 customers across 60+ countries. For product specifications, compliance documentation, or sourcing inquiries, contact the team at salescontact@yourlite.com or visit yourlite-smart.com.
