Home » Blogs » Industry Blog » AC T1 Surge Protective Device Explained: Iimp, 10/350μs Waveform and SPD Selection
2026-08-17
Why AC T1 Surge Protective Device Is Required for Direct Lightning Current Protection
Electrical systems are exposed to different types of transient overvoltage events, including switching surges, grid disturbances, and lightning-induced surges. However, direct lightning current entering a building through power lines, external lightning protection systems, or incoming cables creates a much higher energy stress than ordinary surge events.
An AC T1 Surge Protective Device is designed for the first protection stage and is installed at the service entrance or main distribution board. It diverts extremely fast, high-energy impulses that are carried by lightning from downstream equipment to ground before they reach equipment.
Unlike Type 2 or Type 3 surge protection devices, an AC T1 Surge Protective Device is tested with the 10/350 μs lightning current waveform, which simulates the characteristics of a direct lightning strike.
The selection of a Type 1 SPD should therefore focus on:
•Lightning current carrying capability
•Iimp impulse current rating
•Voltage protection level (Up)
•System voltage compatibility (Uc)
•Coordination with downstream SPDs
What Does the 10/350 μs Lightning Current Waveform Mean?
The 10/350 μs waveform is the standard impulse current waveform used to evaluate the lightning current capability of a Type 1 SPD.
It represents a high-energy lightning event with:
| Parameter | Explanation |
| 10 μs | Time required for current to rise to peak value |
| 350 μs | Time for current to decay to 50% of peak value |
| Main application | Simulation of direct lightning current |
The energy contained in a 10/350 μs impulse is much higher than an 8/20 μs surge impulse. This is why an AC T1 Surge Protective Device requires different testing criteria compared with Type 2 SPD products.
A Type 1 SPD is not selected simply because it has a high discharge current value. It must be capable of handling the energy characteristics of lightning current itself.
Iimp: The Key Parameter of an AC T1 Surge Protective Device
What Is Iimp?
An AC T1 Surge Protective Device (SPD) can discharge a maximum peak current of 10/350μs called Iimp.
Iimp is measured in kA and is directly proportional to a SPD's lightning current withstand capability.
Typical factors in the selection of Iimp are:
•Building and external lightning protection systems
•Exposure of incoming power lines
•Importance of the electrical installation
•Local lightning density
An Iimp with a higher rating allows for more lightning current to be dissipated. The rating selected should reflect a risk assessment and not simply a large Iimp rating.
Relationship Between Iimp, Up and Uc in AC T1 Surge Protective Device Selection
A reliable AC T1 Surge Protective Device must balance several electrical parameters.
| Parameter | Function | Selection Consideration |
| Iimp | Handles lightning impulse current | Match expected lightning energy |
| Uc | Maximum continuous operating voltage | Must match electrical system voltage |
| Up | Residual voltage after surge discharge | Lower Up provides better equipment protection |
| Short-circuit withstand | Safety under fault conditions | Match installation fault level |
These parameters work together:
•Iimp determines energy capacity
•Up determines protection performance
•Uc determines operating reliability
A common engineering mistake is selecting an SPD only according to kA rating while ignoring voltage protection level and system compatibility.
AC T1 Surge Protective Device vs Type 2 SPD: Different Protection Roles
Although both devices protect against transient overvoltage, their design objectives are different.
| Feature | AC T1 Surge Protective Device | Type 2 SPD |
| Protection level | Primary lightning protection | Secondary surge protection |
| Installation position | Main distribution entrance | Sub-distribution panels |
| Test waveform | 10/350 μs | 8/20 μs |
| Main parameter | Iimp | In / Imax |
| Main purpose | Discharge partial lightning current | Reduce residual surge voltage |
In a complete electrical protection system:
Type 1 SPD → Type 2 SPD → Type 3 SPD
creates a coordinated protection structure.
The first stage handles high-energy lightning current, while later stages protect sensitive electronic equipment.
Spark Gap and MOV Technologies Used in AC T1 Surge Protective Device Design
Different AC T1 Surge Protective Device designs use different protection technologies.
Spark Gap Technology
Spark gap devices offer unique advantages for applications needing high energy discharges along a path of lightning current.
Advantages include:
•High energy current discharge
•Direct connection of the lightning current
•Low leakage current during normal operation
Disadvantages may include:
•The need for downstream coordination
•More restrictive voltage limits
MOV and Hybrid Protection Technology
Metal Oxide Varistor (MOV) technology has a high response rate and is useful for voltage limiting.
Advantages include:
•Fast transient response
•Compact design
•MOV technology lends itself well to the design of voltage limitation surge protection systems.
In modern surge protection design, MOV and Gas Discharge Tube (GDT) technologies can be combined to provide both rapid response and high discharge capability.
Telebahn combines both MOV technology and GDT technology in surge protection systems employing the combination of characteristics of rapid response and high discharge capability, respectively.
Selecting AC T1 Surge Protective Devices for Various Electric Systems
When considering the use of a T1 Surge Protective Device, the entire electrical system must be evaluated.
OF paramount importance are:
•Power system configuration (single phase or three phase)
•TN-S, TN-C-S or TT grounding system
•Operating voltage
•Required level of lightning protection
•Available space for installation
•Coordination with backup protection
Incorrect SPD selection can lead to insufficient lightning protection in an application resulting in overprotection and unnecessarily high costs.
Installation and Maintenance Requirements for AC T1 Surge Protective Devices
The performance of an AC T1 Surge Protective Device is dependent on the quality of the installation as well as the SPD itself.
Important installation practices include:
Short Connection Paths
•Short lengths of SPD-grounding conductor
•Reduction of surge event inductive voltage
Correct Grounding Connection
•Confirm grounding resistance
•Assure SPD and earth system contact
Regular Checks
Inspect:
•SPD status indicators
•Mechanical damage
•Surge event logs
•Check condition of connections
Surge protection solutions considering practical maintenance are offered by Telebahn. Design features of its products incorporate clear LED status indication, and thus, easier identification of the status of surge protection and grounding connection.
How to Choose the Right AC T1 Surge Protective Device Supplier
Selecting an AC T1 Surge Protective Device supplier goes beyond assessing product ratings.
Other factors worthy of consideration include:
•Knowledge of lighting protection applications
•Testing of products
•Consistency in manufacturing
•Paperwork for compliance
•Technical support
In terms of surge protection solutions for the different types of environments that make up an electrical system, Telebahn focuses on reliable solutions.
Telebahn offers customers an "easy-to-install" manual for the AC T1 Surge Protective Devices, along with application support, and additional installation support. Telebahn continues to support surge protection and provides engineering-based support for modern power protection services.
FAQs
Q1. What is an AC T1 Surge Protective Device?
An AC T1 Surge Protective Device is designed to offer primary surge protective equipment for service at the main distribution board, protecting equipment from high energy surges caused by lightning.
Q2. What is the difference between Type 1 and Type 2 surge protection devices?
Type 1 surge protection devices (SPDs) use a 10/350 μs lightning waveform and can defend against a partial lightning current. In contrast, Type 2 SPDs defend against switching transients and induced surges using an 8/20 μs waveform.
Q3. What does Iimp mean in surge protection products?
Iimp stands for impact current capability in lightning current testing. In such a situation, it indicates the limit of a Type 1 surge protective device.
Q4. Does Telebahn have surge protection solutions for other voltages?
Telebahn has surge protection solutions for an array of electrical environments fulfilling various AC voltage and application requirements.
Q5. What kind of technology does Telebahn use in its surge protection devices?
Combined surge protection technologies, including technologies built around MOV and GDT, are used to provide high discharge capability and fast transient response to a wide range of surges.
