Transient Voltage Surge Suppressor TVSS: 8/20 vs. 10/350

When engineers compare a Transient Voltage Surge Suppressor TVSS, the largest kA value on the datasheet often attracts the most attention. However, surge-current ratings only become meaningful when the test waveform, current parameter, protection level, installation point, and power-system configuration are considered together.

A TVSS rated at 100 kA under an 8/20 μs waveform is not automatically superior to a device rated at 25 kA under a 10/350 μs waveform. These ratings describe different electrical stresses and serve different protection functions.

For Telebahn, this distinction is fundamental when specifying surge protection for main distribution boards, sub-distribution panels, industrial control systems, and other low-voltage installations.

Why 8/20 μs and 10/350 μs Cannot Be Compared by Peak Current Alone

The 8/20 μs and 10/350 μs waveforms are standardized impulse-current shapes used to evaluate different surge conditions.

Parameter	8/20 μs	10/350 μs
Typical Rating	In, Imax	Iimp
Main Test Purpose	Surge discharge capability	Lightning-current capability
Current Tail	Relatively short	Significantly longer
Typical SPD Class	Type 2	Type 1
Main Electrical Stress	Repetitive/residual surge stress	High charge and energy stress

The first number relates to the current front, while the second describes the decay toward half-value. The 10/350 μs waveform therefore maintains substantial current for much longer.

This matters because the stress on a Transient Voltage Surge Suppressor TVSS is not determined by peak current alone. Engineers must also consider:

•Charge transfer, Q

•Specific energy, W/R

•Thermal loading of protection components

•Mechanical stress on current-carrying paths

•Electrode and contact stress

•Repetitive impulse capability

This is why a 25 kA Iimp rating cannot be directly compared with a 100 kA Imax rating.

Iimp, In and Imax Describe Different TVSS Capabilities

A professional TVSS specification should clearly distinguish these three values.

Iimp: Lightning Impulse Current

Iimp is normally associated with the 10/350 μs waveform and is especially relevant where a portion of direct lightning current may enter the low-voltage system.

Typical applications include:

•Service entrances

•Buildings with external lightning protection systems

•Main distribution boards

•Installations requiring Type 1 surge protection

In: Nominal Discharge Current

In is normally tested using an 8/20 μs waveform. It represents a standardized nominal surge-current level used to evaluate repeatable discharge performance.

In practical selection, In gives engineers a better indication of regular surge-duty capability than simply looking at a headline maximum current.

Imax: Maximum Discharge Current

Imax is the maximum specified discharge current under an 8/20 μs impulse.

It is useful for comparing the upper discharge capability of Type 2 protection, but:

•Imax is not the same as Iimp.

•Imax does not directly indicate service life.

•A higher Imax does not automatically mean a lower residual voltage.

•Imax should not be used alone to select a service-entrance SPD.

Type 1, Type 2 and Type 1+2 Must Match the Installation Point

A Transient Voltage Surge Suppressor TVSS should be selected according to the expected surge environment.

Type 1 at the Main Incoming Supply

Where lightning current can enter through the supply system, Type 1 protection is designed to handle high-energy current impulses.

Key parameters include:

•Iimp

•10/350 μs test waveform

•Up

•Uc

•Follow-current behavior where applicable

•Required backup protection

Type 2 at Sub-Distribution Level

Type 2 protection is normally used to reduce residual lightning effects and switching surges further downstream.

Important ratings include:

•In

•Imax

•Up

•Uc

•SCCR or short-circuit capability

•Protection mode

Coordinated Type 1+2 Protection

In some applications, a combined Type 1+2 Transient Voltage Surge Suppressor TVSS can provide both lightning-current handling and downstream voltage limitation in one coordinated design.

However, one high-current device should not automatically be treated as sufficient for every sensitive load. PLCs, drives, instrumentation, servers, and communication equipment may still require additional downstream protection.

MOV, GDT and Hybrid Designs Have Different Roles

The internal protection technology influences how a TVSS reacts to impulse current.

Technology	Engineering Strength	Design Consideration
MOV	Fast voltage limiting, compact design	Aging, leakage current, thermal protection
GDT / Spark Gap	High surge-energy capability	Ignition behavior and follow current
MOV + GDT	Combines energy handling and voltage limiting	Requires careful coordination

Telebahn uses protection architectures such as MOV and GDT combinations in relevant surge-protection designs. The objective is not simply to maximize kA rating, but to balance discharge capability, voltage protection level, thermal safety, and system compatibility.

Up, Uc and SCCR Matter as Much as Surge Current

A high-current Transient Voltage Surge Suppressor TVSS can still be poorly matched if other electrical parameters are ignored.

Voltage Protection Level, Up

Up indicates the maximum residual voltage under specified test conditions.

For sensitive equipment, a lower suitable Up can be more important than selecting the highest possible Imax.

Maximum Continuous Operating Voltage, Uc

Uc must be compatible with the actual system voltage and possible long-duration overvoltage conditions.

An incorrectly selected Uc can lead to:

•Excessive leakage

•MOV heating

•Accelerated aging

•Premature disconnection

SCCR Is Not a Surge Rating

SCCR relates to short-circuit safety, not lightning or surge-current capability.

Therefore:

Imax ≠ Iimp ≠ SCCR

Each parameter addresses a different electrical risk.

Installation Quality Directly Affects Protection Performance

Even a correctly specified TVSS may provide poor equipment protection if the wiring layout is unsuitable.

During a fast surge, conductor inductance creates additional voltage. This means the voltage experienced by downstream equipment can exceed the laboratory Up value.

Good installation practice therefore includes:

•Keep line and PE conductors short.

•Avoid loops and unnecessary bends.

•Maintain low-impedance bonding.

•Use the specified conductor cross-section.

•Confirm N-PE and L-PE protection paths.

•Verify the required upstream fuse or MCB.

•Match SCCR to the available fault current.

For a Transient Voltage Surge Suppressor TVSS, installation geometry is part of the protection system, not just a mechanical detail.

What Buyers Should Verify Before Comparing TVSS Products

A professional procurement review should answer these questions:

•Is the current value Iimp, In or Imax?

•Is the test waveform 8/20 μs or 10/350 μs?

•Is the value per pole or total?

•What are the Up and Uc values?

•Which protection modes are included?

•What is the short-circuit rating?

•What backup protection is required?

•Which standard supports the declared ratings?

•Are supporting test reports available?

Telebahn separate surge-current parameters for its Transient Voltage Surge Suppressor TVSS solutions, including 8/20 μs In/Imax data and 10/350 μs Iimp data where applicable. This gives engineers a more useful basis for system-level comparison than relying on a single headline kA number.

For projects involving main distribution, industrial power systems, or coordinated surge protection, engineers can review Telebahn's Transient Voltage Surge Suppressor TVSS options here.

FAQs

Q1. What surge-current parameters should buyers check when selecting a Telebahn Transient Voltage Surge Suppressor TVSS?

Buyers should review Iimp, In, Imax, Up, Uc, protection mode, short-circuit capability, and required backup protection rather than comparing only the highest kA value.

Q2. Does Telebahn provide both 8/20 μs and 10/350 μs surge ratings?

Telebahn separate 8/20 μs In/Imax and 10/350 μs Iimp ratings on relevant surge protection products, allowing engineers to distinguish nominal surge duty from lightning-current capability.

Q3. What is the difference between Telebahn's In and Imax ratings?

In represents nominal discharge-current performance under an 8/20 μs waveform, while Imax represents the specified maximum discharge current under the same waveform.

Q4. Why should Iimp be checked separately from Imax on a Telebahn TVSS?

Iimp is associated with the 10/350 μs lightning-current waveform, while Imax is normally based on 8/20 μs testing. Because the waveforms impose different charge and energy stresses, the values cannot be directly compared.

Q5. What protection technologies are used in Telebahn surge protection products?

Relevant Telebahn products use protection architectures including high-energy MOV and GDT elements, depending on the product and protection configuration.