Surge Suppressor Protector Response to 8/20 μs and 10/350 μs Surge Currents

A Surge Suppressor Protector should never be selected by comparing kA values alone. A 60 kA rating measured with an 8/20 μs current impulse does not represent the same electrical, thermal, or mechanical stress as a 25 kA rating measured with a 10/350 μs impulse.

The waveform determines how much charge passes through the Surge Suppressor Protector, how much energy its internal components must withstand, and where the device can be safely installed. A reliable assessment, therefore, requires engineers to examine:

•Surge-current waveform;

•SPD classification and installation location;

•Iimp, In, Imax, Q, W/R, and Up;

•Protection technology and circuit topology;

•Earthing system and protection mode;

•Installation conductor length;

•Test standard and certification scope.

Why High-kA Surge Protection May Still Fail

A high-capacity Type 2 Surge Suppressor Protector can suppress induced lightning and switching surges, but it may fail if installed where partial direct lightning current is expected.

Common causes include:

•Installing Type 2 protection at an LPZ boundary requiring Type 1 protection;

•Comparing an 8/20 μs Imax value with a 10/350 μs Iimp value;

•Using an aggregated multi-pole rating instead of a per-mode rating;

•Selecting the wrong Uc for the operating voltage or earthing system;

•Allowing long connection conductors to increase the equipment-side voltage;

•Protecting the power supply while leaving communication and control lines exposed.

A failed Surge Suppressor Protector should not automatically be blamed on insufficient kA capacity. Temporary overvoltage, poor N-PE coordination, incorrect backup protection, and excessive short-circuit current can produce similar failure symptoms.

What 8/20 μs and 10/350 μs Actually Represent

The first value describes the approximate current rise time. The second describes the time required for the current to decay to half of its peak value.

Technical factor	8/20 μs waveform	10/350 μs waveform
Typical SPD duty	Induced lightning and switching surges	Partial lightning current
Common ratings	In and Imax	Iimp
Typical classification	Type 2	Type 1
Current tail	Relatively short	Significantly longer
Charge transfer	Lower at the same peak current	Substantially higher
Main component stress	Fast impulse and repetitive duty	High thermal and mechanical stress

The total charge is expressed as:

Q = ∫ i(t) dt

The specific energy is expressed as:

W/R = ∫ i²(t) dt

Charge indicates how much electrical quantity passes through the Surge Suppressor Protector. Specific energy is closely related to conductor heating, internal connection stress, and the thermal loading of protection elements.

This is why a 25 kA 10/350 μs impulse cannot be treated as weaker than a 60 kA 8/20 μs impulse simply because its peak current is lower.

How Different SPD Technologies Respond

The internal protection technology determines how a Surge Suppressor Protector manages voltage, current, leakage, ageing, and follow current.

Technology	Technical advantage	Main limitation	Typical role
MOV	Fast voltage limiting and relatively low residual voltage	Ages under repeated impulses and TOV exposure	Type 2 L-N protection
Spark gap	High charge and lightning-current capability	Requires reliable follow-current interruption	Type 1 protection
GDT	Low leakage and high-energy discharge capability	Higher operating voltage and possible follow current	N-PE protection
MOV + GDT	Combines voltage limiting and energy discharge	Requires coordinated internal design	TT/TN 2+1 circuits

In a coordinated 2+1 Surge Suppressor Protector, MOV elements can limit L-N voltage while a high-energy GDT manages the N-PE discharge path. Telebahn applies this arrangement in pluggable Type 2 devices intended for TT and TN sub-distribution systems.

How to Interpret Iimp, In, Imax, and Up

Iimp Describes 10/350 μs Lightning-Current Duty

Iimp is primarily associated with Type 1 testing. It should be reviewed together with:

•Charge Q;

•Specific energy W/R;

•Rating per pole or protection mode;

•Number of applied impulses;

•Post-test operating condition;

•Follow-current interruption capability.

An Iimp value cannot be derived from Imax using a general conversion factor.

In Represents Nominal 8/20 μs Duty

In indicates the nominal discharge current under a defined test sequence. It provides more information about repeated Type 2 impulse duty than a maximum rating alone.

Imax Represents the Upper 8/20 μs Test Level

Imax is the maximum tested discharge current for a Type 2 Surge Suppressor Protector. It should not be presented as an indefinitely repeatable operating current.

Telebahn Type 2 designs provide separate L-N and N-PE ratings:

Protection path	In, 8/20 μs	Imax, 8/20 μs
L-N	30 kA	60 kA
N-PE	40 kA	65 kA

These values describe high-capacity Type 2 performance. They do not represent 10/350 μs Iimp capability.

Up Must Be Linked to Test Current and Wiring

Up is the voltage protection level measured at the Surge Suppressor Protector terminals under specified test conditions. It is not necessarily the voltage that appears at the protected equipment.

A practical relationship is:

Equipment-side voltage ≈ SPD Up + conductor inductive voltage + induced cable voltage

Because:

ΔU ≈ L × di/dt

even a short conductor can add significant voltage during a steep current front. Telebahn configurations provide an Up of no more than approximately 1.5 kV at In for both L-N and N-PE paths, but installation layout must still be included in the protection assessment.

Match the Waveform to the Installation Zone

Service Entrance and LPZ Boundary

Type 1 or Type 1+2 protection should be evaluated when:

•The building has an external lightning protection system;

•Power conductors enter from LPZ 0;

•Partial lightning current may flow through the installation;

•Risk assessment requires 10/350 μs capability.

Sub-Distribution Board

A Type 2 Surge Suppressor Protector is normally used to reduce residual induced and switching surges. Selection should include:

•Uc and system nominal voltage;

•In, Imax, and Up;

•TOV capability;

•TT or TN earthing arrangement;

•Coordination with the upstream SPD;

•Available short-circuit current.

Telebahn's Type 2 configuration is intended for LPZ 0B–1 or higher locations in 230/400 V TT or TN systems. Its 2+1 topology provides defined L-N and N-PE protection rather than relying on a general multi-pole rating.

Equipment and Control Level

Sensitive equipment may require additional protection close to the load, particularly when:

•The subpanel is far from the machine;

•PLC or drive impulse withstand levels are low;

•Cables run outdoors or between buildings;

•Ethernet, RS-485, sensor, or control lines cross LPZ boundaries.

Installation Determines Actual Surge Suppressor Protector Performance

The field performance of a Surge Suppressor Protector depends heavily on conductor routing.

Installation priorities include:

•Keep L, N, and PE connections short and direct;

•Minimize conductor loops and sharp bends;

•Reduce the total surge-current path;

•Use the specified conductor cross-section;

•Maintain low-impedance equipotential bonding;

•Coordinate the approved fuse or circuit breaker;

•Confirm that the installation-point fault current is acceptable.

Remote signalling indicates operating or fault status; it should not be interpreted as a measurement of remaining service life.

Verify Test Conditions Before Comparing Suppliers

Before approving a Surge Suppressor Protector, request documentation confirming:

•Whether the rating is Iimp, In, or Imax;

•The applied 8/20 μs or 10/350 μs waveform;

•Per-pole, per-mode, or total-system rating;

•Up and its corresponding test current;

•L-N and N-PE performance;

•TOV and backup protection requirements;

•Complete-device rather than component-only testing;

•Exact model and configuration covered by the report.

Select a Surge Suppressor Protector by Actual Surge Duty

An effective Surge Suppressor specification must include waveform handling based on the application. Devices of Type 1 handle 10/350 μs currents. Type 2 devices handle 8/20 μs surges. The protection of residual voltage is achieved at the end of a circuit near sensitive equipment.

For pluggable Type 2 protection coordinated in L-N and N-PE within TT or TN sub-distribution boards, Telebahn offers solutions with a combination of high-energy MOVs, GDTs, thermal disconnector, visible condition indicator, and a remote monitoring option. Reviewing the Telebahn technical data against the project's LPZ boundary, earthing arrangement, Up target, conductor routing, and expected surge waveform can support a more defensible protection design.

FAQs

Q1. Which surge-current waveform is referenced for the rating of Telebahn Type 2 Surge Suppressor Protector products?

The specified Telebahn Type 2 products are rated using the 8/20 μs waveform for nominal and maximum discharge current. Imax and In are not to be considered as 10/350 μs Iimp ratings.

Q2. What are the main distinctions of the 8/20 μs and 10/350 μs waveforms?

The 8/20 μs waveform relates primarily to induced lightning and switching surges. The 10/350 μs waveform represents a current duty of lightning and as such, conveys more charge and energy at the same peak current compared to the 8/20 μs waveform.

Q3. Can a Telebahn Type 2 Surge Suppressor Protector be used in place of a Type 1 SPD?

Not as a direct replacement. Type 2 protection is designed for 8/20 μs surge duty, and in the case of partial lightning current entering the installation, Type 1 or Type 1 + 2 protection with 10/350 μs Iimp ratings needs to be considered.

Q4. What discharge-current ratings do Telebahn Type 2 protection products offer?

For the specified Telebahn Type 2 protection, the following ratings are offered for the different paths: L-N with In of 30 kA, N-PE with In of 40 kA, and Imax of 60 kA and 65 kA, respectively.

Q5. Why does Telebahn combine MOVs and GDTs in one Surge Suppressor Protector?

For voltage limiting in the L-N paths, MOVs are the primary elements, whereas GDTs are the primary elements for the N-PE discharge path. This combination is effective for certain TT and TN 2+1 systems.