Type 2 Surge Protective Device: TOV Risks and Selection Guide

Devices designed to be protected from Type 2 Surges can still fail from over-volt conditions, even in the absence of lightning. Voltage surges can occur in a power distribution system for a number of reasons including earth faults or system faults in upstream power equipment.

When evaluating surge protective devices, engineers must consider the over-voltage level (TOV), the level of surge protection (SP Level), the thermal stability of the MOV, and the design of the protective device to clear system faults. They must also compare the short circuit current that the devices can withstand.

1. Temporary Overvoltage vs Transient Surge: Different Electrical Stresses

A Type 2 Surge Protective Device primarily limits short-duration transient overvoltages. TOV, in contrast, is a power-frequency voltage rise that persists beyond a typical surge impulse.

Parameter	Transient Surge	Temporary Overvoltage
Typical causes	Lightning, switching	Neutral loss, network faults
Duration	Microseconds	Fault-dependent; potentially minutes
Primary stress	Impulse current and energy	Sustained voltage and heating
Evaluation	In, Imax, Up	Uc, UT, TOV test outcome
Main concern	Surge limitation	Withstand or safe failure

The distinction is important: a Type 2 Surge Protective Device rated at 40kA Imax is not automatically capable of withstanding prolonged abnormal mains voltage. The 8/20μs discharge test and TOV test evaluate fundamentally different operating conditions.

Where Does TOV Originate?

Three conditions merit further discussion:

•Neutral interruption: With three-phase, un-balanced power systems, with the neutral conductor removed, the phase to neutral voltages become un-defined and can take on any value.

•Earth faults: In some earth systems, normal phase conductor potentials can increase above earth potential.

•HV/LV faults: Faults in high voltage systems can introduce abnormal conditions to a low voltage system.

TOV exposure therefore depends on system configuration, fault magnitude, network impedance, and clearing time.

2. Why TOV Can Cause MOV Thermal Runaway

Most MOV-based Type 2 Surge Protective Device designs rely on the nonlinear voltage-current characteristics of metal oxide varistors.

MOV leakage current is low under normal operating conditions. MOVs can quickly make conduction paths as applied voltage increases.

MOVs are designed to limit voltage. If an overvoltage condition is sustained, the MOV will quickly dissipate energy and potentially fail.

The failure condition of MOVs is as follows:

• An increase in mains voltage conduction causes Joule heating.

• Joule heating increases the MOV temperature.

• An increase in temperature further increases the MOV conduction.

• Thermal runaway is the final condition before the MOV fails.

• An internal thermal disconnector must isolate the stressed component when its operating threshold is reached.

TOV Withstand vs Safe Failure

These terms should not be confused.

TOV withstand indicates that the SPD survives the specified test while satisfying the applicable operational acceptance criteria.

TOV safe failure permits loss of surge protection, provided the failure meets the applicable safety requirements.

A disconnected MOV may prevent further internal overheating, but it no longer provides its original protection function.

3. Uc, UT and Up: Three Critical Selection Parameters

At Telebahn, we consider voltage coordination a critical part of evaluating a Type 2 Surge Protective Device.

Engineers should distinguish three parameters:

•Uc (Maximum Continuous Operating Voltage): The maximum RMS voltage continuously applicable to the SPD.

•UT (Temporary Overvoltage Test Value): The abnormal voltage associated with a specified TOV test duration.

•Up (Voltage Protection Level): The declared voltage limitation under specified surge test conditions.

Telebahn Technical Comparison for 230V Systems

The following values are taken from Telebahn's published Type 2 SPD specifications.

Uc	UT (5s)	UT (120min)	Up at In
275V	335V	440V	≤1.25kV
320V	335V	440V	≤1.50kV
385V	385V	440V	≤1.80kV

All three configurations share:

•Nominal discharge current (In): 20kA (8/20μs).

•Maximum discharge current (Imax): 40kA (8/20μs).

Does Higher Uc Mean Better TOV Protection?

Not necessarily.

A higher Uc changes the MOV operating threshold but may also increase Up. As the table illustrates, the 385V configuration has a higher declared protection level than the 275V configuration.

Moreover, the published UT values alone do not identify whether each test concludes with continued operation or safe disconnection. The associated test results should be verified.

The engineering objective is to balance continuous voltage compatibility, TOV behavior, and downstream equipment insulation coordination—not simply maximize Uc.

4. Thermal Disconnection and SPD Failure Monitoring

A Type 2 Surge Protective Device must address internal degradation as well as transient discharge performance.

Telebahn incorporates several relevant protection and maintenance features:

Design Feature	Engineering Purpose
High-energy MOV	Transient voltage limitation
Double thermal disconnector	Isolation of thermally stressed protection elements
Red failure indicator	Local fault identification
Remote signaling contact	External status monitoring
Pluggable module	Maintenance and replacement access

The remote signaling contact can support centralized monitoring in distribution cabinets.

However, thermal disconnection does not eliminate the original TOV condition. A Type 2 Surge Protective Device is not a substitute for a dedicated sustained-overvoltage monitoring and disconnection system.

If the indicator turns red, qualified personnel should investigate the supply condition and replace the failed module according to manufacturer instructions.

5. System Compatibility and Installation Requirements

Before selecting a Type 2 Surge Protective Device (SPD) a number of considerations must be taken into account.

These include verification of:

•Network conditions, in particular the Voltage, Frequency, Transformer-Type (TN/TT/IT) and Protection Mode

•Coordination of Voltage, e.g. Uc, Tolerable Overvoltage (TOV) and the Time Duration

•Equipment Impulse Level

•Short Circuit Level and the Protective Device used upstream

•Earthing and Wiring, e.g. the size and length of Protective Conductors

•Maintenance Accessibility

•Failure Indication and Remote Signaling

The SPD, referenced by Telebahn, is of a type which may be used in 220-240 V, 1 phase TN Networks and is of a protection level of 2+0.

The SPD is of a sub-distribution level and is of a DIN Rail mounting type.

The length of the conductor must also be considered. Long conductor lengths may cause an increase in the Overvoltage level at the SPD.

In accordance with the requirements of the relevant standard, all conductor connections must be kept as short as possible.

6. IEC Standards and Verification of TOV

There are a number of relevant International Electrotechnical Commission (IEC) documents, i.e., IEC 61643 series. IEC 61643-11, detailing testing and assessment of AC Surge Protective Devices, describes the behavior of surge protective devices during Temporary Overvoltage conditions (TOV). IEC 61643-12 and IEC 60364-5-53 describe the application of surge protective devices and relevant installation conditions, respectively.

The second edition of IEC 61643-11 has recently been published; however, evaluation is based on the first edition, as well as the Harmonized Standard EN 61643-11.

Prior to acceptance, the following documentation shall be provided:

•The test conditions and results for TOV.

•Uc, Up, In and Imax.

•The condition of protective devices and upstream circuit conditions for short circuits.

•Drawings showing installation.

•Relevant documents demonstrating compliance.

A meaningful supplier comparison must establish not only how much surge current an SPD can discharge, but also what happens when abnormal mains voltage persists.

Engineering Support from Telebahn

Selecting a Type 2 Surge Protective Device requires coordination between electrical network conditions, surge limitation, and internal fault protection.

Telebahn has modular Type 2 SPDs with TOVs, double thermal disconnection, and fault status monitoring. Review Telebahn's Type 2 SPDs for your next low-voltage distribution project to protect your system.

FAQs

Q1. What is Telebahn's Type 2 Surge Protective Device used for?

It protects low-voltage equipment against transient overvoltages. The referenced design is intended for single-phase 220–240V TN systems using a 2+0 circuit.

Q2. Can Telebahn's Type 2 Surge Protective Device protect against TOV?

It is primarily designed for transient surge protection. Its declared TOV characteristics describe behavior under specified abnormal voltage conditions, not continuous voltage regulation.

Q3. What Uc ratings does Telebahn offer?

The range includes 150V, 275V, 320V and 385V Uc options. The 275V, 320V and 385V configurations are listed for 230V systems.

Q4. What are Telebahn's declared TOV test values?

For the 275V, 320V and 385V Uc configurations, the published 5-second UT values are 335V, 335V and 385V, respectively. All three list 440V for the 120-minute TOV test.

Q5. Does a 440V TOV rating mean the SPD remains operational for 120 minutes?

Not necessarily. The declared test voltage must be considered alongside the specified test outcome, distinguishing TOV withstand from safe failure.