Understand the difference between 175 V and 275 V SPDs, what Uc means, its relationship with 127/220 V systems, L-N/L-PE modes, TN/TT/IT, TOV, Up and selection criteria.

Check it out!

The choice between a 175 V and 275 V SPD should not be made only by looking at the nominal voltage printed on a panel or repeating a rule of thumb. These values are mainly associated with Uc, the maximum RMS continuous operating voltage that may remain applied to the SPD protection mode without causing unintended operation or degradation incompatible with its function.

Uc must be compatible with the voltage that actually appears between the terminals to which the SPD is connected. This depends on network voltage, earthing arrangement, mode of protection and device position — phase-neutral, phase-PE, neutral-PE or phase-phase. The same system can therefore require different values for different modes of protection.

There is also a trade-off. Excessively low Uc may make the SPD more susceptible to temporary overvoltages, or TOV. Very high Uc can increase continuous-operation margin, but does not guarantee better voltage limitation and may move the selection away from the protected equipment’s actual need.

The correct technical question is not “175 V or 275 V, which is better?” but rather: which Uc is compatible with the continuous voltage and TOV of the protection mode while maintaining Up and Up/f suitable for the equipment’s Uw withstand?

What is Uc in an SPD?

ABNT NBR IEC 61643-11 defines Uc as the maximum RMS voltage that can be continuously applied to the SPD mode of protection.

This definition contains two important points:

1. Uc is a continuous operating voltage, not the voltage level that the SPD limits during a surge; 2. the value applies to a specific mode of protection.

An SPD may have modes between:

  • phase and neutral;
  • phase and PE;
  • neutral and PE;
  • phase and phase.

The voltage present in each pair can be different, and the manufacturer must declare Uc for the intended modes.

For an overview of the other parameters, see Surge Protective Devices (SPDs).

Uc is not Up

Uc and Up answer different questions.

ParameterQuestion it answers
Ucwhat RMS voltage may remain continuously applied to the SPD?
Upwhat protection voltage level is declared during the surge test?
Up/fwhat is the effective level in the installation, including connection voltage drops?
Uwwhat impulse voltage can the protected equipment withstand?

A 275 V SPD may have lower or higher Up than a 175 V SPD depending on the product design. There is no universal equivalence such as “lower Uc = lower Up.”

Why do 175 V and 275 V SPDs exist?

These values are used because different networks and connection modes require different maximum continuous operating voltages.

In practical terms:

  • networks with phase-neutral voltage around 127 V may allow SPDs with lower Uc in certain modes;
  • networks with phase-neutral voltage around 220 V normally require higher Uc;
  • phase-phase connections may require still higher values;
  • phase-PE or neutral-PE connections depend on the earthing arrangement and possible overvoltages.

Selection must nevertheless be based on the voltage between the terminals of the mode of protection, not merely on the overall network label.

What NBR 5410 establishes for Uc

ABNT NBR 5410:2004, corrected version 2008, establishes minimum Uc values as a function of the earthing arrangement and the points between which the SPD is connected.

The logic uses:

  • Uo: phase-neutral voltage;
  • U: phase-phase voltage.

According to connection mode, the standard requires minimum Uc related to 1.1 Uo, √3 Uo, Uo or U.

This shows why there is no single universal value for every network.

Example using Uo

If phase-neutral voltage is 127 V, 1.1 Uo is approximately 140 V. An SPD with Uc = 175 V can, in certain modes and arrangements, provide sufficient margin above this minimum.

If phase-neutral voltage is 220 V, 1.1 Uo is approximately 242 V. In this case, Uc = 175 V would be below the continuous voltage required for a typical phase-neutral mode, while 275 V may be compatible.

This does not mean that “127 V = 175 V” and “220 V = 275 V” is a complete rule. Arrangement, mode and TOV must be considered.

175 V SPD on a 127 V network

On a network with 127 V phase-neutral voltage, an SPD with Uc = 175 V is frequently found in compatible phase-neutral or phase-PE applications.

The potential advantage is operation with Uc closer to nominal voltage, which in some technologies and product families may allow a lower limiting level.

But selection must check:

  • actual network tolerance;
  • permanent voltage rises;
  • neutral loss;
  • TOV due to faults;
  • TN, TT or IT arrangement;
  • connection mode;
  • Up;
  • backup protection.

A 175 V SPD should not be placed on every “127 V” circuit without analyzing these conditions.

275 V SPD on a 127 V network: can it be used?

A technically acceptable application may exist, but that does not mean it is always the best choice.

Higher Uc increases margin against continuous voltage and certain TOV, but may come with a different Up. The designer must compare the final protection offered to the equipment.

If a 275 V SPD has sufficiently low Up and meets the other requirements, it can be used. But choosing 275 V only “so the SPD will not burn out” may unnecessarily sacrifice performance.

175 V SPD on a 220 V network: why it is normally problematic

If the SPD is connected between phase and neutral on a network whose continuous voltage is approximately 220 V, a Uc of 175 V is below the normal voltage of the mode itself.

This can cause:

  • unintended conduction;
  • heating;
  • accelerated degradation;
  • disconnector operation;
  • device failure.

Selection must always keep Uc above the permissible continuous voltage expected for the mode of protection.

275 V SPD on a 220 V network

On networks with approximately 220 V phase-neutral, Uc = 275 V is common because it provides margin above continuous voltage and can meet minimum values derived from 1.1 Uo in certain configurations.

Even so, it is necessary to verify:

  • utility voltage variation;
  • TOV;
  • earthing arrangement;
  • Up;
  • class;
  • surge current;
  • ISCCR;
  • manufacturer instructions.

“275 V” is not a complete specification.

SPD on a 220/380 V network

On a 220/380 V network:

  • phase-neutral: approximately 220 V;
  • phase-phase: approximately 380 V.

An SPD installed phase-neutral sees a different voltage from one installed phase-phase.

A 275 V device may therefore be suitable for phase-neutral mode, but not necessarily for phase-phase mode.

The architecture must define modes of protection according to the earthing arrangement.

SPD on a 127/220 V network

On a 127/220 V network:

  • phase-neutral: approximately 127 V;
  • phase-phase: approximately 220 V.

This explains why different Uc values may exist within the same installation depending on how the SPDs are connected.

The solution should not be selected only from the statement “the network is 220 V,” because this may mean 220 V phase-phase with 127 V phase-neutral or 220 V phase-neutral in another system.

The importance of the earthing arrangement

NBR 5410 distinguishes connections in:

  • TN-S;
  • TN-C;
  • TN-C-S;
  • TT;
  • IT with neutral;
  • IT without neutral.

The arrangement changes:

  • which modes of protection are used;
  • which voltages appear between terminals;
  • fault behavior;
  • TOV;
  • relative position to RCDs.

The correct Uc therefore also depends on the grounding architecture.

SPD between phase and neutral

In a phase-neutral connection, the SPD is continuously subjected to phase-neutral voltage.

Selection must consider:

  • Uo;
  • tolerance;
  • harmonics and network conditions;
  • neutral voltage rise;
  • neutral loss;
  • TOV.

In many systems, this is where the 175 V versus 275 V comparison originates.

SPD between phase and PE

Phase-PE voltage depends on the system and on neutral/earth conditions.

In TN-S, for example, phase-PE tends to follow phase-neutral under normal operation. In TT and IT systems, fault and temporary-overvoltage conditions may differ.

Selection must consider the continuous voltage and TOV expected for this mode.

SPD between neutral and PE

The neutral-PE mode has particular behavior.

In normal operation, N-PE voltage may be small, but faults and system events can increase this potential difference.

The N-PE SPD should therefore not be selected only from normal voltage. TOV and conduction capability are also decisive.

In specific NBR 5410 connection arrangements, the N-PE device may receive the sum of currents coming from multiple phase conductors during a surge, requiring its own current assessment.

SPD between phases

In a phase-phase connection, Uc must be compatible with phase-phase voltage U, not with Uo.

This makes a 275 V SPD unsuitable in many phase-phase circuits at 380 V or 440 V.

In such cases, products with higher Uc are required.

What is TOV?

TOV means Temporary Overvoltage.

Unlike a microsecond surge, a TOV remains for much longer. It can result from:

  • neutral loss;
  • earth faults;
  • medium-voltage network faults;
  • neutral-point displacement;
  • abnormal operating conditions.

An SPD must withstand specified TOV or fail safely according to the applicable tests.

Why Uc and TOV are linked

If Uc is very close to continuous voltage, a temporary rise may cause the SPD to conduct for too long.

A varistor, for example, can heat progressively under TOV and enter thermal degradation.

On the other hand, selecting excessively high Uc only to withstand TOV may increase the limiting level in some product families.

The design must balance:

  • continuous operation;
  • TOV;
  • surge protection.

Are there recurring SPD failures, neutral loss or abnormal network behavior?

In this scenario, changing 175 V to 275 V — or the reverse — without diagnosis may only mask the cause. SPD and SPM Inspection, Diagnosis and Upgrade checks actual voltage, TOV, connections, grounding, device condition and compatibility with the installation.

Neutral loss and SPDs

Neutral loss is a critical condition in three-phase systems with unbalanced loads.

Phase-neutral voltages can shift significantly. An SPD with Uc selected too close to nominal voltage can be subjected to severe temporary overvoltage.

The solution must verify the TOV behavior declared by the manufacturer, not only Uc.

Does higher Uc always increase safety?

Not in every sense.

Higher Uc may reduce the probability of operation during temporary rises, but surge protection must not be assessed independently of residual voltage.

In some technologies, increasing Uc may result in higher Up.

The correct question is whether the Uc + TOV + Up + Up/f combination meets system requirements.

What is Up and why does it matter when choosing 175 or 275 V?

Up is the declared voltage protection level.

Two SPDs with similar currents and different Uc may have different Up.

To protect a load, Up must be compatible with Uw and with the physical installation.

NBR 5419-4:2026 establishes the concept of Up/f, the effective level including voltage drops in the branch.

Up/f: actual voltage can be higher than the catalog value

For voltage-limiting SPDs:

Up/f = Up + ΔU

ΔU includes voltage drop in conductors, connections and associated branch elements.

NBR 5419-4 presents as a reference that 1 m of connection carrying 10 kA can add approximately 1 kV.

Thus, discussing 175 V versus 275 V without controlling connection length can mean optimizing one parameter while losing protection in another.

Uw: load withstand

Uw represents equipment impulse withstand voltage.

SPD selection must ensure that effective voltage remains within load capability, considering distance and criticality.

For sections no longer than 10 m, NBR 5419-4 uses margins such as Up/f ≤ 0.8 Uw, reaching 0.5 Uw in critical systems.

If distance exceeds 10 m, additional measures may be necessary.

How to select Uc: an eight-step method

1. Identify actual system voltage

Do not use only “127,” “220” or “380” without knowing whether the value is phase-neutral or phase-phase.

Record:

  • Uo;
  • U;
  • operating tolerance;
  • alternative sources.

2. Identify the earthing arrangement

Define TN-S, TN-C, TN-C-S, TT or IT.

3. Define the modes of protection

Specify phase-neutral, phase-PE, N-PE and/or phase-phase.

4. Determine continuous voltage in each mode

Use the actual topology, not a generic table.

5. Check the minimum normative Uc

Compare with NBR 5410 criteria for the corresponding arrangement and mode.

6. Evaluate TOV

Consider neutral loss, faults and manufacturer data.

7. Compare Up and Up/f with Uw

Do not select Uc in isolation.

8. Document the decision

Record the value and rationale for each mode of protection.

Example: 127/220 V TN-S network

Consider a network with:

  • 127 V phase-neutral;
  • 220 V phase-phase;
  • TN-S.

For a phase-neutral SPD, the reference calculation based on 1.1 Uo gives a minimum near 140 V. Uc = 175 V may be technically compatible provided TOV, Up and manufacturer data are suitable.

For a phase-phase mode, voltage is 220 V. A 175 V device would not be compatible with the mode’s continuous voltage.

The example shows why one “175 V SPD for a 127/220 V network” does not describe every mode.

Example: 220/380 V TN-S network

Here:

  • Uo = 220 V;
  • U = 380 V.

For phase-neutral, 1.1 Uo is approximately 242 V. Uc = 275 V can meet the minimum requirement in many applications.

For phase-phase, 275 V is below the continuous 380 V and is not suitable for that mode.

Example: TT system

In TT, the relationship between neutral, PE and local earth requires special attention.

SPD position relative to the RCD and TOV resulting from faults must be analyzed.

NBR 5410 provides specific conditions for SPDs upstream of the RCD in TT and for the N-PE mode.

The simple 175/275 V choice does not resolve the architecture.

Does the installation operate in TT, IT or have complex interfaces between neutral, PE and RCDs?

Defining Uc must consider the protection mode and temporary overvoltages possible in each arrangement. In complex installations, the Low-Voltage Electrical Design should coordinate SPDs, RCDs, grounding, panels and operating conditions.

Example: IT system

In IT, phase-earth voltage can change significantly during the first fault.

NBR 5410 therefore establishes different Uc requirements for certain modes in IT with or without neutral.

This is one of the scenarios in which applying a residential rule based only on 127 or 220 V is particularly unsuitable.

440 V and 460 V SPDs

In installations with higher voltages, such as 380/440 V phase-phase or 440 V in certain modes, SPDs with Uc of 440 V, 460 V or other values are available.

The same methodology remains valid:

  • identify voltage between terminals;
  • verify TOV;
  • compare Up;
  • verify class and current;
  • confirm the connection arrangement.

The principle does not change merely because the number is higher.

Uc and SPD class are independent

You can find:

  • Class I 275 V;
  • Class II 275 V;
  • Class II 175 V;
  • combined T1+T2 devices with different Uc.

Class describes the energy/test regime. Uc describes continuous voltage.

Mixing the concepts leads to errors such as “Class I must be 275 V.”

For more detail on classification, see Class 1, Class 2 and Class 3 SPDs.

Uc and discharge current are also independent

A 175 V SPD can exist with 20 kA, 40 kA or other capacities. The same applies to 275 V.

Surge current relates to energy stress. Uc relates to continuous operation.

The current comparison is detailed in 20 kA, 40 kA, 45 kA or 60 kA SPD.

Uc does not define ISCCR

ISCCR represents the rated short-circuit current associated with the SPD and specified disconnectors.

A 275 V 40 kA SPD may have ISCCR unsuitable for a main LV switchboard with high short-circuit current.

Voltage and surge current therefore do not replace short-circuit verification.

The interface with circuit breakers and fuses is covered in SPD and Circuit Breaker.

Uc and coordination between SPDs

In a cascade, devices may have different Uc and Up.

Coordination must ensure:

  • stability of each stage at continuous voltage;
  • compatible energy capability;
  • progressive protection levels;
  • absence of unintended operation;
  • final load protection.

Not every SPD in an installation needs exactly the same Uc, provided each mode and point is correctly specified.

Common mistakes when choosing 175 V versus 275 V

Selecting from phase-phase voltage without identifying Uo

A “220 V” network may be 127/220 V or may have 220 V phase-neutral in another configuration.

Using 175 V in a mode permanently near 220 V

This can cause conduction and degradation.

Always using 275 V “for safety”

This can increase TOV margin, but must be checked against Up and required protection.

Ignoring neutral loss

Severe TOV can destroy an SPD correctly sized only for normal operation.

Ignoring TN, TT and IT

The earthing arrangement changes voltages and modes.

Confusing Uc with Up

They are different quantities.

Confusing Uc with class

Class I, II or III does not determine maximum continuous voltage.

Not checking the datasheet by mode

Multipole SPDs may behave differently between L-N, L-PE and N-PE.

How to specify Uc in a design memorandum

A technical memorandum should record at least:

  • network nominal voltage;
  • Uo and U;
  • earthing arrangement;
  • mode of protection;
  • minimum calculated or required Uc;
  • selected Uc;
  • TOV considered;
  • Up;
  • estimated Up/f;
  • load Uw or applicable category;
  • class;
  • In/Iimp;
  • ISCCR;
  • backup protection;
  • reference model.

This turns “275 V SPD” from a purchase line into a verifiable specification.

How to audit an installed 175/275 V SPD

In existing installations, check:

1. actual panel voltage; 2. connection mode; 3. earthing arrangement; 4. model Uc; 5. Up; 6. indicator condition; 7. backup protection; 8. available short-circuit current; 9. connection length; 10. history of TOV or neutral loss; 11. network modifications; 12. coordination with other SPDs.

An SPD that has operated for years is not automatically correctly specified. It may simply not yet have been subjected to the critical event or may be operating with inadequate margin.

For a field assessment, see SPD and SPM Inspection, Diagnosis and Upgrade.

When Uc selection requires SPM design

The analysis should be treated as a design when there are:

  • LPS;
  • multiple panels;
  • complex TN/TT/IT networks;
  • critical equipment;
  • multiple power sources;
  • generators;
  • UPS systems;
  • photovoltaic systems;
  • signal lines;
  • history of failures;
  • long distances;
  • interconnected structures.

The Surge Protection Measures Design consolidates Uc, TOV, Up, Up/f, classes, currents, LPZ, coordination, grounding and documentation.

Do you need to specify SPDs in a critical or multi-panel installation?

When the decision involves more than choosing “175 V or 275 V,” specification should move from component level to system engineering. See the Surge Protection Measures (SPM) Design.

Summary: 175 V or 275 V SPD?

The decision should follow this sequence:

1. identify Uo and U; 2. identify the earthing arrangement; 3. define the mode of protection; 4. calculate/check the applicable minimum Uc; 5. consider TOV; 6. compare Up; 7. verify Up/f against Uw; 8. verify class, current and ISCCR; 9. coordinate with other SPDs.

On many networks with 127 V phase-neutral, 175 V may be appropriate for certain modes. On many networks with 220 V phase-neutral, 275 V may be the coherent choice. But these associations are valid only when the mode, arrangement, TOV and other parameters confirm the application.

Correct selection is not “175 V for 127 V and 275 V for 220 V.” Correct selection is Uc compatible with continuous voltage and TOV while preserving the protection level required by the equipment.

Technical references

[1] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. ABNT NBR IEC 61643-11:2021, corrected version 2022 — Dispositivos de proteção contra surtos de baixa tensão — Parte 11. Available at: ABNT Catalog.

[2] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. ABNT NBR 5410:2004, corrected version 2008 — Instalações elétricas de baixa tensão. Available at: ABNT Catalog.

[3] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. ABNT NBR 5419-4:2026 — Proteção contra descargas atmosféricas — Parte 4. Available at: ABNT Catalog.

[4] INTERNATIONAL ELECTROTECHNICAL COMMISSION. IEC 61643-11:2011 — Low-voltage surge protective devices — Part 11. Available at: IEC Webstore.

Frequently asked questions
What is the difference between a 175 V and 275 V SPD?

The main difference is maximum continuous operating voltage Uc. The value must be compatible with the voltage between the terminals of the protection mode and with temporary overvoltages on the network.

Is a 175 V SPD suitable for a 127 V network?

It can often be appropriate in certain modes of protection on 127 V phase-neutral networks, but selection depends on earthing arrangement, TOV, Up and connection mode.

Is a 275 V SPD suitable for a 220 V network?

On many 220 V phase-neutral networks, Uc of 275 V is compatible with the continuous-voltage requirement, but the design must also verify TOV, Up, class and other parameters.

Can I use a 275 V SPD on a 127 V network?

It may be technically possible, but it is not automatically the best choice. Check whether Up and final performance remain appropriate for the load.

Can I use a 175 V SPD on a 220 V network?

In phase-neutral mode with approximately 220 V continuously present, Uc of 175 V is normally below operating voltage and tends to be unsuitable.

What is Uc in an SPD?

Uc is the maximum RMS voltage that may be continuously applied to an SPD mode of protection.

Is Uc the same as Up?

No. Uc concerns permissible continuous voltage; Up is the protection voltage level during a surge.

What is TOV?

TOV is a temporary overvoltage lasting much longer than a transient surge and may occur, for example, because of neutral loss or system faults.

Can neutral loss damage an SPD?

It can subject the SPD to high TOV. Suitability depends on Uc, the product’s TOV behavior and network architecture.

Can a 275 V SPD be used phase-to-phase at 380 V?

Not if continuous voltage between its terminals is 380 V, because Uc of 275 V would be below the normal voltage of that mode.

Do TN, TT and IT change Uc selection?

Yes. The earthing arrangement changes connection modes, terminal voltages and behavior under faults and TOV.

How do I choose between 175 V and 275 V?

Identify Uo, U, earthing arrangement and mode of protection; verify the applicable minimum Uc, TOV, Up, Up/f, class, current and ISCCR.

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