275 V vs 320 V vs 385 V SPD: How to Choose the Correct Uc

275 V, 320 V or 385 V: The Short Answer

For a 230/400 V AC installation, a 275 V SPD is often the first candidate when its protection mode normally sees about 230 V, but it is not an automatic choice for every network. Use 320 V when the approved design needs more continuous-voltage or temporary-overvoltage margin, and use 385 V only when the actual protection mode, earthing arrangement and manufacturer data justify that higher rating. Do not select from the nominal system voltage alone.

The correct U_c is the lowest approved maximum continuous operating voltage that remains above the highest voltage the SPD can experience continuously in its connected mode. It must also survive the temporary overvoltage conditions expected at that point. After selecting U_c, verify that the resulting voltage protection level U_p is still low enough for the protected equipment.

That last check matters because U_c is not a surge-current rating. A 385 V cartridge is not a stronger version of a 275 V cartridge in every respect. It may tolerate a higher sustained power-frequency voltage, but it can begin limiting at a higher voltage and may have a higher declared U_p. The best choice therefore balances service stability against protection level.

What U_c Actually Means on an AC SPD

U_c is the maximum continuous operating voltage that may be applied across the SPD terminals under the manufacturer’s stated conditions. On an AC product, it is normally an RMS value at the declared frequency. During normal operation, the SPD must remain stable at this voltage without behaving as though every mains crest were a surge.

This definition explains why U_c must be read across one specific protection mode:

• L–N: phase to neutral;
• L–PE: phase to protective earth;
• N–PE: neutral to protective earth; or
• L–L: phase to phase in an arrangement designed for that mode.

A three-phase network may be described as 230/400 V, yet the voltage across an individual SPD module can be 230 V or 400 V depending on where that module is connected. The system label and the cartridge label are therefore not interchangeable.

Do not confuse U_c with the other figures printed on an SPD:

Marking	What it describes	What it does not prove
Uc	Maximum voltage that may remain continuously across the declared protection mode	Surge discharge capacity or the voltage delivered to the load during a surge
Up	Declared voltage protection level under specified impulse-test conditions	Continuous operating-voltage tolerance
In	Nominal discharge-current duty using the 8/20 µs waveform	Direct-lightning capability
Imax	Maximum discharge current for a Type 2 SPD under the stated 8/20 µs test	Repeated duty at that maximum value
Iimp	Impulse-current duty associated with Type 1 testing and the 10/350 µs waveform	A substitute for Uc or Up
UT / TOV data	Behaviour under a specified temporary power-frequency overvoltage for a stated duration	Permission to operate continuously at that voltage

For a broader explanation of SPD types, discharge-current ratings and installation positions, use the JUTRION surge protective device guide. This article stays with the narrower decision: choosing among 275 V, 320 V and 385 V U_c values.

Why All Three Ratings Appear in 230/400 V Projects

The three ratings exist because a low-voltage system does not present one perfectly fixed voltage under all operating and fault conditions. Supply tolerance, voltage regulation, harmonic distortion, neutral displacement, earth faults and the earthing arrangement can change the voltage applied to a protection mode. Product families also differ in their TOV behaviour and internal technology.

For a nominal 230 V phase-to-neutral supply, the simple ratio is:

275 / 230 = 1.20, 320 / 230 = 1.39, and 385 / 230 = 1.67.

These ratios are useful for seeing the increasing continuous-voltage margin, but they are not a complete selection formula. They do not describe the site’s maximum steady-state voltage, the protection mode, or how the SPD behaves during a TOV. Two SPDs with the same U_c can also have different U_T, U_p, backup-protection and disconnection data.

Uc class	Typical reason it enters the shortlist	Main benefit	Main check before approval
275 V AC	The protection mode normally sees approximately 230 V and the product’s TOV data suits the network	Tighter voltage margin often supports a relatively low protection level	Can it remain stable at the site’s highest continuous voltage and survive the specified TOV?
320 V AC	The design needs additional operating-voltage margin, or the manufacturer assigns this class to the required network arrangement	More tolerance to sustained voltage elevation than a 275 V class	Is its Up still coordinated with the equipment impulse withstand voltage?
385 V AC	The connected mode or approved TOV strategy requires substantially more voltage margin	Reduced risk of unwanted operation from elevated power-frequency voltage	Does the higher class leave a sufficiently low effective protection level at the load?

This is a screening table, not a universal substitution chart. A 320 V device cannot automatically replace a 275 V device, and a 385 V device cannot be approved merely because it is less likely to react to an elevated mains voltage.

Start with the Voltage Across Each Protection Mode

Draw the SPD connection before choosing the cartridge. Mark the normal voltage across every mode and ask what voltage can appear there during the credible operating and fault states defined by the project.

Single-Phase 230 V, L–N Protection

An SPD connected from line to neutral normally sees the phase-to-neutral voltage. In an ordinary, correctly regulated 230 V system, 275 V is commonly shortlisted. The approval still depends on the permitted supply range, site measurements where relevant, TOV requirements and the exact datasheet.

If the site regularly operates near or above the upper boundary assumed by the design, selecting 275 V from the nominal label alone can cause accelerated ageing or unnecessary thermal disconnector operation. The engineering response is not automatically “fit 385 V.” First determine whether the elevated voltage is a valid operating condition, a supply-quality problem, a neutral fault or an application outside the original design.

Three-Phase 230/400 V, L–N or L–PE Protection

In a 230/400 V wye system, each phase is about 230 V to neutral and about 400 V to another phase. A module connected L–N is therefore not continuously exposed to 400 V during normal operation. A module connected in another mode may face a different requirement.

This distinction is why pole count does not select U_c. “Four-pole SPD for a 400 V panel” is incomplete. The specification must state the network configuration and protection circuit, such as 3+1 or 4+0, and identify the voltage across each module.

Earthing System and Circuit Arrangement Change the Answer

The same 230/400 V nominal supply can require a different SPD assembly depending on whether it is TN-C, TN-S, TT or IT. The important question is not only how many modules are installed, but which components are connected between which conductors.

TN Systems

TN-C and TN-S systems have different PEN or N/PE connection points, so use an assembly declared for the specific TN arrangement. A 275 V cartridge is common for a 230 V protection mode, but its TOV performance still has to pass; 320 V or 385 V variants are used only where the required continuous-voltage or TOV margin supports them.

TT Systems and 3+1 Circuits

A 3+1 arrangement commonly places voltage-limiting components from phase to neutral and a spark-gap-based component from neutral to PE. The phase cartridges are therefore selected for L–N duty; the N–PE component has different data. This arrangement can control TOV consequences in TT systems, but it must follow the applicable installation standard and manufacturer instructions.

IT Systems

In an IT system, healthy conductors can rise relative to earth after the first earth fault. Do not specify a 275 V L–PE cartridge solely because the loads are called 230 V. Use an assembly declared for the IT arrangement and verify conductor-to-earth voltage, protection modes and TOV behaviour.

The Real Trade-Off: Nuisance Ageing Versus Protection Level

Choosing U_c too low and choosing it too high create different problems.

If U_c Is Too Low

A metal-oxide varistor is voltage-dependent. Repeated or sustained operation too close to its continuous limit can increase leakage, temperature and ageing. A temporary overvoltage outside the declared capability can cause the internal thermal disconnector to operate, leaving the downstream circuit energized but no longer surge-protected. In a severe mismatch, the failure can be unsafe unless the product and backup protection clear it correctly.

A red indicator shortly after commissioning does not necessarily prove that the site experienced an exceptional lightning event. The investigation should include steady-state voltage, neutral integrity, wiring, TOV exposure, backup protection and whether the installed U_c and circuit match the network.

If U_c Is Too High

A higher-U_c varistor normally begins significant conduction at a higher voltage. Depending on the product design, this can raise U_p. For example, one manufacturer’s otherwise comparable single-pole Type 2 family declares U_p at no more than 1.5 kV for its 275 V and 320 V versions, but no more than 1.75 kV for its 385 V version. That is a product example, not a universal conversion rule, but it shows why the higher number is not automatically superior.

The relevant protection at the load is not just the catalogue U_p. Connection-lead inductance adds voltage while surge current rises. Long conductors can erase the apparent advantage of a lower U_p. U_c, U_p, conductor length and equipment impulse withstand voltage must therefore be coordinated together.

Selection principle: choose the lowest U_c class that is approved for the maximum continuous voltage, protection mode, earthing arrangement and TOV duty. Then confirm that its U_p and installed lead voltage remain below the protected equipment’s impulse withstand requirement with an adequate margin.

Field Case: Why Moving Straight to 385 V Left the VFDs Exposed

An anonymized cold-chain packaging facility in Southeast Asia experienced repeated SPD disconnector operation on secondary 230/400 V distribution panels during the first three months after commissioning. The indicators on several installed 275 V Type 2 cartridges changed to red. The local contractor assumed that frequent lightning surges were exhausting the SPDs and replaced the cartridges with 385 V versions, treating the higher U_c as a heavier-duty rating.

Two months later, a grid-switching event was followed by dielectric failure in three variable frequency drives and two digital controller cards. The 385 V cartridges remained serviceable, forcing the investigation to separate the original nuisance disconnection from the later failure of protection coordination.

Why the 275 V Cartridges Were Disconnecting

Recorded phase-to-neutral voltage repeatedly reached 258–262 V AC RMS and contained notable fifth-harmonic distortion. The investigation attributed rising varistor leakage and temperature to prolonged power-frequency stress rather than repeated lightning impulses. This does not mean every 275 V SPD will fail at 262 V: the response also depends on TOV data, waveform, ambient temperature and ageing. Here, the site measurements and product data had not been reviewed together.

Why the 385 V Replacement Was Not an Equivalent Upgrade

The 385 V cartridges tolerated the elevated supply, but their declared U_p was 1.8 kV versus no more than 1.35 kV for the original cartridges. The 60 cm connection path was estimated, for the assessed surge current and geometry, to add about 0.6 kV. The resulting 2.4 kV first-pass estimate did not exceed the drives’ stated 2.5 kV impulse withstand, but it left almost no margin for tolerance, waveform and installation effects.

Resolution: Use the Middle Voltage Class and Shorten the Current Path

The replacement used a JUTRION 320 V Type 2 configuration declared at U_p ≤ 1.4 kV. Its U_c provided approximately 22% margin over the recorded 262 V RMS, subject to confirmation against the product documentation and site TOV assessment. Busbar comb connections and optimized V-wiring reduced the total path from about 60 cm to less than 25 cm; under the same estimation method, inductive addition fell below approximately 0.25 kV and the effective protection estimate stayed below about 1.65 kV.

No repeat nuisance disconnection or related controller damage was reported during the observation period. The correction combined sufficient continuous-voltage margin, a lower U_p and a shorter discharge path instead of automatically selecting the highest U_c.

A Worked 230/400 V Panel Comparison

For an illustrative 230/400 V commercial board with separate N and PE conductors and a 3+1 SPD, review the candidates in this order:

1. Identify the mode voltage. Each phase module is connected L–N, so it normally sees about 230 V, not 400 V.
2. Establish the highest continuous supply voltage. Use the project’s supply tolerance and any verified site data. Do not treat an abnormal neutral condition as an ordinary continuous operating point.
3. Define TOV duty. Review the earthing system, plausible neutral and earth faults, and the U_T-versus-time data required by the installation design.
4. Compare approved products. A 275 V version may provide adequate continuous and TOV performance. If it does not, the 320 V version may be evaluated. A 385 V version remains a candidate only if the higher margin is actually necessary.
5. Recheck protection performance. Compare U_p for each exact model, then include the effect of the connecting conductors and coordination with downstream protection.

If 275 V passes the continuous-voltage and TOV checks, it normally remains the first candidate. Move to 320 V when additional margin is required and U_p remains acceptable. Use 385 V only when the mode or TOV duty justifies it and the downstream protection calculation still passes.

Read U_c, U_T and U_p as One Set

An SPD datasheet should allow the designer to trace three different voltage questions:

• U_c: What voltage may remain continuously across this protection mode?
• U_T or TOV characteristic: What happens at a stated power-frequency overvoltage and duration—does the SPD withstand it or fail safely?
• U_p: To what declared level does the SPD limit a specified impulse at its terminals?

Do not infer one value from another. After voltage compatibility is established, separately verify Type 1, Type 2 or Type 1+2 duty; I_imp, I_n and I_max; prospective short-circuit current; disconnection; backup protection; and remote indication.

What to Put in an SPD Enquiry or Specification

“Need 4P 385 V SPD” is not enough information for a reliable product match. Send the electrical context that determines the cartridge duty:

• nominal system voltage and frequency;
• single-phase or three-phase conductor arrangement;
• earthing system: TN-C, TN-S, TT, IT or another defined arrangement;
• required protection circuit, such as 1+1, 3+1 or 4+0;
• SPD type and installation position;
• required U_c, TOV and U_p performance;
• I_imp, I_n and I_max requirements;
• prospective short-circuit current and upstream protection;
• remote contact, replaceable module and certification requirements; and
• destination market, quantity and project documentation needs.

The JUTRION AC SPD range includes Type 2 and Type 1+2 options for low-voltage distribution systems. A final model should be matched to the actual network and connection diagram rather than selected from the U_c number in isolation.

Common Questions About 275 V, 320 V and 385 V SPDs

The Practical Selection Rule

Do not choose 275 V, 320 V or 385 V from the front label of the distribution board. First establish the voltage across each SPD protection mode, then verify the highest continuous voltage, earthing arrangement and TOV duty. From the ratings that pass those checks, choose the option that still gives adequate U_p coordination at the protected equipment.

In many ordinary 230 V protection modes, that process begins with 275 V. It moves to 320 V or 385 V only when the network conditions and manufacturer data support the change. Higher U_c is additional voltage margin—not a universal measure of better surge protection.

Technical References

• IEC 61643-11:2025 — Low-voltage surge protective devices connected to AC low-voltage power systems
• IEC 61643-12:2020 — Selection and application principles for low-voltage SPDs
• Schneider Electric — IEC SPD parameters including U_c, U_p and I_n