SPDは過電圧から保護しますか？ はい、ただしその答えはどのような種類の過電圧が関与しているかによって異なります。サージ防護デバイス（SPD）は主に過渡過電圧を制限するように設計されていますが、一時的過電圧（TOV）や持続的な異常電圧については別の分析が必要です。.

「過電圧保護」というフレーズは、 “「過電圧保護」” すべての過電圧が過渡サージであるとは限らないため、誤解を招く可能性があります。.

次の3つの状況を検討してください：

雷サージは、わずか数秒の極めて短い時間だけ配電盤に到達します。.

ネットワークの故障により、供給電圧が数秒間にわたって大幅に上昇します。.

あるいは、中性線の接触不良により、故障が修理されるまで設備の一部に異常電圧が残留することがあります。.

これら3つはすべて 過電圧, 、関連していますが、同一の電気的事象ではありません。.

また、SPDはこれらすべてに対して同じように反応するわけではありません。.

最も重要な違いは以下の通りです。

従来のSPDは、主に過渡過電圧を制限するように設計されています。一時的または持続的な異常供給電圧に対する保護として、自動的に見なすべきではありません。.

この区別は、SPDの選定、故障解析、さらには技術者が損傷したSPDをどのように解釈するかに影響を与えます。.

IEC 61643-01:2024 は、低圧SPDを少なくとも1つの非線形素子を含み、かつ以下の目的を持つ機器と定義しています。 サージ電圧を制限し、サージ電流を分流させること. 。交流低圧電源システムの場合、製品要件は以下に規定されています。 IEC 61643-11:2025.

クイックアンサー：SPDは過電圧を保護するか？

はい。ただし、特定の種類の過電圧に限ります。.

SPDは主に以下の目的で設計されています。 過渡過電圧, 、雷サージや開閉サージなどによって引き起こされるもの。.

通常、供給電圧を調整したり、RMS電圧が高すぎる場合に負荷を遮断したりすることを目的としたものではありません。.

下表に基本的な違いを示します。.

これが、以下の問いが生じる理由です：

“「私の分電盤にはすでにSPDが設置されていますか？」”

これは、以下の問いとは異なります：

“「私の機器はあらゆる異常電圧状態に対して保護されていますか？」”

そうではありません。.

過渡サージ vs 一時的過電圧(TOV) vs 持続的過電圧

「クラス」という言葉 過電圧 は広範なカテゴリーです。.

どの保護デバイスが応答すべきかを決定する前に、事象の持続時間と原因を理解することが不可欠です。.

1. 過渡過電圧

過渡過電圧とは、短時間の電圧外乱のことです。.

一般的な原因は以下の通りである。

• 雷の影響;
• 開閉操作;
• 誘導負荷の開閉;
• コンデンサの開閉;
• 電気システムにおけるその他の過渡現象。.

標準的なSPD試験では、一般的に以下のようなインパルス波形が使用されます。 8/20 μs電流波形 タイプ2アプリケーション用。.

この現象は極めて短時間で発生します。.

これこそが、SPDが設計された環境そのものです。クランプおよび電流分流プロセスの詳細については、以下のガイドをご覧ください。 DCサージ保護デバイスの仕組み.

2. 一時的過電圧 — TOV

TOV（一時的過電圧）は異なります。.

電圧はインパルスよりもはるかに長く上昇した状態が続き、一般的にマイクロ秒単位の単一サージではなく、電力系統に関連しています。.

考えられる原因は以下の通りです：

• 地絡；;
• 中性線の問題；;
• 系統の接地状態；;
• フェロレゾナンス（鉄共振）；;
• 電圧調整の問題；;
• 異常な開閉状態。.

大きさだけでリスクを定義することはできない。.

電圧 + 持続時間 + 保護モード + 接地システム + SPD設計 すべてが重要である。.

例えばABBは、 ウク, 最大連続使用電圧と、 UT, 一時過電圧耐力を区別している。その OVR実務ガイド TOV（一時的過電圧）の挙動をサージ電流性能とは分けて扱い、適用されるストレスがSPDの接続方法やネットワーク構成に依存することを示しています。.

持続的または恒久的な過電圧

異常な供給電圧が一時的な障害イベントのように振る舞うのではなく継続する場合、技術的な課題は再び変化します。.

一般的な並列接続型SPDを電圧調整器として使用してはなりません。.

設計目的が以下の場合、通常、RMS電圧を監視し遮断を開始するように設計されたデバイスが必要です。

供給電圧が許容閾値を超過または下回った状態が続く場合に負荷を遮断する。.

例えば、シュナイダーエレクトリック Harmony 電圧監視リレー 過電圧および不足電圧の監視機能を提供します。これは、SPDの高速サージ放電機能とは異なる機能です。.

Why Can an SPD Handle a Surge but Be Damaged by a TOV?

This seems contradictory at first.

An SPD may discharge a very large surge current measured in kiloamperes.

This is why asking “does an SPD protect against overvoltage?” requires first identifying whether the event is a transient surge, a TOV, or a sustained abnormal supply condition.

So why could a much smaller abnormal AC voltage damage it?

回答は以下の通りです。

Energy is not determined by current magnitude alone. Time matters.

Consider an MOV-based SPD.

Under normal operating voltage, the MOV remains in a high-impedance state and conducts only a very small current.

When a transient surge raises the voltage sufficiently, the MOV becomes conductive.

Surge current is diverted through the SPD and the voltage is limited.

The surge then disappears very quickly.

The MOV returns toward its normal high-impedance state.

A simplified sequence looks like this:

That last condition is the problem.

If the voltage does not disappear quickly enough, the MOV may continue conducting.

Electrical energy then becomes heat.

In simplified form:

abnormal voltage → MOV current → power dissipation → temperature rise → disconnector operation or component damage

This is why:

40 kA Imax does not mean that an SPD can withstand any abnormal supply voltage.

Imax describes a defined surge-current capability.

TOV behavior describes a completely different type of electrical stress.

Uc, Up and TOV: Three Different Questions

Another common source of confusion is mixing voltage specifications together.

Suppose an SPD datasheet contains ウク, 上へ and a TOV/UT specification.

They should not be compared as though they describe the same thing.

The IEC definition of an SPD focuses on limiting surge voltage and diverting surge current. IEC 61643-12:2020 separately covers selection, operation, location and coordination principles for AC power SPDs. If the differences between Uc, Up, In, Imax and Iscpv are not yet clear, see our DC SPD仕様ガイドをご参照ください.

Does Higher Uc Mean Better TOV Protection?

Not automatically.

It is tempting to think:

Higher Uc = safer SPD.

それはあまりに単純すぎます。.

A suitably higher Uc can give more operating-voltage margin, but SPD selection involves a tradeoff.

The device must remain stable at the expected continuous system voltage while still providing an appropriate surge protection level.

NIST researchers have described this design dilemma for decades: an SPD must provide effective surge limitation without being expected to clamp longer-duration power-frequency overvoltages indefinitely.

したがって：

Do not deliberately oversize Uc simply because you are worried about TOV.

Instead, check:

Real Case 1: A Documented TOV Above 150% Lasted About Four Seconds

This is where the distinction stops being theoretical.

A NIST-hosted paper by François Martzloff and Arshad Mansoor discusses actual power-system overvoltage scenarios.

One example cited from an EPRI report involved a power-system switching incident during restoration of power phase by phase.

The recorded condition produced an RMS voltage exceeding 150% of normal voltage for approximately four seconds.

The researchers identified the phenomenon as a temporary overvoltage associated with ferroresonance.

Notice what makes this different from a normal SPD surge event:

Four seconds may sound short to a person.

For an MOV that is dissipating abnormal power continuously, it is extremely long compared with a microsecond-scale surge.

Real Case 2: Commercial SPDs Were Tested Under Simulated TOV Conditions

Another NIST-hosted TOV study by researchers from Eaton Electrical, EPRI Solutions and François Martzloff investigated what happens when commercial SPDs experience several TOV conditions.

The study intentionally exposed commercial devices to conditions representing real power-system abnormalities.

The test program included the following examples:

The values relate to the study’s 120 V system basis; they must ない be copied directly as ratings for another SPD or another electrical system.

The result was especially important.

The researchers found that SPD response varied substantially depending on the product design.

The three cord-connected SPD specimens failed under the simulated 1.5 pu neutral-loss condition, while the two permanently connected specimens used in that study survived the first four test scenarios. Under the extreme 3.0 pu test, the permanently connected specimens experienced partial internal failure but retained some protective capability.

This should ない be interpreted as:

“Hardwired SPDs always survive TOV.”

The study used only a small group of specific products and represents historical designs.

Its real lesson is more useful:

Two products with similar-looking surge ratings may behave very differently under a long-duration abnormal voltage condition.

That is why TOV cannot be judged from Imax alone.

Does an SPD Protect Against Overvoltage Caused by Neutral Loss?

Neutral interruption is one of the most useful examples because it explains why the phrase “the SPD should clamp the voltage” can become dangerous.

In a multi-wire system with unequal phase-to-neutral loads, loss of the neutral can shift the neutral point.

The voltage across individual loads can then become severely unbalanced.

NIST describes loss-of-neutral conditions in North American 120/240 V three-wire systems where one side can approach twice normal voltage depending on the connected load impedances. The paper notes that such neutral problems may result from loose connections, mechanical damage or corrosion.

Now imagine an MOV-based SPD connected across the affected conductors.

If the abnormal power-frequency voltage pushes the MOV into conduction:

it begins conducting continuously.

But an MOV is not supposed to behave like a voltage regulator carrying that current indefinitely.

It may heat rapidly.

Eventually the result may be:

The last point is particularly important.

A typical parallel-connected SPD can disconnect itself while the protected circuit remains energized.

したがって：

SPD failure or disconnection does not necessarily remove the abnormal voltage from the load.

A conventional SPD should therefore not be relied on as the sole protection against neutral-loss overvoltage. Where the design requires abnormal RMS voltage to be detected and the load disconnected, a dedicated voltage-monitoring or protection function may be needed. The neutral fault itself must still be located and corrected.

Why Doesn’t the SPD Simply Clamp the TOV?

Because doing so could require the SPD to absorb or divert power continuously.

Consider a simplified example.

A Type 2 MOV-based SPD is installed across the supply.

A brief transient occurs.

The MOV conducts for a very short time and then stops.

That is normal.

Now assume abnormal mains voltage remains high for many seconds.

If the MOV attempts to hold the voltage down continuously, the SPD essentially becomes part of a power-frequency current path.

The longer the current flows, the greater the thermal stress.

That can lead to:

thermal runaway → disconnector operation → loss of SPD protection or component damage

This is exactly why SPDs require coordinated fault and disconnection behavior rather than simply “clamping everything.”

TOV Withstand Does Not Mean TOV Protection for the Load

This distinction is extremely important when reading datasheets.

Suppose a manufacturer states that an SPD has a certain UT or TOV capability.

That generally describes the SPD’s behavior under specified test conditions.

It does not automatically mean:

“The downstream equipment will remain at normal voltage throughout the TOV.”

These are different functions.

Compare them:

Schneider’s Harmony control relays, for example, monitor overvoltage and undervoltage conditions with threshold and timing functions; this is fundamentally different from the shunt surge-diversion function of an SPD.

Can a Circuit Breaker Protect Against Overvoltage?

A standard circuit breaker should not automatically be treated as an overvoltage protector either.

Its primary job is overcurrent protection.

If voltage rises while load current remains below the breaker trip characteristic, an ordinary breaker may remain closed.

That means four devices can perform four different functions:

This is why replacing one device with another based only on the word “protection” is a mistake. For the separate question of SPD overcurrent coordination, see our guide to SPD backup fuse and circuit-breaker selection.

Can a Bigger kA SPD Solve a TOV Problem?

そうだ。.

Suppose one SPD is rated:

In = 20 kA
Imax = 40 kA

and another says:

Imax = 60 kA

The 60 kA product is not automatically better at surviving a temporary power-frequency overvoltage.

The ratings describe different stresses.

Therefore, if SPDs repeatedly fail without obvious lightning activity, simply purchasing a higher-Imax model may not solve the actual problem.

Repeated SPD Failure: What Should You Investigate?

This is a situation where field diagnosis matters more than changing brands.

Use this investigation table.

NIST’s historical analysis lists power-system faults, poor voltage regulation, conductor problems, generator conditions, ferroresonance and loss of neutral among conditions capable of contributing to SPD overvoltage stress. If a device already shows a red indicator, abnormal heating or visible damage, our guide on SPDの故障を見分ける方法 explains the next inspection steps.

What About TOV in Solar PV DC Systems?

Extra care is required here.

Most discussion of power-frequency TOV relates to AC systems.

A PV DC system has different operating conditions.

Important PV DC concerns include:

IEC 61643-31:2018 specifically covers SPDs intended for the DC side of PV installations up to DC1500V. IEC 61643-41:2025 now covers general DC low-voltage SPDs up to 1500 V DC, but it explicitly excludes PV applications, which remain within IEC 61643-31.

Therefore, do not take an AC TOV number from a 230/400 V AC SPD datasheet and assume the same rule applies to a 1000 V or 1500 V PV SPD.

For PV systems, check the exact:

Ucpv + topology + protection mode + manufacturer documentation + applicable PV SPD standard.

For voltage selection, see our DC SPD voltage selection guide. For grounded versus floating PV arrangements and protection paths, see 2P vs 3P DC SPD for Solar PV.

AC SPD vs PV DC SPD Under Overvoltage

This is another reason AC and DC SPDs should not be interchanged casually.

IEC currently lists IEC 61643-11:2025 for AC low-voltage SPDs, while IEC 61643-31:2018 addresses PV DC SPDs. For a practical explanation of why an AC-only SPD should not simply be moved to a DC circuit, see What Happens If You Use an AC SPD on a DC System?.

Common Mistakes About SPD and Overvoltage

How to Select an SPD When TOV Is a Concern

Instead of starting with:

“How many kA do I need?”

start with the electrical system.

Installation also matters after the product is selected. Long SPD connecting conductors can increase the voltage appearing at the protected equipment during a fast surge; see our guide to SPD distance from the inverter and SPD lead length.

This order avoids a common purchasing mistake:

choosing the largest kA number first and checking voltage compatibility later.

Practical Example: Diagnosing an SPD That Keeps Failing

Assume an electrician installs a new Type 2 SPD.

A few days later the status window indicates replacement.

A new cartridge is installed.

It fails again.

It would be easy to conclude:

“The SPD quality is poor.”

But that is only one possibility.

A better troubleshooting sequence is:

Only after these checks should repeated SPD replacement be treated as a simple product issue.

SPD vs Overvoltage Relay: Do You Need Both?

In some installations, yes.

They solve different problems.

Think of them this way:

SPD:
“What happens if a very fast surge arrives?”

Voltage monitoring protection:
“What happens if the supply voltage remains abnormal?”

A system containing both functions may therefore provide broader protection than either one alone.

But the exact protection design depends on the installation, applicable codes and equipment requirements.

SPD Overvoltage Protection Checklist

Before approving an SPD specification, confirm the following:

よくある質問

Does an SPD Protect Against Overvoltage or High Voltage?

An SPD primarily protects against transient overvoltage, such as short-duration surges caused by lightning effects or switching events. It should not automatically be treated as protection against every temporary or sustained abnormal supply voltage. If the supply voltage remains excessively high, separate voltage monitoring or disconnection protection may be required.

Can an SPD protect against temporary overvoltage?

It depends on the SPD and the specific TOV condition.

The datasheet or manufacturer documentation may specify TOV withstand or behavior for defined voltage, duration and connection conditions.

Do not assume that every SPD will survive every TOV.

What happens if mains voltage stays above the SPD’s Uc?

The result depends on how far the voltage exceeds Uc, how long it lasts and the SPD design.

In an MOV-based SPD, excessive continuous voltage can increase MOV current and heating.

This may eventually cause thermal disconnection or damage.

Is TOV the same as a lightning surge?

そうだ。.

A lightning-related surge is a short transient event.

A TOV lasts much longer and is usually associated with a power-system abnormality.

The stresses placed on an SPD are therefore very different.

Does Imax tell me the TOV capability?

そうだ。.

Imax is a surge-current parameter.

TOV behavior must be evaluated separately.

Does a 40 kA SPD protect against a 400 V overvoltage?

That question cannot be answered from 40 kA.

You need to know the nominal system voltage, Uc, connection mode, TOV declaration, duration of the abnormal voltage and the SPD design.

Can an SPD prevent damage from a broken neutral?

Do not rely on an SPD alone.

A broken or displaced neutral can create dangerous abnormal phase-to-neutral voltages.

Neutral integrity must be restored, and where required a dedicated voltage-monitoring and disconnection function should be used.

Can a circuit breaker replace an SPD?

そうだ。.

A conventional circuit breaker protects primarily against overcurrent and short circuit.

An SPD limits transient overvoltage.

They perform different functions.

Can an overvoltage relay replace an SPD?

そうだ。.

A voltage-monitoring relay can detect abnormal RMS voltage and initiate a switching action.

It does not replace the high-speed surge-diversion function of an SPD.

結論

だから, does an SPD protect against overvoltage?

The technically correct answer is:

An SPD protects against transient overvoltage, but it should not be treated as universal protection against every temporary or sustained abnormal voltage condition.

Surge current ratings such as In, Imax and Iimp describe transient performance.

Uc/Ucpv describes continuous voltage compatibility.

上へ describes surge-voltage limitation.

And TOV behavior describes how the SPD responds to a specified temporary abnormal-voltage condition.

These parameters should never be treated as interchangeable.

Real-world research also shows why the distinction matters. NIST/EPRI investigations found dramatically different SPD responses to temporary-overvoltage conditions, ranging from survival to internal failure depending on the device and the applied stress.

The practical rule is simple:

First identify the electrical event. Then select the protection function.

Use an SPD for transient surge protection.

Use appropriate voltage monitoring or disconnection when persistent abnormal supply voltage must be detected and removed.

And if SPDs repeatedly fail, investigate system voltage, neutral condition, earthing, Uc/Ucpv and TOV conditions before simply replacing the module again.

For PV applications, always verify the exact SPD model against the system’s maximum DC voltage, PV topology and applicable IEC 61643-31 requirements.