Is 1 mA voltage the same as clamping voltage?

No. The 1 mA reference voltage is measured at very low current. During an actual surge, much larger current flows and the voltage across the arrester is described by its residual or discharge voltage.

How should a surge arrester be selected for a transformer?

Arrester selection should consider system voltage, phase-to-ground voltage, grounding configuration, MCOV, rated voltage, temporary-overvoltage capability, residual voltage, transformer BIL, surge duty, and physical arrester placement.

Why does system grounding affect surge arrester selection?

Because surge arresters are commonly connected phase to ground, the voltage imposed on them during normal operation and ground faults depends on the system grounding arrangement. Ungrounded or impedance-grounded systems can experience higher temporary phase-to-ground voltages.

What Is a 1 mA Voltage Rating on a Surge Arrester (9/9/2026)

Q: Is 1 mA voltage the same as MCOV?

No. MCOV is the maximum designated RMS power-frequency voltage that may be continuously applied to the arrester under specified conditions. The 1 mA voltage is a reference point on the metal-oxide varistor voltage-current characteristic.

Q: Which standards apply to medium-voltage metal-oxide surge arresters?

IEEE C62.11-2020 covers metal-oxide surge arresters for AC circuits above 1 kV in North American practice. IEC 60099-4:2014 covers gapless metal-oxide surge arresters for AC systems above 1 kV in IEC-based applications.

The 1 mA voltage is a reference test value on a metal-oxide surge arrester's nonlinear voltage-current curve. It is not the same as MCOV, rated voltage, or residual voltage and should be considered alongside system voltage, grounding, TOV capability, and insulation coordination.

By LarsonElectronics.com, September 09, 2026

The 1 mA voltage rating associated with a metal-oxide surge arrester is a reference point on the arrester's nonlinear voltage-current characteristic. It identifies the voltage measured when a specified small current, often 1 milliampere, flows through the metal-oxide varistor elements under defined test conditions. This value can be useful for testing, comparison, and condition assessment, but it should not be confused with the voltage at which the arrester suddenly turns on or with the voltage that appears across the arrester during a lightning surge.

The 1 mA voltage is a reference test value rather than a simple turn-on threshold

Metal-oxide surge arresters do not behave like an ideal switch that remains completely nonconductive until one exact voltage is reached. Their metal-oxide varistor elements have a highly nonlinear voltage-current relationship. Small leakage current exists at normal operating voltage, and current increases rapidly as voltage rises.

A 1 mA reference voltage therefore identifies one defined point on that nonlinear characteristic. In many arrester and metal-oxide varistor specifications, a manufacturer applies a test voltage and records the voltage at which the specified direct current reaches 1 mA.

This makes the 1 mA value useful as a reproducible reference, but it does not mean that the arrester suddenly begins protecting equipment at exactly that voltage.

The 1 mA voltage should not be confused with MCOV

Maximum Continuous Operating Voltage, commonly abbreviated MCOV, is one of the most important values used when applying a metal-oxide surge arrester to an AC power system.

MCOV represents the maximum designated power-frequency RMS voltage that may be applied continuously across the arrester under specified operating conditions. It is therefore directly related to the normal phase-to-ground voltage that the arrester will experience in service.

The 1 mA reference voltage and MCOV serve different purposes:

Parameter	Meaning	Primary use
1 mA reference voltage	Voltage corresponding to a specified low test current on the MOV voltage-current characteristic	Reference testing, manufacturing comparison, and in some designs condition assessment
MCOV	Maximum continuous RMS power-frequency voltage that can be applied to the arrester as specified	System application and continuous operating-voltage coordination
Rated voltage	Manufacturer-assigned arrester voltage rating associated with defined operating-duty and temporary-overvoltage performance	Arrester classification and application
Residual voltage	Voltage appearing across the arrester while it conducts a specified surge current	Insulation coordination and determination of protective level

The residual voltage is more important when evaluating actual surge protection

When a lightning or switching surge reaches a metal-oxide arrester, current can rise from milliamperes to thousands of amperes. The voltage appearing across the arrester under those conditions is called residual voltage or discharge voltage.

This is the value that is most directly related to the arrester's protective performance during a surge. Residual voltage depends on both the magnitude and waveform of the discharge current.

For example, an arrester may have a reference measurement based on a current of only 1 mA, while its published lightning impulse residual voltage may be specified at discharge currents such as 5 kA, 10 kA, or another standardized test level.

The two measurements describe different locations on the same nonlinear voltage-current characteristic. They should not be treated as interchangeable.

A metal-oxide arrester conducts progressively as voltage increases

The metal-oxide blocks inside a modern gapless surge arrester behave as strongly nonlinear resistive elements. Under normal system voltage, only a relatively small current flows through the blocks. As an overvoltage develops, their effective resistance decreases sharply and substantially more current is diverted through the arrester.

This behavior allows the arrester to limit the voltage imposed on connected insulation while conducting surge energy toward ground.

Once the transient subsides and normal system voltage returns, the arrester returns to its normal high-resistance operating region and limits the continuing power-frequency current.

IEEE C62.11-2020 covers metal-oxide surge arresters intended for AC power circuits above 1 kV and describes devices that repeatedly limit surges by passing discharge current while limiting subsequent system power current.

The 1 mA value can be useful for evaluating MOV condition

Where a manufacturer specifies an applicable 1 mA reference-voltage test, comparison with factory or baseline values can provide information about the electrical characteristics of the metal-oxide elements.

A significant change in reference voltage may indicate that the MOV elements have experienced electrical, thermal, environmental, or aging effects. Interpretation should always follow the manufacturer's test method and acceptance criteria because test voltage polarity, current, temperature, measurement technique, and allowable tolerance can vary by product.

A field-measured 1 mA value should therefore not be judged against a generic industry number. The correct comparison is the manufacturer's specified value or an established baseline for the specific arrester design.

The 1 mA voltage does not determine arrester selection by itself

Engineers selecting a surge arrester for a transformer or medium-voltage system must consider the complete system rather than choosing an arrester from its 1 mA reference voltage.

Important application parameters include:

System nominal voltage
Maximum system voltage
Phase-to-ground operating voltage
System grounding method
Arrester MCOV
Arrester rated voltage
Temporary overvoltage capability
Lightning and switching residual voltage
Equipment basic impulse insulation level
Available surge-current duty
Arrester energy and charge-transfer capability where applicable
Lead length between the arrester and protected equipment
Altitude and environmental conditions

IEEE C62.11-2020 is the active IEEE standard covering metal-oxide surge arresters for AC power circuits above 1 kV, including performance characteristics related to surge limitation and operating duty. IEEE also maintains an active revision project for the C62.22 application guide addressing the application of metal-oxide arresters on systems over 1000 V.

System grounding has a major influence on arrester voltage selection

An arrester is typically connected from phase to ground, so the voltage appearing across it is strongly influenced by the system grounding arrangement.

On an effectively grounded system, the normal phase-to-ground voltage is relatively predictable. During a ground fault, however, systems that are impedance-grounded, resistance-grounded, ungrounded, or otherwise not effectively grounded may experience higher temporary phase-to-ground voltages on the unfaulted phases.

The selected MCOV and rated voltage must be high enough to survive the expected temporary overvoltage while still providing a sufficiently low protective level for the insulation being protected.

Selecting an arrester solely because it has the lowest possible voltage rating can therefore be a serious engineering error. An arrester subjected to sustained voltage above its allowable capability may overheat and fail.

Temporary overvoltage capability must be coordinated with system conditions

Temporary overvoltage, commonly abbreviated TOV, is a power-frequency overvoltage that can persist much longer than a lightning impulse. Possible causes include ground faults, load rejection, resonance, ferroresonance, and certain switching events.

Surge arresters can withstand temporary overvoltages only within defined voltage-versus-time limits. Manufacturers commonly publish TOV curves showing the permissible overvoltage for different durations.

IEC terminology similarly distinguishes rated voltage, continuous operating voltage, residual voltage, and temporary overvoltage performance in the application of gapless metal-oxide surge arresters. IEC 60099-4:2014 applies to gapless metal-oxide arresters for AC systems above 1 kV.

Arrester protective level must coordinate with transformer insulation strength

The primary purpose of a surge arrester installed near a transformer is to keep transient voltage below damaging insulation levels while maintaining adequate protective margin.

Transformers are assigned insulation withstand ratings, including a basic impulse insulation level, commonly called BIL in North American practice. The arrester's applicable discharge or residual voltage should be coordinated with the transformer's insulation capability rather than compared only with normal system voltage.

For example, a medium-voltage industrial transformer connected to an outdoor feeder may experience lightning-induced surges entering from overhead distribution. Properly applied arresters near the transformer terminals can limit the incoming impulse before the transformer's winding insulation is subjected to the full surge magnitude.

Physical installation is also important. Excessive conductor length between the arrester, protected terminal, and grounding connection can add inductive voltage during a rapidly changing surge current. Arresters should therefore be installed with short, direct connections consistent with the equipment design and applicable installation requirements.

A practical medium-voltage example shows why 1 mA voltage alone is insufficient

Consider a 13.8 kV three-phase industrial distribution system supplying a pad-mounted transformer. An engineer reviewing two possible arresters may find that both have published 1 mA reference-voltage values.

Those values alone do not establish which arrester is appropriate.

The engineer must first determine the actual maximum phase-to-ground voltage, grounding configuration, expected ground-fault overvoltage, MCOV requirement, TOV exposure, and transformer insulation level. The arrester's lightning impulse residual voltage must then be evaluated against the transformer's BIL and required protective margin.

The 1 mA reference voltage may remain useful as a product characteristic or diagnostic value, but it is only one piece of the engineering picture.

North American and IEC arrester terminology should not be assumed to be identical

Industrial buyers frequently encounter both IEEE/ANSI-style and IEC-style surge arrester specifications, particularly when equipment is sourced for projects in the United States and Canada or incorporated into internationally designed systems.

Both IEEE C62.11 and IEC 60099-4 address gapless metal-oxide surge arresters used to limit overvoltages on AC systems above 1 kV, but terminology, ratings, test procedures, and preferred application practices are not identical.

Engineers should avoid converting between IEEE and IEC arrester ratings based only on similar-looking numbers. The complete manufacturer's data sheet and applicable standard should be reviewed.

Canadian industrial applications require coordination with the applicable electrical code and utility requirements

For Canadian installations, surge arrester application should be coordinated with the Canadian Electrical Code, provincial or territorial adoption requirements, utility standards, and the applicable CSA or equipment certification requirements.

Many Canadian industrial facilities use medium-voltage distribution architectures similar to those found in the United States, but equipment certification, grounding practices, and installation requirements may differ by jurisdiction.

For equipment serving cross-border projects, engineers should define the governing standards and certification requirements during specification rather than attempting to reconcile them after equipment has been manufactured.

The NEC addresses surge arresters but does not define the 1 mA reference voltage

For U.S. installations, NFPA 70, National Electrical Code, includes requirements for surge arresters and surge-protective devices. Article 242 addresses overvoltage protection, including surge-protective equipment, while other applicable articles govern transformer installations, grounding and bonding, medium-voltage conductors, equipment access, and installation practices.

The NEC does not establish the manufacturer's 1 mA reference-voltage value for a metal-oxide arrester. That characteristic comes from the arrester design, testing standard, and manufacturer data.

NEC compliance therefore does not replace the engineering task of coordinating the arrester's MCOV, rated voltage, TOV capability, and residual voltage with the power system and protected equipment.

A low 1 mA voltage does not automatically mean better surge protection

A lower reference voltage can appear attractive because it may suggest that the MOV conducts more current at a lower applied voltage. In an actual power system, however, lowering arrester voltage ratings without considering continuous operating voltage and temporary overvoltage can reduce operating margin and increase thermal stress.

The engineering objective is not to select the lowest possible arrester voltage. It is to select an arrester that can withstand the system's normal and temporary voltages while maintaining a protective level low enough to coordinate with equipment insulation.

This balance is at the heart of insulation coordination.

Arrester rated voltage and MCOV should always be checked on the manufacturer's data sheet

A complete arrester data sheet normally provides considerably more useful application information than the 1 mA reference voltage alone.

Depending on arrester class and standard, important published data may include:

Rated voltage
MCOV or continuous operating voltage
Nominal discharge current
Lightning impulse residual voltage
Switching impulse residual voltage
Steep-current impulse protective level
TOV capability
Energy or charge-transfer capability
Pressure-relief or short-circuit rating
Housing and creepage characteristics
Mechanical loading capability
Applicable IEEE or IEC classification

For example, surge-arrester technical literature distinguishes rated voltage from residual voltage, with residual voltage describing the voltage developed across the arrester during a specified current impulse.

Transformer surge protection should be engineered as part of the complete insulation system

Surge protection is most effective when transformer insulation, arrester characteristics, cable configuration, grounding, protective-device placement, and system switching behavior are considered together.

For low-voltage, medium-voltage, and high-voltage transformer applications, engineers may need to evaluate surge arresters along with transformer BIL, switchgear insulation ratings, cable impulse capability, grounding transformers, instrument transformers, and other connected equipment.

A specification based on system voltage alone can miss temporary overvoltage or insulation-coordination issues that become apparent only when the complete one-line diagram and grounding system are reviewed.

Frequently asked questions about 1 mA surge arrester voltage

The meaning of a 1 mA surge arrester voltage

The 1 mA voltage is a reference voltage measured when a specified current of 1 milliampere flows through the metal-oxide element under defined test conditions. It identifies one point on the arrester's nonlinear voltage-current characteristic.

The difference between 1 mA voltage and clamping voltage

The 1 mA reference voltage is measured at very low current and should not be treated as the arrester's surge clamping voltage. During an actual surge, thousands of amperes may flow and the resulting voltage across the arrester is described by its residual or discharge voltage.

The difference between 1 mA voltage and MCOV

MCOV is the maximum designated RMS power-frequency voltage that can be continuously applied to the arrester under specified conditions. The 1 mA voltage is a reference point on the MOV voltage-current curve and is not a substitute for MCOV when selecting an arrester.

The importance of arrester rated voltage

Rated voltage is associated with the arrester's ability to operate correctly under defined power-frequency and temporary-overvoltage conditions. It must be coordinated with system voltage, grounding, and expected TOV exposure.

The most important value for transformer surge protection

No single rating determines proper protection. Engineers should coordinate MCOV, rated voltage, temporary-overvoltage capability, residual voltage, and arrester placement with the transformer's BIL and the power system's grounding and overvoltage characteristics.

The reason an arrester should not be selected only by system line-to-line voltage

Surge arresters are commonly connected phase to ground, so actual phase-to-ground voltage and temporary overvoltage depend on the system grounding arrangement. Selecting from line-to-line voltage alone can result in an incorrect arrester application.

The standards commonly associated with medium-voltage metal-oxide surge arresters

IEEE C62.11-2020 covers metal-oxide surge arresters for AC power circuits above 1 kV in North American practice. IEC 60099-4:2014 covers gapless metal-oxide surge arresters for AC systems above 1 kV in IEC-based applications.

For assistance evaluating transformer ratings, insulation coordination, and industrial transformer applications, contact Larson Electronics.

Larson Electronics Building Trust Since 1973.

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