Is MCOV on a surge arrester measured in mA?

No. Maximum Continuous Operating Voltage, or MCOV, is a voltage rating expressed in volts or kilovolts RMS. Milliamperes measure current and may be associated with a reference-voltage test point, but they are not the unit used for MCOV.

What does 1 mA mean on a surge arrester?

A 1 mA value may identify a current used to establish a manufacturer-specified reference voltage or to characterize metal-oxide varistor elements. The resulting voltage is a reference characteristic and should not be confused with MCOV or surge discharge voltage.

Is the 1 mA voltage the clamping voltage of a surge arrester?

No. A 1 mA reference measurement occurs at relatively low current. Discharge or residual voltage describes the voltage appearing across the arrester while it conducts a specified surge current, typically much larger than the reference current.

Is MCOV the voltage where a surge arrester turns on?

No. A gapless metal-oxide surge arrester has a nonlinear voltage-current characteristic and carries some leakage current during normal operation. MCOV defines continuous voltage capability rather than a precise turn-on threshold.

What factors are used to select surge arrester MCOV?

Selection should consider maximum continuous system voltage, arrester connection, grounding configuration, temporary overvoltage magnitude and duration, fault-clearing time, equipment insulation withstand levels and the manufacturer's arrester characteristics.

Is the MCOV Rating on a Surge Arrester Measured in mA? (9/15/2026)

Q: What is MCOV on a surge arrester?

MCOV is the maximum designated RMS power-frequency voltage that may be continuously applied across a surge arrester's terminals under specified operating conditions.

Maximum Continuous Operating Voltage (MCOV) on a metal-oxide surge arrester is a voltage rating expressed in volts or kilovolts RMS, not milliamperes. A current value such as 1 mA may be used to establish a reference voltage on the arrester's nonlinear voltage-current characteristic, but this reference point is different from MCOV, arrester rated voltage, and discharge or residual voltage.

By LarsonElectronics.com, September 15, 2026

Maximum Continuous Operating Voltage (MCOV) is not measured in milliamperes. MCOV is a voltage rating, normally expressed in volts or kilovolts RMS, that identifies the maximum designated power-frequency voltage that may be continuously applied across a surge arrester's terminals under its specified operating conditions. A value such as 1 mA refers instead to a current used in establishing a reference point on the voltage-current characteristic of a metal-oxide surge arrester.

This distinction is important when specifying surge arresters for industrial transformers, switchgear, substations and other medium- and high-voltage equipment. MCOV, reference voltage, arrester rated voltage and discharge voltage describe different characteristics and should not be used interchangeably.

MCOV is a voltage rating rather than a current rating

For metal-oxide surge arresters used on AC power systems, MCOV stands for Maximum Continuous Operating Voltage. IEEE surge-arrester terminology defines MCOV in terms of the maximum RMS power-frequency voltage that can be applied continuously between the arrester terminals.

Therefore, MCOV is specified in volts RMS or kilovolts RMS, depending on the voltage class of the equipment. It is not specified in amperes or milliamperes.

A surge arrester installed phase-to-ground on a medium-voltage system is continuously exposed to the system's phase-to-ground voltage. Its MCOV must be selected so the arrester can withstand the expected continuous voltage at its installation point while also providing suitable overvoltage protection for the equipment.

The 1 mA value refers to current used at a reference-voltage point

The source of confusion between MCOV and milliamperes is often the reference voltage of a metal-oxide surge arrester or its metal-oxide varistor elements.

A metal-oxide arrester has a highly nonlinear voltage-current characteristic. Current does not remain exactly zero below a single turn-on voltage and then suddenly begin flowing above it. Instead, current changes continuously as applied voltage changes.

IEEE C62.11 defines reference voltage using a specified reference current selected so that the resistive component of current is sufficiently large compared with the capacitive component. Depending on arrester design and metal-oxide disk area, this reference current is not necessarily exactly 1 mA. Published engineering literature describing IEEE C62.11 places the reference-current density within a specified low-current range. Therefore, describing every IEEE arrester reference voltage simply as a universal "1 mA voltage" is an oversimplification.

For individual metal-oxide varistors and some manufacturer test procedures, however, a DC voltage measured at 1 mA is commonly used as a convenient characteristic or quality-control reference. When a manufacturer specifies a 1 mA voltage, the 1 mA is the test current and the resulting voltage is the measured characteristic.

MCOV and 1 mA reference voltage describe different electrical characteristics

Parameter	Typical Unit	What It Describes
MCOV	V RMS or kV RMS	Maximum designated power-frequency voltage that may be continuously applied across the arrester.
Reference voltage	V or kV	Voltage corresponding to a defined reference-current condition on the metal-oxide arrester's nonlinear voltage-current characteristic.
1 mA	mA	A current value sometimes used when determining a manufacturer-specified reference voltage or testing individual MOV elements.
Arrester rated voltage	kV RMS	A standardized arrester voltage rating associated with its specified operating-duty requirements; it is distinct from MCOV.
Discharge or residual voltage	kV peak	Voltage appearing across the arrester while it conducts a specified surge current.
Discharge current	A or kA	Surge current conducted by the arrester during standardized impulse conditions.

MCOV should not be described as the voltage where an arrester begins conducting

A common explanation of surge arresters states that the device remains completely nonconductive until a threshold voltage is reached. This is useful as a simplified concept, but it is not technically precise for a gapless metal-oxide arrester.

Zinc-oxide varistor elements have a nonlinear voltage-current characteristic. Some leakage current exists while normal power-frequency voltage is applied. At normal operating stress, the current is predominantly capacitive. As voltage increases into the nonlinear region, the resistive component becomes increasingly significant.

MCOV therefore should not be interpreted as a precise turn-on voltage. It defines the continuous voltage capability of the arrester, not a boundary below which absolutely no current flows.

The 1 mA point should not be treated as the arrester's surge clamping voltage

The reference-voltage region also should not be confused with the voltage developed across an arrester while it is diverting a lightning or switching surge.

A reference measurement uses relatively small current. Actual surge currents can be thousands of amperes. The voltage appearing across an arrester while conducting a specified impulse current is generally described as its discharge voltage or residual voltage.

For example, an arrester may be characterized using a low-current reference measurement, while its lightning protective performance is evaluated using standardized impulse currents measured in kiloamperes. These points occupy very different regions of the arrester's voltage-current characteristic.

IEEE C62.11 provides the primary framework for metal-oxide surge arrester ratings

For U.S. power-system applications, IEEE C62.11 is a principal standard covering metal-oxide surge arresters for AC power circuits above 1 kV. The standard addresses electrical characteristics and standardized testing of surge arresters, including MCOV, reference voltage, discharge-voltage characteristics, impulse-current performance and other design requirements.

IEEE guidance distinguishes MCOV from the reference-voltage and discharge-voltage characteristics. This distinction is essential because each parameter answers a different engineering question:

MCOV addresses continuous voltage exposure.
Reference voltage characterizes a defined point on the nonlinear voltage-current relationship.
Discharge voltage indicates the voltage developed across the arrester while carrying a specified surge current.
Temporary overvoltage capability addresses elevated power-frequency voltage that may be tolerated for a limited duration.

MCOV selection depends on the actual system voltage and grounding configuration

Proper surge-arrester selection requires more than matching the arrester to the nominal line-to-line voltage printed on equipment.

Engineers must determine the voltage that will actually appear across the arrester terminals during normal operation. For a phase-to-ground arrester, this generally requires evaluating the system's maximum phase-to-ground voltage rather than simply using nominal line-to-line voltage.

The system grounding arrangement also matters. During a single-line-to-ground fault, voltage on the unfaulted phases can increase relative to ground. The magnitude and duration of this temporary overvoltage depend on system grounding, fault-clearing time and other system characteristics.

Consequently, selecting an arrester with the lowest possible MCOV is not automatically the best approach. An MCOV that is too low can subject the arrester to excessive continuous or temporary power-frequency stress. An unnecessarily high MCOV, however, can result in a higher protective level. Correct selection balances continuous-voltage capability, temporary-overvoltage capability and insulation coordination.

Temporary overvoltage capability is different from MCOV

An arrester may tolerate voltage above its MCOV for a limited period. This is evaluated through its temporary overvoltage capability, commonly abbreviated TOV.

MCOV describes continuous application. TOV capability describes the arrester's ability to withstand a higher power-frequency voltage for a specified duration without unacceptable damage or loss of thermal stability.

Ground faults, load rejection, resonance and other abnormal system conditions can create temporary overvoltages. Engineers should therefore evaluate both magnitude and duration of expected TOVs when selecting an arrester.

Insulation coordination determines whether an arrester provides adequate equipment protection

Surge-arrester selection is ultimately an insulation-coordination problem. The arrester must tolerate the system's continuous and temporary voltage stresses while limiting transient overvoltages to levels compatible with the insulation withstand capability of the protected equipment.

For transformers, engineers may evaluate the arrester's lightning impulse protective characteristics relative to the transformer's basic lightning impulse insulation level, commonly identified as BIL. Switching impulse requirements may also become important depending on system voltage and application.

Physical installation is part of insulation coordination as well. Long arrester leads can introduce additional inductive voltage during steep-front surge currents. For transformer protection, arresters are generally installed with connections arranged to minimize unnecessary lead length while satisfying electrical clearances and equipment requirements.

Surge arresters protect industrial transformers and switchgear from transient overvoltages

Metal-oxide surge arresters are commonly applied to protect industrial transformers, switchgear, cable terminations, motors, generators and other power-system equipment from transient overvoltages associated with lightning and switching events.

Consider a medium-voltage transformer supplied from an outdoor distribution system. The arrester must withstand the normal phase-to-ground operating voltage continuously, remain thermally stable during anticipated temporary overvoltages and provide a sufficiently low protective level during lightning impulses to maintain an appropriate margin relative to the transformer's insulation withstand capability.

The 1 mA characteristic alone cannot establish whether that arrester is correctly applied. MCOV, rated voltage, TOV capability, discharge-voltage characteristics, energy or charge capability, system grounding and equipment insulation strength all contribute to the engineering decision.

NEC requirements and surge-arrester engineering address different parts of the installation

In U.S. installations, applicable requirements of the National Electrical Code must also be considered. NEC Article 242 addresses overvoltage protection, including surge-protective devices and surge arresters. Transformer installations are additionally subject to applicable requirements of NEC Article 450, while grounding and bonding requirements are addressed primarily in NEC Article 250.

The NEC establishes installation and safety requirements but does not replace detailed surge-arrester application and insulation-coordination engineering. IEEE C62 standards, equipment-manufacturer data, system studies and project specifications should be used as appropriate when selecting medium- and high-voltage arresters.

Canadian installations require coordination with applicable CSA requirements

For Canadian industrial installations, surge-arrester application should be coordinated with the applicable edition of the Canadian Electrical Code, Part I, CSA C22.1, provincial or territorial requirements, equipment certification requirements and the serving utility's standards.

IEEE surge-arrester engineering practices may still be relevant to equipment specification and insulation coordination, particularly for North American power equipment designed and tested using IEEE standards. The governing code, adopted standards and authority having jurisdiction should be confirmed for each project.

The correct interpretation separates voltage ratings from current test points

The simplest engineering distinction is that MCOV is measured in volts or kilovolts RMS, not mA. Milliamperes describe current. When 1 mA appears on an arrester specification or test report, it may identify the current associated with a reference-voltage measurement rather than the arrester's continuous operating voltage.

It is also important not to interpret the 1 mA reference point as a universal turn-on threshold or as the arrester's lightning clamping voltage. Metal-oxide arresters have nonlinear voltage-current characteristics, and their behavior must be evaluated at the appropriate current and voltage conditions for the parameter being considered.

Frequently referenced surge arrester terms have distinct meanings

MCOV is expressed as an RMS voltage

Maximum Continuous Operating Voltage identifies the maximum designated RMS power-frequency voltage that may be applied continuously across the arrester terminals under specified conditions.

A 1 mA value identifies current rather than MCOV

When a manufacturer specifies a voltage at 1 mA, the 1 mA value identifies the current at which the associated voltage characteristic is measured. It does not mean MCOV is rated in milliamperes.

Reference voltage is not a universal turn-on voltage

Metal-oxide varistors conduct current according to a nonlinear voltage-current relationship. Reference voltage establishes a defined characteristic point rather than a hard boundary between zero current and conduction.

Discharge voltage describes arrester behavior during surge current

Discharge or residual voltage is the voltage appearing across an arrester while it conducts a specified impulse current. It is a key parameter for evaluating the protection provided to transformer and equipment insulation.

MCOV and TOV capability are not interchangeable

MCOV applies to continuous voltage. Temporary overvoltage capability describes higher power-frequency voltage that the arrester can withstand for a limited period under specified conditions.

Engineering-grade arrester selection requires the complete system picture

Surge-arrester application should account for maximum system voltage, grounding configuration, expected temporary overvoltages, fault-clearing time, arrester MCOV, rated voltage, protective characteristics, insulation withstand levels and installation geometry. Manufacturer-specific data should be used because arresters with similar nominal voltage ratings can have different electrical characteristics.

For industrial transformer projects, the arrester should therefore be selected as part of the overall insulation-coordination and protection strategy rather than from MCOV or a 1 mA reference value alone.

For assistance with transformer applications, voltage-class selection and industrial power equipment, contact Larson Electronics.

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