What Is the MCOV of a 26.1 kV Surge Arrester (10/1/2026)

Q: What is the MCOV of a 26.1 kV surge arrester?

A 26.1 kV value alone is insufficient to determine MCOV. The value must first be identified as system voltage, arrester rated voltage, MCOV, or a protective characteristic. The correct MCOV should be obtained from manufacturer specifications and verified against system voltage, grounding, and temporary overvoltage requirements.

Q: Can MCOV be calculated as 80 to 90 percent of arrester voltage?

A percentage relationship may be approximately true for some designs, but it should not be used to specify an arrester. Manufacturer-published MCOV and system application requirements should be used.

Q: Why does grounding affect surge arrester selection?

Grounding affects the line-to-ground voltage that can appear on healthy phases during a ground fault. Systems that are not effectively grounded can experience higher temporary overvoltages and may require a different arrester MCOV or rated voltage.

Q: Does a higher MCOV always provide better transformer protection?

No. A higher MCOV increases continuous voltage capability but can also increase the arrester protective voltage. Selection requires coordination between system voltage requirements and the insulation withstand capability of the protected transformer.

A 26.1 kV value does not by itself establish a surge arrester's MCOV. MCOV is a specific manufacturer-assigned rating based on the maximum continuous line-to-ground voltage the arrester can withstand. For example, common IEEE arrester ratings include a 27 kV arrester with a 22 kV MCOV and a 30 kV arrester with a 24.4 kV MCOV. Always identify whether 26.1 kV refers to system voltage, arrester rating, MCOV, or discharge/protective voltage before selecting an arrester.

By LarsonElectronics.com, October 1, 2026

The maximum continuous operating voltage (MCOV) of a surge arrester cannot be reliably calculated by simply taking a percentage of a 26.1 kV value. MCOV is a specified characteristic of a metal-oxide surge arrester and represents the maximum designated RMS power-frequency voltage that may be applied continuously across the arrester under its specified operating conditions.

For medium-voltage systems protecting industrial transformers and related equipment, arrester selection should consider system voltage, maximum line-to-ground voltage, grounding configuration, temporary overvoltage (TOV), arrester MCOV, arrester rating, and insulation coordination.

A 26.1 kV value does not automatically mean a 21 to 23 kV MCOV

It is not technically appropriate to assume that the MCOV of a device identified as 26.1 kV is simply 80% to 90% of 26.1 kV. The first step is determining what the 26.1 kV value represents.

A value appearing on a drawing, specification, data sheet, or equipment label could represent system line-to-line voltage, line-to-ground voltage, arrester duty-cycle rating, MCOV, or a protective characteristic such as discharge voltage. These values are not interchangeable.

For example, published IEEE-style arrester data commonly pairs a 27 kV arrester rating with approximately 22 kV MCOV, while a 30 kV arrester may have approximately 24.4 kV MCOV. The exact values must be confirmed from the manufacturer's data for the specific arrester design.

MCOV defines the continuous power-frequency voltage capability of the arrester

MCOV stands for maximum continuous operating voltage. For a metal-oxide surge arrester, it identifies the designated RMS power-frequency voltage that can be applied continuously across the arrester under specified operating conditions.

MCOV is important because the arrester is normally connected between phase and ground. During normal operation, the arrester must withstand the system's continuous line-to-ground voltage without conducting significant surge current or experiencing unacceptable thermal stress.

When a lightning or switching surge raises the voltage substantially above normal operating conditions, the metal-oxide elements become conductive and divert surge current while limiting the voltage imposed on the protected equipment.

MCOV and arrester rated voltage are different values

One of the most common sources of confusion is treating arrester rated voltage and MCOV as the same specification.

Term	Meaning
System voltage	The nominal or maximum operating voltage of the electrical system.
Arrester rated voltage	An arrester voltage rating associated with its design and power-frequency overvoltage capability.
MCOV	The designated RMS power-frequency voltage that can be continuously applied across the arrester under specified conditions.
Discharge voltage	The voltage appearing across the arrester while it conducts a specified surge current.

These distinctions matter when specifying surge protection for medium-voltage transformers because selecting an arrester solely from the transformer's nominal system voltage can result in an improperly applied device.

System line-to-ground voltage is a starting point for MCOV selection

For a balanced three-phase system, nominal line-to-ground voltage can be estimated from the line-to-line voltage:

V_LG = V_LL / v3

For example, if 26.1 kV represents the nominal line-to-line voltage of a three-phase system:

26.1 kV / v3 ˜ 15.1 kV line-to-ground

This calculation is useful as a starting point, but it does not by itself determine the correct arrester MCOV. Maximum system voltage and the voltage that can appear from phase to ground during abnormal system conditions must also be considered.

System grounding affects the required arrester MCOV

Grounding configuration is a major factor in medium-voltage arrester selection. On an effectively grounded system, phase-to-ground voltage on the unfaulted phases is constrained during a single-line-to-ground fault.

On impedance-grounded, ungrounded, resonant-grounded, or otherwise non-effectively grounded systems, the healthy phases can experience substantially higher line-to-ground voltage during a ground fault. An arrester selected only for normal line-to-ground voltage may therefore experience excessive temporary overvoltage.

This is why the same nominal line-to-line system voltage can require different arrester ratings depending on grounding and fault conditions.

Temporary overvoltage capability must be checked in addition to MCOV

MCOV addresses continuous voltage, while temporary overvoltage capability addresses elevated power-frequency voltage that may persist for a limited duration.

Ground faults, load rejection, resonance, ferroresonance, and other system events can expose an arrester to voltage above its MCOV. Manufacturers publish TOV capability curves showing the magnitude and duration of temporary overvoltage that a particular arrester design can withstand.

Engineering selection should therefore verify both continuous MCOV requirements and expected temporary overvoltage magnitude and duration.

IEEE C62.11 provides the primary U.S. framework for metal-oxide surge arresters

IEEE C62.11 addresses metal-oxide surge arresters for AC power circuits above 1 kV. It covers arresters designed to repeatedly limit voltage surges by conducting surge current and then limiting power-frequency follow current. IEEE C62.22 provides application guidance for using metal-oxide surge arresters to protect electric power equipment against abnormal overvoltages.

For cable systems, IEEE C62.22.1-2024 provides guidance for connecting surge arresters to protect insulated shielded electric power cable systems up to 46 kV.

For Canadian installations, equipment selection should also account for applicable CSA requirements, utility standards, provincial electrical requirements, and manufacturer application data. The underlying engineering principles of matching MCOV, TOV capability, system grounding, and equipment insulation withstand remain essential.

NEC requirements complement but do not replace arrester engineering

For U.S. installations, the National Electrical Code contains requirements relevant to surge arresters and medium-voltage systems. NEC Article 242 addresses overvoltage protection, including surge-protective devices and surge arresters, while other NEC requirements may apply to equipment grounding, conductors, clearances, wiring methods, and installations over 1,000 volts.

The NEC should not be used as a substitute for an arrester application study. Correct medium-voltage arrester selection requires coordination with system characteristics, equipment ratings, manufacturer data, and applicable IEEE guidance.

Transformer insulation coordination is part of arrester selection

The purpose of the arrester is not simply to survive the system voltage. It must also provide an appropriate protective level for the insulation of the transformer or other equipment being protected.

For medium-voltage and high-voltage transformers, engineers should evaluate the transformer's insulation withstand characteristics, including applicable basic lightning impulse insulation level (BIL), against the arrester's published protective characteristics.

Arrester location and lead length also matter. Additional voltage can develop across connection inductance during fast-rising surge currents, so arresters are generally installed as close as practical to the equipment being protected with appropriately designed connections.

A 26.1 kV reading on an arrester data sheet may not be its voltage rating

A value such as 26.1 kV can appear in manufacturer data as a discharge or switching-surge characteristic rather than an arrester rating or MCOV. This is another reason not to infer MCOV from a single voltage number.

Before selecting a replacement arrester, identify the manufacturer's catalog number and locate the specific entries for arrester rating and MCOV. If only the electrical system voltage is known, determine the maximum system voltage, grounding method, expected temporary overvoltage, and equipment insulation requirements before selecting the arrester.

A practical surge arrester selection process uses multiple system parameters

For transformer applications, a basic engineering selection process should identify:

The nominal and maximum system line-to-line voltage.
The normal maximum line-to-ground voltage.
The system grounding configuration.
The expected temporary overvoltage magnitude and duration.
The required arrester MCOV and rated voltage.
The transformer's BIL and other applicable insulation withstand levels.
The arrester's published discharge and protective characteristics.
The physical location and connection length between the arrester and protected equipment.
The applicable IEEE, NEC, CSA, utility, and manufacturer requirements.

This approach avoids selecting an arrester from nominal voltage alone and provides a more defensible basis for transformer insulation coordination.

Frequently asked questions about surge arrester MCOV

What is the MCOV of a 26.1 kV surge arrester

A 26.1 kV value alone is insufficient to determine MCOV. First determine whether 26.1 kV represents system voltage, arrester rated voltage, MCOV, or a discharge/protective voltage. The correct MCOV should be taken from the manufacturer's specifications and verified against the system's maximum line-to-ground voltage, grounding method, and TOV requirements.

Is MCOV the same as surge arrester rated voltage

No. MCOV and arrester rated voltage are separate specifications. For example, published IEEE-style arrester ratings commonly include a 27 kV arrester with approximately 22 kV MCOV and a 30 kV arrester with approximately 24.4 kV MCOV.

Can MCOV be calculated as 80 to 90 percent of arrester voltage

A percentage relationship may appear approximately true for some arrester designs, but it should not be used as the engineering method for determining MCOV. Use the manufacturer's published MCOV and verify its suitability for the electrical system.

Why does grounding affect surge arrester selection

Grounding determines how much phase-to-ground voltage can rise on healthy phases during a ground fault. Systems that are not effectively grounded can expose arresters to higher temporary power-frequency voltage and may require a higher MCOV or arrester rating.

What information is needed to select a transformer surge arrester

Selection typically requires nominal and maximum system voltage, system grounding, maximum continuous line-to-ground voltage, expected temporary overvoltage, transformer insulation withstand level, arrester protective characteristics, and manufacturer application data.

Does a higher MCOV always provide better transformer protection

No. Increasing MCOV can improve the arrester's ability to withstand system voltage and temporary overvoltage, but it can also increase its protective voltage. Arrester selection balances operating-voltage capability with the protective level required by the transformer's insulation system.

For assistance with transformer voltage requirements, insulation coordination, and industrial transformer applications, contact Larson Electronics. Larson Electronics Building Trust Since 1973.

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