Understanding Milliampere Measurements on Surge Arresters: Leakage Current, 1 mA Reference Voltage, and Transformer Protection (10/9/2026)

Q: Can leakage current be used to determine surge arrester service life?

Leakage-current trends can support condition assessment but cannot independently predict remaining service life. Other diagnostic findings, operating history, and manufacturer criteria must also be considered.

Milliampere (mA) measurements on surge arresters help evaluate leakage current, reference voltage, and potential deterioration. Understanding these measurements, along with MCOV and residual voltage, supports proper surge arrester selection, testing, and maintenance for industrial transformers and medium-voltage power systems.

By LarsonElectronics.com, October 9, 2026

Surge arresters protect transformers, switchgear, and electrical distribution equipment by limiting transient overvoltages caused by lightning, switching operations, and other electrical disturbances. In modern metal-oxide surge arresters, milliampere (mA) measurements are associated primarily with leakage current and specified reference-current testing.

Although these measurements provide useful information about arrester characteristics and condition, they must be interpreted alongside maximum continuous operating voltage (MCOV), temporary overvoltage capability, discharge current ratings, and protective characteristics. A 1 mA measurement alone does not establish whether an arrester is suitable for a particular electrical installation.

What Milliampere Measurements Represent in Surge Arresters

Milliamperes measure electrical current, with one milliampere equal to 0.001 ampere. For metal-oxide surge arresters, mA values commonly appear in two different contexts:

Leakage current: Current flowing through the arrester while it is energized under normal operating voltage.
Reference current: A specified current used to determine a reference voltage on the nonlinear voltage-current characteristic of the metal-oxide varistor (MOV) elements.

These measurements serve different engineering purposes. Leakage-current testing can support condition assessment, while reference-voltage testing characterizes the electrical behavior of the MOV elements under defined test conditions.

How Leakage Current Is Measured in a Surge Arrester

Metal-oxide surge arresters conduct a small amount of current during normal energized operation. This current is called leakage current and consists of capacitive and resistive components.

Capacitive Leakage Current

The capacitive component results from the electrical characteristics of the arrester and its relationship to the applied alternating voltage. In many healthy metal-oxide arresters, capacitive current accounts for a substantial portion of the total AC leakage current.

Capacitive leakage current does not necessarily indicate deterioration. Its magnitude depends on arrester design, applied voltage, and installation conditions.

Resistive Leakage Current

The resistive component is associated with conduction through the MOV elements and contributes to electrical losses and heating.

An increase in resistive leakage current under comparable operating conditions may indicate MOV deterioration, moisture-related problems, contamination effects, or other abnormal conditions.

However, resistive current also depends on applied voltage, temperature, and arrester design. Engineers should evaluate trends using manufacturer-specific limits and appropriate measurement techniques rather than relying on a universal mA threshold.

Leakage Current Testing Methods

Common assessment methods include:

Measuring total leakage current using a suitable arrester monitoring device.
Separating or estimating the resistive component through approved diagnostic techniques.
Comparing measurements with historical baseline values under similar operating conditions.
Reviewing manufacturer recommendations for energized testing and condition monitoring.

Engineering consideration: Total leakage current alone may not reliably identify MOV deterioration because the capacitive component can mask changes in resistive current. Surface contamination and installation configuration may also influence measurements.

What the 1 mA Reference Voltage Means

The 1 mA reference voltage is the voltage measured across a metal-oxide varistor element or defined arrester assembly when a specified reference current of 1 milliampere flows through it under controlled test conditions.

This measurement identifies a defined point on the nonlinear voltage-current characteristic of the MOV material.

As applied voltage increases, MOV current increases nonlinearly. Reference-voltage testing provides a repeatable method for evaluating electrical characteristics and comparing them with manufacturer specifications.

However, 1 mA is not a universal reference current for every complete surge arrester. The specified reference current depends on the applicable standard, manufacturer, arrester design, and test method.

Under IEEE C62.11, reference-voltage characteristics are established using specified reference-current procedures. Engineers should not assume that every IEEE-rated distribution or station-class arrester uses a 1 mA reference-current test.

Why Reference Voltage Matters

Reference-voltage measurements can help identify changes in MOV characteristics during manufacturing, qualification, or appropriate diagnostic testing.

A measured reference voltage outside the manufacturer's specified tolerance may indicate a nonconforming or deteriorated component. However, reference-voltage measurements must be compared with the correct original specifications and test conditions.

Reference voltage is not equivalent to the voltage remaining across an arrester during a high-current lightning or switching surge.

Differences Between Leakage Current, Reference Voltage, MCOV, and Residual Voltage

These terms describe different aspects of arrester performance and should not be used interchangeably.

Parameter	Meaning	Engineering Application
Leakage Current	Current flowing during normal energized operation	Condition monitoring and diagnostic evaluation
1 mA Reference Voltage	Voltage measured at a specified 1 mA reference current when that test is applicable	MOV characterization and specified testing
MCOV	Maximum designated RMS power-frequency voltage continuously applicable across an arrester	Continuous operating voltage selection
Rated Voltage	Arrester voltage designation associated with specified duty and operating requirements	Arrester classification and application
Residual Voltage	Voltage appearing across the arrester while discharging a specified surge current	Insulation coordination and protective-level evaluation
Discharge Current	Transient current conducted during surge operation, commonly expressed in kA	Surge-duty and protective-characteristic assessment

For transformer protection, residual voltage and the resulting insulation coordination are particularly important because they determine the voltage stress experienced by protected equipment during specified surge conditions.

How Milliampere Measurements Relate to Transformer Protection

Surge arresters are commonly installed on medium-voltage transformer primary connections, substation equipment, and distribution circuits.

During normal operation, a properly selected arrester remains in its high-impedance operating region while conducting a relatively small leakage current.

During a lightning or switching surge, the MOV elements conduct substantially greater current, limiting the voltage appearing across the arrester terminals.

Effective transformer protection requires coordination between the arrester's protective characteristics and the transformer's insulation withstand capability.

Important engineering considerations include:

System nominal voltage and maximum operating voltage.
Grounding configuration and maximum continuous phase-to-ground voltage.
Temporary overvoltage magnitude and duration.
Arrester MCOV and rated voltage.
Lightning and switching impulse protective levels.
Transformer basic lightning impulse insulation level (BIL).
Installation lead lengths and their effects on protective performance.
Expected surge environment and arrester energy-handling requirements.

For example, a pad-mounted transformer connected to a grounded medium-voltage distribution system requires arresters selected for the actual system grounding arrangement and expected overvoltage conditions.

An arrester with an unsuitable MCOV may experience excessive stress during normal or abnormal system operation, while an arrester with an unnecessarily high protective level may provide reduced insulation protection.

Industrial buyers evaluating transformer configurations can review available industrial transformers and coordinate surge protection requirements with the equipment manufacturer and electrical design engineer.

How Engineers Evaluate Abnormal Leakage Current

An increase in leakage current does not automatically establish that an arrester has failed. Accurate interpretation requires consideration of the electrical and environmental conditions during testing.

Applied System Voltage

Leakage current varies with applied voltage. Measurements taken at different operating voltages may not be directly comparable without appropriate correction or interpretation.

Arrester Temperature

MOV conduction characteristics are temperature-dependent. Temperature changes can affect resistive leakage-current measurements.

Surface Contamination and Moisture

Outdoor arresters exposed to pollution, salt deposits, or moisture may exhibit surface leakage that affects total measured current.

Age and Electrical Stress

Repeated electrical stress, abnormal temporary overvoltages, or excessive energy exposure can contribute to MOV deterioration.

Measurement Method

Different monitoring instruments and diagnostic methods may produce results that are not directly interchangeable. Measurements should be evaluated against the appropriate manufacturer procedures and baseline data.

Maintenance recommendation: When abnormal readings are identified, qualified personnel should review operating conditions, historical trends, visible condition, and manufacturer guidance before determining whether additional testing or replacement is necessary.

IEEE Standards Applicable to Surge Arrester Evaluation

Surge arrester selection, testing, and insulation coordination in North American industrial installations commonly reference the IEEE C62 series.

IEEE C62.11

IEEE C62.11 addresses metal-oxide surge arresters for AC power circuits above 1 kV and establishes applicable performance and testing requirements.

It provides a framework for evaluating characteristics such as reference voltage, protective performance, operating duty, and other arrester requirements.

IEEE C62.22

IEEE C62.22 provides application guidance for metal-oxide surge arresters on AC systems above 1 kV.

Its application principles support arrester selection based on operating voltage, temporary overvoltages, protective levels, and insulation coordination.

IEEE C62.82 Series

The IEEE C62.82 series addresses insulation coordination, including principles relevant to selecting insulation withstand levels and coordinating protective devices with equipment insulation.

Engineers should use the applicable edition and relevant portions of these standards for the equipment and system being evaluated.

NEC and Canadian Electrical Code Considerations

For U.S. installations, NFPA 70, National Electrical Code (NEC), Article 280 addresses surge arresters on circuits operating above 1,000 volts, subject to the applicable adopted Code edition.

Article 280 includes requirements concerning arrester installation and connections. These installation requirements are separate from the IEEE performance tests used to characterize arresters.

NEC Article 242 addresses overvoltage protection and surge-protective devices within its applicable scope, including lower-voltage applications. Engineers should distinguish low-voltage surge-protective devices from medium- and high-voltage metal-oxide surge arresters.

In Canada, installations should be evaluated under the applicable edition of CSA C22.1, Canadian Electrical Code, Part I, including the relevant surge arrester and overvoltage protection requirements.

Equipment certification, local amendments, utility specifications, and authority-having-jurisdiction requirements must also be considered.

Practical Example of Surge Arrester Testing on a Medium-Voltage Transformer

Consider a 15 kV-class industrial transformer installation equipped with metal-oxide surge arresters at the incoming medium-voltage terminals.

During routine maintenance, technicians identify an increase in measured leakage current on one arrester compared with its previous readings.

Rather than immediately assuming failure, the maintenance team should:

Verify that the measurement was performed using an approved method and suitable instrument.
Compare the applied voltage and operating temperature with previous measurement conditions.
Determine whether the increase is associated primarily with resistive current or total current.
Inspect for contamination, moisture intrusion, physical damage, or abnormal indications where safe and appropriate.
Compare the findings with the arrester manufacturer's maintenance criteria.
Determine whether additional testing, continued monitoring, or replacement is warranted.

This approach helps distinguish potentially significant electrical deterioration from variations caused by measurement conditions.

Testing on energized medium-voltage equipment must be performed only by qualified personnel using approved procedures, suitable instrumentation, and applicable electrical safety practices. Offline reference-voltage tests should not be performed on installed arresters unless expressly supported by the manufacturer and the approved test procedure.

Frequently Asked Questions About Surge Arrester mA Measurements

What Does mA Mean on a Surge Arrester

mA means milliamperes, a unit of electrical current. On a surge arrester, it may identify leakage current or a specified reference current used during electrical testing.

What Is the 1 mA Reference Voltage of a Surge Arrester

The 1 mA reference voltage is the voltage measured at a current of 1 milliampere under defined test conditions. It is commonly associated with MOV characterization, but the applicable reference current must be verified against the manufacturer's specification and relevant standard.

Does Higher Leakage Current Mean a Surge Arrester Is Failing

Not necessarily. Increased resistive leakage current under comparable conditions may indicate deterioration, but applied voltage, temperature, contamination, and measurement methods must also be considered.

Is the 1 mA Reference Voltage the Same as MCOV

No. MCOV identifies the maximum designated continuous RMS power-frequency voltage, while reference voltage identifies a point on the MOV voltage-current characteristic under specified test conditions.

Can Leakage Current Be Used to Determine Surge Arrester Service Life

Leakage-current trends can support condition assessment, but they cannot independently predict remaining service life. A complete evaluation considers electrical stress, environmental exposure, operating history, manufacturer criteria, and other diagnostic findings.

What Standards Apply to Medium-Voltage Surge Arresters

IEEE C62.11 establishes applicable performance and testing requirements, while IEEE C62.22 provides application guidance. Installation requirements may also be governed by NEC Article 280 in the United States or applicable Canadian Electrical Code provisions in Canada.

Key Engineering Considerations for Surge Arrester Selection and Maintenance

Milliampere measurements provide useful information about metal-oxide surge arrester behavior, but they represent only part of a complete engineering evaluation.

Leakage current helps assess operating condition, while reference-current measurements help characterize MOV electrical properties. Neither measurement should be confused with discharge current, MCOV, or residual voltage.

For industrial transformers and medium-voltage distribution equipment, proper arrester selection requires consideration of system voltage, grounding, temporary overvoltage conditions, protective characteristics, and insulation coordination.

Maintenance decisions should rely on repeatable measurements, manufacturer criteria, and qualified engineering assessment rather than a single current reading.

For assistance with transformer specifications, medium-voltage equipment configurations, and coordinated electrical protection requirements, contact Larson Electronics.

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