The purpose of online testing for HV lightning arresters is to evaluate their health and operational condition while they remain energized and connected to the high-voltage system. This non-intrusive testing monitors critical parameters—especially leakage current—to detect early signs of insulation degradation, moisture ingress, or internal damage. By performing these tests without disconnecting the arrester, utilities can ensure continuous protection against transient over voltages, prevent unexpected failures, and schedule maintenance proactively, thereby enhancing system reliability and safety.

Online testing of lightning arresters is performed while the arrester is energized and connected to the live high-voltage system. It monitors parameters like leakage current and voltage in real time to assess the arrester’s condition without interrupting power supply or removing the arrester from service.

Offline testing, on the other hand, requires disconnecting the arrester from the system and applying controlled test voltages or impulses in a laboratory or test environment. This allows detailed diagnostic tests, such as insulation resistance, dielectric withstand, and energy absorption capability, but causes downtime and power disruption.

In summary, online testing enables continuous condition monitoring without service interruption, while offline testing offers more comprehensive diagnostics but requires taking the arrester out of operation.

During online testing of lightning arresters, the key parameters measured include:

Leakage current: Both resistive and capacitive components are monitored to detect insulation deterioration or moisture ingress.

Discharge current: Measures the arrester’s response to transient overvoltages.

Voltage across the arrester: To correlate leakage current with operating voltage.

Power factor (dissipation factor): Indicates the level of insulation losses and ageing.

Harmonic content of leakage current: Helps identify partial discharges or defects.

Continuous monitoring of these parameters helps identify early signs of failure and evaluate arrester condition in real time.

In surge arresters (typically metal oxide varistor or MOV-based), leakage current patterns can offer early warning signs of deterioration or failure. Here are the key types of leakage current patterns that indicate a failing arrester:

1. Increasing Total Leakage Current Over Time

What it means: The arrester is gradually losing its insulation resistance.

Cause: Ageing of the zinc oxide blocks or moisture ingress.

Warning: A consistent upward trend is a red flag — especially under normal system voltage.

2. High Resistive Leakage Current

What it means: An increase in resistive (non-linear) current indicates internal degradation.

Why it's critical: Unlike capacitive leakage (which is normal), resistive leakage is a sign of arrester deterioration.

How it shows up: Measured using third-harmonic analysis or waveform separation techniques.

3. Sudden Jumps or Spikes in Leakage Current

What it means: Possible internal flashover, moisture ingress, or external contamination.

Typical sign: A sharp increase without a gradual trend.

Next step: Immediate inspection or replacement is usually advised.

4. Leakage Current with Strong Daily Variation

What it means: Leakage current rises during daytime due to heating and falls at night — abnormal if variation is large.

Potential cause: Moisture or contamination interacting with thermal cycles.

5. Phase Shift Changes in Leakage Current

What it means: The phase angle between voltage and leakage current shifts — especially the third harmonic.

Used in: Online monitoring systems.

Why it matters: Indicates the balance between capacitive and resistive components is shifting unfavourably.

6. Leakage Current under Wet Conditions

What it means: If leakage current increases dramatically during rain or fog, it may indicate surface tracking or contamination.

Action: Cleaning or replacing the arrester may be required.

The IEC 60099-4 standard (from the International Electrotechnical Commission) is the most widely accepted global standard for metal-oxide lightning arresters. It defines performance criteria, testing procedures—including online monitoring methods—and requirements for high-voltage arresters used in power systems. The B2 method for assessing leakage current in metal-oxide surge arresters is detailed in the IEC 60099-4 standard — specifically in IEC 60099-4:2013, titled "Surge arresters — Part 4: Metal-oxide surge arresters without gaps for AC systems — Methods of test".

This standard outlines how to measure and analyze leakage current components, including the resistive part used in the B2 method, for condition assessment and online monitoring of high-voltage lightning arresters.

In the B2 method, leakage current measurement is used to evaluate the condition of a metal-oxide lightning arrester by analyzing its resistive (non-capacitive) leakage current component while the arrester is energized at operating voltage.

Here’s how it works:

Measurement: The total leakage current flowing through the arrester is measured using sensitive instruments. This current consists of a capacitive component (normal and stable) and a resistive component (indicative of arrester aging or damage).

Resistive component isolation: The B2 method focuses on isolating the resistive part of the leakage current, as an increase in resistive current usually signals degradation, moisture ingress, or damage to the arrester’s internal varistors.

Analysis: By comparing the measured resistive leakage current to baseline or manufacturer’s reference values, the condition of the arrester can be assessed. A rising resistive leakage current over time indicates deteriorating insulation and the potential for failure.

Trend monitoring: Periodic measurements allow trend analysis, helping predict the arrester’s remaining life and plan maintenance before catastrophic failure occurs.

This method is widely used because it is sensitive, non-invasive, and can be performed online without disconnecting the arrester.

In the B2 method, leakage current measurement is used to evaluate the condition of a metal-oxide lightning arrester by analyzing its resistive (non-capacitive) leakage current component while the arrester is energized at operating voltage.

Here’s how it works:

Measurement: The total leakage current flowing through the arrester is measured using sensitive instruments. This current consists of a capacitive component (normal and stable) and a resistive component (indicative of arrester aging or damage).

Resistive component isolation: The B2 method focuses on isolating the resistive part of the leakage current, as an increase in resistive current usually signals degradation, moisture ingress, or damage to the arrester’s internal varistors.

Analysis: By comparing the measured resistive leakage current to baseline or manufacturer’s reference values, the condition of the arrester can be assessed. A rising resistive leakage current over time indicates deteriorating insulation and the potential for failure.

Trend monitoring: Periodic measurements allow trend analysis, helping predict the arrester’s remaining life and plan maintenance before catastrophic failure occurs.

This method is widely used because it is sensitive, non-invasive, and can be performed online without disconnecting the arrester.