Metal-oxide surge arresters (MOA) protect transformers, circuit breakers and busbars from overvoltage. They are installed in large numbers, they look identical from the ground, and they fail in a way that is hard to predict: moisture ingress and varistor ageing slowly change the electrical behaviour of the arrester long before a visible defect appears. Measuring the leakage current through the arrester is the standard way to track this ageing, and a lightning strike counter adds the operational history that explains why the arrester is stressed. This article explains the HUWOR surge arrester leakage current sensor (model HMJ1011-WB).
Why leakage current reveals arrester health
Under normal power-frequency voltage, an arrester conducts a small leakage current. This current has two components:
- The capacitive component, which is relatively stable and carries most of the current.
- The resistive component, which is small in a healthy varistor but rises sharply as the zinc-oxide valve blocks age, absorb moisture or suffer thermal stress.
It is the resistive component that is the early-warning signal. A rising resistive current indicates that the varistor is losing its blocking capability, and that the arrester may be approaching thermal runaway. Because the total leakage current is dominated by the capacitive part, an operator who only watches total current can miss the change; monitoring the resistive component (or both) is what makes early detection possible.
What the HMJ1011-WB measures
The HMJ1011-WB is a high-precision sensor designed for measuring microcurrents under high voltage. According to the published specification, the total current measurement range is 100 uA – 5 mA, which covers healthy arresters and arresters at an advanced degradation stage. The sensor measures the total leakage current and derives the resistive component, giving the monitoring platform a direct view of varistor condition.
The sensor is installed in the earthing conductor of the arrester, so it does not disturb the high-voltage circuit and can be fitted without taking the arrester out of service in most cases. The output feeds the online monitoring system, where the values are trended and alarmed.
Lightning strike counting
In addition to leakage measurement, the HMJ1011-WB includes a lightning strike counter. Every strike that discharges through the arrester is recorded, so the operator knows how many events the unit has absorbed and when they happened. This data matters for two reasons: a rapid succession of strikes may justify a closer look at the arrester, and the strike count supports life-cycle decisions such as replacement planning and insurance reporting.
Where surge arrester monitoring pays off
Online leakage monitoring is most valuable where arresters are difficult to inspect or where a failure is expensive:
- Substations with many MOA units, where routine visual inspection cannot detect internal ageing
- Transmission line arresters on towers, which are only inspected at long intervals
- Arresters protecting capacitor banks and converter stations, where an arrester failure can take a whole bay out of service
- Areas with high lightning activity, where the strike counter provides the operational record
Integration with the monitoring platform
The HMJ1011-WB connects to the same HUWOR online monitoring system as the partial discharge sensors. The platform logs leakage current, resistive current and strike counts, sets thresholds for warning and alarm, and can export the history for maintenance planning. For retrofit projects, the sensor’s interface works with the standard gateway architecture used across the substation.
Summary
Surge arresters fail without warning unless their electrical condition is tracked. The HMJ1011-WB measures the leakage current over a 100 uA – 5 mA range, extracts the resistive component that signals varistor ageing, and counts every lightning strike. Installed in the earthing conductor and feeding the monitoring platform, it converts a hidden degradation process into a trend that maintenance teams can act on. HUWOR supplies the sensor with OEM/ODM support; contact the HUWOR sales team for a specification sheet or a pilot project.