Metal oxide (zinc oxide) surge arresters protect substations, transformers and lines against lightning and switching overvoltages. At normal operating voltage they conduct only a small leakage current; during a surge their resistance collapses and they divert the surge energy to earth. The risk is gradual degradation. Moisture getting past seals, repeated surge duty and ageing of the zinc oxide blocks can increase the leakage current, which heats the arrester and can end in thermal runaway and failure.
Total Current vs Resistive Current
The leakage current of a healthy arrester at operating voltage is mostly capacitive. It depends on the arrester's geometry and stray capacitance and changes little as the arrester ages. The small resistive part is what heats the blocks, and it is the part that grows with degradation. Because the capacitive part dominates, a change in the resistive component may barely show in the total current. That is why total-current milliammeters on arresters give only a coarse indication, and why diagnostic guidance such as IEC 60099-5 focuses on the resistive component.
How Resistive Current Is Separated
- Voltage reference method: the instrument takes a reference voltage signal, typically from the secondary of the voltage transformer on the same bus. Knowing the voltage phase, it splits the leakage current into the component in phase with voltage (resistive) and the component leading it by 90° (capacitive).
- Field (induction plate) reference: where connecting to the VT secondary is impractical, a plate near the arrester base picks up the electric field and provides the phase reference instead.
- Harmonic analysis: the zinc oxide characteristic is non-linear, so the resistive current contains a third harmonic. Analysing the current's harmonic content gives an estimate of the resistive component without any voltage reference, at lower accuracy.
MEWOI's MEWOI8203D/8204E testers offer all three methods, measure three phases at once with automatic compensation for interphase coupling, and use FFT analysis to report total current, resistive current and the peak of the resistive current's fundamental. The supplier gives a 0–10 mA range, with resistive-fundamental accuracy of ±(5% + 5 µA) using a voltage reference and ±(15% + 5 µA) using current-only analysis. The MEWOI1100D is a simpler wireless clamp on an insulated pole, reading total leakage current from 0 to 30 mA at 1 µA resolution.
Interpreting Results
Arrester leakage current depends on applied voltage, temperature, surface pollution and humidity, and on the arrester's design. The most reliable approach is comparative:
- Compare each arrester with its own earlier readings at similar system voltage.
- Compare the three phases of the same set.
- Investigate an arrester whose resistive current is rising steadily or is clearly above identical units nearby.
MEWOI's guidance for its clamp tester suggests that total leakage above about 500 µA, when system voltage is not abnormally high, suggests dirt, moisture or ageing worth investigating. Treat this as a screening threshold rather than a pass/fail limit, since normal values vary between arrester designs.
Arresters and Component-Level Surge Protection
MEWOI also supplies zinc oxide arresters for distribution, station and line applications, specified to IEC 60099-4. At the circuit-board end of surge protection, the same incoming-inspection logic applies to suppression components. Varistors are characterised by the voltage they reach at a defined DC current, commonly 1 mA. TVS diodes are characterised by their reverse breakdown voltage at a specified test current and by leakage at their stand-off voltage. A dedicated tester such as the MEWOI4850 checks these parameters on incoming parts.
Frequently Asked Questions
Does the arrester need to be disconnected? No. Resistive current methods are designed for measurement in service, using the arrester's own earth lead or monitor.
Why measure all three phases together? In a three-phase arrester set, stray capacitance between phases distorts each phase's apparent resistive current. Measuring all three together lets the instrument compensate for it.