10.4 Surge Arrester Acceptance & SPD Verification

Surge Arrester vs. SPD - Know the Difference

Both devices clamp transient overvoltages, but they sit at different points in the system and are tested differently:

A surge arrester keeps a lightning or switching surge from flashing over a transformer bushing or insulator. An SPD clamps the residual surge that reaches a panelboard so it does not reach sensitive downstream electronics. Both must be verified at acceptance and maintained.

Surge Arrester Acceptance Test (Gapless MOV)

Modern medium- and high-voltage arresters are gapless metal-oxide varistor (MOV) stacks. With no series gap, the MOV blocks continuously conduct a small leakage current at system voltage - that is normal. The diagnostic question is whether the resistive component of that leakage has grown, which signals degradation.

Required Electrical Tests (NETA MTS Section 7.19.2)

1. Bolted-connection resistance - measure across each bolted connection with a micro-ohmmeter; investigate any value deviating more than 50% from the lowest similar connection.
2. Insulation resistance - megohmmeter test from phase terminal to case, 1 minute. Compare to manufacturer data or NETA Table 100.1. Note: a megohmmeter gives limited diagnostic value on MOV arresters because both healthy and degraded units can show very high insulation resistance - do not rely on it alone.
3. Grounding connection test - resistance between the arrester ground terminal and the ground grid should be less than 0.5 ohm.

Watts-Loss / Leakage-Current Test (The Diagnostic Test)

The most informative field test applies rated power-frequency voltage (MCOV) to the arrester and measures total leakage current, then separates the resistive component from the capacitive component (using a wattmeter or a third-harmonic method):

• In a healthy arrester the capacitive component dominates (the MOV behaves mostly as a capacitor at system voltage), and the resistive component is small.
• As MOV blocks degrade - from moisture ingress, contamination, repeated surges, or thermal stress - the resistive component grows.

NETA MTS acceptance criteria:

• Watts loss is evaluated by comparison with similar units or with the factory/baseline value. Investigate if the measured value exceeds 150% of the factory or prior test value.
• Resistive leakage current: investigate if it increases more than 50% from baseline or if it exceeds 20-25% of total leakage current.
• Power factor: should not exceed the manufacturer's published value; investigate if it exceeds 150% of factory or prior test.

Reference Voltage Test (Offline)

With the arrester de-energized and isolated, a hipot can apply voltage to the phase terminal and measure the voltage at a specified reference current (typically 1 mA DC or the manufacturer's value). The reference voltage should match the nameplate within manufacturer tolerance. A shift indicates a changed MOV characteristic.

Optional Partial Discharge

Partial-discharge detection is of limited value on MOV arresters in the field and is typically a factory test. Some maintenance programs use it as a secondary check on polymer-housed arresters where internal voids can develop.

What Rising Leakage Means

An arrester whose resistive leakage current or watts loss is climbing is on its way to thermal runaway - the failure mode where leakage heats the MOV, heat lowers its resistance, lower resistance draws more current, and the cycle accelerates until the arrester shorts internally. Causes include:

• Degraded MOV blocks - cumulative surge energy has damaged grain boundaries.
• Moisture ingress - a breached seal (especially on older porcelain-housed units) lets water in, which raises leakage and corrodes internal components.
• Contamination - external pollution on the housing creates a resistive film that alters the voltage distribution along the stack.

A rising trend does not mean the arrester has failed today, but it should be flagged, rechecked at the next outage, and scheduled for replacement if it crosses the 150%-of-baseline threshold.

SPD (Surge Protective Device) Status Check

Panelboard and equipment-level SPDs (UL 1449, Types 1, 2, or 3) contain MOVs with an internal thermal disconnect that opens when an MOV overheats - preventing the MOV from becoming a fire hazard after it has sacrificed itself. Once the thermal disconnect operates, the SPD no longer provides any surge protection, even though the panelboard keeps energized.

How to Verify an SPD

1. Visual indicator - most SPDs have a status LED (green = OK, red or off = failed) on the front face. Newer units may also provide a remote dry-contact signal (Form C) that can be wired to a BAS or SCADA for remote annunciation.
2. Check at acceptance and at every maintenance interval. A failed indicator means the MOV stack has disconnected and the SPD must be replaced - there is no field repair.
3. Verify per manufacturer - some SPDs have a test port or a communications module that reports surge count and remaining MOV health; follow the manufacturer's procedure.
4. Record the model and voltage rating to ensure the replacement matches the original specification.

NETA MTS Section 7.19.1 (Low-Voltage SPD)

• Visual and mechanical inspection of the status indicator.
• Verify the SPD is properly connected and grounded.
• Record any surge counter readings and compare to prior values.

Worked Example - 15 kV Class Arrester

A distribution substation has three 15 kV class gapless MOV arresters on the incoming line. At acceptance, the watts-loss test on phase B reads 2.1 W at MCOV; phases A and C read 2.0 and 2.2 W - all within the comparison band. Five years later, the maintenance test reads phase B at 3.6 W (a 71% increase from the 2.1 W baseline), while A and C read 2.1 and 2.3 W.

Per NETA MTS, 3.6 W is 172% of the baseline (2.1 W) - above the 150% investigation threshold. The resistive component of leakage has also grown from 8% to 14% of total current, approaching the 20-25% flag. The arrester is not yet failed, but it is flagged for replacement at the next planned outage and for more frequent monitoring (infrared thermography, leakage-current trending) until then.