8.6 Surge Arresters: Types, MCOV, Application, and Field Evaluation

Surge Arresters: Types, MCOV, Application, and Field Evaluation

Quick Answer: Level II task 2.1h.1 requires visual inspection "for corona and thermal discoloration, and state of connections and mountings," and 2.1h.2 requires electrical tests on "capacitors, reactors, and surge arresters." Level III 3.1h.2 raises this to "perform and evaluate mechanical and electrical tests." The rating that governs application is MCOV — Maximum Continuous Operating Voltage — and the number it must be compared against depends on how the system is grounded.

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1. What an arrester does

A surge arrester is a voltage-limiting device. In normal operation it is effectively an insulator, drawing only a small leakage current. When a transient overvoltage arrives — a lightning stroke, a switching surge, a capacitor-switching transient — the arrester conducts, diverting the surge energy to ground and clamping the voltage across the protected equipment to a level the equipment's insulation can survive.

It is not a short circuit and it is not a fuse. It clamps, passes the surge, and then must return to its non-conducting state against the power-frequency voltage still present. That recovery requirement is what MCOV is about.

2. Two generations of technology

The MOV block's extreme non-linearity is the whole design: over a modest voltage range its resistance falls by many orders of magnitude. Below MCOV it is effectively an insulator; above the clamping threshold it conducts hard. No gap is needed because the material itself stops conducting when the voltage falls.

The MOV failure mode matters for field work. Moisture ingress or repeated energy absorption raises the block's leakage current at operating voltage. Leakage produces heat, heat further reduces resistance, and the process can run away to a violent failure. This is why leakage current monitoring is the primary MOV diagnostic and why a thermographic survey of arresters is genuinely useful — a hot arrester is a failing arrester.

3. The ratings and what each means

4. Selection: the grounding-method interaction

This is the most examinable point in the section.

On a solidly grounded system, a ground fault on one phase pulls that phase to earth and the unfaulted phases rise only modestly. An effectively grounded system is defined by a coefficient of grounding of 80 % or less — the highest line-to-ground voltage during a fault stays at or below 80 % of the line-to-line voltage, which works out to roughly 1.4 times the normal line-to-ground value. MCOV is selected against roughly the normal line-to-ground voltage with margin.

On an ungrounded or impedance-grounded system, a sustained ground fault on one phase raises the two unfaulted phases to the full line-to-line voltage with respect to ground — a factor of √3, or 1.732. And on an ungrounded system, that condition is allowed to persist, because the whole point of ungrounded operation is to keep running while the fault is located.

Therefore: an arrester on an ungrounded or high-resistance-grounded system must have an MCOV at or above the full line-to-line voltage, because it must withstand that condition continuously rather than momentarily.

Worked example. A 4,160 V system.

• Normal line-to-ground: 4,160 / √3 = 2,402 V
• Solidly grounded: select MCOV comfortably above 2,402 V — a 2.55 kV MCOV unit is typical.
• Ungrounded or high-resistance grounded: during a sustained ground fault the unfaulted phases sit at 4,160 V to ground, so MCOV must be at least 4,160 V — commonly a 4.36 kV MCOV unit.

Applying the solidly grounded selection to an ungrounded system produces an arrester that will thermally run away and fail during the first sustained ground fault — a failure that looks like an arrester defect but is actually a selection error. Level IV failure-analysis questions are built on exactly this distinction.

5. Protective margin

$\text{Protective margin} = \frac{\text{Equipment BIL} - \text{Arrester discharge voltage}}{\text{Arrester discharge voltage}} \times 100\%$

A margin of at least about 20 % is the conventional minimum. Two factors erode it in practice:

• Lead length. The arrester's connecting leads have inductance, and a fast-rising surge current develops a voltage across that inductance (v = L di/dt) which adds to the arrester's discharge voltage as seen by the protected equipment. With surge currents rising in microseconds, even a few feet of lead can add a significant voltage. This is why arrester leads are kept as short and as straight as physically possible, on both the line side and the ground side, and why a neatly looped "service loop" in an arrester ground lead is a defect rather than good workmanship.
• Separation distance. The further the arrester is from the equipment it protects, the higher the voltage that can appear at the equipment due to travelling-wave reflection. Arresters are mounted close to the protected apparatus — at the transformer bushings, not at the property line.

6. Field inspection and testing

Visual and mechanical (Level II 2.1h.1):

• Corona and thermal discoloration — the outline names both explicitly.
• Tracking, chipping, cracking of the porcelain or polymer housing.
• Contamination — salt, cement dust, industrial deposits — which promotes external flashover.
• Grading ring presence and alignment on higher-voltage units.
• Mounting and connections: tight, correctly torqued, and with short direct leads.
• Ground connection integrity and continuity to the grid.
• Pressure relief / vent path unobstructed and not aimed at equipment or a walkway.
• Surge counter and leakage current monitor readings recorded — the counter reading trended over time indicates surge activity.

Electrical tests:

• Insulation resistance at the manufacturer's recommended test voltage, arrester terminal to ground with the ground lead disconnected. This is a gross check for contamination and moisture, not a precise diagnostic. Apply the manufacturer's value — an excessive megohmmeter voltage can conduct the MOV blocks and give a meaningless reading.
• Leakage current / watts-loss measurement at operating voltage, which is the meaningful MOV diagnostic. Third-harmonic resistive leakage current analysis separates the resistive component from the much larger capacitive component and is the sensitive technique.
• Power factor / watts loss on station-class units, trended.
• Thermographic survey with the unit energized — a rising temperature relative to its neighbours is a direct indicator of increasing leakage and impending failure.

Safety: disconnect the ground lead before an insulation resistance test, and restore it afterward. An arrester left with its ground lead disconnected offers no protection at all and gives every appearance of being in service.

Exam trap: A question gives a 4,160 V high-resistance-grounded system and offers an arrester with 2.55 kV MCOV — the correct choice for a solidly grounded 4,160 V system. On a high-resistance-grounded system the unfaulted phases sit at the full 4,160 V to ground for the duration of a sustained ground fault, so an MCOV of at least 4.36 kV is required.