Jul. 21, 2025
Surge Arrester Currents
By Waymon P. Goch
Discharge current is the surge current that flows through a surge arrester during discharge of an overvoltage surge (and discharge voltage is the voltage that appears across the terminals of an arrester during that time). There are four additional currents that are of significance in the design, application, and performance of a surge arrester. Those currents can be defined as follows:
The difference in fault and power follow current is timing. The low arrester impedance during discharge is, for all practical purposes, a short circuit but the arrester must interrupt and reseal against power follow current.
Fault current is dictated by the power system and the available current at the arrester location. Power follow current is dictated by the power system and the surge arrester design. The others depend upon the arrester age, class, rating, and design [gapped silicon-carbide (SiC), gapped or gapless metal oxide (MOV)].
Surge arresters manufactured before about are gapped SiC and there are many distribution, riser pole, intermediate, and station class arresters in service today that are of that design. The majority of these arresters were manufactured from through . They represented state of the art at the time they were installed and for the most part their service history has been satisfactory.
The design of the earliest SiC arresters consisted of a simple multigap structure in series with non-linear SiC valve blocks. In those arresters, all system voltage was applied to the gap structure. The gap structure sparked over in response to an overvoltage surge to prevent damage to line or equipment insulation and the resulting power follow current flowed through the series gap-valve block combination. The non-linear blocks limited the follow current to a level that the gap structure could typically interrupt on the next voltage zero crossing (although restrikes were not uncommon). Following successful reseal the arrester returned to normal operation.
A major improvement in that design, primarily for station and intermediate class arresters, occurred with the introduction of the current-limiting gap in . The current-limiting gap helped limit system follow current by generating a back EMF which, in combination with the non-linear SiC blocks, allowed
current interruption without reliance on a voltage zero crossing.
Most gapped SiC arresters also utilized resistive (R), capacitive (C) or resistive-capacitive (RC) grading circuits to grade the system voltage and obtain uniform voltage distribution across the gap structure. These grading circuits were electrically connected external to the gaps and blocks so that grading current flowed only through the grading circuit. Typical grading circuits resulted in a few milliamps of line to ground current.
A concern with gapped SiC arresters was operation in severely contaminated environments. Severe external contamination and the resulting leakage currents could couple and upset weaker internal grading circuits and alter the voltage distribution over the gap structure.
One method of monitoring grading and leakage currents as well as the number of line to ground discharges primarily through station arresters is a discharge counter with a leakage/grading current meter, Counters are used with gapped SiC and gapped and ungapped MOV arresters to assist in monitoring their duty and condition. Installation of discharge counters requires grounding the arrester through the discharge counter. This is typically done by mounting the arrester on an insulating sub base, as shown in Figure 1. (Many years ago, one manufacturer offered a discharge counter that also contained a mirrored replica gap in the discharge path. By examination of the replica gap and the copper mirror electrodes one could theoretically judge the condition of the arrester internal gaps and determine the duty to which they had been exposed).
The introduction of metal-oxide [primarily zinc-oxide (ZnO)] semiconductors for use in MOV surge arresters in was the second major advancement in surge arrester design and performance. The metal-oxide varistor is characterized by an extremely non-linear current-voltage relationship resulting in a much higher voltage exponent over the nonlinear portion of the volt-amp curve than SiC. This characteristic is what allows the design of gapless surge arresters. It also requires the introduction of another current called the reference current (Iref), which is an AC current specified by the surge arrester manufacturer in conjunction with a reference AC voltage (Vref) that essentially defines the point at which the arrester elements go into conduction. Below that point (and when energized at normal line to ground operating voltage) MOV elements can be characterized as lossy capacitors with current leading voltage by almost 90⁰. As voltage is increased above Vref the MOV elements become more resistive and at full conduction almost purely resistive with current and voltage in phase.
The significant improvement in operating characteristics and protective levels afforded by gapless MOV surge arresters also renders them virtually immune to the effects of contamination and external leakage currents.
The IEEE C62 family of standards covers surge arresters and their application. For example, C62.11 is titled, “IEEE Standard for Metal-Oxide Surge Arresters for AC Power Circuits (>1kV)”.
In our previous post, we deep-dived into the various techniques and methodologies that are widely adopted for assessing the healthiness of Lightning/Surge Arresters.
In this post, we will answer the 10 frequently asked questions (FAQs) on the various aspects of testing and measuring Lightning/Surge Arresters. Let’s begin!
FAQs:
1. What are Third Harmonics?
A. In the context of electrical power systems, ‘harmonic’ refers to the voltage or current expressed as a multiple of the system’s fundamental frequency. Considering the fundamental frequency to be 50 Hz, Third Harmonic means 3 times 50 Hz i.e. 150 Hz.
Monitoring the Third Harmonic Resistive Component (THRC) of Lightning/Surge Arrester’s leakage current becomes important as THRC is directly related to the extent of degradation of the Arresters’ ZnO blocks.
2. Is the leakage current a 50Hz component or sum of other components?
A. The total leakage current component is the sum of all harmonic components. But out of all the harmonic components, THRC is the most important as it is directly responsible for the degradation of LAs/SAs.
3. What if the THRC value is more than the IR value?
A. Practically, this is not possible because as per IEC -5 standard, the THRC value is 10-40% of the IR value, i.e. the resistive component of the leakage current. So if the THRC value increases, the IR value will increase correspondingly.
4. What happens to the resultant value after measurement if the system voltage itself has harmonic content?
A. The system’s harmonic components are compensated while conducting the THRC measurement with system harmonic compensation as per the IEC -5 B2 method. Hence there is no effect of system harmonics in the THRC measurement methodology.
5. Could you please elaborate, how SCOPE’s SA 30i+ Leakage Current Analyser kit distinguishes system voltage harmonics from leakage current harmonics?
A. SA 30i+ is an advanced Leakage Current Analyser that includes an accessory called ‘Field Probe’ that automatically measures the harmonics present in the system voltage and then algorithmically compensates for the same.
6. What should be the exact position of the Field Probe?
A. It is recommended that the Field Probe be placed at the base of the Lightning/Surge Arrester.
7. Does SCOPE offer a different kit for testing and measurement of GIS Lightning Arresters (LAs)?
A. Both SA 30i and SA 30i+ offered by SCOPE can be used for GIS LA THRC measurement.
8. In the SA 30i T&M variants, will we get separate readings for IR, IC and THRC?
A. SA 30i kit measures the below four parameters:
1) Third Harmonic resistive leakage current at ambient temperature
2) Third Harmonic resistive leakage current at 200 Celsius temperature with the voltage correction factor
3) Total leakage current, and
4) Ambient Temperature.
9. Is the leakage current same for all ratings of LA of the same make?
A. No. The value of the leakage current depends on the following five parameters:
1) Rated voltage of the Lightning/Surge Arrester
2) OEM’s design parameters
3) Ambient temperature
4) Absorption/entry of moisture, and
5) Entry of solid micro particles.
10. How can we determine the healthiness of the LA by only observing the leakage current meter for differently rated LAs?
A. It is not always possible to determine the healthiness of a LA/SA only by observing the leakage current meter reading. Leakage current meters display only the value of the total leakage current whereas the most important parameter that needs to be measured is the THRC of the leakage current.
This is where SCOPE’s SA 30i/SA 30i+ comes handy. Unlike leakage current meters, SCOPE SA 30i variants not only measure the total leakage current but also measure the THRC at ambient temperature and at 200 Celsius thereby giving better insights into a LA/SA’s healthiness.
Summing Up
Regularly testing Lightning/Surge Arresters is pivotal to ensure their operational healthiness and safeguard your electrical infrastructure.
Globally, there are approximately 40 to 50 flashes of lightning every second, or nearly one and a half billion annually. Not only is the amount of strikes alarming, but each strike can have between 100 million and 1 billion volts and consumes billions of watts.
Such voltage and frequency cause irreparable personal injury and property damage, as well as unexpected equipment downtime, costly replacements and breaks in the production schedule. Lightning strikes may never be part of the schedule, but creating a defense for your facility with a lightning protection system should be.
In this post, we provide a brief overview of who needs a lightning protection system and the steps required to reduce the risk posed by lightning.
There is no known method of preventing the occurrence of a lightning discharge. The purpose of a lightning protection system, therefore, is to control the passage of a discharge in such a manner that prevents personal injury or property damage.
For architects, designers, developers and engineers, the need to provide protection should be assessed in the early stages of the structure design. Although no strict rules can be given, it is possible to use broad guidelines to arrive at the degree of protection required.
The general factors to consider include the level of risk lightning poses to personnel, equipment, structural damage and the consequential problems of a lightning-produced failure. Although not a strict science, assessment of these factors is one of judgment in comparing risks, economics and aesthetics.
The first mention of a traditional lighting rod—during the infancy stage of lightning protection systems—was published by Benjamin Franklin in in Gentleman’s Magazine. A year later he recommended the use of lightning rods to protect houses and other structures from lightning.
Although it varies on a case-by-case basis, Franklin’s recognition that the highest point of a facility is the most vulnerable to a direct lightning strike still forms the basis for many protection systems today.
However, fast-forward 250 years to see that the advanced lightning protection systems of today claim 84% to 99% effectiveness, based on the desired level of protection.
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Below, we outline the basic steps and the corresponding components required to successfully prevent damage caused by lightning strikes.
As Franklin noted, the highest point of a facility is the most vulnerable to a direct lightning strike. Lightning rods or air terminals capture the strike at a preferred point, and help to conduct the energy to the ground to minimize the risk of damage.
The two keys to effective strike termination devices (i.e. lightning rods or air terminals) are the type and placement.
Types of air terminal can be one of the following, depending on the application:
Rods — typically copper or aluminum
Masts — can be copper, aluminum, fiberglass, or stainless steel
Meshed conductors (on building surface or elevated)
Catenary wires
Natural components
To best capture lightning strikes, the following considerations should be made for air terminal placement, often dependent on material type:
Install as close as practical to roof edges.
Secure per requirements.
Select materials to reduce risk of corrosion.
Do not introduce trip hazards upon roof surface.
Do not locate in areas where water may pool (e.g. gutters).
Avoid penetrations into roof for fixing of conductors.
The components required to do this are known as down conductors, which provide the interconnection of the air terminations to the earth-termination system. They generally follow the profile of the structure, without being positioned where safety to individuals could be compromised.
Down conductors should provide multiple parallel paths for the discharge of energy from the lightning to the ground. Doing so lowers the risk of current density, thus reducing the risk of side flashing. This also reduces electromagnetic radiation effects of the impulse current at points inside the structure.
In general, a down conductor system should:
Provide multiple paths for lightning current.
Be as short and straight as practical.
Be spaced and use equipotential bonding rings.
Be a direct continuation of the air-termination system.
Not be installed in gutters or down spouts (even if PVC covered).
Connect via a test joint to the earth termination network.
Be fitted with external protection to reduce exposure to accidental damage or vandalism.
Be fitted with three-millimeter, cross-linked polyethylene insulation where there is risk of danger due to touch potential.
The reliable performance of the entire lightning protection system is dependent upon an effective earthing or grounding system.
Consideration for earthing systems must be given to:
Providing a low impedance network to dissipate the fast-rising lightning impulse.
Minimizing potential of touch and step hazards.
Long-term performance of the system – i.e. quality of materials and connections.
Grounding systems can be comprised of:
Ground rods
Perimeter (ring) bare wire
Radials
Ground plates
Concrete (rebar)
Equipotential bonding is required to eliminate voltage gradients, which reduces the possibility of electric shock or electrical equipment fault.
Each product, from the air termination system to the conductors to the grounding system, must work in tandem to effectively transfer discharge from interception to dissipation—without posing risks to people or the building.
In achieving overall effectiveness, the lightning protection system must:
Reduce thermal or mechanical damage to the structure.
Avoid sparking, which may cause fire or explosion.
Limit step and touch voltages to control the risk of injury to occupants.
Avoid damage to internal electrical and electronic systems.
To ensure proper protection, you must know which type of lightning arrester to use and what its rating means.
Lightning arresters (LAs) are among the most misunderstood and misapplied protective devices in our industry. Yet, with the increasing use of sensitive electronic equipment, they're almost a "must," because they divert the effects of extremely short-term overvoltages on an electrical system to ground. These overvoltages usually stem from lightning strikes.
How do lightning arresters divert the energy associated with lightning strikes? Lightning arresters are made up of varistors whose resistance reduces as the implied voltage increases. This reduction in resistance continues until the lightning arrester acts just like a direct short to ground. Upon reaching this condition, the lightning energy diverts to ground away from the protected equipment, thus reducing the effect of the overvoltage. That said, how do you know which type of LA to use? And what do their ratings mean?
What's in a class? ANSI/IEEE C62.1 (IEEE Standard for Gapped Silicon-Carbon Surge Arresters for AC Power Circuits) and C62.11 (IEEE Standard for Metal-Oxide Surge Arresters for Alternating Current Power Circuits) separate LAs into four classes: station, intermediate, distribution, and secondary.
Each type provides different levels of protection and energy diversion. The station class offers the best protective level and is capable of diverting the most energy. The intermediate class has the next best level; but a lower energy diversion capability than station-type LAs. The distribution class provides the worst protective levels and lowest energy diversion. Because the secondary-type LA doesn't overlap in voltage range with any of the other classes, it's difficult to make a direct comparison. Keep in mind: As you progress from station to distribution class, the cost significantly declines, but so does the protective level. By comparing cost to benefit, you can get the most efficient arrester for the application.
What's in a rating? A metal oxide varistor (MOV) arrester has two voltage ratings: duty cycle and maximum continuous operating voltage, unlike the silicon carbide that just has the duty cycle rating.
Duty cycle rating. The silicon carbide and MOV arrester have a duty cycle rating (in kV), which duty cycle testing established. This testing subjects an arrester to an AC rms voltage equal to its rating for 24 min, during which the arrester must withstand lightning surges at 1-min intervals. The magnitude of the surges is 10kA (10,000A) for station class arresters and 5kA for intermediate and distribution class arresters. The surge waveshape is an 8/20, which means the current wave reaches a crest in 8 ms (8 microseconds or 0. sec) and diminishes to half the crest value in 20 ms.
Maximum continuous operating voltage rating (MCOV). The MCOV rating is usually 80% to 90% of the duty cycle rating. Table 2 lists the MCOV ratings of various MOV arresters.
The MCOV rating of an MOV arrester is important because it's the recommended magnitude limit of continuously applied voltage. If you operate the arrester at a voltage level greater than its MCOV, the metal oxide elements will operate at a higher-than-recommended temperature. This may lead to premature failure or shortened life.
A closer look. The conductive elements of most LAs are made of either silicon carbide or metal oxide. An insulated housing (either porcelain or polymer-based rubber) surrounds the conductive elements.
Silicon carbide LAs. This design uses nonlinear resistors made of a bonded silicon carbide placed in series with gaps. The function of the gaps is to isolate the resistors from the normal steady-state system voltage. One major drawback is the gaps require elaborate designs to ensure a consistent spark-over level and positive clearing (resealing) after a surge passes. This design has lost popularity due to the emergence of the MOV arrester.
MOV LAs. The MOV design usually does not require series gaps to isolate the elements from the steady-state voltages because the material (zinc oxide) is more nonlinear than silicon carbide. This trait results in negligible current through the elements when you apply normal voltage. This leads to a much simpler arrester design.
An insulated housing surrounds series disks of zinc oxide in an MOV arrester. The disks have a conducting layer (generally aluminum) applied to their flat faces to ensure a proper contact and uniform current distribution within the disk. This design results in no "gaps;" thus, the reference to the MOV arrester as the "gapless" arrester. The MOV arrester design has become the most preferred because of its simplicity and resulting reduced purchase cost.
Are you interested in learning more about Zinc oxide arrester? Contact us today to secure an expert consultation!
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