Summary Highlights
- How arresters limit transient overvoltage from lightning and switching events
- MOV behaviour, normal leakage current and thermal recovery after a surge
- Distribution and station duty, line applications and LV SPD classifications
- Reading continuous and rated voltage, TOV, residual voltage, current and charge ratings
- Coordinating insulation, connection layout, separation distance and supporting documents
Article Details
A surge arrester limits transient overvoltage caused by lightning or switching events in an electrical system. A modern metal-oxide arrester conducts surge current through a designed protective path while limiting the voltage across its terminals. Correct selection and installation help reduce the stress applied to transformer, cable termination and switchgear insulation. Performance depends on the arrester ratings, the power system and the actual connection layout.
What does an arrester protect, and how does it fit into the protection system?
A transformer can experience a surge from an overhead line, a cable-to-overhead transition or a switching event as well as from direct lightning effects. The arrester addresses the resulting voltage stress. Fault detection and interruption, external lightning protection and equipotential bonding provide other functions within the installation.
| Device or arrangement | Principal duty | How its application is defined |
|---|---|---|
| MV/HV surge arrester | Limits transient voltage stress on power-system equipment. | System earthing, insulation coordination and the applicable arrester data. |
| Low-voltage surge protective device / SPD | Limits transient overvoltage in an appropriate low-voltage power or signal circuit. | Circuit type, protection mode, product standard and coordinated stages. |
| Air terminal / external lightning protection | Participates in a designed arrangement for intercepting and conducting lightning current. | The structure’s lightning protection design. |
| Fuse, circuit breaker and protection relay | Provide their assigned overcurrent/fault detection and circuit interruption duties. | Settings, the trip chain and the specified interrupting or isolating conditions. |
Terminology varies between markets: “lightning arrester” can refer to a power-system surge arrester, while an air terminal is a different component. Identify the voltage level, circuit and product family before comparing names. The public scope of IEC 60099-4:2014 concerns gapless metal-oxide arresters for AC systems. An LV Type 1, Type 2 or Type 3 marking does not supply the equivalent MV/HV duty classification.
How do MOV blocks limit voltage?
MOV stands for metal-oxide varistor. The active blocks, commonly based on zinc oxide, have a nonlinear voltage–current characteristic. A small current flows at normal operating voltage. As the voltage increases, the conducted current rises much more rapidly. This characteristic enables the arrester to conduct surge current while limiting terminal voltage.
A small normal current does not mean zero leakage. Capacitive and resistive components, temperature and applied voltage matter when assessing behaviour. An MOV arrester is not a mechanical switch waiting for a control command: voltage limitation follows its characteristic. Gapless and series-gapped designs also have different constructions and application details.
In a power circuit, the arrester normally forms a shunt protective path across the insulation being protected; phase-to-earth MV/HV installation is a common example. LV devices can use different protection modes, including phase-to-neutral and neutral-to-protective-conductor arrangements. The circuit and product documentation determine the appropriate connections. Signal-line protection must be assessed against its own connection diagram.
After a surge within the device’s duty limits, current can return to its small operating value. Thermal recovery matters: power-frequency temporary overvoltage, successive surges or an unsuitable continuous-voltage rating can impose a different stress from a single impulse. Both voltage limitation and the ability to recover thermally belong in the selection.
How do distribution, station, line and LV classifications differ?
Distribution and station designations are read within the applicable standard and product family. A line arrester describes an application intended to improve a line’s lightning performance. Porcelain or polymer housing describes another design feature. These are separate selection axes rather than a universal ranking from a basic device to the “best” arrester.
| Description | What it describes | Supporting information |
|---|---|---|
| Distribution or station duty class | Electrical duties and tests defined within the relevant framework. | Standard edition, class designation, current, charge-transfer and energy declarations. |
| Line application | Reducing insulation stress and lightning-related line interruptions at selected locations. | Line arrangement, earthing, installation positions and an appropriate product. |
| Porcelain or polymer housing | External insulation and mechanical construction. | Pollution, creepage, mechanical loading, environment and mounting limits. |
| LV Type 1 / 2 / 3 or combined type | The applicable LV SPD test and application framework. | Protection modes, system arrangement, ratings and coordination between stages. |
For a concrete example, the October 2022 revision of TEDAŞ-MLZ/96-019.B specifies particular distribution and station classes with different current, charge and energy requirements. Its procurement scope covers specified gapless metal-oxide silicone-composite phase-to-earth arresters for systems up to 36 kV. The table does not establish the ratings of every other voltage level or application.
What do continuous voltage, rated voltage, TOV and residual voltage mean?
Different kV and kA values on a nameplate describe different conditions. A technical comparison should retain the RMS or peak basis, impulse waveform, duration and product standard alongside each value. This separates normal operating voltage, fault-related stress and protection during an impulse.
| Parameter | Meaning | Comparison conditions |
|---|---|---|
| Uc / MCOV | The RMS power-frequency voltage that may be continuously applied under the relevant definition. | Actual continuous voltage across the protection mode and service conditions. |
| Ur / rated voltage | A reference rating for the specified duties and tests. | The standard and manufacturer definition, separately from Uc. |
| TOV capability | Withstand of power-frequency temporary overvoltage for a defined duration. | Voltage–time curve, prior energy duty and thermal conditions. |
| Ures / Upl and other residual or protective voltages | Peak terminal voltage during the specified impulse-current duty. | Impulse waveform, current magnitude and test conditions. |
| In / nominal discharge current | A peak-current quantity used for the specified impulse duty and classification. | Waveform and test sequence, rather than continuous load current. |
| Qrs / Qth | Repetitive or thermal charge-transfer quantities under the applicable definition. | Coulomb units and the stated duty and test. |
| Wth / energy declaration | A specified thermal-energy duty. | kJ or kJ/kV units, whether the voltage base is Ur or MCOV, and test conditions. |
| Short-circuit behaviour or withstand declaration | The stated safety and test duty under fault conditions. | RMS/peak current, duration, installation and test scope, separately from In. |
The TEDAŞ definitions distinguish Uc, Ur, discharge current and residual voltage. The duration used in that document’s Ur definition is not a substitute for every manufacturer’s TOV selection curve. A continuous-voltage RMS value and a residual-voltage peak value also represent different quantities.
Why does selection start with the system earthing arrangement?
First establish the highest continuous voltage across the arrester’s connected terminals. Then assess the voltage on healthy phases during an earth fault and the time taken to clear it. Solidly earthed, resistance-earthed and other neutral arrangements can produce different stresses. A nominal system-voltage label alone does not determine a suitable continuous-voltage or rated-voltage choice.
A concrete example is Table 7 of the 2022 TEDAŞ specification. For a maximum system voltage of 36 kV, it gives Ur 30 kV / Uc 24 kV for directly earthed service and Ur 36 kV / Uc 28.8 kV for resistance-earthed service. These are minimum values within that procurement scope and the document’s earth-fault clearing condition of less than one second. Transferring the numbers alone does not approve a product for a particular installation.
- Nominal and maximum operating voltage, frequency and protection mode.
- Neutral earthing arrangement, expected earth-fault voltage and clearing time.
- Protected equipment insulation withstand and the actual layout.
- Expected lightning/switching duty, successive events and energy/charge requirements.
- Altitude, pollution, temperature, mechanical loading and outdoor conditions.
- Voltage, TOV, residual-voltage, class and installation data for the exact product.
The selection discussion in Siemens’ 2017 US HP-AR 25 catalogue connects MCOV selection to phase-to-earth voltage, neutral arrangement and temporary overvoltage. Its US rating tables and factors belong to that product and standards context. Use the actual product’s voltage–time data rather than transfer a fixed catalogue ratio to another system.
Choosing a higher continuous-voltage rating may provide a different continuous/TOV capability, while also affecting the impulse protective voltage within a product family. Selection therefore balances operating withstand and protection of equipment insulation. Neither the lowest residual voltage nor the highest rated voltage alone resolves that balance.
How is residual voltage used in insulation coordination?
Compare the arrester’s residual voltage with the protected equipment’s impulse withstand using the relevant waveform and assessment framework. Steep-current, lightning-current and switching-current duties can have different results. The smallest kV number in a catalogue does not define the voltage at the equipment terminals under every field condition.
Additional voltage along connections, earthing and equipotential arrangements, and travelling-wave effects between the arrester and equipment contribute to the terminal stress. A protective margin is assessed with these effects and the design uncertainties. There is no single percentage, or subtraction of two nameplate values, that completes selection for every installation.
The public scope of IEC 60099-5:2018 treats selection and application of metal-oxide arresters as a separate guide topic. In practice, equipment insulation data, waveform-dependent arrester data and the installation model are evaluated together. This explanation is not a completed insulation-coordination study or approval of a product for a particular site.
Why are current, energy and charge-transfer ratings separate?
Nominal discharge current describes an impulse-current duty; it does not state continuous load current or remaining service life. Waveform and duration also affect charge transfer and energy duty. Two arresters with the same kA rating can have different repetitive-charge or thermal-energy declarations.
For example, Hitachi Energy’s POLIM-H table lists nominal discharge current, charge-transfer and energy properties separately. The Siemens discussion of energy duty also makes the voltage base relevant to the unit. Values expressed in kJ/kV Ur and kJ/kV MCOV cannot be ranked as if their denominators were identical.
LV SPD markings such as Iimp, In and Imax likewise require their own test definitions. A larger Imax does not by itself establish a lower voltage protection level or the same energy duty as an MV arrester. These ratings are not a counter stating how many surges may occur before replacement.
How do connection length, equipment distance and earthing affect protection?
Unnecessary length and loops in the line and protective connections can add voltage during a rapidly changing surge current. Separation from the protected equipment raises another question: wave propagation and reflection. Connection lead length and the arrester-to-equipment protective distance are different measurements.
| Check | Design issue | Supporting information |
|---|---|---|
| Connection routing | Length, loops and shared connections in the surge path. | Manufacturer diagram, product connection limits and installation layout. |
| Distance to protected equipment | Wave propagation, reflections and equipment terminal voltage. | The relevant insulation-coordination assessment and line/cable data. |
| Earthing and equipotential arrangement | Shared return paths and potential differences during a surge. | The complete connection arrangement, including impulse behaviour alongside power-frequency measurements. |
A specific LV example appears in the installation figures of the October 2022 version of TEDAŞ-MLZ/2022-077. For figure-a, the preferred combined l1 + l2 length is at most 0.5 m; the document prefers figure-b and considers figure-c where that arrangement is unavailable. This is a combined lead length rather than the PE conductor alone. The LV diagram conditions do not establish one universal distance for every MV/HV connection.
Conductor cross-section, backup protection, short-circuit behaviour and terminal torque are checked against the applicable product instructions. A backup protective device required for an LV SPD does not mean that the same fuse is required ahead of every MV/HV arrester. A single earth-resistance value also cannot describe the complete impulse performance of the installation.
What do indicators, disconnectors and inspections tell you?
Some LV products provide a visual status indicator and remote contact; some MV line applications use a device that disconnects a failed arrester. Surge counters and leakage-current monitoring provide different information. Check which features exist on the exact model and what each indicates. One visible indication does not verify every electrical characteristic.
Inspection considers housing and connection condition, pollution, mechanical stress, any indicator/disconnector, and the conditions of previous measurements. Leakage-current or thermal observations are interpreted with applied voltage, temperature, environment and method. Test voltage, disconnection steps and acceptance limits come from the product instructions and authorised work plan rather than this definition article.
Which documents help define a technical assessment or quotation?
- Exact model, nameplate photograph, product identification and applicable standard/edition.
- Current single-line diagram, neutral earthing arrangement and fault-clearing time.
- Uc/Ur or MCOV data, TOV curves and waveform/current-dependent residual voltages.
- Nominal discharge current, duty class, charge-transfer and energy declarations.
- Protected equipment insulation withstand, layout and connection drawings.
- Environmental/mechanical conditions, relevant test reports and maintenance/fault history.
Matching a model name, catalogue photograph or one kA value does not establish technical equivalence. Link the offered product documentation to the installation’s voltage and surge duty. Following a failure, examine event records, voltage conditions, the protected equipment and connection arrangement as well as the replacement arrester.
The surge arrester testing and maintenance article addresses condition assessment; this article explains the device and selection concepts. A site assessment may involve HV/MV testing, maintenance and repair, LV/MV/HV project design and consultancy, and the relevant operation-responsibility scope. Product and system information helps define the assessment and quotation accurately.
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Frequently Asked Questions
Is an arrester with the same kV label as the system automatically suitable?
Identify which voltage the label represents. System voltage, rated voltage and continuous-voltage rating are different data. Neutral earthing, earth-fault duration, the TOV curve, equipment insulation and connection layout must be assessed together; a matching kV number alone is insufficient.
How should Uc and MCOV be compared across catalogues?
Both relate to continuous operating voltage within their applicable frameworks. Similar terminology does not establish equivalence of every test or product class. Read the standard and manufacturer definitions, RMS voltage basis, protection mode and service conditions together, and check ratios referenced to rated voltage separately.
Does a 10 kA arrester continuously carry 10 kA of load current?
Nominal discharge current is a specified impulse and classification quantity. The arrester is not the series conductor carrying the power circuit’s continuous load current. The kA marking also does not directly state short-circuit withstand or remaining life after a given number of events.
Can an LV Type 1+2 SPD replace an MV station arrester?
These belong to different voltage and product frameworks. A combined LV type marking does not supply the continuous voltage, TOV, insulation and duty ratings needed for MV service. Suitability is established from the exact product, applicable standard and system conditions.
Must every arrester be replaced after a severe storm?
A storm alone does not establish that every arrester has reached end of life. Assess indicator/disconnector condition, damage, event and maintenance records, and manufacturer criteria. Isolation and replacement of a failed or ineffective device follow the authorised work plan.
Does a surge counter show remaining arrester life?
A counter records events according to its sensing threshold and arrangement. It does not independently determine every event’s waveform, energy or the arrester’s thermal condition. Its count is not a universal remaining-life measure or a direct count of lightning strikes.
Does low earth resistance make lead length unimportant?
Connection geometry and additional voltage from rapidly changing impulse current still matter. A power-frequency resistance measurement does not by itself represent complete impulse impedance or potential differences along shared connections. Routing and equipotential arrangements are assessed together.
Will an arrester interrupt sustained overvoltage from a lost neutral?
An arrester limits transient overvoltage; it does not open the power circuit like a voltage relay and circuit breaker. Its power-frequency overvoltage capability has duration and condition limits. Such an event requires assessment of arrester TOV duty alongside the installation’s separate protection arrangements.
Can a standard AC SPD be installed on the DC side of a PV system?
The DC/PV side requires a product with appropriate documented voltage, pole arrangement and fault behaviour. An AC product’s type or kA marking does not establish PV suitability. Array voltage, inverter requirements and product instructions are checked within the relevant design.
Does a good insulation-resistance reading prove complete arrester performance?
One measurement does not verify all impulse-protection and thermal-withstand characteristics. Method suitability, test conditions and manufacturer evaluation criteria matter. Condition assessment combines relevant measurements, records and physical findings; this article is not a test procedure.