IEEE standards for medium-voltage capacitor banks and harmonic filter banks establish requirements and guidance for component ratings, switching duty, protection, insulation coordination, harmonic limits, and equipment construction. VarStec applies IEEE 18, IEEE 1036, IEEE 1531, IEEE 519, and the applicable IEEE C37 series standards throughout the design and application of its equipment, together with applicable UL, ANSI, NFPA, CSA, IEC, and other requirements. This Technical Q&A explains how these standards apply to capacitors, reactors, switches, circuit breakers, fuses, surge arresters, harmonic filters, and metal-enclosed equipment.

Standards for Medium-Voltage Capacitor Banks and Harmonic Filters

VarStec designs its medium-voltage capacitor banks and harmonic filter banks using the applicable IEEE, UL, ANSI, and NFPA standards and codes. No single standard governs the complete assembly; different standards address capacitor ratings, harmonic filter design, switching duty, circuit breakers, fusing, protection, surge arresters, insulation coordination, metal-enclosed construction, control panels, enclosures, safety labeling, and installation requirements.

IEEE Standards and Guides

  • IEEE 18-2025 — Standard for Shunt Power Capacitors
  • IEEE 1036-2020 — Guide for the Application of Shunt Power Capacitors
  • IEEE 1531-2020 — Guide for the Application and Specification of Harmonic Filters
  • IEEE 519-2022 — Standard for Harmonic Control in Electric Power Systems
  • IEEE C37.20.3-2023 — Standard for Metal-Enclosed Interrupter Switchgear
  • IEEE C37.20.4-2013 — Standard for Indoor AC Switches for Use in Metal-Enclosed Switchgear
  • IEEE C37.011-2019 — Guide for the Application of Transient Recovery Voltage
  • IEEE C37.04-2018, with applicable amendments, Standard for Ratings and Requirements for AC High-Voltage Circuit Breakers
  • IEEE C37.66-2021 — Standard Requirements for Capacitor Switches
  • IEEE C37.012-2022 — Guide for Capacitive Current Switching
  • IEEE C37.99-2012 — Guide for the Protection of Shunt Power Capacitor Banks
  • IEEE C37.48-2020 — Guide and Tutorial for the Application of High-Voltage Fuses
  • IEEE C62.22-2009 — Guide for the Application of Metal-Oxide Surge Arresters
  • IEEE C37.20.7-2024 — Recommended Practice for Testing Switchgear for Internal Arcing Faults
  • IEEE PES-TR16 — Transient Limiting Inductor Applications in Shunt Capacitor Banks

UL Standards

  • UL 508A — Standard for Industrial Control Panels. Applicable to the low-voltage control-panel portions of VarStec equipment, including control wiring, components, overcurrent protection, disconnecting means, and panel construction where UL 508A compliance is specified.
  • UL 50 / UL 50E — Enclosures for Electrical Equipment. Applicable to enclosure construction and environmental considerations such as outdoor exposure, water ingress, corrosion, gasketing, and enclosure environmental ratings.

ANSI Standards

  • ANSI Z535.4-R2023 — Product Safety Signs and Labels. Establishes requirements for safety labels, signal words, colors, symbols, and hazard messaging applied to VarStec equipment.

NFPA / National Electrical Code

  • NFPA 70 — National Electrical Code.
    • Article 460 — Capacitors
    • Article 490 — Equipment Over 1,000 Volts, Nominal

VarStec Perspective: These standards are not applied independently. A properly engineered capacitor bank or harmonic filter must be designed with the applicable provisions of multiple standards evaluated together. For example, the capacitor units are rated under IEEE 18, the filter applied using IEEE 1531, harmonic performance evaluated under IEEE 519, the switching device selected under the applicable IEEE C37 standards, protection developed using IEEE C37.99 and C37.48, and the complete assembly constructed and labeled using the applicable IEEE, UL, ANSI, and NEC requirements.

For equipment supplied into Canada, VarStec applies the corresponding CSA standards and Canadian Electrical Code requirements, including standards such as CSA C22.2 No. 190 where applicable. Many U.S. and Canadian electrical standards are closely aligned or harmonized, which allows similar engineering principles and equipment designs to be applied while addressing the specific certification and code requirements of each country.

For projects in Europe, South America, and other international markets where IEC standards are specified or preferred, VarStec applies the applicable IEC standards and local national requirements. Many IEEE and IEC standards have undergone substantial technical alignment and harmonization, particularly in areas such as switchgear, circuit breakers, insulation coordination, and power-system performance; however, the requirements are not always identical. VarStec therefore evaluates the governing standards specified for each project rather than assuming that compliance with one standards system automatically establishes compliance with another.

Capacitor and Reactor Ratings

Capacitor ratings are governed directly by IEEE 18, while IEEE 1036, IEEE 1531, and IEEE PES-TR16 provide application guidance for how capacitors, reactors, and other filter components are selected and applied within capacitor banks and harmonic filter banks. IEEE C57.16 and IEEE C57.21 do not directly govern the iron-core reactors typically used in harmonic filters, but applicable portions of these standards are commonly used as guidance for reactor design, construction, rating, and testing. The principal IEEE references are:

  • IEEE 18-2025 — IEEE Standard for Shunt Power Capacitors. This is the primary capacitor-unit standard and establishes the electrical ratings, operating limits, and performance requirements applicable to the power capacitors used in VarStec capacitor banks and harmonic filter banks.
  • IEEE 1036-2020 — IEEE Guide for the Application of Shunt Power Capacitors. Extends the component requirements of IEEE 18 into practical capacitor-bank application, including equipment ratings, protection, switching, and the reliable application of shunt power capacitors.
  • IEEE 1531-2020 — IEEE Guide for the Application and Specification of Harmonic Filters. Provides guidance for the application and specification of passive harmonic filters and their individual components.
  • IEEE PES-TR16-2014 — Transient Limiting Inductor Applications in Shunt Capacitor Banks. Provides application guidance for reactors used to limit capacitor-bank inrush and outrush currents. It is particularly important because the selection and placement of these reactors can affect fault current, circuit-breaker duty, and transient recovery voltage (TRV).
  • IEEE C57.16-2025 — IEEE Standard for Requirements, Terminology, and Test Code for Dry-Type Air-Core Series-Connected Reactors. Establishes requirements, terminology, ratings, and test procedures for dry-type air-core series-connected reactors and specifically includes filter reactors among its covered applications. VarStec uses this standard as a primary reference when specifying applicable reactor ratings and tests, while recognizing that iron-core filter reactors may require additional engineering considerations not specifically addressed by the standard.
  • IEEE C57.21-2021 — IEEE Standard Requirements, Terminology, and Test Code for Shunt Reactors Rated Over 500 kVA. This standard applies to shunt reactors and specifically excludes filter reactors from its scope. However, portions addressing such subjects as temperature rise, losses, impedance, dielectric testing, insulation levels, and reactor construction provide useful engineering guidance when developing specifications and test requirements for iron-core filter reactors.
  • IEEE C57.32-2015 / IEEE C57.32a-2020 — IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices / Amendment 1: Neutral Grounding Resistors Clause. There is no single IEEE component standard dedicated specifically to the construction and testing of harmonic filter damping resistors. Because neutral-grounding resistors and harmonic filter resistors share many fundamental thermal, dielectric, insulation, and resistor-construction considerations, VarStec uses applicable portions of IEEE C57.32 and C57.32a as engineering guidance when specifying filter resistors. IEEE currently lists these editions as inactive-reserved while a revision of C57.32 is under development.

Harmonic Filter Design and Harmonic Limits

Two IEEE documents have distinctly different but complementary roles in harmonic-filter applications: IEEE 519 establishes the harmonic-performance limits, while IEEE 1531 addresses how passive harmonic filters should be applied and specified. Supporting capacitor standards are then used to verify the individual component ratings.

  • IEEE 1531-2020 — IEEE Guide for the Application and Specification of Harmonic Filters. This is the primary filter-design application guide. It addresses filter components, protection, control, and the application of passive shunt harmonic filters on low-, medium-, and high-voltage power systems.
  • IEEE 519-2022 — IEEE Standard for Harmonic Control in Electric Power Systems. Establishes harmonic voltage- and current-distortion limits at the point of common coupling. IEEE 519 defines the required system performance; it does not prescribe how the harmonic filter itself must be designed.
  • IEEE 18-2025 — IEEE Standard for Shunt Power Capacitors. Applies to the power capacitors used within the filter and establishes the capacitor rating and operating limits that must remain satisfied after fundamental and harmonic quantities are combined.
  • IEEE 1036-2020 — IEEE Guide for the Application of Shunt Power Capacitors. Provides additional application and protection guidance for the capacitor portion of the filter bank.

For VarStec, compliance therefore requires both sides of the problem: the filter must provide the required harmonic performance at the system level while every capacitor, reactor, resistor, switching device, and protective component remains within its individual electrical and thermal ratings.

Capacitor Switching, Circuit Breakers, and TRV

Capacitor bank and harmonic filter bank switching subjects the substation to unique, high-frequency transients, including severe inrush currents, outrush currents to adjacent parallel banks, and extreme Transient Recovery Voltages (TRV). To ensure long-term system reliability and prevent catastrophic insulation flashovers or restrike-induced equipment failure, the specific capacitive switching and TRV duties must be calculated and evaluated at the transient level. The IEEE standards governing these complex phenomena form a highly specialized matrix of rating structures, application guides, and testing procedures:

  • IEEE C37.66-2021 — IEEE Standard Requirements for Capacitor Switches for AC Systems (1 kV to 38 kV). Applies to switches specifically designed for routine capacitor switching and establishes ratings, construction requirements, and test requirements for that duty.
  • IEEE C37.012-2022 — IEEE Guide for the Application of Capacitive Current Switching for AC High-Voltage Circuit Breakers Above 1000 V. Provides the application guidance for circuit breakers switching capacitor banks, filter banks, cables, and other capacitive loads and addresses restrikes, reignitions, and related phenomena.
  • IEEE C37.04-2018, with IEEE C37.04a-2025 — IEEE Standard for Ratings and Requirements for AC High-Voltage Circuit Breakers with Rated Maximum Voltage Above 1000 V. Establishes the circuit-breaker rating structure, including capacitive-current switching and TRV capability. The 2025 amendment includes additional clarification related to capacitive switching.
  • IEEE C37.011-2019 — IEEE Guide for the Application of Transient Recovery Voltage for AC High-Voltage Circuit Breakers with Rated Maximum Voltage Above 1000 V. Provides the methodology for comparing the actual system TRV with the breaker TRV capability and evaluating mitigation where the system duty exceeds the standard rating.
  • IEEE PES-TR16-2014 — Transient Limiting Inductor Applications in Shunt Capacitor Banks. Addresses an especially important capacitor-bank condition: a reactor installed to limit switching current can create severe TRV during certain reactor-limited faults. This report is therefore a key VarStec reference when applying transient-limiting reactors.
  • IEEE Std C37.100.2-2018 — IEEE Standard for Common Requirements for Testing of AC Capacitive Current Switching Devices over 1000 V.  Is the absolute core test procedure standard that unified capacitive current testing requirements previously scattered across C37.09, C37.66, and IEEE 1247. It formally defines the Class C1 and Class C2 restrike performance classes and contains the exact testing procedures for both the 104-shot targeted test program (for time-controllable devices) and the 1,200-shot random test program. Any Class C2 rating on a switch or breaker must be proven against this standard.
  • IEEE Std C37.09-2018 (with Corrigendum 1-2021) — IEEE Standard Test Procedures for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis. While C37.04 defines breaker rating structures, C37.09 defines the actual design test procedures for circuit breakers. It details the terminal fault tests (like the T60 duty) required for contact preconditioning prior to Class C2 testing.
  • IEEE Std 1036-2020 — IEEE Guide for the Application of Shunt Power Capacitors provides details on calculating transient inrush/outrush magnitudes and is foundational in the development of VarStec’s Capacitor Bank Switching Peak Inrush Current Analysis tool, Capacitor Bank Outrush & Close-in Fault Analysis Tool, and Reactor-Limited Fault TRV and Breaker Duty tool.
  • IEEE Std C37.010-2016 — IEEE Guide for AC High-Voltage Circuit Breakers greater than 1000 Vac Rated on a Symmetrical Current Basis. This is the parent application guide for breakers, outlining general system calculations, short-circuit current evaluations, close-and-latch limits, and general outrush current considerations.

Recap of Obsoleted and Rolled-In Standards

  • IEEE Std C37.06-2009 (Preferred Ratings & Required Capabilities) ⇒ Officially Withdrawn / Absorbed into IEEE Std C37.04-2018.
    • What happened: C37.06 previously housed all the preferred North American physical rating tables (such as continuous, short-circuit, and capacitive currents). To align with international IEC standards, the IEEE completely eliminated C37.06 as a standalone document and rolled its rating tables directly into IEEE C37.04-2018. Standard rules dictate that any specification still referencing C37.06-2009 must now be legally interpreted as referencing C37.04.
  • Testing Standards Consolidation ⇒ Absorbed into IEEE Std C37.100.2-2018
    • What happened: Capacitive switching testing procedures were historically a messy, fragmented landscape. In 2018, the IEEE Switchgear Committee gathered the capacitive testing sections from three distinct standards and merged them into the unified C37.100.2-2018
      1. IEEE Std C37.09a-2005 (Circuit Breaker Capacitive Switching tests).
      2. IEEE Std C37.66-2005 (Capacitor Switch testing).
      3. IEEE Std 1247-2005 (Interrupter Switch capacitive tests). This consolidation created a single, harmonized standard for testing all capacitive switching devices, regardless of their technology.
  • IEEE Std 1036-2010 ⇒ Superseded by IEEE Std 1036-2020
      • What happened: The shunt capacitor application guide was fully revised to align with modern distribution practices and coordination with IEEE Std 18-2012 (now revised to IEEE 18-2025), adding extensive calculations for bank rating considerations and transient overvoltage behaviors.

VarStec Perspective:
Capacitive-switching standards have evolved significantly, with rating requirements consolidated into newer documents and testing procedures reorganized into common standards such as IEEE C37.100.2. As a result, many existing utility and industrial specifications still reference superseded standards or requirements that no longer align cleanly with current IEEE practice. VarStec helps clients reconcile these legacy requirements with the applicable current standards.

VarStec reviews the technical basis behind switching-device ratings and vendor test claims for switches they install in their equipment. This includes confirming that the correct test standard, test sequence, and preconditioning requirements were applied for the specific device technology and claimed performance class. For critical duties such as Class C2 capacitor switching, the rating should be supported by the appropriate design-test evidence, not simply by a nameplate or catalog claim.

Protection, Fusing, and Surge Protection

The principal IEEE references used by VarStec are:

IEEE C37.99-2012 — IEEE Guide for the Protection of Shunt Capacitor Banks. Specifically addresses protection methods for both shunt power capacitor banks and filter capacitor banks and is the principal IEEE reference for capacitor-bank protection philosophy. The 2012 document is the latest published edition; IEEE presently lists it as inactive-reserved while a revision is underway.
IEEE C37.48-2020 — IEEE Guide and Tutorial for the Application of High-Voltage (>1000 V) Fuses and Accessories. Provides guidance for selecting and coordinating medium-voltage current-limiting, expulsion, and other high-voltage fuse types. It is particularly relevant to capacitor-unit and bank fusing applications.
IEEE C62.22-2009 — IEEE Guide for the Application of Metal-Oxide Surge Arresters for Alternating-Current Systems. Provides the application and insulation-coordination framework for metal-oxide surge arresters used to protect the capacitor bank, filter bank, switching devices, and associated medium-voltage equipment from transient overvoltages. The 2009 edition remains the latest published base guide while a new revision is under development.
IEEE 1036-2020 — IEEE Guide for the Application of Shunt Power Capacitors. Supplements the dedicated protection standards with capacitor-bank application, protection, and equipment-rating guidance.

IEEE C62.22 mandates a minimum Protective Margin (PM) of 20% (Protective Ratio) when coordinating surge arresters with equipment insulation.

  • LPL Margin (PML2): Must be ≥ 20% between the equipment’s Basic Lightning Impulse Insulation Level (BIL) and the arrester’s Lightning Protective Level (LPL).
  • FOW Margin (PML2): Must be ≥ 20% between the equipment’s Chopped Wave Withstand (CWW) and the Front-of-Wave (FOW) protective level.
  • Calculated Stress: Both protective margins must legally account for the connecting lead inductive voltage drop.

Engineering Tip: The VarStec Capacitor Bank Altitude Derating Tool explicitly calculates PML2 and PML2 and will automatically flag a “FAIL” if either drops below the 20% threshold.

Altitude, Insulation Coordination, and Environmental Ratings

Altitude and environmental conditions affect different components in different ways. Reduced air density primarily affects external insulation, while temperature, contamination, transient overvoltages, and equipment construction impose additional requirements. VarStec therefore evaluates both the equipment specific IEEE standards and the applicable insulation coordination standards.

  • IEEE C37.20.3-2023 — IEEE Standard for Metal-Enclosed Interrupter Switchgear Rated above 1 kV AC up to and Including 48.3 kV AC. Establishes the ratings and service-condition framework for metal-enclosed interrupter switchgear assemblies and is a primary reference when applying VarStec metal-enclosed equipment at non-standard site conditions.
  • IEEE C37.20.2-2025 — IEEE Standard for Metal-Clad Switchgear. Applies when metal-clad circuit-breaker switchgear construction is involved and establishes service conditions, insulation withstand requirements, ratings, and testing requirements.
  • IEEE C37.010-2016 — IEEE Application Guide for AC High-Voltage Circuit Breakers >1000 Vac Rated on a Symmetrical Current Basis. Addresses circuit-breaker application under varied service conditions, including reduced dielectric capability, making it particularly relevant to high-altitude breaker applications.
  • IEEE C62.22-2009 — IEEE Guide for the Application of Metal-Oxide Surge Arresters for Alternating-Current Systems. Provides the IEEE methodology used to coordinate surge-arrester protective levels with equipment insulation capability.
  • IEC 60071-1-2019 — Insulation Co-ordination — Part 1: Definitions, Principles and Rules. Establishes the fundamental IEC insulation-coordination framework and selection of rated withstand voltages for equipment above 1 kV.
  • IEC 60071-2-2023 — Insulation Co-ordination — Part 2: Application Guidelines. Provides the detailed application methodology for insulation coordination and is the current IEC companion to IEC 60071-1. This is the edition I would use going forward rather than the superseded 2018 edition.
  • IEEE 18-2025 and IEEE 1036-2020 also remain applicable to capacitor-specific service conditions and ratings and must be considered alongside the insulation requirements of the overall metal-enclosed assembly.

You can directly cite IEEE C37.010, IEEE C37.20.2, and IEEE C37.20.3 to formally validate that properly coordinated surge arresters can safely mitigate the need for physical altitude derating.

  • IEEE C37.010, Section 3.2.2: Directly challenges blind upsizing by stating that derating the dielectric capabilities is “not always necessary and usually not the most economical approach”. It confirms that if at least a 20% margin exists between the arrester protective level and the equipment’s Basic Lightning Impulse Insulation Level (BIL), “no derating should be necessary”.
  • IEEE C37.20.2, Section 8.1.3 & IEEE C37.20.3, Section 8.1.4: Both switchgear standards explicitly mandate the evaluation of this mitigation strategy, stating that for applications above 1,000 meters (3,300 feet), the use of surge arresters on each circuit selected to “keep transient voltages below the reduced levels should be considered”.
  • Economic Impact: Referencing these specific clauses prevents the automatic penalty of specifying artificially oversized equipment and ensures a highly optimized, cost-effective design.

Engineering Tip: These standard citations are dynamically generated in the “Justification Section” of the Executive Summary within the VarStec Capacitor Bank Altitude Derating Tool to provide documented mathematical proof for easy client submission

You can directly cite IEC TR 62271-306, Section 4.5.1.2 and IEC 60871-1, Section 18.1. These documents formally validate that overvoltage limiting devices can be used to lower the required insulation level of a substation.

  • IEC TR 62271-306: States that the “Application of surge arresters with the intent to lower the insulation level of the substation may result in a more advantageous choice of equipment.”.
  • IEC 60871-1: Confirms that insulation choice must account for “the type of overvoltage limiting devices” utilized in the system.
  • Economic Impact: Referencing these clauses prevents the automatic penalty of specifying artificially oversized equipment.

Engineering Tip: These standard citations are dynamically generated in the Executive Summary of the VarStec Capacitor Bank Altitude Derating Tool for easy client submission.

The primary difference between IEEE C62.22 and IEC 60071-2 lies in the direction of the mathematical evaluation: IEEE utilizes an “Equipment-Down” method, while IEC uses a “Surge-Up” method. Both ensure that transient overvoltages do not exceed the altitude-derated dielectric strength of the equipment.

  • IEEE Equipment-Down: Derates the equipment’s sea-level BIL using an altitude correction factor, then verifies a strict protective margin against the calculated transient surge.
  • IEC Surge-Up: Multiplies the arrester’s coordination withstand voltage (UCW) by a safety factor (KS=1.15) and an atmospheric altitude correction factor (Ka) upward to calculate a required sea-level rating (Urw).

Validation: A rigorous design verifies both methodologies to ensure global compliance.

Engineering Tip: VarStec Capacitor Bank Altitude Derating Tool processes both engines simultaneously to provide a unified PASS/FAIL status for the EPC.

No. Surge arresters are designed strictly to clamp fast-rising transient overvoltages (like lightning and switching surges), not to limit long-duration power-frequency voltages.

  • Thermal Runaway: Arresters exposed to continuous clamping conditions during prolonged 50/60Hz overvoltages will absorb excessive energy and catastrophically fail.
  • Direct Check: Altitude derating for PFW must be checked directly. The derated PFW at altitude must remain higher than the maximum continuous system voltage (Um).
  • Standard Rule: Both IEEE and IEC methods require an unmitigated “Direct Check” for power-frequency compliance.

Engineering Tip: The VarStec Capacitor Bank Altitude Derating Tool evaluates PFW independently of the arrester, displaying an “N/A (Direct Check)” in the Calculated Margin column.

According to IEC 60071-2 and IEEE C37.20.2, standard environmental service conditions cap at 1,000 meters (3,300 feet). Above this altitude, the reduced air density degrades the dielectric withstand capabilities of external insulation.

  • Unmitigated Rule: Without mitigation, equipment installed above 1,000 meters must be derated using standard-defined Altitude Correction Factors (ACF).
  • Mitigated Rule: Applying coordinated surge arresters allows standard equipment to operate safely beyond 1,000 meters without artificially upsizing the BIL. Engineering

Tip: The VarStec Capacitor Bank Altitude Derating Tool uses 1,000 meters as the baseline; entering any target altitude above this triggers the automated derating and arrester verification engines.

Unlike the IEEE “Equipment-Down” margin approach, IEC 60071-2 mandates a “Surge-Up” methodology. It requires engineers to calculate a Required Withstand Voltage (Urw) based on the arrester’s protective level and verify that the equipment’s standard Nameplate Rating is greater than or equal to this calculated value.

  • Coordination Withstand Voltage (Ucw): The baseline transient stress at the equipment is established using the arrester’s Lightning Impulse Protective Level (Upl). This calculation must account for the added voltage drop of the connecting lead wires and separation distance effects.
  • Required Withstand Voltage (Urw): The Ucw is then multiplied upward by a mandatory Safety Factor (Ks) to account for insulation aging and manufacturing dispersion. IEC 60071-2 recommends Ks = 1.15 for internal insulation and KS = 1.05 for external insulation. For high-altitude installations, an Atmospheric Correction Factor (Ka) is also multiplied into this value.

Urw = Ucw x Ks x Ka

  • Verification: To achieve a compliant design, the equipment’s standard Lightning Impulse Withstand Voltage (LIWV or BIL) must be strictly greater than or equal to the calculated Required Withstand Voltage (Urw).

Engineering Tip: The VarStec Capacitor Bank Altitude Derating & Insulation Coordination Tool automatically processes this IEC “Surge-Up” method simultaneously with the IEEE margin method, ensuring that Urw remains below the equipment’s standard Nameplate BIL for global compliance.

When a transient duty evaluation fails at high altitude, specific standard-backed mitigation strategies can be evaluated to achieve compliance before artificially increasing the equipment’s base insulation level.

  • Reduce Arrester Lead Length: Minimize the separation distance and connecting wires to reduce the inductive voltage drop, which adds directly to the arrester’s clamping voltage.
  • Improve Arrester Characteristics: Select an arrester with lower Front-of-Wave (FOW) and Lightning Protective Level (LPL) clamping voltages.
  • Partially Increase BIL: If the above steps fail to achieve the required margins, step up the equipment BIL to the next minimum standard rating.

Engineering Tip: Adjusting the arrester lead length in the VarStec Capacitor Bank Altitude Derating Tool is often the most cost-effective way to transition a “FAIL” to a “PASS”.

No, only the external insulation (air clearances and exposed insulator surfaces) requires altitude derating. Internal insulation is unaffected by changes in external air density.

  • Internal Insulation: Dielectric characteristics remain identical at any altitude; no altitude correction factor is applied.
  • External Insulation: Air density decreases at high altitudes, reducing dielectric strength and requiring an Altitude Correction Factor.
  • Coordination: Surge arresters are coordinated strictly against the derated external insulation strength. Engineering Tip: The VarStec Altitude Derating Tool automatically isolates the atmospheric correction calculations to external insulation parameters.

Metal-Enclosed Equipment, Construction, and Safety Standards

The complete metal-enclosed assembly is governed by a different set of requirements than the individual capacitors, reactors, switches, or protective devices installed within it. VarStec applies the appropriate IEEE equipment standards together with UL, ANSI, and NEC requirements where they are applicable to the specific equipment and project.

  • IEEE C37.20.3-2023 — IEEE Standard for Metal-Enclosed Interrupter Switchgear Rated above 1 kV AC up to and Including 48.3 kV AC. This is the principal IEEE metal-enclosed equipment standard applicable to assemblies containing switches, fuses, circuit breakers, controls, instrumentation, and protective equipment.
  • IEEE C37.20.7-2024 — IEEE Recommended Practice for Testing Switchgear Rated Up to 52 kV for Internal Arcing Faults.
  • IEEE C37.20.4-2013 — IEEE Standard for Indoor AC Switches (1 kV to 38 kV) for Use in Metal-Enclosed Switchgear. Applies to indoor medium-voltage switches used within metal-enclosed equipment. IEEE presently lists the 2013 edition as inactive-reserved and has an active revision project underway, so I would describe it on the website as the latest published edition rather than as a current active standard.
  • IEEE C37.20.2-2025 — IEEE Standard for Metal-Clad Switchgear. Applies where the equipment incorporates metal-clad switchgear with drawout electrically operated circuit breakers. It should not be described as a general “enclosure category” standard.
  • UL 508A-Standard for Industrial Control Panels. Applies to the control and protection panel of the capacitor bank and harmonic filter bank; it does not govern the complete medium voltage portions of the capacitor banks and harmonic filter banks.
  • UL 50 / UL 50E — Enclosures for Electrical Equipment, Non-Environmental and Environmental Considerations. Provide enclosure construction and environmental-performance requirements, including considerations such as corrosion, UV exposure, and maintaining environmental seals.
  • ANSI Z535.4-2023 — American National Standard for Product Safety Signs and Labels. Provides the standardized system for safety signs and labels applied to equipment and is applicable to VarStec equipment hazard labeling.
  • NFPA 70-2026 — National Electrical Code. Articles 460 and 490 provide requirements applicable to capacitors and equipment over 1000 V, respectively. The actual project must follow the edition adopted by the governing jurisdiction or specified by the contract.

Related Technical Topics

VarStec’s Practical Experience and Standards-Based Design

VarStec brings more than 30 years of practical, hands-on experience in the design, application, and manufacturing of medium-voltage capacitor banks and harmonic filter banks. We use the applicable IEEE, IEC, ANSI, and related industry standards as core engineering references throughout the design and application process, including component ratings, insulation coordination, switching duty, protection, environmental considerations, and equipment construction. The result is a standards-based, experience-driven design approach focused on equipment that is properly applied, properly manufactured, and built for reliable long-term operation.