Skip to main content

Under what circumstance is D.C. Resistance Measurement needed?

Current from a D.C. resistance bridge is applied to the transformers windings to determine the D.C. resistance voltage of the coils. This test is important for the calculation of transformer winding heat losses used for winding temperature testing, and as base data for future assessment in the field.

  • Under what circumstance is D.C. Resistance Measurement needed?

      Current from a D.C. resistance bridge is applied to the transformers windings to determine the D.C. resistance voltage of the coils. This test is important for the calculation of transformer winding heat losses used for winding temperature testing, and as base data for future assessment in the field.

  • What Seismic ratings Does HPS Use?

      HPS units meet Risk Category IV (Ip=1.5) for Sds=2.0 per ASCE 7-16 (and equivalent factors in NBCC 2015) for ground-level installations only (z/h=0) for all locations in North America.

      HPS units can be designed to meet California OSHPD (Office of Statewide health Planning and Development) requirements. Please see the California OSHPD site for a listing of HPS transformers which have been tested and certified.

  • What is an Applied Voltage test?
  • What is ANSI C57.12.91?
  • What is ANSI C57.12.51?

      IEEE Standard for Ventilated Dry- Type Power Transformers, 501 kVA and Larger, Three-Phase, with High- Voltage 34.5 kV to 601 V and Low- Voltage 208Y/120 V to 4160 V covering General Requirements. The current standard was updated in 2008.

      This standard is intended to set forth characteristics relating to performance, limited electrical and mechanical interchangeability, and safety of the equipment described, and to assist in the proper selection of such equipment. Specific rating combinations are described in the range from 750/1000 to 7500/10 000 kVA inclusive, with high-voltage 601 to 34 500 volts inclusive and low-voltage 208Y/120 to 4160 volts inclusive. Part I of this standard describes certain electrical and mechanical requirements and takes into consideration certain safety features of 60-Hz, two-winding, three-phase, ventilated dry-type transformers with self-cooled ratings 501 kVA and larger, generally used for step-down purposes. Part Il describes other requirements or alternatives which may be specified for some applications and lists forced-air-cooled ratings for certain sizes.

  • What is an Induced Voltage test?

      The induced voltage test is applied for 7200 cycles or 60 seconds whichever is shorter. The voltage applied is twice the operating voltage, and confines the integrity of the insulation

  • What are Impedance Voltage and Load Loss tests?

      The voltage required to circulate the rated current under short-circuit conditions when connected on the rated voltage tap, is the impedance voltage. Rated current is circulated through the windings with the secondary short-circuited. The impedance voltage and load loss is measured. They are corrected to rise +20°C reference temperature.

      Note: This is a standard test only on units over 500kVA. It will only be carried out on lower kVA units when specifically requested.

  • What is a Polarity and Phase-Relation test for?

      Polarity and phase-relation tests are made to determine angular displacement and relative phase sequence to facilitate connections in a transformer. Determining polarity is also essential when paralleling or banking two or more transformers.

  • What is NEMA TP2?

      NEMA TP2 defines how energy efficiency is measured. Typically, it uses a sinusoidal wave with no harmonics at unity (1.0) power factor at 35% load for 600 volt class units and 50% load for medium voltage units.

      This regulation has been replaced by similar test standards described in DOE 2016 and NRCan 2019 regulations.

  • What is NEMA TP3?
  • What is NEMA ST 20?
  • What are Dielectric tests?

      The purpose of dielectric tests is to demonstrate that the transformer has been designed and constructed to withstand the voltages associated with specified insulation levels.

  • What is U.L. 1562?

      U.L. 1562 covers medium voltage dry-type transformers:

      1.1 These requirements cover single-phase or three-phase, dry-type, distribution transformers, including solid cast and resin encapsulated transformers. The transformers are provided with either ventilated or non-ventilated enclosures and are rated for a primary or secondary voltage from 601 to 35000 V.

      1.2 These transformers are intended for installation in accordance with the National Electrical Code, ANSI/NFPA 70.

      1.3 These requirements do not cover the following transformers:

      1. Instrument transformers
      2. Step-voltage and induction voltage regulators
      3. Current regulators
      4. Arc furnace transformers
      5. Rectifier transformers
      6. Specialty transformers (such as rectifier, ignition, gas tube sign transformers, and the like)
      7. Mining transformers
      8. Motor-starting reactors and transformers

      1.4 These requirements do not cover transformers under the exclusive control of electrical utilities utilized for communication, metering, generation, control, transformation, transmission, and distribution of electric energy regardless of whether such transformers are located indoors, in buildings and rooms used exclusively by utilities for such purposes; or outdoors on property owned, leased, established rights on private property or on public rights of way (highways, streets, roads, and the like).

  • What is U.L. 1561?

      UL1561 covers 600 Volt Class Transformers:

      1.1 These requirements cover:

      1. General purpose and power transformers of the air-cooled, dry, ventilated, and non-ventilated types to be used in accordance with the National Electrical Code, ANSI/NFPA 70. Construction types include step up, step down, insulating, and autotransformer type transformers as well as air-cooled and dry-type reactors


      1. General purpose and power transformers of the exposed core, air-cooled, dry, and compound-filled types rated more than 10 kVA to be used in accordance with the National Electrical Code, ANSI/NFPA 70. Constructions include step up, step down, insulating, and autotransformer type transformers as well as air-cooled, dry, and compound-filled type reactors.

      1.2 These requirements do not cover ballasts for high intensity discharge (HID) lamps (metal halide, mercury vapor, and sodium types) or fluorescent lamps, exposed core transformers, compound-filled transformers, liquid-filled transformers, voltage regulators, general use or special types of transformers covered in requirements for other electrical equipment, autotransformers forming part of industrial control equipment, motor-starting autotransformers, variable voltage autotransformers, transformers having a nominal primary or secondary rating of more than 600 volts, or overvoltage taps rated greater than 660 volts.

      1.3 These requirements do not cover transformers provided with waveshaping or rectifying circuitry. Waveshaping or rectifying circuits may include components such as diodes and transistors. Components such as capacitors, transient voltage surge suppressors, and surge arresters are not considered to be waveshaping or rectifying devices.

  • What is ANSI C57.12.01?
  • What is Zone Classification?

      Obsolete versions of seismic standards used to classify seismic areas ranging from zone 0 to zone 4, where zone 0 indicates the weakest earthquake ground motion and zone 4 indicates the strongest. The zone classification is no longer used. The current standards specify the Sds design earthquake spectral response acceleration parameter as described above.

      Please refer link to determine the Sds criteria for a specific location.

  • Who Needs Seismic?

      Healthcare facilities and emergency response locations, including police stations and other vital government facilities, will often include a Seismic Certification requirement. Power generation stations may also have this requirement as well as facilities handling hazardous, toxic or explosive materials.

      To determine the Sds criteria for a specific location, the U.S. Geological Survey provides a utility on their website, which can be viewed at

  • What are some of the Tests performed on transformers?

      Normal, routine production tests include:

      1. core loss;
      2. load loss – winding or copper loss;
      3. Impedance;
      4. hi-pot – high voltage between windings and ground;
      5. induced – double induced two times voltage.

      Optional special tests include:

      1. heat run – temperature testing;
      2. Noise tests – sound level measurement;
      3. impulse tests – BIL tests:
      4. partial discharge

  • How are Seismic Units Rated?

      Three criteria are typically defined for seismic units:  Sds, Ip, z/h.

      Sds = Design earthquake spectral response acceleration parameter at short periods (ASCE 7-16 Section 11.4.4 Design Spectral Acceleration Parameters).  The required motion coefficient is dependent on the facility’s location and soil type. Most of the United States requires Sds = 0.05 to 1.5g. Specific regions require an Sds = 2.0g such as along the Missouri state line south of Illinois and parts of California.

      Ip = Component Importance Factor (ASCE 7-16 Section 13.1.3 Component Importance Factor).  Ip is dependent on the function of the building in which the transformer is installed. Typically, an Ip is assumed to equal 1.5 for transformers expected to function continuously through and after an earthquake.

      z/h = A ratio of the height in the structure that the component has been anchored, to the overall height of the structure.  A value of z/h of 1.0 states that the component is capable of being installed anywhere within the structure (ASCE 7-16 Section 13.3.1 Seismic Design Force).

      z/h - a ratio of the height of the structure

  • What is Seismic Certified?

      A fair amount of construction projects require components to be “Seismic Certified.” A Seismic Certification ensures the component will withstand and operate after an event such as an earthquake. In addition to requiring structural components to meet specific seismic regulations, most jurisdictions also require non-structural components – including electrical systems – to be “Seismic Certified.”

      Seismic requirements are defined by the International Building Code 2018 and the California Building Code (2019). ASCE 7-16 is the base standard for many building codes, and is referenced by both IBC and the CBC.

      OSHPD, the Office of State-wide Health Planning and Development, requires actual “shake-testing” of products prior to allowing products to be specified for construction or retrofit projects anywhere in the state of California. This testing must be reviewed by a California state certified structural engineer. Without a widespread nationwide approval process, many other jurisdictions require the OSHPD Special Seismic Certification Preapproval (OSP) for projects.