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Electromagnetic Measurements

Precision Ratio Measurements

Inductive Dividers

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Technical Contact:
Scott Shields
Tel: 301-975-4232
E-mail: scott.shields@nist.gov

Denise D. Prather
Administration and Logistics
Tel: 301-975-4221
E-mail: denise.prather@nist.gov

Please contact the administration and logistics staff before shipping instruments or standards to the address listed below.

Mailing Address:
National Institute of Standards and Technology
100 Bureau Drive, Stop 8170
Gaithersburg, MD 20899-8170

Service ID
Number
Description of Services Fee ($)
54110S Special Ratio Measurements and Tests of Inductive Voltage Dividers, by Prearrangement At Cost
54120C Inductive Voltage Dividers - (Single Frequency, Voltage to be Specified, Each Setting of 3 Most Significant Dials) 4939
54121C Additional Frequency Points 4939
54130C Inductive Voltage Dividers - (Single Frequency, Voltage to be Specified, Each Setting of Most Significant Dial Only) 3097
54131C Additional Frequency Points 3097
Fees are subject to change without notice.

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Special Ratio Measurements and Tests of Inductive Voltage Dividers, by Prearrangement (54110S)

This service category provides for the measurement and/or evaluation of prototype ratio devices and inductive voltage dividers based on new principles, and for unique ratio measurements at the highest accuracy levels, such as the determination of the ratios of Hamon resistance transfer devices or Silsbee-type voltage ratio standards. Such measurements are undertaken at the discretion of NIST technical staff and only when the need for them can be clearly demonstrated.

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Inductive Voltage Dividers (54120C-54131C)

Inductive voltage dividers (decade transformer dividers) are accepted for calibration only at (50, 60, 100, 120, 400, 1000, 5000, and 10 000) Hz. The most significant dial only can be calibrated at 15 kHz and 20 kHz.

Calibration voltages may be specified up to 100 V or the manufacturer's specified limit, whichever is lower. The largest contribution to instability in undamaged inductive voltage dividers is wear or dirt in the decade switches. Variable contact resistance in these switches sometimes affects the stability of voltage-ratio measurements to a significant extent but is most evident by its effect on the phase angle. When a decade inductive voltage divider exhibits large changes in phase angle for repeated measurements after the switches have been disturbed, the divider should no longer be considered satisfactory for use as a voltage-ratio reference standard. Inductive voltage dividers that use pushbutton switching or incorporate a resistive divider as a fine adjustment usually are not accepted for calibration.

Corrections to the separate decades of an inductive divider, in general, cannot be simply combined. However, the correction to a step setting of one of the higher decades usually is independent of the setting of the lower decades. The effects of stray impedances must be corrected by connecting the case to the divider at one point, and unless otherwise specified, the case will be connected to one of the "common" terminals, typically marked "GRD," "Case GND," or "Case GRD." Decade inductive voltage dividers are calibrated at NIST at room temperature (22 °C to 24 °C) by comparison with a two-stage, three-decade transformer of known ratios.

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References-Inductive Dividers

American National Standard for Decade Transformer Dividers (Voltage Type), ANSI C100, 1-1972 Amer. Natl. Stand. Inst., New York, NY (Jan. 1972).

Instructions for the Use of the NBS Reference Inductive Divider, Wilbur C. Sze, Natl. Bur. Stand. (U.S.), NBSIR, unpublished (1970). (Available from NIST.)

Two-Stage, Guarded Inductive Voltage Divider for Use at 100 kHz, D. H. Hamon and T. L. Zapf, ISA Transactions, 9, 3, Instrum. Soc. of Amer. Res. Triangle Park, NC (1970).

Comparator for Calibration of Inductive Voltage Dividers from 1 to 10 kHz, W. C. Sze, ISA Transactions 6,4, Instrum. Soc. of America, Res. Triangle Park, NC (1967).

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Resistive Dividers

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Technical Contact:
Marlin Kraft
Tel: 301-975-4239
email: marlin.kraft@nist.gov

Gerald FitzPatrick
Tel: 301-975-8922
email: gfitzpatrick@nist.gov

Denise D. Prather
Administration and Logistics
Tel: 301-975-4221
email: denise.prather@nist.gov

Please contact the administration and logistics staff before shipping instruments or standards to the address listed below.

Mailing Address:
National Institute of Standards and Technology
100 Bureau Drive, Stop 8170
Gaithersburg, MD 20899-8170

Service ID
Number
Description of Services Fee ($)
54210C Resistor and Resistive Dividers, Total Resistance or Voltage Ratio, Two Direct Voltage Levels Between 10 kV and 150 kV 3565
54211S Special Tests of Resistor and Resistive Dividers at Direct Voltage Levels, by Prearrangement At Cost
Fees are subject to change without notice.

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Resistor and Resistive Dividers, DC Measurements (54210C-54211S)

A calibration service is maintained at NIST to determine the dc voltage ratio of resistive dividers. The routine calibration service is available for applied voltages from 10 kV; to 150 kV. The calibrations are performed with a measurement system which has a relative expanded uncertainty of 0.006% of the voltage ratio. To assure adequate sensitivity at the lowest applied voltage levels, calibrations are performed routinely only on dividers with ratios of 105:1 or smaller. The routine calibration service is also restricted to dividers with nominal ratios of 105:1, 104:1, or 103:1.

Resistive dividers are accepted for calibration only if they are nearly corona free at the rated operating voltage and are designed to have small temperature and voltage coefficients. Specifically, a device is not generally suitable for calibration by NIST if these coefficients produce a change in the ratio of 0.1% over the normal range of operating voltages. At a given voltage, dividers should not exhibit instabilities in their ratio value in excess of 0.005%. NIST staff can provide some assistance in the identification of other calibration laboratories capable of certifying the response of less accurate dividers.

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References-Resistive Dividers

High-Voltage Divider and Resistor Calibrations, M. Misakian, Natl. Bur. Stand. (U.S.), Tech. Note 1215 (July 1985).

Special Shielded Resistor for High-Voltage Measurements, J. H. Park, J. Res. Natl. Bur. Stand. (U.S.), 66C (1), 19 (Jan.-Mar. 1962).

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Current Transformers

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Technical Contacts:
Thomas L. Nelson
Tel: 301-975-2986
E-mail: thomas.nelson@nist.gov

Gerald FitzPatrick
Tel: 301-975-8922
E-mail: gfitzpatrick@nist.gov

Denise D. Prather
Administration and Logistics
Tel: 301/975-4221
E-mail: denise.prather@nist.gov

Please contact the administration and logistics staff before shipping instruments or standards to the address listed below.

Mailing Address:
National Institute of Standards and Technology
100 Bureau Drive, Stop 8170
Gaithersburg, MD 20899-8170

Service ID
Number
Description of Services Fee ($)
54520C Current Transformer, Ratio & Phase Angle, 1 Range at 1 Frequency, 1 Burden, Secondary Currents (0.5, 1, 2, 3, 4, 5) A, Primary Current Not Over 12 000 A 4576
54521C Current Transformer, Ratio & Phase Angle, 1 Secondary Current, Additional Combination of Range, Frequency, and Burden, Primary Current Not Over 12 000 A 431
54522C Current Transformer, Ratio & Phase at Each additional Secondary Current, Same Combination of Range, Frequency, and Burden as 54520C or 54521C 350
Fees are subject to change without notice.

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Current Transformers (54520C-54522C)

Normally NIST calibrates only current transformers of high quality for use as reference standards. The NIST equipment is designed to test current transformers with a rated secondary current of 5 A, with test points chosen to be one or more of the following values: (0.5, 1, 2, 3, 4, 5) A.

Routine tests are carried out at 50 Hz, 60 Hz, and 400 Hz. For measurements at 50 Hz or 60 Hz, the results are generally reported with expanded uncertainties of 0.01% in ratio and 0.1 mrad in phase angle. For measurements at 400 Hz, the reported expanded uncertainty is 0.03% in ratio and 0.3 mrad in phase angle.

The customer must specify the test frequency, the secondary currents, and the secondary burdens for each transformer or for each range of a multirange transformer. Current transformers should be tested with burdens equivalent to those which are imposed when the device is used as a transfer standard. Routine calibration using the burdens specified in the American National Standards Institute (ANSI) Standard C-57.13 is not recommended unless these burdens are subsequently used in the customer's factory or laboratory. Large errors can result if the values of ratio and phase angle obtained with an ANSI recommended burden are used for the transformer when it is connected to a different burden.

The burden is preferably specified in terms of the measured resistance and inductance. These values should include the effects of the leads used to make a connection to the transformer secondary. An alternative, which is sometimes feasible, is to submit the transformer together with its normal leads and connected burden for calibration as a unit. If neither of the above are possible, the burden may be stated in terms of the voltampere product and the power factor of the secondary circuit at the test frequency. For reference, it should be noted that the test equipment regularly used at NIST represents a minimum test burden of about 0.03 Ω with a inductance of about 10 µH.

Because of contact resistance and current rectification, loose or dirty primary and secondary terminations may affect the measurement results. These surfaces should be tight and clean when the transformer is shipped to NIST to minimize this source of error.

Unless otherwise specified, current transformers are demagnetized prior to calibration. If it is desired to have a transformer tested as submitted (without de-magnetization), this requirement should be stated on the purchase order and NIST staff should be informed by telephone before the transformer is shipped.

Many current transformers are not designed to be used as transfer standards, and most of these do not require calibration at NIST. NIST staff can provide some assistance in the assessment of the appropriateness of the device for NIST calibration and in the identification of alternative calibration sources. If NIST is required to perform laboratory measurements to determine whether or not a particular device can be calibrated, a charge for the cost of these measurements will be made.

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References-Current Transformers

A Calibration Service for Current Transformers, J. D. Ramboz and O. Petersons, NIST Spec. Publ. 250-36 (June 1991).

An Electronic Ratio Error Set for Current Transformer Calibrations, R. L. Kahler, IEEE Trans. Instrum. Meas. IM-28 (2), 162 (June 1979).

Wide-Band Two-Stage Current Transformers of High Accuracy, T. M. Souders, IEEE Trans. Instrum. Meas. IM-21 (4), 340 (Nov. 1972).

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Program questions: Calibrations
Phone: 301-975-2200, Fax: 301-975-2950
NIST, 100 Bureau Drive, Stop 8363, Gaithersburg, MD 20899-8363