NBS

349

The Design and Operation of A

High Voltage Calibration Facility

WINSTON W. SCOTT, JR.

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U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards
THE NATIONAL BUREAU OF STANDARDS

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UNITED STATES DEPARTMENT OF COMMERCE • John T. Connor, Secretary

NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

NBS TECHNICAL NOTE 349

ISSUED NOVEMBER 10, 1966

THE DESIGN AND OPERATION OF A

HIGH VOLTAGE CALIBRATION FACILITY

WINSTON W. SCOTT, JR.

Radio Standards Laboratory

National Bureau of Standards

Boulder, Colorado

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means tor making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C., 20402

Price: 25 cents.

Contents Page

Abstract 1

1. Introduction 1

2. The Design 2

2. 1 Safety Enclosure 2

2. 2 Remote Reading System 5

3. Calibration Techniques 6

3. 1 A-V Measurements 6

3. 2 D-V Measurements 7

4. Discussion 11

5. Conclusion 14

6. Acknowledgements 14

7. References 14

8. Appendix. Derivation of Equation used in Calibration of

High-Voltage Resistive Dividers ..." 15

List of Figures

Figure 1. Control Selector Unit and Interlock Circuits .... 18

Figure 2. A-V Control Box 19

Figure 3. A-V Calibration of Electrostatic Voltmeters .... 20

Figure 4. Calibration of Potential Transformers 21

Figure 5. D-V Calibration of Electrostatic Voltmeters .... 22

Figure 6. D-V Calibration of HV Resistive Dividers 23

Figure 7. Electrostatic Voltmeter Drift 24

Figure 8. High-Voltage Calibration Facility 25

Figure 9. Some Sources of Error in Calibration of HV

Resistive Dividers 26

Figure 10. Potential Difference and Current Loop Diagram ... 26

The Design and Operation of a High-Voltage Calibration FacilityWinston W. Scott, Jr.

The high-voltage calibration facility, in operation for

seven years at the National Bureau of Standards Radio

Standards Laboratory (RSL) but now at NBS Washington,

is described with emphasis on several novel construction

features and calibration techniques. The more usual

calibration techniques are also outlined for the assistance

of those personnel in standards laboratories assigned the

task of calibrating with high accuracy, electrostatic voltmeters, resistive dividers, and potential transformers

at high voltage. Precautions, based on experience in the

calibration and use of high voltage standards, are given

so that certain errors may be avoided by the users.

Key Words: alternating -current, calibration, direct-current,

high-voltage, ratio, safety, standards enclosure,

transformer, voltage -divider , voltmeter.

1. Introduction

Visitors have asked many technical questions of the Radio Standards

Laboratory which, over a period of time, have indicated a need for a published record of the design, construction, and operation of a highvoltage calibration facility. On the conclusion of the transfer of most of

the equipment to the Washington, D. C. high-voltage laboratory of the

National Bureau of Standards, it seems especially appropriate to describe

the calibration equipment and methods for the benefit of others who have

an interest in high -voltage calibration.

The facility was used for the calibration of potential transformers,

electrostatic voltmeters, and high-voltage resistive dividers. Potential

transformers were calibrated with uncertainties within ±0. 05 percent in

ratio and ±2 minutes in phase-angle at 60 Hz. Electrostatic voltmeters
were calibrated within ±0. 5 percent of full scale reading at d-c or 60 Hz.

Most high-voltage resistive dividers could be calibrated within ±0. 03

percent using direct current.

2. Design

The facility has a safety enclosure with control circuitry, a telescopic reading -aid for electrostatic voltmeter calibration, and some standards including a high-voltage resistive divider, a potential transformer,

and a potential-transformer test set.

2. 1 Safety Enclosure

The safety enclosure was designed for the protection of both the

operator and the standards. On several occasions previous to the construction, there was exposure (although fortunately not contact) to high

voltage when observers were not aware of its presence. Furthermore,

because of the need to wire control circuits prior to each calibration,

there was danger of miswiring and applying sudden substantial voltage to

a supply transformer. Undesired transient effects could occur which

could harm both the supply and potential transformers. Therefore, two

design objectives for construction of a safety enclosure were formulated:

1. Equipment which contains voltages above 120 volts would be

placed within a grounded metal screen.

2. Control circuitry would have features preventing unexpected

occurrences of high voltage.

The first objective was accomplished by designing the a-c and d-c

power supplies so that only the low-voltage circuits are outside the

enclosure. All high-voltage components, including transformers, rectifiers, filters, and standards (including those under calibration) are

placed behind a grounded screen for protection of the operator and others

in the vicinity. A screen built from 3/4-inch No. 13 expanded steel (on
a 1-1/4-inch steel angle frame) proved satisfactory and is recommended

for future constructions. Entry into the enclosure is controlled by means

of a sliding door which has appropriate interlocks to help prevent harmful situations. Figure 1 shows the interlock circuits and the control

selector unit, which provides power to either the alternating -voltage (a-v) or the direct -voltage (d-v) control box. (Alternating -voltage and directvoltage are designated a-v and d-v in this paper. ) In order to accomplish the second objective, four possibly harmful

situations were recognized and steps were taken for their prevention.

The four situations are:

Situation 1. Supply -main voltage is applied to either the a-v or d-v highvoltage supply when the sliding door is open. This is especially dangerous when personnel are working on high-voltage

equipment within the enclosure.

Situation 2. Supply-main voltage is suddenly applied to either highvoltage supply when the control settings are off zero.

Situation 3. Voltage is suddenly removed from a high-voltage supply

when the control setting is off zero.

Situation 4. Supply -main voltage is suddenly removed from one highvoltage supply and applied to the other supply with controls

of either or both supplies set off zero. Each high-voltage supply uses a control box to control the highvoltage supply components within the enclosure. Figure 2 is a schematic

of the a-v control box, a simple item which ordinarily is not commercially available. The isolation transformer eliminates the possibility of a ground duplication or cross ground through the supply main. The

coarse and fine controls are variable autotransformer s. The fine control is used with a step-down transformer to add an adjustable small

voltage to the voltage provided by the coarse control. The microswitch

is closed when the a-v coarse control is set at zero. The d-v control
box is commercially available as part of a d-v high-voltage supply and

therefore is not shown. However, a microswitch was added to the d-v

coarse control, as in the a-v control box.

It was considered very important to avoid situation 1, and therefore,

as shown in figure 1, an interlock was provided which inserts two contactor bars between the supply main and the control boxes. The contactor bars, one for each side of the supply line, are mounted on the sliding

door. The electrical contacts, which the bars short when the door is

closed, are mounted on a wall of the enclosure. In addition, a patch

cable is fastened to the door so that when the door is open, the supply

line must be electrically open. The interlock is primarily for the protection of high-voltage equipment, while the patch cable provides positive

protection to personnel.

Situations 2, 3, and 4 endanger the reliability of the transformers

and other standards. For example, a standard might not be dependable

if excessive voltage were applied. In figure 1, a solenoid-operated

latch prevents the sliding door from opening or closing the interlock

until the solenoid is energized. Proper control of the solenoid helps

prevent situations 2 and 3. Each coarse control has a rotatable voltagepickup arm which can be made to operate microswitches. The microswitches, as previously described, close whenever the coarse controls

are at zero setting. This arrangement insures that the solenoid-operated

latch does not allow the door to be opened or closed unless high voltage

within the enclosure is absent. The patch cable, which can be disconnected only after the interlocks are opened, serves to provide the same

certain protection as pulling an electrical outlet plug.

Situation 4, which occurs when switching high-voltage supplies with

high voltage present, is prevented with the help of the control selector

unit shown schematically in figure 1. The control-selector unit contains

an av-dv transfer relay, a control selector switch, a microswitch, and
lamps that indicate which control box is energized. The a-v control box

has power when the av-dv transfer relay is energized. But the relay

will not energize unless the control selector switch is closed and the a-v

coarse control indicates zero. The microswitch is mechanically arranged so that it is in the #1 position when the a-v coarse control indicates zero, and is in the #2 position when the control indicates other

than zero. Ordinarily, once the relay is energized, a holding circuit

keeps it energized for all non-zero settings. However, should a power

failure occur on the supply main, and the potential transformer is energized, the relay will de-energize. When the failure is corrected, as most often happens within a few seconds, the relay will not energize

until the a-v coarse control is turned to zero, thereby preventing possible further damage to the transformers. Unfortunately, no way has been

found to prevent the initial sudden voltage change. This points out the

advisability of keeping a reference standard available so that if a working

standard is suspected of change, an inter comparison can be made.

2. 2 Remote Reading System

In order to enclose fully all high-voltage equipment, including

electrostatic voltmeters, (objective 1), a system had to be devised to

read the electrostatic voltmeters from outside the enclosure. The

electrostatic voltmeters referred to are portable high-voltage standards

in the 1 percent and 1/2 percent accuracy classes and have scales

marked on a cylindrical surface which covers an arc of about 80 degrees.

The major problem in accurately reading this type of scale is to reduce

satisfactorily the effect of parallax error. The following system was

devised for remote reading. The electrostatic voltmeter is placed on a turntable which can be rotated at about one -half rpm in either a clockwise or counterclockwise sense. Two tapping keys control the sense and

amount of rotation. A telescope with a magnification of about 20 is used
to read the scale from outside the screen. If the screen is within about

one -half meter of the telescope objective, the view of the scale is not

blocked, although the scale must be artificially illuminated for good

visibility. Most electrostatic voltmeters built today have mirrored

scales for the reduction of parallax error. To obtain an accurate reading, it is necessary to focus the telescopic image of the scale and to

rotate the turntable so as to place the pointer over the pointer reflection.

The system is most convenient to use when the telescope is aimed at the

axis of rotation of the turntable and the moving -system axis of the

electrostatic voltmeter is approximately both collinear with the turntable

axis and vertical.

3. Calibration Techniques

3. 1 A-V Measurements

The high -voltage calibration facility permits two types of a-v measurements: alternating -voltage and alternating -voltage ratio. Figure 3

is the schematic used for a-v calibration of an electrostatic voltmeter.

The standard transformer is a supply transformer and an instrument

potential transformer in one container and therefore has two sets of

windings. One set of windings transforms low voltage (obtained through

the a-v control box from the power main) to high voltage for supply purposes while the other set accurately transforms the high voltage to a low

level suitable for reading with a low-range standard voltmeter. The high

voltage is accurately determined from the exact potential -transformer

winding ratio (corresponding to the burden imposed by the standard voltmeter) and the standard voltmeter correction. This voltage is then

compared with the indication of the electrostatic voltmeter to obtain the

correction. Correction (kv) = Actual Voltage - Voltmeter Indication.

When the electrostatic voltmeter is used for measurement, the actual

voltage is the sum of the voltmeter indication and the correction.
Alternating-voltage ratio measurements are used in the calibration

of instrument potential transformers. Figure 4 shows the interconnection between a potential transformer under calibration and the standard

transformer, burden box, and a potential transformer test set. The potential

transformer test set [Harris, 1952] is used to determine differences in

ratio and phase angle (at a given frequency and burden) between the transformer under calibration and the standard transformer. The burden box

may be adjusted for resistive (or resistive and inductive burdens) up to

the volt-ampere rating of the transformer under calibration. In using

the two transformers, one side of both high -voltage and low-voltage

windings is connected to ground potential so that, if insulation breakdown

between the high voltage and low voltage circuits should occur, the observer is protected. Furthermore, certain undesirable electrostatic

effects are avoided. The ratios and phase angles of potential transformers are generally very stable with time and if the standard is treated

with normal care, a typical recalibration interval might be about five

years. However, as suggested earlier, it is always wise for the user

to intercompare standards in his own laboratory more often, to check

his measurement technique occasionally and to discover whether his

standards are changing with time.

3. 2 D-V Measurements

Both direct-voltage measurements and direct-voltage ratio measurements are of interest in high-voltage calibration laboratories. Figure 5

is the schematic of the circuit used in the direct -voltage calibration of

electrostatic voltmeters. The high-voltage resistive divider (or Park

divider) is of special construction [Park, 1962] to control leakage and

temperature errors to within about ±50 ppm under laboratory conditions.

The design incorporates 100 shielded one-megohm wire-wound resistors

connected in series and arranged in the form of a vertical helix between
a ground plane and a toroidally shaped corona cap. Connections are

provided to allow application of convenient voltages up to 100 kv, with

rated or near rated current in selected resistor -pair s on the helix.

Each resistor -pair effectively has zero temperature -coefficient because

of matching one resistor of positive coefficient with another of negative

coefficient [Park, 1962]. The four -terminal resistor is of sufficient

value (often 100 ohms) to develop convenient voltage (100 millivolts) for

the d-c potentiometer.

Two observers are used to calibrate an electrostatic voltmeter.

One observer adjusts the d-v control box until the pointer is deflected to

the desired scale division at which time the second observer adjusts the

d-c potentiometer and obtains the voltage reading. The current through

the four -terminal resistor is the voltage drop, accurately determined by

the d-c potentiometer, divided by the actual four -terminal resistance.

If, as determined by current "in"-"out" tests [Park, 1962], the current

is sufficiently uniform throughout the H-V resistor, then the product of

the current and the resistance gives the voltage. This is the voltage

applied to the electrostatic voltmeter.

Direct-voltage -ratio measurements are used in calibrating highvoltage resistive dividers. The divider under calibration is compared

with another divider which serves as a standard. The standard divider

consists of the H-V resistor, described earlier, and a conveniently

selected four -terminal resistance standard connected in series. The

NBS standard divider has been carefully studied for sources of systematic

and random errors [Park, 1962]. For example, a Direct Reading Ratio

Set [Brooks, 1962] was used in conjunction with two one-megohm resistance standards to help determine the value for each resistor -pair of the

H-V resistor. These measurements were periodically performed using

low voltage (approximately 90 volts). The four -terminal resistance

standard was calibrated at the same time. A check for current uniformity
and related current leakage was made. Finally, temperature coefficients

were investigated for the H-V resistor. The study showed that the standard divider, after low-voltage resistance corrections had been applied,

was suitable for measurements within ±0. 01 percent uncertainty without

applying further corrections. If the divider under calibration is carefully constructed, then the low-voltage resistance determinations will

remain valid for high voltage use. However, some dividers are constructed with large-valued resistors which have large temperature coefficients. In those cases, because of significant resistance changes

resulting from joule heating, several measurements of the divider are

necessary to determine the ratio as a function of time after "turn-on".

To compare high-voltage resistive dividers at rated or near rated

voltage, one of two simple methods was used. For both methods, the

ratio of the NBS standard was conveniently adjusted to approximate the

ratio of the divider under calibration by selecting the proper number of

resistor -pair s and choosing a convenient nominal value for the four

terminal resistor. The first method (later abandoned) consisted of connecting each divider input across the high-voltage supply and determining the divider output voltages separately with a d-c precision potentiometer. Disadvantages were that a highly precise potentiometer was required to obtain enough significant places, and a well-regulated highvoltage source was required to obtain a stable galvanometer null.

The second method makes use of a very simple and inexpensive

Lindeck potentiometer [Oliver, 1955] and does not require a highly

regulated high-voltage source for precise measurements. As shown in

figure 6, a Lindeck potentiometer generates a voltage which, at galvanometer balance, equals the voltage at the terminals of the selector

switch. For two dividers which have the same nominal ratio, the voltage

will be small (usually millivolts). The voltage polarity necessary for

galvanometer balance is controlled by the reversing switch. The Lindeck
balance voltage°

is the product of the four -terminal resistance R and

p

the milliammeter indication I. For the two positions of the selector

switch, the corresponding Lindeck voltages are designated e and e .

If the voltage drops across the leads from resistors D and B to ground

are equal then e = 0. The voltages e and e can be obtained very

precisely, even in the presence of short-time power supply variations.

If the voltages V, and V (fig. 6) are increasing or decreasing in the

same manner, their difference tends to remain constant. The more

nearly equal the two voltages are, the better the stability of their differ- ence. The stability is directly observable on the galvanometer. If the

galvanometer indication drifts, the ratio of the voltage divider under

calibration is changing, provided the standard voltage divider is stable.

From appendix 1, the exact ratio X for the voltage divider under calibration is:

6

1 " 6

2 X = X ( 1 + S + -i-rnV V/S

n

where X is the voltage ratio V./V of the voltage divider under

calibration.

X is the nominal ratio of the voltage divider under calibration.

n

&

s is the correction to the standard voltage divider in ppm.

e , e are the difference voltages as determined by the Lindeck

potentiometer

.

V is the voltage applied by the high -voltage power supply.

S is the nominal ratio of the standard voltage divider,

n 6

10
4. Discussion

Various experiences in the use and calibration of high-voltage standards are discussed here so that the user may properly account for certain sources of measurement error. Especially considered are some of

those sources not usually accounted for in the calibration process.

In the calibration of electrostatic voltmeters, the practice is to zero

the instrument and then take two measurements at each of five cardinal

scale divisions spaced conveniently across one range. One set of measurements is taken while increasing the voltage and then the other set is

taken while decreasing the voltage. The results are then averaged. For

a calibration using direct -voltages, the first and second determinations

occasionally differ by an amount which seems related to the time interval

between determinations. Further study usually reveals that this drift in

voltage indication is caused within the electrostatic voltmeter. Figure 7

illustrates the change of power supply voltage with time necessary to

keep a constant indication for a particular electrostatic voltmeter. One

exceptionally faulty electrostatic voltmeter drifted about 1 percent in 17

minutes. This defect, when observed, can occur with either polarity of

d-v and generally is in a downscale direction with constant voltage. The

defect creates no problem if the voltmeter is used on a-v. Such drift can

be eliminated by replacing a feedthrough insulator, which insulates from

case ground a conductor bringing high voltage to the stator within the

electrostatic voltmeter. Some insulators apparently become charged

thereby distorting the electrostatic field within the voltmeter. The user

can check his own electrostatic voltmeter by connecting in parallel another voltmeter or, for more precise results, a high voltage resistive

divider. Any drift in the second voltmeter or in the divider may be

effectively eliminated beforehand by applying the test voltage for 30 to

60 minutes. Then any drift which occurs after connection can be attributed

11
entirely to the electrostatic voltmeter being tested. This procedure is

of particular interest to those who desire to use an electrostatic voltmeter to help hold a particular direct-voltage constant for an interval.

It is good operating practice, during calibration, to gently tap those

instruments having jewel and pivot bearings in order to avoid possible

error from pivot friction. Because the voltmeter under calibration is

placed inside the safety enclosure, a special method was devised to tap

the instrument. This was accomplished by placing a tennis ball at one

end of an insulated rod and inserting this combination through the mesh

of the enclosure. The mesh served as a fulcrum. In one instance,

excessive pivot friction caused a change of nearly 2 percent of full scale

deflection when the voltmeter was tapped. Instruments with a large

amount of pivot friction should, of course, be repaired.

A less noticeable source of measurement error in an instrument is

an unbalanced deflection system. The voltmeter reading should not

depend on the degree of instrument leveling. Sometimes this occurs,

however, because a balance weight has shifted position. On one occasion,

a faulty electrostatic voltmeter was first calibrated at full scale while

level. Then one side was elevated 5 mm, and it was recalibrated, with

the zero reset in each case. The two calibrations differed by 1. 2 percent. The instrument calibration made under one condition of leveling,

is not valid under other conditions. Repeated measurements in the same

position would not reveal this source of systematic error.

Other sources of systematic error may exist, such as the rotational

position or exact in-out position of the slide used to change ranges in the

instrument. However, in the author 1 s experience, these effects are less

significant than the d-v drift problem. It is emphasized that many electrostatic voltmeters do not have these problems. However, unless each

instrument is checked for sources of systematic error and the large

sources are accounted for, the user may be seriously misled in regard

to the measurement accuracy achieved.

12
One problem in calibrating high-voltage resistive dividers is that of

resistance change due to self -heating. In particular, the resistance

change between "cold" and "warm" conditions depends on the temperaturecoefficient of the resistance material used. Most older high-voltage

resistive dividers were built of material which had a sizable temperature

coefficient, and therefore these dividers were difficult to calibrate. Some

of these dividers changed as much as 0. 2 percent in ratio in 30 to 45

minutes before reaching temperature equilibrium. In these cases, it is

necessary to relate the resistance of the divider to both the applied voltage level and the time from initial turn -on. It is obvious that a calibration at low-voltage, such as that obtained using a direct-reading ratio

set, would not be valid at rated high voltage. The high -valued resistors

in one divider had been immersed in oil, apparently in an effort to reduce

temperature effects. However, in thirty minutes the ratio changed by

about 0. 03 percent, and temperature equilibrium was not obtained even

after two hours of operation.

Other high-voltage dividers of more successful design have been

seen recently. One divider, built by a company laboratory following the

Park design, had heating effects of less than 0. 005 percent in thirty

minutes of operation. Another divider, using unshielded construction,

showed a similar lack of ratio dependence on heating but was rated for

use at only 10 kv.

Figure 8 is a photograph of the high-voltage calibration facility.

The laboratory personnel are calibrating an electrostatic voltmeter using

direct-voltage. The standard transformer and Park divider are within

the enclosure. The d-v and a-v control boxes are on the bench. The

potentiometer, standard voltmeter, and potential transformer test set

are inset just below the bench.

13
5. Conclusion

A high-voltage calibration laboratory should provide for the safety

of its personnel and standards. Methods and aids can be developed for

accurate comparisons of high-voltage standards. The National Bureau

of Standards provides calibration services for suitable standards and

enables the calibrator to tie his comparative measurements to the

NBS standards. However, even an NBS calibrated standard must be

used correctly by observant personnel who are alert to conditions which

could unknowingly alter the calibration.

6. Acknowledgements

The author gratefully acknowledges the work of David S. Bailey and

Kenneth V. Ballard. Mr. Bailey designed and placed into operation the

remote reading system. Mr. Ballard wired the control circuitry and

contributed to its design.

7. References

Brooks, P. P. B. (March 1962), Calibration procedures for directcurrent resistance apparatus, NBS Monograph 39, 13-26.

Harris, F. K. (1952), Electrical Measurements, 599-615 (John Wiley

and Sons, Inc., New York, N. Y. ).

Oliver, D. W. (1955), Lindeck potentiometer, Rev. Sci. Instr. 2^,

No. 11, 1078.

Park, J. H. (Jan. - March 1962), Special shielded resistor for highvoltage d-c measurements, J. Res. NBS 66C, No. 1, 19-24.

14
8. Appendix

Derivation of Equation used in Calibration

of High. -Voltage Resistive Dividers

This derivation is presented to help the reader more fully understand the method described in the text for calibrating a high-voltage

resistive divider by comparison with a standard resistive divider.

Figure 9 illustrates certain sources of error inherent in the bridge

circuit components of figure 6. Resistors A and C are one megohm or

larger, depending on rated voltage and ohms -per -volt criteria, while

resistors B and D are in the order of 100 ohms. The distributed leakage resistance to ground from resistor C is represented by rj. This

leakage effect, as well as corona effects, makes the current through

the high resistance nonuniform. Current uniformity can be determined

at high voltage by the current "in" vs. "out" method described by Park

[1962]. In standards using modern insulating materials and electrode

designs, this current leakage usually is negligible. Leakage current

between the turns of resistor C also occurs and is represented in figure

9 by Ty. The effect of r^ on the adjustment of resistor C is accounted

for in the calibration of the divider. For a stable divider resistance r?

must be stable. The resistances r represent lead resistances which

can be treated as part of the internal resistance of the power supply and

are of no practical concern. The lead resistances r-, are small (usually

much less than an ohm) compared with the megohms of resistor C and

if neglected will cause an error in measurement of less than one part

per million (ppm) . The primed quantities can be treated in a similar

manner. However, lead resistances x a and r^.', if they differ by 0. 01

ohm or more, are significant compared to resistances D and B. In

fact they could cause a measurement error of 0. 01 percent (100 ppm)

or larger unless accounted for. The described calibration method

eliminates this source of error and does not restrain r^ and rV.

15
The exact ratio X of the voltage divider under calibration can be

expressed in terms of the nominal ratio X , the voltages e

1 and e shown in figure 10, the correction s to the standard voltage divider,

the nominal ratio of the standard S , and the applied voltage V. If D

n

and B are high quality four -terminal resistors, such as the Thomas

type, and if voltages e and e are determined using a Lindeck potentiometer, then voltages e., e , and V can be expressed in terms of

bridge elements as follows (see note 1):

e

l =Di

l + r 4

i l - r 4 'V Bi

2

or

e

l " e

2

=Di

l " Bi

2

But

*i = cTd and h

= aT¥ ' Thus

e

l " e

2 D B

V C+D A+B S X

where S and X are the exact ratios of the standard and "unknown"

dividers. Solving for X,

x —s^vK1 *^) (1)

i -

, v/s

As mentioned in the text, the nominal ratios of the two dividers must be

equal (X = S ) and therefore e - e is very small compared with

V/S (see note 2).

16
Since S = S (1 + s), substituting into (1) and simplifying (see note 3) gives

n

e

l ' e2 X = X ( 1 + s + ——77:

nV V/S

n

(2)

Equation (2) is useful for dividers of ratio 10,000:1 or larger because

resistances r can be neglected.

Note 1

In figure 10, e and e are considered positive when the highvoltage supply is connected as shown and the terminals connected to

the standard are more positive than similar terminals connected to

the divider under calibration.

Note 2

From the algebra of small numbers, if

e

l - 62

V/S u i 0. 001,

then

1 - u V 1 - uy V 1 + u

1 + u\ 1 + u

1 - u

= 1 + u

because u contributes a very small error (1 ppm or less\ to the

result if it is neglected.

Note 3

If |u|^ 0.001 and | s | 5 0. 001, then (1 + s)(l + u) = 1 + u +

s + us = l+u+s because us contributes a very small error (1 ppm

or less) to the result if it is neglected.

17
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STANDARD TRANSFORMER

SYSTEM GROUND

SAFETY ENCLOSURE

HV

ELECTROSTATIC VOLTMETER

GND

A A

FROM AV

CONTROL BOX

STANDARO

VOLTMETER

FIG. 3 AV CALIBRATION OF ELECTROSTATIC VOLTMETERS

20
1

STANDARD I TRANSFORMER

J -o- ±

SYSTEM GROUND

SAFETY ENCLOSURE

HV

POTENTIAL

TRANSFORMER

UNDER CALIBRATION

SEC

ADJUSTABLE

BURDEN BOX

A A

FROM AV

CONTROL BOX STD TEST

NULL

DETECTOR

DET

POTENTIAL TRANSFORMER

TEST SET

VM

REFERENCE

VOLTMETER

FIG. 4 CALIBRATION OF POTENTIAL TRANSFORMERS

21
HV

RESISTIVE

DIVIDER

HV RESISTOR

FOURTERMINAL

RESISTOR

HV

ELECTROSTATIC VOLTMETER

GND

SYSTEM GROUND

SAFETY ENCLOSURE

ELECTRICAL

CONTROL

CABLE

FROM CONTROL

SELECTOR UNIT

FIG. 5 DV CALIBRATION OF ELECTROSTATIC VOLTMETERS

22
VOLTAGE

DIVIDER

UNDER

CALIBRATION

A+B

Xn = B

STANDARD

VOLTAGE

DIVIDER

C + D

I

#1

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V

2

- SAFETY ENCLOSURE

SELECTOR SWITCH

LINDECK POTENTIOMETER

I 1 Rr I

*2

TO

CONTROL

SELECTOR

UNIT

I

FIG. 6 DV CALIBRATION OF HV RESISTIVE DIVIDERS

23
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VOLTAGE

DIVIDER

UNDER

CALIBRATION

X

STANDARD

VOLTAGE

DIVIDER

S

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FIG. 9 SOME SOURCES OF ERROR

IN CALIBRATION OF HV RESISTIVE DIVIDERS

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—< r-

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+

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FIG. 10 POTENTIAL DIFFERENCE AND

CURRENT LOOP DIAGRAM

26

U.S. DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20230

POSTAGE AND FEES PAID

U.S. DEPARTMENT OF COMMERCE

OFFICIAL BUSINESS