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Calibration & Measurement Facilities forAC High Current & High Voltage Ratio Standards at NPL

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Calibration & Measurement Facilities for AC High

Current & High Voltage Ratio Standards at NPL

S.R. GUPTANational Physical Laboratory, New Delhi - 110012

e-mail : [email protected]

[Received : 12.01.2009; Accepted : 23.03.2009]

Abstract

Regular calibration of the Instrument Transformers (ITs) is very essential for the accurate measurementof power and energy at high voltages. ITs are also used in the measurement of high currents & highvoltages at power frequencies. The nominal transformation of an IT depends only on the number ofturns of its windings but due to various losses there are always errors in the Instrument Transformers'ratios. The correct knowledge of their errors requires calibrations of the ITs. In this paper the methodsused in calibrating the Current Transformers (CTs) & Potential Transformers (PTs) and the requiredaccuracy have been discussed.

Metrology Society of India, All rights reserved.

1. Introduction

AC ratio measurements play a vital role in the

power/ energy metering as well as in the developmentof good quality electrical products. For this, oneessentially requires a laboratory in every electricalgoods manufacturing company which is equippedwith the precision type AC ratio measuringinstruments along with other instruments [1]. Metersinstalled for billing purposes determine the quantumof revenues received by electrical authorities. Themeasurement of power and energy does not form thesubject of the present paper, yet it is relevant and usefulto mention that in the metering of electrical energy theinstrument transformers i.e. the current and voltagetransformers are widely used. The determination ofthe corresponding ratio errors and phasedisplacement in CTs & PTs is carried out NationalPhysical Laboratory, India. It is the statutaryresponsibility of NPLI to maintain the NationalStandards of all parameters [2-7].

MAPAN - Journal of Metrology Society of India, Vol. 24, No. 1, 2009; pp. 29-39

This paper describes the calibration of AC HighCurrent Ratios from 5A/1A,5A up to 5000A/1A,5Aand the calibration of Voltage Transformers of anyratio with 110V and 110/3 V secondary outputs upto 100kV at 50 Hz at the required burdens at NPLI.

2. Theoretical Analysis

2.1 Current Transformers

Results of detailed mathematical analysis [8, 9]with the help of a vector diagram (not shown here)are as follows

For Current Transformers

C T R II

nI

I

nI

I

p

s

w

s

w

p

. . . =

= +

= +

1

REPORTS

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Phase Displacement

=ImIp

wheren = Turns RatioI

m= Magnetizing Current

Iw

= Iron loss component of exciting CurrentI

P= Primary Current

Is= Secondary Current

2.2 Voltage Transformers

Results of detailed mathematical analysis with thehelp of a vector diagram (not shown here) are asfollows

P T R VV

nI R X

V

p

s

w p p

s

. . .

Im

=

= ++

Phase Displacement

=Im R I X

nV

p w p

s

wheren = Turns Ratio

Im = Magnetizing CurrentIw = Iron loss component of exciting CurrentVs = Secondary VoltageVp = Primary VoltageRp = Resistance of the primary windingXp = Reactance of the primary winding

3. Demagnetization

To ensure reliable calibration, currenttransformers / voltage transformers should not haveany residual magnetization in the core. Generally thedemagnetization is a simple and quick process which

makes certain that no residual magnetic flux exists inthe core.

Adopting the already laid down proceduresensures the demagnetization of the CT/VT.

4. Polarity Check

The calibration of the instrument transformers is

conducted by comparison method [10], which isbasically a relative method. In this method the ratioerror and phase angle error of the CT / VT under testare determined by comparing it against the ReferenceStandard CT / VT having the same nominal ratio. If

the balance in the CTTS / VTTS comes within theexpected values of the ratio error and the phase angleerror, then the polarity checking is automatically done.If the CTTS/VTTS goes "into the maximum of thereadings" condition or out of the balance, then it isconsidered that the polarity mismatch is there betweenthe CT / VT and the CT / VT Under Test. To bring thecorrect balanced condition the secondary terminalsof the test CT / VT or of the Std. CT / VT areinterchanged. By this method the polarity check ofthe CT / VT is accomplished.

5. Burden Selection

The process of selecting and setting the burdenfor CT / VT under calibration must be completed priorto calibration. The customer specifies the burden orthe burden requirement is seen from the 'Name Plate'.The selection of the burden as per the requirement ismade in the required burden box. The burden isconnected in series in case of CT calibration and inparallel in case of VT calibration. The Digital PowerMeter does the accurate determination of the burdenincluding the contribution from the leads.

Care should be taken in selecting the burden for

calibration of the current / voltage transformers. Ifthe secondary output of the CT is 1A or 5A then theburden box should be of 1A or 5A rating. If thesecondary output of the VT is 110V then the burdenbox shall be of 110V rating and if it is of 110/3Vsecondary output then the burden box shall be of110/3V rating. The proper burden is seldomachieved in the first setting from the measurementsindicated by Digital Power Meter. The amount ofburden is adjusted with the help of the burden boxand the lengths of the connecting leads and theburden measurement process is again repeated.Usually after a few iterations the proper value isobtained. The final value of the measured burden isrecorded.

6. Precautions

1. Current Comparator secondary terminals shallbe short circuited in off condition.

2. All the power equipment shall be earthed

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properly.

3. The humidity and temperature shall bemaintained as per the calibration requirements.

4. All the connections shall be tight.

7. Traceability

In order to ensure that the measurementtraceability of the equipments being used as ReferenceStandards and other auxiliary instruments are withinthe specified limits of uncertainty, therefore, they mustbe linked up with higher accuracy standards at theNational and International Level. Our measurementtraceability lies with PTB, Germany.

8. Calibration of Current Transformers (CTs)

8.1 Principle

The accurate and precise calibration of CurrentTransformers (CT) is accomplished by comparisonmethod i.e. by comparing the customer's currenttransformer to a Reference Standard CurrentTransformer (RSCT) [11, 12] known as CurrentComparator (CC) whose accuracy is so high that thecorrections to be applied are negligibly small. TheComparison Method basically relies on a calibratedCurrent Comparator nominally of the same ratio asthat of the current transformer under calibration. Asuitable Current Transformer Test Set (CTTS) [12] isused for this purpose to compare the output of the

current transformer under calibration to that of theCurrent Comparator.

8.2 Standards Used for Calibration

Current Comparator: The standard used for thecalibration of current transformer [13], is a CurrentComparator having 155 standard ratios right fromthe lowest 5A/1A, 5A to the highest 5000A/1A, 5A.In order to avoid corrections it is again desirable tohave a Current Comparator whose errors arenegligibly small. The uncertainty of this referencestandard is 30ppm and phase displacementuncertainty is 0.1 min. The Current Comparator istraceable to PTB, Germany.

8.3 Current Transformer Test Set (CTTS)

For the determination of the errors of the currenttransformer under calibration, an AutomaticInstrument Transformer Test Set (AITTS) is commonlyrequired and is being used as a CTTS. The CTTS shall

be capable to operate in the operating range from 1%to 200% of the rated secondary currents of 1A and 5Aat 50Hz. The CTTS must indicate the current ratioerror in % and the phase displacement error in minutesor centi-radians. The uncertainty of the CTTS is

20ppm in ratio and 0.1 minutes in phasedisplacement. The AITTS is traceable to PTB,Germany.

8.4 Support Equipment Required for Calibration

A.C. Current Source: The A.C. High Current Sourceshould be able to supply the required current for theaccuracy test up to 120% (as per IS 2705, 1992) of thehighest rated current. Since the rated primary currentincludes a range of 5A to 1000A and some times evenup to 5000A, therefore, the current source should beprovided with tappings to allow the current to be

adjusted to the required value. Our lab is equippedwith one current source of 5000AX3V

To connect the current source to the CT and theCurrent Comparator a bifilar bus bar arrangement hasbeen chosen.

8.5 Clamp Meter

A clamp meter is an instrument up to 1000 Awhich is required in the lab for making measurementsof the flowing current in the conductors withoutbreaking the circuit. This instrument as such playsno role in the calibration process.

8.6 Current Transformer Burden

Standard burdens are required for the calibrationof current transformers as per IS 2705, 1992 and IEC60044-1:1996 The rated burdens which are normallyrequired are 2.5VA, 5VA, 7.5VA, 10VA, 15VA, 20VA,30VA, 40VA for the rated secondary currents of 1Aand 5A at a power factor of 0.8P.F. Lag.

8.7 Cables

Cables of different sizes and lengths are

essentially required in the calibration of CTs.9. Methodology

9.1 Inspection

Up on the receipt of the Current Transformer froma customer the transformer is visually inspected. It isensured that no apparent damage has occurred in

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the process of transportation. In case the damage isobserved then the customer is immediately notifiedthrough CFCT (Centre For Calibration & Testing) andthe necessary steps are taken for applying thecorrective action. Terminals' condition is also

inspected carefully. If the terminals are not clean thenthe cleaning is done with the help of emery paper andsome cleaning agents like alcohol. If excessive oxideis present or corrosion is severe then the customer isintimated and the CT is returned un-calibrated. Repairsor major cleaning is normally not done by NPLI.

Since differential comparison method is used theeffect of temperature variation on both the primaryand secondary windings of the under test get nullifiedand hence is not taken into account.

10. Calibration Set-up & Procedure

The schematic block diagram of the calibrationset-up in its simplest forms is shown in Fig. 1 forcurrent transformers.

Before measurement first we select the currentaccording to the requirement of the customer in theCurrent Injection Set. After that we select theconductors of proper current rating. The secondaryterminals of the Current Comparator and Under TestCT should be selected according to the secondary

current. Judicious selection is done of the currentrange on the Current Comparator and on the CT UnderTest. Before making connections we should see theburden of the CT Under Test on the nameplate or asper customer's requirement.

The Current Comparator and the Under Test CTprimaries are connected in series. The secondaryterminals of the Current Comparator are connected tothe Standard port of the CTTS and the secondaryterminals of the Under Test CT are connected in serieswith the burden to the Test port of CTTS. Afterensuring that connections are proper, the calibrationis started.

11. Voltage Transformers Calibration

11.1 Principle

The accurate and precise calibration of VoltageTransformers (VT) is accomplished by comparisonmethod i.e. by comparing the customer's Voltagetransformer to a Reference Standard VoltageTransformers (RSVT) whose accuracy is so high thatthe corrections to be applied are negligibly small. TheComparison Method basically relies on a calibratedVoltage Transformer nominally of the same ratio asthat of the Voltage Transformer under calibration. A

Fig. 1. Calibration of current transformer by comparison technique

C.T. BurdenC.T. Under TestSTD C.T.

STD

Load (%) Ratio Error(in %)

PhaseError (in Min)

Under Test

High Current SourceAC

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suitable Voltage Transformer Test Set (VTTS) is usedfor this purpose to compare the output of the VoltageTransformers under calibration to that of the VoltageTransformer under calibration.

11.2 Standards Used For Calibration

Standard Voltage Transformer: The standardused for the calibration of voltage transformer isStandard Voltage Divider. It comprises of a CapacitiveVoltage Divider and an Electronic Device. In order toavoid corrections it is desirable to have a StandardVoltage Divider whose errors are negligibly small. Theuncertainty of this reference standard is 50ppm andphase displacement uncertainty is 0.2 minutes. TheStandard Voltage Divider is traceable to PTB, Germany.

Voltage Transformer Test Set (VTTS): Regarding

the calibration of Voltage Transformers (VT) it is alsoaccomplished by comparison method i.e. bycomparing the customer's Voltage Transformer to aHigh Voltage Ratio Measuring System (StandardVoltage Divider) which basically comprises theCapacitive Voltage Divider (CVD) C1/C2 and HighPrecision Electronic Voltage Divider (EVD) whoseaccuracy is so high that corrections need not to beapplied. The Comparison Method basically relies onthe calibrated voltage transformer i.e. in our case is aCVD and the Programmable EVD nominally of thesame ratio as that of the voltage transformer under

calibration. A suitable Voltage Transformer Test Set(VTTS) is used for this purpose to compare the outputof the voltage transformer under calibration to that ofthe Standard Voltage Divider.

11.3 Support Equipment Required for Calibration

A.C. Voltage Source: The A.C. High Voltage Sourceshould be able to supply the required voltages for theaccuracy test up to 120% of the highest rated voltages.Two high voltage sources are available in our lab. Onesource is up to 50kV, 5000VA with digital indicationand the other source is 150kV, 10,000VA with

analogue indication.

Voltage Transformer Burden: Standard burdens arerequired for the calibration of voltage transformers asper IS 3156 and IEC 60044-2:1997. The rated burdens,which are normally required, are 2.5VA, 5VA, 7.5VA,10VA, 15VA, 20VA, 40VA for the rated secondary voltageof 110V and 110/ 3V at a power factor of 0.8P.F Lag.

Cables : Proper taut wires are used for makingconnections to the primary side of Standard VoltageDivider and VT Under Test from the AC High VoltageSource. Proper flexible cables are used for thesecondary side connections to the voltage transformer

Test Set.

Environmental Conditions : The temperature of the labis maintained at (25 2) C and humidity (50 10) %.

12. Methodology

12.1 Inspection

Up on the receipt of the Voltage Transformer froma customer the transformer is visually inspected. It isensured that no apparent damage has occurred inthe process of transportation. In case the damage is

observed then the customer is immediately notifiedthrough CFCT (Centre For Calibration & Testing) andthe necessary steps are taken for applying thecorrective action. Terminals' condition is alsoinspected carefully. If the terminals are not clean thenthe cleaning is done with the help of emery paperand some cleaning agents like alcohol. If excessiveoxide is present or corrosion is severe then thecustomer is intimated and the VT is returned un-calibrated. Repairs or major cleaning is normally notdone by NPL.

13. Calibration Set-up

The schematic block diagram of the calibrationset-up in its simplest form is shown in Fig. 2 for voltagetransformers.

In case of VT calibrations first we select the voltageaccording to the requirement of the customer in the

Voltage Injection Set. The secondary terminals of the

Standard Voltage Divider and Under Test VT should

be selected according to the secondary voltage.

Judicious selection is done of the voltage range on the

Standard Voltage Divider and on the VT Under Test.

Before making connections we should see the burdenof the VT Under Test on the nameplate or as per

customer's requirement.

The Standard Voltage Divider and Under Test

VT primaries are connected in parallel. The

secondary terminals of the Standard Voltage Divider

are connected to the Standard port of the VTTS and

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Calibration and Measurement Capabilities (CMC)

S. No. Instrument / Parameter Expanded uncertainty

1. AC High Voltage: Source 130 ppm2. AC High Voltage: Meter 130 ppm3. AC High Voltage: Real Component 60 ppm4. AC High Voltage: Imaginary Component 70 ppm

5. AC High Current: Source 120 ppm6. AC High Current: Meter 120 ppm7. AC High Current: Real Component 40 ppm8. AC High Current: Imaginary Component 50 ppm

Fig. 2. Voltage transformer calibration by using programmable E.V.D.

the Under Test VT secondary terminals are connectedin parallel with the burden to the Test Port of the VTTS.The connections must be tight. Taut wires are used forprimary side of the VTs.

After ensuring that the connections are properthen the Calibration Set-up is switched on.

14. Results with Uncertainty Budget for CT and VTCalibrations

After the data has been collected and reviewed forconsistency, the averages of the current and voltageratio errors and the averages of phase displacementsare determined. Values of estimated measurementuncertainty at 50Hz are calculated as per theguidelines given in NABL 141 [14]. A sample

calculations are given in the Annexures I - IV.

15. Conclusion

Instrument Transformers are widely used devicesin the electric power industries. They are relativelyinexpensive, but accurate and stable. However likeall the other measuring devices, they need to becalibrated particularly when they are used for bulkpower/ energy metering where revenues are involved.Due to the development of the calibration facilities inthe area of ITs NPLI is now in a position to renderinternationally traceable calibrations.

Acknowledgements

I am grateful to Dr. Vikram Kumar, Director, NPLI,

V.T. Under Test

Compressed

Gas STD. CapacitorElectronicVoltageDevice

Air Capacitor

VoltageSupply

UnderTest

Burden Box

ST D

Load %

100.00

Phase Error(in minute)

Ratio Error(in %)

-0.001+1.0

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New Delhi and Dr. P. Banerjee, Head, Electrical andElectronic Standards Division, NPLI, for their supportand encouragement for this work. I extend my sincerethanks to my colleagues Sh. L. Sridhar, Sh. DaleepSingh Yadav and Sh. Manish Kumar Tamrakar, who

have extended their entire support and help is thepreparation of this paper.

References

[1] Sharwan Kumar and S.R. Gupta, Calibrationof A.C. Electrical Measuring Instruments,Presented at All India Seminar of MetrologySociety of India, New Delhi, November 8-10(1985).

[2] Saood Ahmed, V.K. Rustagi, R. Aggarwal andBijendra Pal, Development of an AutomatedData Acquisition Systems for AbsoluteDetermination of Effective of RF Power Mountsusing Coaxial Microcalorimeter, MAPAN-

Journal of Metrology Society of India, 22 (2007)63-68.

[3] Naveen Garg, Mahavir Singh, Omkar Sharmaand V. Mohanan, Current Status of AcousticMeasurement Standards at National PhysicalLaboratory of India (NPLI), New Delhi - Part 1:Sound Pressure, MAPAN-Journal of MetrologySociety of India, 22 (2007) 77-90.

[4] Naveen Garg, Mahavir Singh, Omkar Sharmaand V. Mohanan, Current Status of AcousticMeasurement Standards at National PhysicalLaboratory of India (NPLI), New Delhi - Part 2:Acceleration Amplitude, MAPAN-Journal ofMetrology Society of India, 22 (2007) 91-101.

[5] A.K. Bandyopadhyay, Sanjay Yadav and NitaDilawar, Current Status of Pressure Standardsat NPLI and our Experiences with the Key

Comparison Data Base (KCDB), MAPAN-Journal of Metrology Society of India, 21 (2006).

[6] Tripurari Lal, Goutam Mandal and C.K.Gopan, Re-establishment of NationalStandards of Mass at NPL India, MAPAN-

Journal of Metrology Society of India,23 (2008)139-158.

[7] Kamlesh K. Jain, S.K. Jain, J.K. Dhawan andAnil Kumar, Realization of Force Scale upto50kN through Dead Weight Force Machines atNPL, India, MAPAN-Journal of MetrologySociety of India, 20 (2005) 249-257.

[8] F.K. Harris, Electrical Measurements, WileyEastern Private Limited, New Delhi, Chapter13 - Instrument Transformers (1968).

[9] V. Karapetoff and B.C. Dennison, ExperimentalElectrical Engineering, I (1937), Chapter XVIII.

[10] P.J . Betts and H.A. Smith, NationalMeasurement Laboratory Technical Paper No.3 Commonwealth Scientific and IndustrialResearch Organization, Australia, CurrentTransformer Testing at NML, Australia (1976).

[11] A. Braun, H. Danneberg and W.J.M. Moore, AnInternational Comparison of 50 - 60Hz CurrentRatio Standards at Currents upto 60,000A,IEEE Trans on Instrumentation andMeasurement, IM-27 (1968).

[12] P.N. Milijanic, Custers and W.J.M. Moore, TheDevelopment of the Current Comparator a HighAccuracy AC Ratio Measuring Device , AIEEEProceedings, (1962) 359-368.

[13] E. Zinn, An Electronic Self Balancing,Instrument Transformers. IEEE Trans. onInstrumentation and Measurement, IM- 21(1971).

[14] Guide to Estimation of MeasurementUncertainty, NABL 141.

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Uncertainty Budget for AC High Current Ratio of 3000A/5A Imaginary Component at 100%

1. NMI Service Identifier: 67

2. Calibration Service: AC High Current Ratio, Imaginary Component

3. Methodology/Principle: Comparison Method

4. Uncertainty of Standards Used: (i) Current Comparator 0.1 minutes

(ii) AITTS (Used as CTTS) 0.1 minutes

Calculation of Measurement Uncertainty In 3000a/5a Ratio

Type A Input no. of readings n = 6

Readings (Xi) Measured Values(minutes)

X1 -0.001

X2 -0.001

X3 -0.001

X4 -0.001

X5 -0.001

X6 -0.001

Average -0.001

Standard Deviation 0.000

Standard Uncertainty 0.000

Degrees of freedom 5

Uncertainty Budget

Sources of error limits Probabilitydistribution

Factor Standarduncertainty

Sensitivitycoefficient

Uncertaintycontribution

Degree offreedom

(min) (min) (min)

Repeatability(U1)

--Normal (Type

A)1 0.00 1 0.00 5

U2 (CC) 0.1Normal, Type -

B2 0.05 1 0.05 infinity

U3(AITTS)0.1

Normal, Type -B

2 0.05 1 0.05 infinity

U4(Reso) -- Rect, Type -B -- -- -- -- --

Combined Uncertainty in measurement : 0.0707 min

Effective Degree Of Freedom : Infinity #DIV/0!

Coverage Factor k = 2

Expanded uncertainty (U) k=2 U = 0.1414 min

U = 41.12 radRoundedoff U = 50 rad

The reported uncertainty is at coverage factor k=2 which corresponds to a coverageprobability of approximately 95% for a normaldistribution

Calculation of Measurement Uncertainty in 3000A/5A Ratio

Annexure - II

n = 6

U= 0.1414 minU= 41.12 rad

U= 50 rad

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Uncertainty Budget for AC High Voltage Ratio of 3000V/100V Real Component at 100%


2. Calibration Service: AC High Voltage Ratio, Real Component

3. Methodology/Principle: Comparison Method4. Uncertainty of Standards Used: (i) HVRMS 0.005 %

(ii) AITTS (Used as PTTS) 0.002 %

Calculation of Measurement Uncertainty in 3000v/100v Ratio


Readings (Xi) Measured Values (%)

X1 0.0010

X2 0.0010

X3 0.0010

X4 0.0010

X5 0.0010

X6 0.0010

Average 0.0010




UncertaintyBudget

Sources of error limits Probability

distribution

Factor Standard

uncertainty

Sensitivity

coefficient

Uncertaintycontribution

Degree offreedom

(%) (%) (%)

Repeatability(U1)

--Normal (Type

A)1 0.0000 1 0.0000 5

U2 (HVRMS) 0.005Normal, Type -

B2 0.0025 1 0.0025 infinity

U3(AITTS)0.002

Normal, Type -B

2 0.0010 1 0.0010 infinity

U4(Reso) -- Rect, Type -B -- -- -- -- --

Combined Uncertainty in measurement : 0.0027 %

Effective Degree Of Freedom : Infinity


Expanded uncertainty (U) k=2 U = 0.0054 %

Roundedoff U = 0.0060 %Roundedoff U = 60 ppm

The reported uncertainty is at coverage factor k=2 which corresponds to a coverage

probability of approximately 95% for a normaldistribution

Calculation of Measurement Uncertainty in 3000V/100V Ratio

Annexure - III

0.002% 0.005%

n = 6

U= 0.0054%

U= 0.0060%

U= 60 ppm

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Uncertainty Budget for AC High Voltage Ratio of 3000V/100V Imaginary Component at 100%


2. Calibration Service: AC High Voltage Ratio, Imaginary Component

3. Methodology/Principle: Comparison Method4. Uncertainty of Standards Used: (i) HVRMS 0.2 minutes

(ii) AITTS (Used as PTTS) 0.1 minutes

Calculation of Measurement Uncertainty in 3000v/100v Ratio


Readings (Xi) Measured Values(minutes)

X1 0.032

X2 0.032X3 0.032

X4 0.032

X5 0.032

X6 0.032

Average 0.032




Uncertainty Budget

Sources of error limits Probability

distribution

Factor Standard

uncertainty

Sensitivity

coefficient

Uncertainty

contribution

Degree of

freedom

(min) (min) (min)

Repeatability(U1)

--Normal (Type

A)1 0.00 1 0.00 5

U2 (HVRMS) 0.2Normal, Type -

B2 0.1 1 0.1 infinity

U3(AITTS)0.1

Normal, Type -B

2 0.05 1 0.05 infinity

U4(Reso) -- Rect, Type -B -- -- -- -- --

Combined Uncertainty in measurement : 0.1118 min

Effective Degree Of Freedom : Infinity


Expanded uncertainty (U) k=2 U = 0.2236 min

U = 65.01 radRoundedoff U = 70 rad

The reported uncertainty is at coverage factor k=2 which corresponds to a coverageprobability of approximately 95% for a normaldistribution

Calculation of Measurement Uncertainty in 3000V/100V Ratio

Annexure - IV

0.2 minutes

0.1 minutes

n = 6

U= 70 rad

U= 65.01 rad

U= 0.2236 min

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