Procedure
Calibration Procedure
Instrument :INSULATION TESTERNo: CI/01/I2
Revision :3
Written By : Eby Joy
QA/QC EngineerSign :
Date :
Distributed to : TD,LM,MR
All users. Original : QA File
Checked By : Miyad Hydrose
Technical SupervisorSign :
Date : Issued Date :
11/12/2014
Effective from :
15/12/2014
Approved By : Izzuddin T Mohammed
Technical DirectorSign :
Date :
Calibration Procedure
Instrument :INSULATION TESTERNo: CI/01/I2
Revision :3
Written By : Eby Joy
QA/QC EngineerSign :
Date :
Distributed to : TD,LM,MR
All users. Original : QA File
Checked By : Miyad Hydrose
Technical SupervisorSign :
Date : Issued Date :
11/12/2014
Effective from :
15/12/2014
Approved By : Izzuddin T Mohammed
Technical DirectorSign :
Date :
1. Purpose Inorder to provide the written calibration procedure
for insulation testers. Insulation testers are used to verify that
electrical installations are safe, to test windings on motors and
to test insulation resistance on various appliances. It is
essential that the tester provides the correct test voltage as the
voltage may influence the measurement results.
2. ScopeThe calibration requirements specified in this document
apply to all portable didital insulation testers. Insulation Test
Voltage, Insulation Test Resistance, Insulation Test current would
come under the scope of calibration in an inulation tester.
Uncertainty budgeting of insulation tester calibration will be
explained in this procedure.3. LocationThe calibration will be
carried out in the Lab or customer premises depending on the
customer requirement. For better accuracy, lab calibration is
preferred.4. Environmental conditions to be maintained Temperature:
22C 2C Humidity: 50%RH 10%RH5. Master Equipment Used
5.1 Transmille 3200A Electrical Test Calibrator(for Insulation
Resistance Voltage, I. Resistance, I. current.)
5.2 IET labs Decade Resistance box for continuity resistance
measurement or any calibrated resistance standard.5.3 Standard
Multimeter
5.4 Multiproduct calibrator which can source voltage (if the
insulation tester has a voltage measurement funtion)
5.5 Guard Function checker for checking the Guard terminal
functioning.
5.6 Calibration Capability5.7 Insulation Resistance- 10k to 100
G (Upto 5000 V insulation tester can be verified, if using the
adpator)
5.8 Insulation Volatge- 50V, 100V, 250V, 500V, 1000V, 2500V,
5000V
5.9 Insulation Current-0.5mA and 1 mA
5.10 Continuity Resistance6. Reference international
standards/guidelines/Manuals6.1 Guidance on the calibration of
Insulation Testers, TG-02-01, South African Accreditation body6.2
Transmille 3200 Electrical Test calibrator manual.
7. Calibration Procedure 7.1 Precautions before and during
calibrationa. Set the function switch to the appropriate position
before measuring.b. Warm-up time for the reference instrument is 30
minutes and for the UUT, refer the operation manual of the
instrument. Calibration can only carried out after the warm-up
time.
c. Do not try to calibrate the insulation resistance if it is
damamged or if metal parts are exposed. Also check the meter for
cracks or missing plastic.d. Ensure that the test leads are placed
and firmly connected in the proper input terminals. Connect the
common test lead before the live test lead and remove the live test
lead before the common test lead.e. Examine the test leads for
damaged insulation or exposed metal. Check test lead continuity.
Replace damaged leads.
f. Ensure that the insulation tester is switched off when
opening the battery compartment.
g. Always remove the test leads before replacing the battery in
the tester. h. The instrument must not be low battery while
carrying out the calibration.
i. Remove the teast leads and the range selector switch to the
OFF position after use.
7.2 Insulation Resistance Measurementa. A resistor of known
value and voltage coefficient should be connected to the insulation
tester for calibration(Refer figure 1). Connect the insulation
tester(unit under test) to the Black & Red 4mm terminals of
Transmille 3200A(which acts as the standard resistor).b. Set
insulation voltage range on the tester. Calibrate the instrument on
at least two points along the scale of each range.c. Enter the
required resistance from 10k to 10,000M on the keyboard followed by
Enter in transmille 3200 to set the output. An alternative way to
select the required resistance is to use the digital Control to
increment / decrement the digit indicated by the cursor. The Left
and Right arrow keys allow the selected digit to be changed.d.
Measure the insulation Resistance set by Transmille 3200 by
pressing the test button in the insulation tester for 1 minute at
the selected voltage range. At the end of the 1 min record the
resistance indicated by the unit under test. Repeat the measurement
for 5 times at each point.e. Care should be taken not to exceed the
voltage and current limits marked on the specification of the
calibrator when applying voltage from the insulation tester. While
applying voltages above 5kV, an voltage adaptor needs to be used
along with the electrical test calibrator inorder to test
insulation tester above 1kV voltage capacity.
Figure 1
7.3 Insulation Voltage Measurement (Figure 2)-Method 1a. The
output voltage is measured on each range to ensure that the tester
delivers at least the minimum specified voltage. b. This
measurement should be conducted at the load recommended by the
manufacturer, normal case we take the load as 1G. The output
voltage measured as well as the value of the load shall be stated
on the calibration certificate.c. The voltage applied by the
insulation tester can be measured by Transmille 3200 Electrical
Test calibrator.
d. To measure the insulation voltage, Connect the insulation
tester to the 4mm sockets at the top of the calibrator using the
set of silicon test leads, or the leads supplied with the
instrument.
e. From the Insulation Resistance menu displayed on the 3200A,
select the VOLTAGE function using the soft key.f. Press the test
button in the insulation tester to inject voltage to the
calibrator. Record the measured value in the transmille 3200 and
the applied value from the tester. Record the error.
7.4 Insulation Voltage Measurement - Method 2a. To measure
voltages above 1000 V the following method shall be used.b. The
insulation Tester is to be connected to 1 G load(CB 101). Across
the load a mulimeter will be connected through a high voltage probe
as the voltage rating for multimeters cannot exceed 1000V.
c. Start applying the voltage across the load and measure the
voltage across the load using the mulimeter.d. Check the error
between the insulation tester voltage and the voltage measured by
the reference multimeter.
7.5 Insulation current Measurement (Figure 3)e. Select the
required voltage range to be measured using either the digital
Control or the up / down arrow keys in Transmille 3200 Electrical
Test Calibrator. Ranges include 50V, 100V, 250V, 500V and 1000V.The
impedance of each range is automatically set to give the correct
load 1mA / 0.5mA (see test current below) at the applied nominal
voltage range. Figure 2 Screenshot of transmille 3200 display
Figure 3g. Select the required measurement current using the
Soft Keys. The default setting for current load is 0.5mA nominal
which is thecorrect load/test current for the 17th Edition
equipment -e.g. must be able to supply 0.5mA at the specified test
voltage.Older insulation testers (16th Edition) produced 1mA
current at the specified test voltage. The 3200A should be set to
1mA current for these instruments. Very old testers may only
produce a very small current and the voltage will collapse under
any load these should be tested using additional equipment.7.6
Insulation Tester with Continuity functiona. Calibrate the low
value ohms/continuity ranges by applying at least two low ohm
resistance values.b. Note that insulation testers are 2-terminal
and are often calibrated with the leads supplied. This shall be
reported on the calibration certificate.7.7 Voltage Measurement
Accuracya. Calibrate the voltage measuring ranges at a minimum of
two points along the range
using transmille 3041 following accredited procedure
(AC/QSP/029). 7.8 Guard Terminal function Checkinga. Where a guard
terminal is incorporated on an insulation tester, the functionality
of this terminal shall be verified. This can be done in the
following way by connecting two suitable series resistors in
parallel with the resistance being measured. (See figure 1
below).
Figure:
Step 1 - Measure without the guard terminal connected (173 M
expected)
Step 2- Measure with the guard terminal connected (1 G
expected)8. Accuracy Criteriaa. If the UUT reading exceeds and
adjustment of the same is not possible to make it within the
manufacturer recommended tolerance limit,then the instrument need
to be rejected without calculating the uncertainty limits.
b. If the UUT reading exceeds the manufacturer recommended
tolerance limit and adjustment is possible to make it within
tolernace limit as specfied by the manufacturer, then the same need
to be approved, certified.The uncertainty Estimation needs to be
done and given in the certificate.
c. If the UUT is within the manufacturer tolerance limit without
any adjustment, then the instrument is approved,certified. The
uncertainty Estimation needs to be done and given in the
certificate.9. Intermediate Check for the Reference Instrumenta.
Intermediate check of the reference instrument shall be done every
6 months. The reference instrument is used for calibrating an
Insulation Tester @1G load under different voltages (50V, 100V,
250V, 500V, 1000V). The results would be recorded in a work
sheet.b. The same nsulation Tester is calibrated again with the
Megger Calibration box-CB101 @1G load under same voltages (50V,
100V, 250V, 500V, 1000V).
c. Both the calibration results shall not have a variation of
more than 1% from each other at any of the points.
d. If a varaition is found more than the acceptable limit, a
corrective action form will be raised by the concerned technician
and investigation to be done to find out the root cause of the
problem. A corrective action will have to be taken to rectify the
error. Until the case is resolved the calibration would be stopped
using the instrument in question.10. Uncertainty Budgeting For
Insulation Tester Calibration
10.1 Type A Uncertainty (uA)
a. Type A uncertainty is determined by the statistical analysis
of a series of measurements. Ten measuremnts are taken to find this
random uncertainty.
10.2 Type B uncertainty (uB1, uB2, uB3, uB4)a. All the other
uncertainties that cannot be determined statistically, during the
measurement process. They fall in to the category of Type B
uncertainties
Calibrator Uncertainty , uB1 Measurement error due to
limitations of the UUTs resolution uB2 Uncertainty due to thermal
emf of test leads, uB3 (Only taken in Voltage measurement ) I year
specifications of the reference instrument, uB4 Uncertainty due to
Drift, uB510.3 Combined Uncertainty (uc)a. It is a summation of
Type A and Type B uncertainties ( U1+UB1+UB2+UB3+UB4.) using law of
error propagation.10.4 Expanded uncertainty (U)
a. Expanded measurement uncertainty (or expanded uncertainty) is
defined as the product of a combined standard measurement
uncertainty and a factor larger than one to provide a certain level
of confidence. It is combined uncertainty multiplied by 2 (coverage
factor k) to provide a level of confidence of 95%.
11. Model Equation
Yi = Ym +Ymc UB1+UB2+UB3+UB4
Ym= Value from the master instrument
Yi = Indicated value in the UUTYmc= Correction from the master
instrument
UB1= Calibration of the reference meter
UB2= Resolution of The UUT
UB3= Thermal EMF in the Leads (Only applicable if a Voltage
measurement fearure is available in the unit)UB4= Drift in the
master instrument.
12. Formula used for estimating uncertainty 12.1 Type A
Uncertainty (uA) Stanard deviation (S) Where n Total Number of
Measurements
S = n 2 Xi value of individual measurements
Xa Average of total number of measurement
Xi - Xa
i = 1
n 1
Estimated standard uncertainty uA = S ------------------- Eq(1)
n
where S estimated standard deviation.
Probability Distribution is Normal 12.2 Type B uncertainties
a. Uncertainty from Master Equipment Certificate (uB1) uB1 =
Expanded Uncertainty
2 ------------------- Eq(2)
Probability Distribution is Normal b. Uncertainty due to UUTs
resolution (uB2)
Resolution ------------------- Equ(3) uB2 = 2 X 3 Probability
Distribution is Rectangularc. Uncertainty due to thermal emf of
test leads (uB3) uB3 = Average of +ive and ive voltage of test
leads ------------------- Equ(4) 2
This uncertainty contribution will be considered only in voltage
measurement function in the Insulation tester. Probability
Distribution is Rectangulard. Uncertainty Due to Drift (uB4) Drift
in direct reading instruments is defined for a specific time
interval of interest. The standard uncertainty for drift is uB4=
|Y0-Yt|/3 ---------------------- Equ(5)
Y0- Measurement at Time 0
Yt- Mesaurement at Time t Probability Distribution is
Rectangular12.3 Formula used for Estimation of standard combined
uncertainty
Standard combined uncertainty, uc = uA + uB12+ uB12+ uB12+ uB12+
uB12 ----------Equ(6)12.4 Formula used for calculating expanded
uncertainty
U= uc x 2 (k is the coverage factor which is =2)
----------Equ(7)13. Procedure
13.1 Type A Uncertainty (Random Errors)
a. Estimation of Type A uncertainty limit
1. Ensure the indicated reading of UUT is within the tolerance
limit given by the manufacturer
2. Take minimum 5 readings on each parameter and range.
3. Calculate the average value for the series of measurement for
all ranges and parameter
4. By using the above mentioned formula,calculate the standard
deviation ( S ).
5. Apply estimated standard deviation to Eq(1) above to
calculate the Type A uncertainty limit ( U1 ).
13.2 Type B Uncertainties (Systematic Errors)
a. Estimation of Imported Uncertainty (uB1)
1. Calibrators uncertainty is mentioned in the calibration
certificate of the Master instrument as expanded uncertainty hence
divide the value by 2 to get the combined uncertainty.
2. Use the above mentioned formula Eq(2) to get UB1b. Estimation
of Uncertainty due to Limited Resolution of the UUT (uB2)
1. Resolution of the UUT is taken into consideration. The limit
to the ability of an instrument to indicate small changes in the
quantity being measured, referred to as resolution or digital
rounding error, is treated as a systematic component of
uncertainty. Half scale of the resolution is taken and the
probability distribution is rectangular, therefore the divisor is 3
. 2. Use the equation, (Equ3) to get uB2.c. Estimation of
Uncertainty due to Thermal EMF (uB3)
1. Refer to the technician guide to get the value of uB3. 2. If
an electrical conductor passes through a temperature gradient then
a potential difference will be generated across that gradient. This
is known as the Seebeck effect and these
unwanted, parasitic voltages can cause errors in some
measurement systems in particular, where small dc voltages are
being measured.3. Take the leads which are used for the
calibration. Short circuit one end of the cables and other ends are
connected to the voltage terminals. Take readings with the leads
connected to the positive and negative of the voltage terminal and
readings taken with the leads switched at the voltage terminal.4.
The probability distribution is rectangular, therefore the divisor
is 3 . 5. Use the equation, Equ(4) to get uB3.d. Estimation of
Uncertainty due to Drift (uB4)
1. Refer to technician guide to get the values of uB4.
2. Electronic instruments are expected to have drift. Drift can
be identified from the certificates of two consecutive calibration
done to the instrument.
3. The probability distribution is rectangular, therefore the
divisor is 3 . 4. Use the equation Equ(5) to get uB4e. Estimation
of Combined Uncertainty (UC)
1. Take the individual uncertainty values of Type A, uB1, uB2,
uB3 and uB42. Add all uncertainties above, using the Law of Error
propagation.3. The probability distibution is normal because it is
a summation of many standard probability distributions. The divisor
is 1
4. Use Equ(6) to get the combined Uncertaintyf. Estimation of
Expanded Uncertainty
1. Although the combined standard uncertainty is used to express
the uncertainty of many measurements, some commercial, industrial
and regulatory applications may require a measure of uncertainty
that defines an interval about which the measurement value will lie
with a certain level of statistical certainty. This type of measure
is termed expanded uncertainty, and is determined by multiplying
the combined standard uncertainty by a coverage factor, k. The
coverage factor is usually in the range of 2 to 3. That is the
expanded uncertainty of the measurement which is multiplied by a
coverage factor k=2,providing the level of confidence of
approximately 95%.
e. Uncertainty Budgeting for Insulation Tester
Calibration.SymbolSource of UncertaintyProbability
DistributionDivisorSensitivity Coefficient(Ci)
uARepeatabilityNormal11
uB1Calibration of the Master InstrumentNormal21
uB2Resolution of the UUTRectangular31
uB3Thermal Emf(Error due to leadsRectangular31
uB4DriftRectangular31
ucCombined UncertaintyNormal1-
UExpanded UncertaintyNormal(k=2)1-
14. Documentation
a. Record all calibration points (includes applied value and
measured value) in the work sheet (AC/WF/003) before and after ( if
required ) adjustment. b. If the instrument is out of tolerance and
not possible to adjust , it is forwarded to the technical
supervisor with a fault report. c. Readings are enterd in to the
central suystem along with all the details of the instrument ( if
calibration completed )d. Certificate of calibration is suued to
the customer.
15. Data to be recorded.
a. Applied readings and indicated readings to be recorded in
work sheet.
b. Applied readings and indicated readings after adjustment ( if
need be ) to be recorded in the same worksheet ( May use more than
one page as required but need to bind together and the assigned
certificate number to be mentioned in each page)c. Calculation of
tolerance limit and calculation of uncertainty values are to be
performed.
Prepare calibration certificate / service report as per the
procedure16. Affixing the identification marka. The due date of
calibration will be mentioned as per the clients requirment or will
be mentioned as one year.b. The calibration sticker must be affixed
in the instrument in such a way that it doesnt block any of
operation of the instrument.
17. Reason for Revision Revision 1 : 1.Changes in the format.
2.Inclusion of documentation chapter.Revision 2 : 1.Inclusion of
RangeRevision 3 : The whole procedure was revised to meet the
requirements of ISO/IEC 17025 standard.
Inclusion of Uncertainty procedure.
Date : 11/Dec/2014****END OF PROCEDURE****
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