TO

1
INTRODUCTION

• TO
• CURRENT TRANSFORMER
• PERFORMANCE ANALYSIS

Hands on workshop developed for field relay techs
practical approach
2
Yellow Brick Road

• INTRODUCTION
• DEFINITIONS
• PERFORMANCE CALCULATIONS
• RATIO SELECTION CONSIDERATIONS
• VARIOUS TOPICS
• TEST

3
Z V/I --- accurate value of I DISTANCE
Z
4
INTRODUCTION

• IEEE Standard Requirements for Instrument
  Transformers C57.13
• IEEE Guide for the Application of Current
  Transformers Used for Protective Relaying
  Purposes C37.110

5
INTRODUCTION

• Bushing, internal to Breakers and Transformers
• Free standing, used with live tank breakers.
• Slipover, mounted externally on
  breaker/transformers bushings.
• Window or Bar - single primary turn
• Wound Primary
• Optic

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MAGNETO-OPTIC CT

• Light polarization passing through an optically
  active material in the presence of a magnetic
  field .
• Passive sensor at line voltage is connected to
  substation equipment by fiber cable.
• Low energy output used for microprocessor relays
• Eliminates heavy support for iron.

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DEFINITIONS

• EXCITATION CURVE
• EXCITATION VOLTAGE
• EXCITATION CURRENT
• EXCITATION IMPEDANCE

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DEFINITIONS

• EQUIVALENT CIRCUIT/DIAGRAM
• POLARITY
• BURDEN
• TERMINAL VOLTAGE
• CLASSIFICATIONS T AND C

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DEFINITIONS

• KNEE POINT
• RELAY ACCURACY CLASS
• MULTI-TAPS ACCURACY
• SATURATION ERROR - RATIO/ANGLE

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EXCITATION CURVE
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EQUIVALENT DIAGRAM
f
Ve EXCITATION VOLTAGE Vef Ie CURRENT (read
a few values) Ze IMPEDANCE Vt TERMINAL
VOLTAGE Vgh POLARITY - next
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TYPICAL EXCITATION BBC CURRENT vs VOLTAGE
V (volts) Ie(amps) Ze(ohms) 3.0 0.004 750 7.
5 0.007 1071 15 0.011 1364 42 ------ -
---- 85 ------ ----- 180 ------ ------ 310
------ 3100 400 0.25 1600 425 ------
------ 450 ------ ------ 500 5.0 100.0 520
10.0 52.0
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CURRENT vs VOLTAGE
V (volts) Ie(amps) Ze(ohms) 3.0 0.004 750 7.
5 0.007 1071 15 0.011 1364 42 0.02 210
0 85 0.03 2833 180 0.05 3600 310 0.1 3
100 400 0.25 1600 425 0.5 850 450 1.00
450 500 5.0 100.0 520 10.0 52.0
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Rsec
Zint
I2
I1
IeI2

Ie
RB
EXTERNAL BURDEN
N1 N2
I1
Ze
LB
POLARITY
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DEFINITIONS

• EXCITATION CURVE
• EXCITATION VOLTAGE
• EXCITATION CURRENT
• EXCITATION IMPEDANCE
• EQUIVALENT CIRCUIT/DIAGRAM
• BURDEN - NEXT

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BURDEN

• The impedances of loads are called BURDEN
• Individual devices or total connected load,
  including sec impedance of instrument
  transformer.
• For devices burden expressed in VA at specified
  current or voltage, the burden impedance Zb is
• Zb VA/IxI or VxV/VA

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EXTERNAL BURDEN
Burden 0.27 VA _at_ 5A .. Ohms 2.51 VA _at_ 15A
.. Ohms

RB
BURDEN VA / I²
LB
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QUIZ
I2
RB
CT winding resistance 0.3 ohms Lead length
750 ft 10 wire Relay burden 0.05 ohms
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DEFINITIONS

• CLASSIFICATIONS T AND C

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ANSI/IEEE STANDARD FOR CLASSIFICATION T C

• CLASS T CTs that have significant leakage flux
  within the transformer core - class T wound CTs,
  with one or more primary-winding turns
  mechanically encircling the core. Performance
  determined by test.

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CLASS C

• CTs with very minimal leakage flux in the core,
  such as the through, bar, and bushing types.
  Performance can be calculated.

KNEE POINT
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DEFINITIONS

• KNEE POINT IEEE IEC - effective saturation
  point
• Quiz- read a few knee point voltages and also at
  10 amps Ie.

23
45 LINE

ANSI/IEEE KNEE POINT
Excitation Volts
Knee Point Volts
QUIZ READ THE KNEE POINT VOLTAGE
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KNEE POINT OR EFFECTIVE POINT OF SATURATION

• ANSI/IEEE as the intersection of the curve with
  a 45? tangent line
• IEC defines the knee point as the intersection of
  straight lines extended from non saturated and
  saturated parts of the excitation curve.
• IEC knee is higher than ANSI - ANSI more
  conservative.

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IEC KNEE POINT
ANSI/IEE KNEE POINT
EX READ THE KNEE POINT VOLTAGE
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DEFINITIONS

• EQUIVALENT CIRCUIT/DIAGRAM
• EXCITATION VOLTAGE, CURRENT, IMPEDANCE
• TERMINAL VOLTAGE
• BURDEN
• CLASSIFICATIONS T AND C
• EXCITATION CURVE
• KNEE POINT IEEE IEC
• ACCURACY CLASS

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CT ACCURACY CLASSIFICATION

• The measure of a CT performance is its ability
  to reproduce accurately the primary current in
  secondary amperes both is wave shape and in
  magnitude. There are two parts
• Performance on symmetrical ac component.
• Performance on offset dc component. Go over the
  paper

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ANSI/IEEE ACCURACY CLASS

• ANSI/IEEE CLASS DESIGNATION C200 INDICATES THE
  CT WILL DELIVER A SECONDARY TERMINAL VOLTAGE OF
  200V
• TO A STANDARD BURDEN B - 2 (2.0 ?) AT 20 TIMES
  THE RATED SECONDARY CURRENT
• WITHOUT EXCEEDING 10 RATIO CORRECTION ERROR.
  Pure sine wave
• Standard defines max error, it does not
  specify the actual error.

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ACCURACY CLASS CSTANDARD BURDEN

• ACCURACY CLASS C100, C200, C400, C800 AT
  POWER FACTOR OF 0.5.
• STANDARD BURDEN B-1, B-2, B-4 AND B-8 THESE
  CORRESPOND TO 1?, 2?, 4? AND 8?.
• EXAMPLE STANDARD BURDEN FOR C100 IS 1 ?, FOR C200
  IS 2 ?, FOR C400 IS 4 ? AND FOR C800 IS 8 ?.
• ACCURACY CLASS APPLIES TO FULL WINDING, AND ARE
  REDUCED PROPORTIONALLY WITH LOWER TAPS.
• EFFECTIVE ACCURACY
• TAP USEDC-CLASS/MAX RATIO

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AN EXERCISE

• 2000/5 MR C800 tap usedc-class/max
  ratio
• TAPS KNEE POINT EFFECTIVE ACCURACY
• 2000/5 .. ...
• 1500/5 .. ...
• 1100/5 .. ...
• 500/5 .. ...
• 300/5 .. ...

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AN EXERCISE

• 2000/5 MR C800 tap usedc-class/max
  ratio
• TAPS KNEE POINT EFFECTIVE ACCURACY
• 2000/5 590 800
• 1500/5 390 600
• 1100/5 120 440
• 500/5 132 200
• 300/5 78 120

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AN EXERCISE

• 2000/5 MR C400 tap usedc-class/max
  ratio
• TAPS KNEE POINT EFFECTIVE ACCURACY
• 2000/5 .. ...
• 1500/5 .. ...
• 1100/5 .. ...
• 500/5 .. ...
• 300/5 .. ...

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AN EXERCISE

• 2000/5 MR C400 tap usedc-class/max
  ratio
• TAPS KNEE POINT EFFECTIVE ACCURACY
• 2000/5 220 400
• 1500/5 170 300
• 1100/5 125 220
• 500/5 55 100
• 300/5 32 60

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CT SELECTIONACCURACY CLASS
POINT OF SATURATION KNEE POINT IT IS DESIRABLE
TO STAY BELOW OR VERY CLOSE TO KNEE POINT FOR THE
AVAILABLE CURRENT.
Recap
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ANSI/IEEE ACCURACY CLASS C400

• STANDARD BURDEN FOR C400 (4.0 ?)
• SECONDARY CURRENT RATING 5 A
• 20 TIMES SEC CURRENT 100 AMPS
• SEC. VOLTAGE DEVELOPED 400V
• MAXIMUM RATIO ERROR 10
• IF BURDEN 2 ?, FOR 400V, IT CAN SUPPLY MORE THAN
  100 AMPS SAY 200 AMPS WITHOUT EXCEECING 10 ERROR.

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Rsec
Isec 100
Zint
I1
IeIsec
Ie lt10
RB
EXTERNAL BURDEN
N1 N2
I1
Ze
LB
ACCURACY ACLASS C200 RATED SEC CURRENT
5 A EXTERNALBURDEN STANDARD BURDEN 2 .0
OHMS Ve200 V Isec 100 A Ie lt10 Amps.
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PERFORMANCE CALCULATIONS
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BUT

• THE REST OF US
• SHOW US THE DATA

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PERFORMANCE CRITERIA

• THE MEASURE OF A CT PERFORMANCE IS ITS ABILITY TO
  REPRODUCE ACCURATELY THE PRIMARY CURRRENT IN
  SECONDARY AMPERES - BOTH IN WAVE SHAPE AND
  MAGNITUDE . CORRECT RATIO AND ANGLE.

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CT SELECTION AND PERFORMANCE EVALUATION FOR PHASE
FAULTS
600/5 MR Accuracy class C100 is selected
Load Current 90 A Max 3 phase Fault Current
2500 A Min. Fault Current350 A STEPS CT
Ratio selection Relay Tap Selection Determine
Total Burden (Load) CT Performance using
ANSI/IEEE Standard CT Performance using
Excitation Curve
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PERFORMANCE CALCULATION
STEPS CT Ratio selection Relay Tap
Selection Determine Total Burden (Load) CT
Performance using ANSI/IEEE Standard CT
Performance using Excitation Curve
STEPS CT Ratio selection - within short time
and continuous current thermal limits - max
load just under 5A Load Current 90 A CT ratio
selection 100/5
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PERFORMANCE CALCULATION
STEP Relay Tap Selection O/C taps min pickup
, higher than the max. load 167, 150 of
specified thermal loading. Load Current 90 A for
100/5 CT ratio 4.5 A sec. Select tap higher
than max load say 5.0 How much higher relay
characteristics, experience and judgment. Fault
current min 350/20 17.5 Multiple of PU
17.5/5 3.5 Multiple of PU 17.5/6 2.9
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PERFORMANCE CALCULATION
STEP Determine Total Burden (Load) Relay 2.64
VA _at_ 5 A and 580 VA _at_ 100 A Lead 0.4
Ohms Total to CT terminals (2.64/55 0.106)
0.4 0.506 ohms _at_ 5A (580/100100 0.058)
0.4 0.458 ohms _at_ 100 A
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PERFORMANCE CALCULATION
STEPS CT Ratio selection Relay Tap
Selection Determine Total Burden (Load) CT
Performance using ANSI/IEEE Standard CT
Performance using Excitation Curve

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PERFORMANCE CALCULATION
STEP CT Performance using ANSI/IEEE Standard
Determine voltage _at_ max fault current CT must
develop across its terminals gh
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PERFORMANCE CALCULATION
STEP Performance ANSI/IEEE Standard Vgh
2500/20 0.458 57.25 600/5 MR C100 CT
used at tap 100/5 -- effective accuracy
class (100/600) x 100 ? CT is capable of
developing 16.6 volts. Severe Saturation. Cannot
be used.
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PERFORMANCE CALCULATION
STEP Performance ANSI/IEEE Standard
For microprocessor based relay Burden will
change from 0.458 to o.4 Vgh 2500/20 0.4
50.0 600/5 MR C100 CT used at tap 100/5 --
effective accuracy class (100/600) x 100
? CT is capable of developing 16.6 volts.
Severe Saturation. Cannot be used.
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PERFORMANCE CALCULATION
STEP Performance ANSI/IEEE Standard
Alternative use 400/5 CT tap Max Load 90
A Relay Tap 90/80 1.125 Use 1.5 relay
tap. Min Fault Multiples of PU(350/804.38,
4.38/1.5 2.9) Relay burden at this tap 1.56
ohms Total burden at CT terminals 1.56 0.4
1.96 Vgh 2500/80 1.96 61.25 600/5 MR C100
CT used at tap 400/5-- effective accuracy
class is (400/600) x 100 ? CT is
capable of developing 66.6 volts. Within CT
capability
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PERFORMANCE CALCULATION
STEP CT Performance using Excitation Curve
ANSI/IEEE ratings ballpark. Excitation curve
method provides relatively exact method. Examine
the curve
Burden CT secondary resistance lead
resistance relay burden Burden 0.211 0.4
1.56 2.171 For load current 1.5 A Vgh
1.5 2.171 3.26 V Ie 0.024 Ip
(1.50.024) 80 123 A well below the min If
350 A (350/1232.84 multiple of pick up)
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PERFORMANCE CALCULATION
STEP CT Performance using Excitation Curve

For max fault current Burden CT secondary
resistance lead resistance relay
burden Burden 0.211 0.4 1.56
2.171 Fault current 2500/80 31.25 A Vgh
31.25 2.171 67.84 V Ie
0.16 Beyond the knee of curve, small amount 0.5
does not significantly decreases the fault
current to the relay.
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TEST
I2
RB
Determine CT performance using Excitation Curve
method
CT winding resistance 0.3 ohms Lead length
750 ft 10 wire Relay burden 0.05 ohms as
constant Fault current 12500A/18000A CT CLASS
C400/C800 2000/5 MR current transformer CT
RATIO 800/5
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AN EXAMPLE C400

• CT RESISTANCE 0.3 OHMS
• LEAD RESISTANCE 1.5 OHMS
• IMPEDANCE OF VARIOUS DEVICES 0.05 OHMS
• FAULT CURRENT 12500 AMPS
• CT RATIO 800/5
• ACCURACY CLASS C400
• supply curves C400/800

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CALCULATIONS for 12500 A C400

• BURDEN ( Z-LEAD Z - CT SEC D - DEVICES)
• Ve (1.5 0.3 0.05 ) 12500/160
• Ve 144.5 VOLTS Plot on curve
• Plot on C400

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CALCULATIONS for 18000 C400

• BURDEN ( Z-LEAD Z - CT SEC D - DEVICES)
• Ve (1.5 0.3 0.05 ) 18000/160
• Ve 209 VOLTS Plot on curve
• Plot on C400

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ANOTHER EXAMPLE C800

• CT RESISTANCE 0.3 OHMS
• LEAD RESISTANCE 1.5 OHMS
• IMPEDANCE OF VARIOUS DEVICES 0.05 OHMS
• FAULT CURRENT 12500 AMPS
• CT RATIO 800/5
• ACCURACY CLASS C800
• supply curves C400/800

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CALCULATIONS for 12500 A C800

• BURDEN ( Z-LEAD Z - CT SEC D - DEVICES)
• Ve (1.5 0.3 0.05 ) 12500/160
• Ve 144.5 VOLTS Plot on curve
• Plot on C800

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CALCULATIONS for 18000 A C800

• BURDEN ( Z-LEAD Z - CT SEC D - DEVICES)
• Ve (1.5 0.3 0.05 ) 18000/160
• For 18,000 A (Ve 209 V) Plot on curve
• Plot on C800

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FAULT CURRENT MAGNITUDES

• 25 -33 KA 8
• 20 - 25 KA 10
• 12.5 -20 KA 46
• 20 - 25 KA 35
• 10 -12.5 KA 35
• lt10 KA 150

REFER TO PAGE 6 OF PAPER
61
RED DELICIOUS
C400
ZONE1
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Z V/A DISTANCE Z
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STANDARD DATA FROM MANUFACTURER

• ACCURACY
• RELAY CLASS C200
• METERING CLASS, USE 0.15
• 0.3, 0.6 1.2 AVAIALABLE BUT NOT RECOMMENDED
• 0.15 MEANS /- 0.15 error at 100 rated
  current and 0.30 error at 10 of rated current (
  double the error)

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STANDARD DATA FROM MANUFACTURER

• CONTINUOUS (Long Term) rating
• Primary
• Secondary, 5 Amp ( 1Amp)
• Rating factor (RF) of 2.0 provides Twice Primary
  and Secondary rating continuous at 30degrees

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STANDARD DATA FROM MANUFACTURER

• SHORT TIME TERMINAL RATINGS
• Transmission Voltage Applications
• One Second Rating 80 Imax Fault, based on
  IxIxTK where T36 cycles IMax fault current
• Distribution Voltage Applications
• One Second Rating Maximum Fault Current level

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RATIO CONSIDERATIONS

• CURRENT SHOULD NOT EXCEED CONNECTED WIRING AND
  RELAY RATINGS AT MAXIMUM LOAD. NOTE DELTA
  CONNECTD CTs PRODUCE CURRENTS IN CABLES AND
  RELAYS THAT ARE 1.732 TIMES THE SECONDARY CURRENTS

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RATIO CONSIDERATIONS

• SELECT RATIO TO BE GREATER THAN THE MAXIMUM
  DESIGN CURRENT RATINGS OF THE ASSOCIATED BREAKERS
  AND TRANSFORMERS.

68
RATIO CONSIDERATIONS

• RATIOS SHOULD NOT BE SO HIGH AS TO REDUCE RELAY
  SENSITIVITY, TAKING INTO ACCOUNT AVAILABLE RANGES.

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RATIO CONSIDERATIONS

• THE MAXIMUM SECONDARY CURRENT SHOULD NOT EXCEED
  20 TIMES RATED CURRENT. (100 A FOR 5A RATED
  SECONDARY)

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RATIO CONSIDERATIONS

• HIGHEST CT RATIO PERMISSIBLE SHOULD BE USED TO
  MINIMIZE WIRING BURDEN AND TO OBTAIN THE HIGHEST
  CT CAPABILITY AND PERFORMANCE.

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RATIO CONSIDERATIONS

• FULL WINGING OF MULTI-RATIO CTs SHOULD BE
  SELECTED WHENEVER POSSIBLE TO AVOID LOWERING OF
  THE EFFECTIVE ACCURACY CLASS.

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TESTING

• Core Demagnetizing
• The core should be demagnetized as the final test
  before the equipment is put in service. Using the
  Saturation test circuit, apply enough voltage to
  the secondary of the CT to saturate the core and
  produce a cecondary currrent of 3-5 amps. Slowly
  reduce the voltage to zero before turning off the
  variac.

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TESTING

• Saturation
• The saturation point is reached when there is a
  rise in the test current but not the voltage.

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TESTING

• Flashing
• This test checks the polarity of the CT
• Ratio
• Insulation test

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