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Fall–Winter 2004

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As test equipment becomes more sophisticated with better features

and accuracy, we risk turning our field personnel into test-set opera-

tors instead of skilled field service technicians. A test-set operator

connects the leads, pushes the buttons, and records the results, hoping

the numbers he records are good. A test technician connects the leads,

pushes the buttons, and records the results. However, the test technician

understands what the test set was doing while all the lights were flash-

ing, and why. The technician can also evaluate the results and determine

if a retest is necessary with different connections or substitute external

equipment for tests when the test equipment malfunctions. The purpose of

this article is to help a test-set operator understand the tests he performs,

reinforce a test technique, or add it to his repertoire.

Description of Operation

During normal operation, a current transformer (CT) transforms higher

current into a more manageable secondary current. This transformation

is made possible by copper coils wrapped around an iron core, with the

ratio between primary and secondary currents determined by the ratio

between the number of primary and secondary turns. Bar- and window-

type CTs do not have a physical primary winding and are considered to

have one primary turn. When current flows through the primary winding,

the following actions occur:

• The iron core inside the transformer is magnetized.

• The magnetic field in the iron core induces a voltage in the sec-

ondary coils.

• If the secondary circuit is closed, a current flows through the

secondary circuit in proportion to the CT ratio.

The current transformation requires a small amount of energy to mag-

netize the iron core that creates small energy losses such as eddy currents,

and heat caused by current flowing through the windings. Therefore, the

secondary current is not a perfect representation of the primary current.

All CTs that are built to ANSI standards have an accuracy class to outline

what effect the losses have on the secondary current under normal con-

Back to the Basics —

Current Transformer Testing

Chris Werstiuk

Valence USA

The Official Publication of the InterNational Electrical Testing Association

Fall – Winter 2004

Valenceusa@earthlink.net

www.home.earthlink.net/~valenceusa/

neta@netaworld.org

www.netaworld.org

Reprinted with permission of the InterNational

Electrical Testing Association. Copyright 2004.

All rights reserved.

ditions. The accuracy class is the

minimum accuracy guaranteed by

the manufacturer, although the CT

may be built to higher standards.

Like every other transformer,

CTs can only transform a finite

amount of energy. This is usu-

ally the voltampere rating for

normal transformers. The CTs

energy limitation is included in

the accuracy class but is shown as

a maximum burden. The CT bur-

den is the amount of impedance

(ac resistance) connected to the CT

secondaries and is usually rated in

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ohms or volts. The manufacturer only guarantees CT

accuracy up to a maximum burden rating, and CT

performance will degrade if the secondary burden is

larger than rated.

Visual and Mechanical Inspections

Equipment Specifications

While performing visual and mechanical inspec-

tions, always compare equipment nameplate data

with drawings and specifications. Every CT test sheet

should include the following information:

Serial Number (when possible): The serial number

is important to organize test results and allow a refer-

ence when comparing test results to manufacturer’s

specifications. The serial number can also be used

when ordering replacement or new CTs from manu-

facturers.

Model Number (when possible): The model

number is important when comparing test results to

manufacturer’s literature and ordering replacement,

new, or spare CTs from manufacturers.

Ratio: The CT ratio is the most important piece of

information regarding the CT and must be recorded

from the nameplate or the design criteria. The ratio

determines the CT’s operating characteristics and is

used as reference for tests. If the CT has multiple taps

(different possible ratio combinations), all taps should

be recorded for future reference in case a new ratio is

required for the application. This will allow a review

of the options without visiting the site beforehand.

Accuracy Class: The CT accuracy class comprises

a number-letter-number combination and indicates

the CT’s ability to perform accurately under differ-

ent conditions. In other words, the CT accuracy class

indicates the CT’s performance characteristics and the

maximum burden allowable on the CT secondaries.

CTs can be separated into two distinct groups: meter-

ing and protection or relay. A CT can have ratings for

both groups. See Figure 1 for a list of typical accuracy

classes.

The CT accuracy class is made up of three parts,

which are described in conjuction with Figure 2.

Accuracy: The first number in the CT accuracy class

is the rated ratio accuracy in percent. For example, the

ratio of a 0.3B0.1 metering-class CT is accurate within

0.3 percent as long as the CT burden rating is not ex-

ceeded. The ratio of a 2.5C100 protection-class CT is

accurate within 2.5 percent if the CT burden rating is

not exceeded.

Current-Transformer Class: The second part of the

accuracy class is a letter. The letter distinguishes for

what application the CT is rated. A CT can have dual

ratings and be used in metering or protection applica-

tions, if both ratings are listed on the nameplate.

Metering-class CTs have the letter B in the accuracy-

class rating. They are designed for maximum accuracy

from very low currents to the maximum CT rating.

These CTs are often used to record power consumption

and are the basis for electrical bills, hence the need for

accuracy. A typical metering-class CT has an accuracy

class similar to 0.3B0.1

Protection-class CTs have the letters C, H, L, or T

to indicate the protection-class rating.

• A C-rating indicates that the CT accuracy can be

calculated before construction because there is no

appreciable leakage flux in the CT design.

• A T-rating indicates that the CT can have significant

leakage flux, and the CT accuracy must be obtained

by testing in the factory.

• An H-rating indicates that the CT accuracy rating

is applicable within the entire range of secondary

currents from five to 20 times the nominal CT rating.

These are typically wound CTs.

• An L-rating indicates that the CT accuracy applies

at the maximum-rated secondary burden at 20 times

rated only. The ratio accuracy can be up to four

Figure 1a — CT Accuracy Classes and Correction Factors

Figure 1b — CT Accuracy Classes and Correction Factors

CORRECTION FACTOR LIMITS FOR CTs

ACCURACY

100% RATED CURRENT

10% RATED CURRENT

CLASS

MIN

MAX

MIN

MAX

1.2

0.998

1.012

0.976

1.024

0.6

0.994

1.006

0.988

1.012

0.3

0.997

1.003

0.994

1.006

0.5

0.995

1.005

0.995

1.005

STANDARD BURDENS FOR STANDARD 5-AMP / 60 Hz

SECONDARY CURRENT TRANSFORMERS

BURDEN

DESIGNATION

STANDARD SECONDARY BURDEN RATINGS

IMPEDANCE

OHMS

VOLT AMPS

(VA)

POWER

FACTOR

B-0.1

0.1

2.5

0.9

B-0.2

0.2

5

0.9

B-0.5

0.5

12.5

0.9

B-1

1

25

0.5

B-2

2

50

0.5

B-4

4

100

0.5

B-8

8

200

0.5

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METERING CLASS

0.3 B 0.3

ACCURACY IN PERCENT (%)

MAXIMUM BURDEN IN OHMS (0.3 Ohms)

CT CLASS – “B” = METERING

PROTECTION CLASS

2.5 C 100

ACCURACY IN PERCENT (%)

MAXIMUM BURDEN IN VOLTS @ 20* CT AMP RATING (100A)

MAXIMUM OHMS = BURDEN/100 = 100/100 = 1.0 OHMS

CT CLASS

“C” = DONUT CT WITH CALCULATED % ACCURACY

“T” = % ACCURACY TESTED IN FACTORY

“H” = ACCURACY APPLIES UP TO 20x CT NOMINAL AMPS

“L” = ACCURACY CAN VARY UP TO 4x RATING DEPENDING ON LOAD

times greater than the listed val-

ue, depending on the connected

burden and fault current. These

are typically window, bushing,

or bar-type CTs.

Protection-class CTs are not as

accurate as metering-class CTs

but are designed to operate over a

wider range of current. This wider

range is necessary to allow the

protective relay to operate at dif-

ferent fault levels. Protection-class

CTs are typically rated to operate

accurately up to 20 times the CT

rating. A typical protection-class

CT has an accuracy class similar

to 2.5C100.

Burden: The third part of the

accuracy-class rating follows the

letter and indicates the maximum

burden allowed. If the CT second-

ary-burden rating is exceeded, CT

accuracy is not guaranteed.

Metering-class CT burdens are

displayed as secondary ohms. For

example, the ratio of a 0.3B0.1 rated

CT is accurate to 0.3 percent if the

connected secondary burden im-

pedance does not exceed 0.1 ohms.

A 0.6B8 rated metering-class CT

will operate within 0.6 percent

accuracy if the secondary burden

does not exceed 8.0 ohms.

Protection-class CT burdens are

displayed as the maximum sec-

ondary volts allowable if 20 times

(100 amperes with a five-ampere

nominal CT secondary) the CT

rating were to flow through the

secondary circuit. For example,

a 2.5C100 protection-class CT is

accurate within 2.5 percent if the

secondary burden is less than 1.0

ohm (100 volts / 100 amperes).

CT Type: The four typical CT

types include window, bushing,

bar, and wound.

• Window CTs are the most com-

mon. They are constructed with

no primary winding and are in-

stalled around the primary con-

ductor. The electric field created

by current flowing through the

conductor interacts with the CT

Figure 2 — Understanding CT Ratings

core to transform the current to the appropriate secondary output.

Window CTs can be of solid or split core construction. The primary

conductor must be disconnected when installing solid window CTs.

However, split core CTs can be installed around the primary conductor

without disconnecting the primary conductor.

• Bushing CTs are window CTs specially constructed to fit around a

bushing. Usually they cannot be accessed, and their nameplates are

found on the transformer or circuit-breaker control cabinets.

• Bar-type CTs operate on the same principle of window CTs but have

a permanent bar installed as a primary conductor.

• Wound CTs have a primary and secondary winding like a normal

transformer. These CTs are rare and are usually used at very low ratios

and currents, typically in CT secondary circuits to compensate for low

currents, to match different CT ratios in summing applications, or to

isolate different CT circuits. Wound CTs have very high burdens, and

special attention to the source CT burden should be applied when

wound CTs are used.

Voltage Class: The CT voltage class determines the maximum volt-

age with which a CT can be in direct contact. For example, a 600-volt

window CT cannot be installed on or around a bare 2400-volt conductor.

However, a 600-volt window CT can be installed around a 2400-volt cable

if the CT is installed around the insulated portion of the cable and the

insulation is rated correctly.

Physical and Mechanical Condition

Verify the physical and mechanical condition of the CT. When pos-

sible, the CT should be inspected for shipping damage, incorrect bracing,

cracks, and general overall condition. The CT should be permanently

mounted and not supported by the primary conductor.

Correct Connection of Transformers

Verify the correct connection of transformers with system require-

ments. This step is often the most critical step performed. Incorrect CT

connections can cause significant problems on any polarity-sensitive

metering or protection scheme such as generator protection (IEEE 32,

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40, 67), line protection (IEEE 21, 67, 78), or differential

elements (IEEE 87). In many instances, the connection

is as important as the CT polarity. If the primary polar-

ity of a CT is reversed, the secondary wiring can also

be reversed to achieve the desired overall polarity.

Two wrongs often make a right when working with

CT connections.

Adequate Clearances

Verify that adequate clearances exist between

primary and secondary circuit wiring. Inspect the

space between the CT phases, ground, and second-

ary conductor to ensure adequate clearance. (Quick

rule-of-thumb: one inch per kilovolt plus one inch.)

Remember that line-to-line voltages apply between

phases. Also check that the secondary wiring is not

run parallel and/or close to the primary conductor

to prevent any current from being induced into the

secondary path. All wiring above the primary con-

ductor should be permanently affixed to the structure

to prevent the wiring from falling onto the primary

conductors in the future. (Sticky backs are not consid-

ered permanent.)

Grounding and Shorting Connection

Verify that all required grounding and shorting

connections provide contact. CTs are always shipped

with shorting devices installed if the burden circuit

cannot be completed at the factory. Measure the re-

sistance between both sides of the terminal block to

ground before removing the shorting device to ensure

the device is operating correctly. The measured resis-

tance should be negligible. The CTs should remain

shorted until the secondary wiring is complete and

loop-tested. Always check to ensure that the short-

ing devices are removed on in-service CTs just prior

to energization.

CT Electrical Tests

Insulation-Resistance Test

In accordance with NETA specifications, “Perform

insulation-resistance tests to ground of the CT and

wiring at 1000 volts dc. For units with solid-state com-

ponents, follow manufacturer’s recommendations.

When applicable, perform insulation-resistance

and dielectric-withstand tests on the primary wind-

ing with the secondary grounded. Use test voltages

in accordance with Tables 100.5 and 100.9 respectively

in the NETA specifications.”

When electromechanical relays were king, insula-

tion-resistance tests of CTs and wiring were an ac-

ceptable method of testing the entire circuit including

the internal relay wiring. In the age of microprocessor

relays, the risk of damage to the new, more sensitive,

relays must be weighed against the benefit of the test.

I recommend isolating the CTs from the external wir-

ing at the first terminal block or test point and apply-

ing the test voltage between the CT and ground. All

three CTs can be tested simultaneously. Investigate

any measurement below 100 megohms.

Never perform this test on bushing CTs while the

power transformer is under vacuum. Although a

perfect vacuum is a perfect insulator, the transformer

vacuum is not perfect, and there is a strong possibility

of causing a flashover inside the transformer.

Polarity Test

Perform a polarity test of each current transformer.

There are two generally accepted methods of testing a

CT’s polarity using simple meters and connections:

• DC Kick/Flick Test: This is the tried and true meth-

od for testing CT polarity, used before electricity

was discovered by some accounts. For this test, a

dc battery, a dc voltmeter or ammeter (preferably

analog), and test leads are required. This method

is a quick and easy test for polarity, but there is a

possibility that it may leave remnant flux in the

CT. Remnant flux may cause saturation when the

CT is next energized. (See C57.13.1, paragraph 4.3.)

Always perform an excitation/saturation test after

performing this test. The steps of the dc kick/flick

test are as follows:

1. Connect the positive of the voltmeter to the H1

(marked) terminal of the high-voltage side of the

CT and the negative lead to the H2 as shown in

Figure 3.

Figure 3 — DC Polarity Test Circuit Window Type CT

12V

+

-

H1

H2

X2

PRIMARY

CONDUCTOR

V

10V SCALE

12V / (10/5) = 6 V

+

-

TEST

LEAD

PRIMARY

CONDUCTOR

TEST

LEAD

X1

10:5 A CT

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Fall–Winter 2004

5

• If testing a window-type CT, run a wire through the

CT and connect your meter across the wire. Make

note of the wire and meter polarity.

• If the CT is enclosed within a breaker, close the

breaker and connect your voltmeter across the

breaker poles. Review the CT drawing carefully

and note the CT polarity and meter polarity.

• If the CT is enclosed within a generator, connect the

voltmeter across the generator windings. Review

the CT drawing carefully and note the CT polarity

and meter polarity. If the star point is not accessible,

connect the voltmeter across two generator leads

and short the remaining generator lead to one of

the generator leads under test.

• If the CT is enclosed within a transformer, connect

the voltmeter across the transformer bushings asso-

ciated with the CT and short the remaining bushing

to the nonpolarity bushing under test. Review the

CT drawing carefully and note the CT polarity and

meter polarity. (See Figure 4.)

3. Connect the negative terminal of the battery to the

nonpolarity of the CT winding under test. Momen-

tarily touch or connect the battery’s positive termi-

nal to the polarity terminal of the CT winding under

test.

4. Closely watch the needle or analog scale of the

voltmeter. It should jump in the positive direction.

This happens in a fraction of a second, so the meter

must be monitored very closely. If the voltmeter

kicks in the positive direction, the polarity marks

are correct. If it kicks in the negative direction, the

polarity marks are incorrect.

AC Voltage Method: This method digs deep into

transformer theory and can be used with any kind of

transformer. Most transformer polarities are marked

with additive polarity that allows creation of an au-

totransformer by connecting X1 and H2 or H1 and

X2 together.

This method is limited by the accuracy of the meters

and may not be reliable with unstable source voltages

(such as construction power or generator) and high

ratio CTs due to the low H side voltages induced.

To test polarity using ac voltage, a variable trans-

former and voltmeter (preferably two) are required.

Use the following steps to test for CT polarity using

the ac method, as shown in Figure 5.

Figure 4 — DC Polarity Test Circuit Delta Connected Generator

V

0.1V SCALE

+

-

TEST

LEAD

H

1

X

1

X

2

12V

+ -

X2

X1

TEST

LEAD

R

S

T

DELTA CONNECTED

GENERATOR

TEST

LEAD

2000:5 CT

12V / (2000/5) = 0.03V

Figure 5 — AC Voltage Ratio Test Circuit

H1

H2

X2

PRIMARY

CONDUCTOR

PRIMARY

CONDUCTOR

TEST

LEAD

X1

15:5 A CT

AC VARIAC = 15V

V

AC

V

RATIO TEST

VM2 = [VM1/CT ratio]

VM2 = 15V / (15/5)

VM2 = 5V

VM1

VM2

V

VM3

POLARITY TEST

VM3 = VM1 + VM2

VM3 = 15V + 5V

VM3 = 20V

2. Calculate the expected voltage using the battery

voltage and the CT ratio. (Battery voltage/CT

ratio.)

1. Connect a variable transformer (such as a Variac)

across the secondary winding of the CT.

2. Connect a voltmeter (VM1) across the secondary

CT winding and variable transformer.

3. Connect a jumper between the nonpolarity of the

H winding and the polarity of the X winding.

4. Connect a voltmeter (VM3) from the polarity mark

of the H winding to the nonpolarity mark of the X

winding.

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5. Increase the variable-transformer test voltage to a

predetermined value. Calculate the expected value.

([VM1 / CT ratio] + [VM1]). If VM3 displays the

expected result, the CT polarity markings are cor-

rect. If VM3 is less than VM1, the test connection or

the CT polarity markings are incorrect. (Note: VM1

and VM3 can be one voltmeter switching between

positions if the test voltage remains stable.)

Ratio-Verification Test

The NETA specifications state, “Perform a ratio-

verification test using the voltage or current method

in accordance with ANSI C57.13.1 (IEEE Guide for

Field-Testing of Relaying Current Transformers).” The

CT ratio test determines the CT accuracy, and the re-

sults should be compared to the accuracy class. The

easiest and most accurate CT ratio test method for

most applications is the voltage method and is the

method we will discuss here.

It may seem strange to use voltage to test a CT ratio,

but basic transformer theory applies to all transform-

ers, including CTs. One of the first transformer funda-

mentals is that the transformer ratio applies inversely

to current and voltage. A 400:5 CT ratio will convert

400 primary amperes to five secondary amperes and

convert 80 secondary volts to one primary volt. We

apply this principle to CT testing because it is easier

to locate, carry, and apply an 80-volt voltage source

than a 400-ampere current supply. The voltage method

is also more accurate because measurements can be

made directly instead of applying CTs and clip-on am-

meters that add error based on their accuracy. The

applied voltage must be well below the saturation

voltage or there will be a significant error reported.

The steps for a ratio-verification test are as follows:

1. Connect a voltage source (variable transformer) and

voltmeter (VM1) across the CT secondary, as shown

in Figure 5.

2. Connect a voltmeter (VM2) across the CT primary.

3. Apply a voltage to the CT secondary and measure

the secondary (VM1) and primary voltages (VM2)

simultaneously. Calculate the ratio between the two

voltages (VM1 / VM2).

This voltage should match the CT ratio (primary /

secondary). You can repeat the procedure above for

each tap of a multitap CT, but I prefer to treat the CT

taps as I would an autotransformer. I apply a voltage

across the maximum ratio tap and measure all remain-

ing taps to a common point, as shown in Figure 6.

Prove all tap combinations by using the information

recorded from this test.

Excitation Test

In accordance with NETA specifications, “Perform

an excitation test on transformers used for relaying

applications in accordance with ANSI C57.13.1. (IEEE

Guide for Field Testing of Relaying Current Transformers).”

During normal operation, the CT operates as a nearly

perfect machine with very small energy losses neces-

sary for CT operation. The magnetic theory involved

in transformer operation is too complex to address in

this article, but the magnetic circuit can be compared to

a normal electrical circuit: the primary winding (gen-

erator), iron core (transmission line), and secondary

winding (load). During normal CT operation the high-

side winding (generator) supplies energy through the

iron core (transmission line) to the low-side winding

(load) with small losses in the transmission line.

When a CT is saturated, the magnetic path inside

the CT operates like a short circuit on the transmis-

sion line. Almost all of the energy supplied by the

primary winding is shunted away from the secondary

winding and is used to create a magnetic field inside

the CT. Saturated CTs can be very deceiving when

used to supply protective relays as they may operate

normally at low current levels and not operate at all

during fault currents.

Some of the following conditions can cause CT

saturation:

• CT secondary burden greater than rated.

• Extremely high current flowing through the CT

(fault current).

• Current flowing through CT primaries with open-

circuit secondaries.

• DC current flowing through either winding.

The excitation test is used to prove that the CT is not

saturated and will operate within specifications at the

rated burden. It is important to remember when com-

paring test results to the burden rating that the burden

V

H1

H2

X1 X2

V

AC

VM2 = 0.5V

X3 X4 X5

X2-X3 = 100:5

X1-X2 = 200:5

X1-X3 = 300:5

X3-X4 = 400:5

X4-X5 = 500:5

X2-X4 = 600:5

X1-X4 = 800:5

X3-X5 = 900:5

X2-X5 = 1000:5

X1-X5 = 1200:5

TEST 1 =

X1-X5 TO VM2

V

TEST 2 = X2-X5

RATIO TESTS 2-4

V

VM1 = 120V

V

TEST 3 = X3-X5

V TEST 4 = X4-X5

TEST 2 = VM1 * (TEST RATIO) / (FULL RATIO)

TEST 2 = 120 * 1000 / 1200

TEST 2 = 100 V

TEST 3 = VM1 * (TEST RATIO) / (FULL RATIO)

TEST 3 = 120 * 900 / 1200

TEST 3 = 90 V

TEST 4 = VM1 * (TEST RATIO) / (FULL RATIO)

TEST 4 = 120 * 500 / 1200

TEST 4 = 50 V

RATIO TEST 1 = X1-X5 TO VM2

VM2 = VM1 / CT RATIO

VM2 = 120 / 240

VM2 = 0.5V

POLARITY TEST 1

VM3 = VM1 + VM2

VM3 = 120 + 0.5

VM3 = 120.5V

Figure 6 — Multi Ratio CT AC Ratio CT Test Connections

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Fall–Winter 2004

7

rating is a minimum value. The CT

could actually have a higher rat-

ing. This happens often in trans-

former bushing applications.

A saturation test is performed

by applying ac voltage to the CT

secondary and increasing the volt-

age in steps until the CT is in satu-

ration. The test voltage is slowly

decreased to zero to demagnetize

the CT. The test results are plot-

ted on a logarithmic graph and

evaluated based on the transition

period between normal operation

and saturation. This transition is

called the knee of the curve and

is directly related to the voltage

burden rating of the CT. Use the

following steps to perform an ex-

citation test:

1. Obtain the CT accuracy class.

2. Convert the accuracy class to a voltage burden rat-

ing. (See Part I of this article Fall 2004 issue of NETA

World.)

3. Connect the test equipment per Figure 7 to the first

CT scheduled for test.

4. Slowly increase the voltage until saturation. (We

will use 1000 milliamperes for this article, but

saturation could be higher or lower depending on

CT construction.) Watch the current and note the

voltage where the current increase beginsto increase

dramatically. Note the voltage where the current

reaches 1000 milliampere.

Figure 7 — Excitation Test Connection

H1

H2

X2

PRIMARY

CONDUCTOR

PRIMARY

CONDUCTOR

X1

AC VARIAC

V

AC

VM1

800+ VOLTS AS

PER CT RATING

A AM1

TYPICAL MAX = 1000mA

CT RATING

2.5C800

5. Slowly decrease the voltage to zero.

6. Determine your test voltages using four equal steps

to the first-noted voltage and six equal steps be-

tween the first-noted voltage and the second-noted

voltage at 1.0 amperes.

7. Repeat the test using the voltage points determined

in Steps 4 and 6 and record the milliamperes at each

step. Never decrease the voltage until the maximum

test voltage is reached. You must either skip the

voltage step or restart the test from 0 volts if the

voltage must be decreased to record a result.

8. Slowly decrease the voltage to zero volts. If the

voltage is turned off for any reason before the

test is complete, the CT may remain saturated.

To desaturate the CT, increase the voltage to 1000

milliamperes (saturation) and slowly decrease the

voltage to zero.

9. Graph the results on a log-log graph and compare

to the manufacturer’s supplied results. See Figure

8 to help evaluate the results.

Manufacturer’s results are often typical for a class

of CT, although each CT may not have been tested.

Because every CT has different operating character-

istics, the results may not be exactly the same. Look

for similarities and glaringly obvious differences

when evaluating the results. If manufacturer’s re-

sults are not available, the knee of the curve must

be higher than the rated burden voltage.

10. Repeat the test using the same test voltages for all

CTs of the same rating. Start from the beginning

for CTs with another rating.

Determining Total Burden

Measure current circuit burdens at transformer

terminals and determine the total burden. This test

is very important and often missed in today’s world

Figure 8 — Example Excitation Test Results

CT SATURATION CURVE

10

100

1000

10

100

1000

10000

CURRENT (milliAmps)

V

O

L

T

S

(V

)

PHASE A

PHASE B

PHASE C

NORMAL OPERATING REGION WITH A

LINEAR RELATIONSHIP BETWEEN

VOLTAGE AND CURRENT

"KNEE" TRANSITION POINT BETWEEN

NORMAL OPERATION AND SATURATED

REGION - MUST BE ABOVE CT BURDEN RATING

CT BURDEN

RATING "L400"

CT SATURATION

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NETA WORLD

of fast track projects where the testing is broken into

stages or multiple testing companies are used in dif-

ferent parts of the project. This test is the final-proof

test used to ensure that:

• CTs are not energized with shorting devices in-

stalled if used for metering or protection.

• CTs are not left open circuit if not used.

• CTs are connected with a single ground point.

• CT burden ratings are not exceeded.

• All connections are tight.

There are many ways of performing this test, each

with its own merits and pitfalls, but this article is fo-

cused on the basics and will only cover the voltage-

drop method. It is time-consuming but only requires

a voltage/current source and a voltmeter. The test

applies five amperes through the secondary circuit

with a known ground reference while measuring the

voltage drop at each point of the circuit to ground.

Ohm’s law plus the measure of the voltage drop at

the source will give us the burden impedance. Analyz-

ing the voltage-drop patterns throughout the circuit

confirms the wiring is correct.

After all CT testing and secondary circuit wiring is

complete, follow the next steps to perform a second-

ary-loop test via the voltage-drop method.

1. Remove the CT secondary shorting devices and re-

move the ground from the circuit. Every CT circuit

should be grounded at one point only and have its

own path to ground per IEEE standard C57.13.2.

Many manufacturers incorrectly daisy chain all CT

grounds together contrary to this standard.

2. Connect an ac source between the CT secondary

feeder as close to the CT as possible and ground.

Slowly increase the voltage or current dial. A lamp-

ing kit may be used as the source. (A lamping kit is

a 120 volt ac source with a light bulb in series.) No

current should flow and light bulb remains off. If

current flows, a second ground exists somewhere

in the circuit which must be permanently removed.

Investigate possible sources of ground and repeat

this step until no current flows.

3. Connect a temporary ground between the neutral

of the test source and the CT star point. Increase the

source voltage until five amperes flow in the circuit.

If no current flows, the ground is incorrectly applied

or the circuit is incomplete.

4. Measure the voltage between the first CT circuit

terminal and ground. Multiply this number by the

measured current to obtain the voltampere burden

(VA = Volts * Amperes). Divide the voltage by the

current to determine the burden in ohms (Ohms =

Volts / Amperes). Check the CT rating to ensure

the burden is less than the nameplate value.

5. Measure the voltage between ground and every

connection. The voltage should drop incrementally

as you work through the circuit. Any increase in

voltage should be immediately investigated. The

most common problems found are:

• Unstable source voltages.

• Reverse polarity connections.

• Assumptions about what the terminal point

should be instead of using drawing references.

• Assumptions about where the terminal point is

instead of reading the designation.

6. Repeat Steps 2 through 5 for the other two CT

phases. The measured voltage patterns should be

similar to the other two phases in the circuit if they

are identical. Drastic deviations from the voltage

pattern should be investigated for loose connections

or shorts (e.g., single copper strand accidentally

connected between terminals). Additional devices

in one phase will cause higher voltages, but the

difference between devices should be consistent

between phases.

7. After all testing has been satisfactorily completed,

turn the ac source off and remove all test leads.

Reconnect the CT ground and check with an ohm-

meter after it is installed. Record the circuit burdens

on your test sheet.

Conclusion

Although the sophisticated test equipment of

modern test technicians makes all of our lives easier,

we often forget that the technicians before us were

frequently limited to a lantern battery, variable trans-

former, multimeter, and their imaginations. We can

use their example to complete the job when our test

set malfunctions or is damaged.

Reference

NETA 2001 Maintenance Testing Specifications for

Electrical Power Distribution Equipment and Systems

Chris Werstiuk graduated from the Electrical Engineering Tech-

nology program at the Northern Alberta Institute of Technology

and is a Registered Engineering Technologist and Journeyman

Power System Electrician. He has performed low-, medium-, and

high-voltage acceptance testing, commissioning, and maintenance

testing at various locations throughout the Americas. His experi-

ence is varied, and he is comfortable in mining, utility, generat-

ing, distribution, production, institutional, plant, pipeline, and

commercial environments. Chris is currently a Project Manager

for Valence USA, a full service field service and electrical training

organization.