Instrument Transformers

A transformer intended to supply measuring instruments, meters, relays and other similar apparatus is called an instrument transformer. These are, according to the quantity measured / monitored, further classified as:


It is intended for measurement/protection purpose, designed to have its primary connected in series with the circuit carrying current to be measured/protected. The terms associated with the CT are:

  1. BURDEN: Burden of a CT is given in terms of volt amperes that they can carry at rated voltage and frequency without exceeding the specified temperature limitations and accuracy limits.
  2. RATED SHORT TIME CURRENT: It is the rms value of the primary current which a CT is capable of carrying for the rated time without getting damaged by the resultant thermal and dynamic effects due to any fault in the system.
  3. RATED INSULATION LEVEL: It is the capability of a CT of withstanding the dielectric stresses. This is a combination of both power frequency and impulse voltages (peak).
  4. RATIO ERROR: The error which a transformer introduces into the measurement of a current and which arises from the fact that the actual transformation ratio is not equal to the rated transformation ratio.
  5. PHASE DISPLACEMENT: It is the difference in phase between the primary and secondary current vectors, the direction of the vectors being so chosen that the angle is zero for a perfect transformer.
  6. ACCURACY CLASS: It is a classification assigned to CTs, the error of which remains within specified limits under prescribed condition of use.
  7. RATED ACCURACY LIMIT PRIMARY CURRENT: It is the value of current assigned by the manufacturer as the highest primary current which the transformer will comply with the appropriate limits of error under specified condition.
  8. ACCURACY LIMIT FACTOR: It is the ratio of rated accuracy limit primary current to the rated primary current. The standard accuracy limit factors are 5, 10, 15, 20 and 30.

CT FOR MEASURING PURPOSES: CT is required to be accurate within the normal working range say up to 125 % of rated current. For over current conditions beyond this, accuracy is unnecessary and indeed it is best that at higher values saturation should occur, since this tends to relieve the connected instruments of the stresses resulting from heavy over current. Also the choice of a CT should be made depending upon its rated output in relation to the burden connected to its secondary winding and on the degree of accuracy required. A caution is required in choosing a CT with a rated output considerably in excess of the required output since it can result in increased errors. Thus it is desirable that the rated output should be near to (but not less than ) the actual output at which it is to operate. Here output means the load due to the connected meters as well as connecting leads. If the connecting leads are longer the secondary current of CT of 1 A may be more suitable (however it is not recommended, specially when ratio is very high, as it can induce very high voltage if the CT is left open circuited under load which may lead to the destruction of inter-turn and inter-layer insulation along with danger to operating personnel). Typical values of VA burden imposed by meters are given below:

i) Ammeter including recording type3
ii) Current coils of watt-meter, p.f. meter, kW⋅h meters, kvar meters including recording type5
iii) Copper connecting leads0.018 l I² / a

where l is length in meters, a is cross section in mm² and I is rated secondary current of CT.

Thus the copper connecting lead of 1.5 mm² cross section area of 5 m handling current of 5 A shall impose a burden of 1.5 VA.

Accuracy class is an important factor while selecting a CT. It shall be uneconomical to go for a higher class of accuracy than that is actually needed. Following are the general guidelines for selecting a proper accuracy class:

i) For precision testing as a substandard in laboratory0.1
ii) For high accuracy indicating instruments and substandard work0.2
iii) For precision industrial metering0.5
iv) For commercial and industrial metering1.0
v) Where approximate values are acceptable3.0

Standard values of primary current of a CT are 10, 12.5, 15, 20, 25, 30, 40, 50, 60, 75, 80, 100 and their multiples.

CT FOR PROTECTION PURPOSES: The accuracy class for protection CT is as below:

5 P± 1± 605
10 P± 3--10
15 P± 5--15

The class of accuracy required for protective CT depends upon the particular application. For instantaneous over current relays and trip coils, class 15P protective CT is generally sufficiently accurate. Rated accuracy limit factor 0f 5 should be enough. However when the instantaneous over current relays are set to operate for high values of over current say 5 to 15 times the rated primary currents, the accuracy limit factor should have at least the value of setting used.

For IDMT relays, class 10P CT is preferred for system where discrimination is obtained be graded time lag, but when close discrimination is not required, class 15P may be preferred.

As a general guideline, the product of rated burden and the rated accuracy limit factor should approach 150, provided that the earth fault relay setting is not less than 20 % of rated secondary current of associated CT. Class 5P CTs are preferred where accurate time grading and stability are desired.

MINIMUM EARTH FAULT SETTING: PRIMARY CURRENT’s ROLE: A Common fallacy is to specify leakage settings as a percentage of normal full load current; expecting for an instance, a setting of 10 % on a 100 kVA power transformer installed on a system having a fault capacity of 500 MVA at 11 kV. In fact, ratios much below 100/5 on switch gears of high rupturing capacity become an embarrassment, obviously it is not always possible to plan a system having 100 A as a minimum normal load current, but much can be done, if the difficulties are realized at the outset.

As for a 100 kVA transformer, normal full load current is 5.25 A. Let the min ratio selected is 20/5. Keeping in view the heating and mechanical limitations, let the number of primary turns allowed be 5. Thus total primary ampere-turns would be 100. A suitable relay sufficiently sensitive would consume roughly 0.33 VA at a setting value of 1 secondary amperes. The voltage required to operate this relay would be 0.33V and the impedance 0.33 Ω at a pf of 0.14. A suitable annular core could have a cross section area of 12.5 cm² a mean diameter of 10.16 cm and a mean circumference length of about 32 cm. So the induced voltage can be given as:

  e = 4.44 × ϕ × f × T = 222 × B × a × T and so for reasonable no of secondary turns of 18

  B = e / (222 × a × T) = 0.33 / (222 × 12.5 / (100 × 100) × 18) = 0.0660 Tesla

  For steel, H = 19.9 AT/m, so magnetizing current im = 19.9 × 0.8 × 32 / (100 × 18) = 0.338 A and the secondary current

  is = 1.0 + 3 × 0.338 = 2.014 A and primary current = 4 × 2.014 = 8.056 A, which is about 40 % of the nominal 20 A primary current. Thus a setting of 40 % of the nominal 20 A is about the minimum which can be secured.

Now let the CT ratio for earth leakage protection be 60/5, then ampere turns = 300 AT. For a secondary turns of about 56, for the same core B = 0.0213 Tesla and proceeding in similar fashion we get a primary current of 12.8 A, which is 21.3 % of 60 A, therefore setting in terms of primary current can be made reasonably close to 20 %.

As the normal full load current is only 5.35 A, this shows that in the first case, there is no justification for providing leakage protection.


  1. Rated voltage, type of supply and earthing conditions.
  2. Insulation level.
  3. Frequency.
  4. Transformation ratio.
  5. Rated output.
  6. Class of accuracy.
  7. Short time thermal current and its duration.
  8. Accuracy limit factor and any other requirement for CT for protection.
  9. Service condition.
  10. Special feature such as limiting dimensions.


  1. 30/5P10 : 30 VA burden, 5P accuracy class and 10 is standard accuracy limit factor.
  2. 30/1: 30 VA burden and class 1 accuracy.


The common terms in use specially for PT are:

THERMAL LIMIT OUTPUT: The value of the apparent power referred to rated voltage which can be taken from a secondary winding, at rated primary voltage applied, without exceeding the limits of temperature rise specified.

HIGHEST SYSTEM VOLTAGE: The highest rms line to line voltage which can be sustained under normal operating conditions at any time and at any point on the system. It excludes temporary voltage variation due to fault condition and the sudden disconnection of large loads.

RATED INSULATION LEVEL: It is the capability of the transformer to withstand die-electric stresses whether power frequency or impulse.

(All voltage values are rms values in kV)
Nominal System VoltageHighest System VoltagePower frequency withstand voltageLightning Impulse Withstand Voltage
Upto 0.600.663.0--

(The choice between list1 and 2 should be made by considering the degree of exposure to lightening and switching over-voltages, the type of neutral earthing and over-voltage protection.)

RATED OUTPUT: The standard values of rated output (in VA, per phase VA in case of 3 phase) at a p.f. of 0.8 are 10, 15, 25, 30, 50, 75, 100, 150, 200, 300, 400 and 500.

MEASURING PT: The selection of PT for measuring purpose can be made as below. Here voltage and phase error are at 0.9 to 1.1 times rated primary voltage, 0.25 to 1.0 times rated output at 0.8 pf lag.

Accuracy classVoltage error (%)Phase error (min)Application
0.1± 0.1± 5For precision testing or as substandard for testing laboratory
0.2± 0.2± 10For laboratory and test work in conjunction with high accuracy indicating instruments/meters and also substandard for testing industrial PT
0.5± 0.5± 20For precision industrial metering and for use with substandard indicating watt-meters
1.0± 1.0± 40For commercial and industrial metering and for use with indicating and graphic watt-meters and voltmeters
3.0± 3.0± 120For purposes where phase angle is less important e.g. a voltmeter


Accuracy classVoltage error (%)Phase error (min)Application
3P± 3.0± 120Directional over current relay, reverse power relay, directional distance protection
6P± 6.0± 240Under voltage/Over voltage/Over current relays

The burden in VA required for various instruments are as below:

1. Voltmeter, voltage coil of watt-meter, under / over volt relay, power factor meter5 VA
2. Voltage coil of kW·h, kvar meters, frequency meters and recording pf and watt-meters7.5 VA
3. Voltage coil of electromagnetic relays3 – 10 VA
4. Voltage coil of synchroscope15 VA
5. Static relays0.02 - 0.2 VA

The VA rating of the PT should be greater than the burden since the accuracy of PT at very low burdens (25 %) is not guaranteed.

Information to be given for ordering a PT: 

  1. Type of transformer viz single/three phase, indoor/outdoor, resin cast/oil immersed etc.
  2. Highest system voltage, type of supply and earthing conditions.
  3. Rated insulation level.
  4. Frequency.
  5. Rated transformation ratio.
  6. Rated output and corresponding accuracy class for measuring/protective PT.
  7. Rated voltage factor and duration (if earthing condition is not specified).
  8. Service condition.
  9. Specific features such as limiting dimensions.


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