Guide to Selection of Protection Systems

Guide to Selection of protection systems, of which the current transformers constitute a vital part, can be complex. Our suggested rating for these applications should be treated with caution as they may be subjected to variations due to relay characteristics or to components of the scheme. Relay manufacturer's recommendation should be followed.

General Type of Relay

Protection System

Typical C.T. Requirement

Burden (VA)

Class

Accuracy Limit Factor (ALF)

Magnetic Trips

Overcurrent

2.5 - 5

10P

5

Magnetic O/L with dashspot

Motor overcurrent with time relay

5

10P

5 - 10

Some low consumption thermal types

Motor overcurrent with time relay

2.5

10P

10 to 15

Thermal

Motor overcurrent with time delay

7.5

10P

10 to 15

Inverse Definite Min. Time relays (I.D.M.T.)

Overcurrent

15

10P

10 to 15

I.D.M.T. Earth Fault Relay

Unrestricted earth fault with approx. time grading

15

10P

10

I.D.M.T. Earth Fault Relays

Unrestricted earth fault where phase fault stability or accurate time grading required

15

5P

10

Burden Guide for Measuring Instrument

VA

Instrument

0.5

Short scale moving iron ammeters

0.75 to 1.5

240o scale moving iron ammeters

0.1 to 1

Rectified moving iron ammeters

1 to 1.25

Watt/VAr/phase angle meters

2 to 4

Recording ammeters

2 to 3.5

Maximum Demand Indicators

3 to 3.5

Combined MDI & MI ammeters

0.5 to 4

Paladin transducer

0.5 to 4

Protector modules

5 to 10

Electronic control system

Application

Class of Accuracy

As a standard for testing other current transformers

Better than 0.1

Precision testing

0.1

Precision metering

0.1 or 0.2

Tarif metering (bulk supplies)

0.2

Tarif metering (general), transducers, test equipment, control systems

0.5

Watt/VAr/Phase Angle meters, recording meters, protection devices

1

Industrial ammeters, maximum demand indicators

1 to 3

Approximate measurements

5

Secondary Lead Burden

The impedance of the pilot wire between the CT and Relay/Instrument will add to the total burden of the measurement of protection circuit. The impedance and hence burden can be significant for long pilot wire run and must be taken into account. This is particularly critical for class 'X' CTs. It may be advisable to use 1A secondary for extremely long pilot wire run. The chart shows the approximate additional burden that must be added to the total CT burden for various sizes of pilot wires at varying distances.

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