BS 3938:1973

Co-operating organizations
Foreword
Specification
1 General
1.1 Scope
1.2 Definitions
2 Requirements for all current transformers
2.1 Service conditions
2.2 Rating and performance requirements applicable
        to all current transformers
2.3 General test requirements
2.4 Requirements for routine tests
2.5 Requirements for type tests
2.6 Commissioning tests on new current transformers
2.7 Marking
3 Additional requirements for measuring current
        transformers
3.1 General
3.2 Accuracy requirements
3.3 Tests for accuracy
3.4 Marking
4 Additional requirements for protective current
        transformers
4.1 General
4.2 Accuracy requirements
4.3 Special purpose current transformers
4.4 Tests
4.5 Marking
Appendices
A Service conditions
B Guidance in the application of current
        transformers
C Short-time current rating of current
        transformers
D Temperature measurements
E Maximum V A output of bar-primary and bushing
        current transformers
F Information required by the manufacturer
G Use of current transformers for the dual purpose
        of measurement and protection
H Current transformer test windings
J Composite error
K Distinction between low-reactance and high-
        reactance current tranformers
L Determination of turns ratio
Tables
1 Limits of temperature rise of windings
2A Insulation test levels for rated highest
        equipment voltages up to and including 72.5 kV
2B Insulation test levels for rated highest
        equipment voltages of 100 kV and above
3 Permissible current densities
4 Power-frequency and d.c. test voltages on
        primary windings (after erection on site)
5 Power-frequency voltage tests of durations
        exceeding one minute (after erection on site)
6 Limits of error for accuracy Classes 0.1 to 1
7 Limits of error for accuracy Class 3 and Class 5
8 Limits of error for accuracy Class 5P and Class
        10P
9 Selection of class of accuracy of measuring
        current transformers
10A Maximum short-time factors obtainable
        economically, corresponding to rated output,
        accuracy class, accuracy limit factor and rated
        time for wound-primary measuring current
        transformers
10B Maximum short-time factors obtainable
        economically, corresponding to rated output,
        accuracy class, accuracy limit factor and rated
        time for wound-primary protective current
        transformers
11 Relationship between normal practical output and
        rated primary current of bar-primary and bushing
        current transformers
12 Maximum duration of current for transformer X
Figures
1 Single ratio transformer
2 Transformer with an intermediate tapping on
        secondary winding
3 Transformer with primary winding in two sections
        intended for connections either in series or in
        parallel
4 Transformer with two secondary windings each
        with its own magnetic core. (Two alternative
        markings for the secondary terminals)
5 Transformer with an intermediate tapping on
        secondary winding and incorporating test taps
6 - 10 Diagrams for determination of composite error
11 Equivalent circuit for the determination of
        turns ratio (current transformers having fewer
        than approximately 1000 turns)
12 Equivalent circuit for the determination of
        turns ratio (current transformers having more
        than approximately 1000 turns)

Current transformers for frequencies 14 Hz to 100 Hz for use with electrical measuring instruments and electrical protective devices. Ratings; accuracy; tests; rating-plate and terminal markings. Appendices on guidance in application of current transformers: short-time rating; maximum V A output of bar-primary and bushing current transformers; use of current transformers for dual purpose of measurement and protection, composite error, high and low reactance current transformers.