BS 171:1970

Co-operating organizations
Foreword
Specification
Section one. Scope and service conditions
1 Scope
2 Service conditions
3 Provision for unusual service conditions
Section two. Definitions
4 Definitions
Section three. Rating
5 General
6 Rated power
7 Preferred values of rated power
Section four. Tappings
8 General
9 Off-circuit tappings
10 Principal tapping
11 Categories of voltage variation
12 Specification of voltage conditions
13 Tapping range
14 Tapping quantities
Section five. Identification according to cooling method
15 Identification symbols
16 Arrangement of symbols
Section six. Limits of temperature rise
17 Temperature rise limits
18 Reduced temperature rises for transformers designed
    for high cooling medium temperatures
19 Reduced temperature rises for transformers designed
    for high altitudes
Section seven. Insulation levels
20 General
21 Windings with graded insulation
22 Transformers for use at high altitudes
Section eight. Ability to withstand short circuit
23 General
24 Overcurrent conditions
25 Mechanical requirements
26 Thermal requirements
27 Transformers directly associated with other
    apparatus
28 Special transformers
29 Short-circuit tests
30 Auxiliary windings
Section nine. Rating plates
31 Rating plates
Section ten. Vector diagrams, connections and terminals
32 Connections of phase-windings
33 Phase displacement between sets of windings;
    marking and position of terminals
Section eleven. Tolerances
34 Tolerances
Section twelve. Tests
35 General requirements for type, routine and special
    tests
36 Measurement of winding resistance (routine test)
37 Voltage ratio measurement and check of polarity or
    vector-group symbol (routine test)
38 Measurement of impedance voltages (routine test)
39 Measurement of load loss (routine test)
40 Measurement of no-load loss and current (routine
    test)
41 Test of temperature rise (type test)
42 Application of dielectric tests
43 Induced overvoltage withstand test (routine test)
44 Separate source voltage-withstand test (routine
    test)
45 Full wave impulse-voltage withstand test including
    chopped waves (special test)
46 Impulse-voltage withstand test including chopped
    waves (special test)
47 Subsequent dielectric tests
48 Measurement of zero phase-sequence impedance
    (special test)
Section thirteen. Insulating liquids
49 Oil
50 Synthetic insulating liquid
Section fourteen. Fittings
51 Provision of fittings
Appendices
A Information required with enquiry and order
B Selection of insulation levels
C Calculation of voltage drop (or rise) for a
    specified load condition
D Transformer connections, winding designations,
    terminal markings and vector diagrams
E Parallel operation of 3-phase transformers
F Examples of specifications for transformers with
    voltage variation facilities
G Typical connection diagrams for induced voltage
    tests on windings with graded insulation
H Fittings
J Supplementary information concerning high-voltage
    tests
K Impulse-voltage testing of low-voltage windings
Tables
1 Phase factors
2 Preferred values of rated power for 3-phase
    transformers
3 Letter symbols
4 Order of symbols
5 Temperature rise limits for dry-type transformers
6 Temperature rise limits for oil-immersed type
    transformers
7 Insulation levels for windings and connected parts
    not designed for impulse-voltage tests
8 Insulation levels for windings and connected parts
    designed for impulse-voltage tests
9 Categories of graded insulation
10 Insulation levels for the neutral ends of windings
    of transformers having graded insulation
11 Overcurrent limits for transformers with two
    windings
12 Maximum permissible average winding temperatures
    (T2)
13 Values of factor a
14 Tolerances
15 Reference temperatures
16 Correction factors
17 Fittings for oil-immersed type transformers
18 Additional fittings for oil-immersed type
    transformers
19 High-voltage tests at reduced levels
Figures
1 The use of vector-group symbols
2 Method for determining final temperature rise of
       oil
2A Method for determining temperature of winding at
       shut-down
3 Designation of connections of 3-phase
       transformers by vector-group symbols
3A Example of three single-phase transformers
       connected to form a 3-phase bank
3B-3J Examples of designating letters for windings and
       terminals
3K Marking and relative position of terminals for
       two-winding transformers
3L-3P Vector diagrams for 3-phase transformers
3Q Vector diagrams for single, 2 and 3/2-phase
       transformers
3R-3T Terminal marking plates
4-5A Parallel operation of transformers
6-13 Typical connection diagrams for induced voltage
       tests on windings with graded insulation
14 Recommended position of fittings

Performance and test requirements for 1 kV A single-phase and 2 kV A polyphase: no upper limits to rated power or voltage. May also be applied as appropriate to special transformers not at present covered by separate British Standards.