IEC 60027-7:2010

IEC 60027-7 ®
Edition 1.0 2010-05
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Letter symbols to be used in electrical technology –
Part 7: Power generation, transmission, and distribution

Symboles littéraux à utiliser en électrotechnique –
Partie 7: Production, transport et distribution de l’énergie électrique

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IEC 60027-7 ®
Edition 1.0 2010-05
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Letter symbols to be used in electrical technology –
Part 7: Power generation, transmission, and distribution

Symboles littéraux à utiliser en électrotechnique –
Partie 7: Production, transport et distribution de l’énergie électrique

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
U
CODE PRIX
ICS 01.060 ISBN 978-2-88910-921-0
– 2 – 60027-7 © IEC:2010
CONTENTS
FOREWORD.3
1 Scope.5
2 Normative references .5
3 Letter symbols for AC, three-phase AC, and other network quantities.7
4 Letter symbols for space and time .17
5 Letter symbols for numerical values and ratios of quantities .20
6 Subscripts and superscripts.24
6.1 Subscripts for natural quantities and components in three-phase AC systems.24
6.2 Subscripts for operating conditions.25
6.3 Subscripts for electrical equipment.25
6.4 Subscripts for locations, reference points, and fault locations.27
6.5 Superscripts .28
6.6 Multiple subscripts and their succession.28
Bibliography.29

60027-7 © IEC:2010 – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
LETTER SYMBOLS TO BE USED
IN ELECTRICAL TECHNOLOGY –
Part 7: Power generation, transmission, and distribution

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60027-7 has been prepared by IEC technical committee 25:
Quantities and units.
The text of this standard is based on the following documents:
CDV Report on voting
25/391/CDV 25/406/RVC
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

– 4 – 60027-7 © IEC:2010
A list of all parts of the IEC 60027 series, under the general title Letter symbols to be used in
electrical technology can be found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
60027-7 © IEC:2010 – 5 –
LETTER SYMBOLS TO BE USED
IN ELECTRICAL TECHNOLOGY –
Part 7: Power generation, transmission, and distribution

1 Scope
This part of IEC 60027 is applicable to generation, transmission, and distribution of electric
energy. It gives names and letter symbols for quantities and units. In addition, rules for
multiple subscripts and their succession are given.
This part of IEC 60027 is an addition to IEC 60027-1. Therefore letter symbols already given
in IEC 60027-1 are repeated only if they have a special meaning in the field of power
generation, transmission, and distribution or if they are used in this field with special
subscripts.
Guidance on the use of capital and lower case letters, is given in IEC 60027-1, 2.1, and
guidance on the representation of complex quantities,is given in IEC 60027-1, 1.6. Therefore
in many cases only U is given instead of U, U = U or u.
2 Normative references
The following referenced documents are indispensable for the application of this document.
For dated references, only the edition cited applies. For undated references, the latest edition
of the referenced document (including any amendments) applies.
IEC 60027-1:1992, Letter symbols to be used in electrical technology – Part 1: General
Amendment 1:1997
Amendment 2:2005
IEC 60027-2:2005, Letter symbols to be used in electrical technology – Part 2: Telecom-
munications and electronics
IEC 60038:2009, IEC standard voltages
IEC 60050-121:1998, International Electrotechnical Vocabulary – Part 121: Electromagne-
tism
Amendment 1 (2002)
IEC 60050-131:2002, International Electrotechnical Vocabulary – Part 131: Circuit theory
Amendment 1 (2008)
IEC 60050-141:2004, International Electrotechnical Vocabulary – Part 141: Polyphase
systems and circuits
IEC 60050-151:2001, International Electrotechnical Vocabulary – Part 151: Electrical and
magnetic devices
IEC 60050-195:1998, International Electrotechnical Vocabulary – Part 195: Earthing and
protection against electric shock
Amendment 1 (1998)
– 6 – 60027-7 © IEC:2010
IEC 60050-411:1996, International Electrotechnical Vocabulary – Chapter 411: Rotating
machines
Amendment 1 (2007)
IEC 60050-421:1990, International Electrotechnical Vocabulary – Chapter 421: Power trans-
formers and reactors
IEC 60050-441:1984, International Electrotechnical Vocabulary – Chapter 441: Switchgear,
controlgear and fuses
Amendment 1 (2000)
IEC 60050-442:1998, International Electrotechnical Vocabulary – Part 442: Electrical
accessories
IEC 60050-448:1995, International Electrotechnical Vocabulary – Chapter 448: Power
system protection
IEC 60050-466:1990, International Electrotechnical Vocabulary – Chapter 466: Overhead
lines
IEC 60050-601:1985, International Electrotechnical Vocabulary – Chapter 601: Generation,
transmission and distribution of electricity – General
Amendment 1 (1998)
IEC 60050-603:1986, International Electrotechnical Vocabulary – Chapter 603: Generation,
transmission and distribution of electricity – Power system planning and management
Amendment 1 (1998)
IEC 60050-604:1987, International Electrotechnical Vocabulary – Chapter 604: Generation,
transmission and distribution of electricity – Operation
Amendment 1 (1998)
IEC 60050-811:1991, International Electrotechnical Vocabulary – Chapter 811: Electric
traction
IEC 60909-0:2001, Short-circuit currents in three-phase AC systems – Part 0: Calculation of
currents
IEC/TR 60909-1:2002, Short-circuit currents in three-phase AC systems – Part 1: Factors for
the calculation of short-circuit currents according to IEC 60909-0
IEC/TR 60909-2:2008, Short-circuit currents in three-phase AC systems – Part 2: Data of
electrical equipment for short-circuit current calculations
IEC 60909-3:2003, Short-circuit currents in three-phase AC systems – Part 3:Currents
during two separate simultaneous line-to-earth short circuits and partial short-circuit currents
flowing through earth
IEC 62428:2008, Electric power engineering – Modal components in three-phase a.c.
systems – Quantities and transformations
IEC 80000-6:2008, Quantities and units – Part 6: Electromagnetism

60027-7 © IEC:2010                  – 7 –

3 Letter symbols for AC, three-phase AC, and other network quantities
Quantities Units
Item Unit, coherent
IEV and/or
Chief Reserve
number with the SI
IEC Name of quantity Remarks Remarks
symbol symbol
number
Name Symbol
101 line-to-earth capacitance of a line
i = 1, 2, 3 in three-phase AC networks
C C farad F
Li LiE
102 line-to-line capacitance of a line i, k = 1, 2, 3 with i ≠ k in three-phase AC
C C
farad F
LiLk Lik
networks
103 131-14-29 hybrid matrix Names and symbols for the elements are one, ohm,
1,Ω, S
H
given in IEC 60027-2.
60027-2 siemens
104 electric current
The general symbol I is used in case of three-
ampere A
I
phase AC networks if the three currents are
equal or nearly equal.
105 441-17-06 prospective breaking current, Current of the first opening pole of a switching
device (circuit breaker) or a fuse.
I I
441-17-07 breaking current ampere A

b a
60909-0
106 capacitive charging current
U
n
IC
= ω where ω is the angular
C
I ampere A
C
frequency, C is the positive-sequence
capacity, and U is the nominal voltage of the
n
three-phase AC line.
107 capacitive earth-fault current
I is the capacitive single line-to-earth fault
Ce
I ampere A
Ce current in a network with isolated neutral
(IEV 601-02-24).
108 131-11-22 direct current average value, for instance in the case of a
direct current link
I I
ampere A
d DC
For the qualifier DC, see IEC 60050-151,
151-15-02
109 442-01-23 earth fault current See items 107 and 121.
I
ampere A
e
110 field (excitation) current Current in the field winding of a machine.
I I ampere A
F f
– 8 –             60027-7 © IEC:2010

Quantities Units
Item Unit, coherent
IEV and/or
Chief Reserve
number with the SI
IEC Name of quantity Remarks Remarks
symbol symbol
number
Name Symbol
111 no-load field (excitation) current
I I
ampere A
0F f0
112 60909-0 steady-state short-circuit current The steady state short-circuit current in the
at a short-circuit location r.m.s. value of a three-phase short-circuit
current at a short-circuit location in a network,
which remains after the decay of all transient
I I ampere A
phenomena.
k k3
Short-circuit currents with subscript k in solidly
earthed or impedance earthed networks
(IEV 601-02-25, IEV 601-02-26).
113 60909-1 transient short-circuit current at a The transient short circuit current is the r.m.s.
short-circuit location value of a three-phase short-circuit current at
' '
ampere A
I I
k k3
a short-circuit location after the decay of the
subtransient short-circuit current.
114 60909-0 subtransient short-circuit current The initial symmetrical short-circuit current at
at a short-circuit location a short-circuit location is the r.m.s. value of
" "
ampere A
I I
k k3
the AC component of a prospective three-
phase short-circuit current.
115 121-11-13 line conductor current
i = 1, 2, 3 in three-phase AC networks.
I
ampere A
Li
MOD
116 411-48-16 locked rotor current
I is the highest r.m.s. current of an
LR
asynchronous motor with locked rotor at the
60909-0 I I ampere A
LR an
most unfavourable postion fed with rated
voltage and frequency of the rotor.
117 magnetizing current I is the magnetizing current of a machine, a
m
I I ampere A
m μ reactor, a transformer, etc. IEC 60027-1,
Table 6, Subscript m, mag.
118 60909-0 peak short-circuit current at a The peak short circuit current is the maximum
short-circuit location i i possible instantaneous value of the ampere A

p p3
prospective three-phase short-circuit current.
119 thermal continuous permissible
I I
ampere A
per d
current
60027-7 © IEC:2010                  – 9 –

Quantities Units
Item Unit, coherent
IEV and/or
Chief Reserve
number with the SI
IEC Name of quantity Remarks Remarks
symbol symbol
number
Name Symbol
120 411-48-23 peak short-circuit current of a Peak value reached by the current in the
MOD generator armature winding within a half cycle after the
winding has been suddenly short circuited,
i ampere A
pG
when the conditions are such that the initial
value of any aperiodic component of current is
a maximum.
121 421-04-05 rated current Rated current, given from the manufacturer of
electrical equipment, for a generator, motor,
I ampere A
r
transformer, reactor, etc. If necessary with an
additional subscript from 6.3.
122 rated current of a current
I ampere A
rCT
transformer
123 411-54-07 rated field (excitation) current
I I ampere A
rF fr
124 60909-0 rated current of a generator
I
ampere A
rG
125 earth-fault residual current Current at the fault location of a resonant
I I ampere A
rsd Rest
earthed network (see IEV 601-02-27)
126 60909-0 rated current at the high-voltage
Use I and I , respectively, at the
rTMV rTLV
I ampere A
side of a transformer rTHV
medium-voltage and the low-voltage side.
127 448-11-30 inrush current Inrush current of a transformer.
I
ampere A
rush
128 rated current of a winding Transformer winding at the high-voltage side
(W) or the low-voltage side (w).
I I ampere A
rW, rw
In the case of three-winding transformers, see
item 253.
– 10 –             60027-7 © IEC:2010

Quantities Units
Item Unit, coherent
IEV and/or
Chief Reserve
number with the SI
IEC Name of quantity Remarks Remarks
symbol symbol
number
Name Symbol
129 60909-0 thermal equivalent short-circuit r.m.s. value of a current having the same
current thermal effect and the same duration as the
actual short-circuit current, which may contain
a DC component and may subside in time,
"
I=+Im n ,
th k
I ampere A
′′
where I is the subtransient short-circuit
th
k
current (item 113), m is the factor for the heat
(thermal) effect of the DC component in a
short-circuit current (item 229), and n the
factor for the (thermal) effect of the AC
component in a short-circuit current (item
232).
130 currents at the terminals of three- Subscripts for the high-voltage side: U, V, W
I I I
, ,
U V W
phase AC transformers with two
ampere A
Subscripts for the low-voltage side: u, v, w
windings
I I I
, ,
u v w
131 currents at the terminals of the The subscripts x, y, z should be used in case
I I I
tertiary winding of a three-phase of transformer windings in delta connection. ampere A
, ,
x y z
AC transformer
132 winding current i = 1, 2, 3 in three-phase AC networks
For instance index W for the high-voltage side
I , I
ampere A
Wi wi
and index w for the low-voltage side (see
item 252).
133 harmonic current component
I ν = (2), 3, (4), 5, . ≠ 1; f = ν f ampere A

ν ν
134 131-11-42 active power In a three-phase AC network with symmetric In practice mostly
sinusoidal voltages and currents: in kW or MW Is
P watt W
used.
P = 3UI cosϕ
135 DC power
PU= I ,
ddd
P watt W
where U is the line-to-line voltage of of a SC-
P d
d
DC
line (item 149) and I the DC-current at the
d
same location (item 108).
136 121-12-11 dielectric loss
watt W
P P
diel ε
60027-7 © IEC:2010                  – 11 –

Quantities Units
Item Unit, coherent
IEV and/or
Chief Reserve
number with the SI
IEC Name of quantity Remarks Remarks
symbol symbol
number
Name Symbol
137 421-06-03 total load loss in transformer In case of a two-winding three-phase
MOD windings at rated current transformer (T) one side is short circuited (k)
and the other side is fed with the rated current
(r) of this side of the main tapping.
P watt W
krT
In case of three-winding transformers three
measurements or calculations are necessary
(see IEC 60909-0 and IEC 60909-2).
138 surge-impedance load of a line Surge-impedance load of a three-phase AC
line in case of U
n
PU= /Z
nat n W
P watt W
nat
where U is the nominal value of the line-to-
n
line voltage (item 159) and Z = Z is the
W W1
surge impedance of a line in the positive-
sequence system (item 186).
139 rated mechanical power of a For a three-phase asynchronous motor use:
motor
PU=3cIosϕ η
rM rM rM rM rM
P
watt W
rM
where U is the rated voltage, I the rated
rM rM
current, ϕ the phase difference and η
rM rM
the efficiency of the motor.
140 131-11-43 non-active power
2 2
Q
= S − P
~
80000-6,
Q , Q′
volt ampere VA
~
6-61
where S is the apparent power (item 142) und
P the active power (item 134)
– 12 –             60027-7 © IEC:2010

Quantities Units
Item Unit, coherent
IEV and/or
Chief Reserve
number with the SI
IEC Name of quantity Remarks Remarks
symbol symbol
number
Name Symbol
141 131-11-44 reactive power Shall be used only in three-phase AC In practice mostly
networks with symmetric sinusoidal voltages var, kvar, Mvar is
used.
and currents:
QU==3sIinϕϕSsin
where S is the apparent power (item 142) and
Q volt ampere VA
ϕϕ=−ϕ the phase difference with ϕ as
ui u
the initial phase of the voltage and ϕ the
i
initial phase of the current (IEC 80000-6, 6-
48).
142 131-11-41 apparent power In a symmetrical three-phase AC network use: In practice mostly
kVA, MVA is used.
S = 3UI , where U is the line-to-line
S volt ampere VA
voltage (item 147) and I the line current
(item 104).
143 601-01-14 short-circuit power The short-circuit apparent power in a sym- In practice mostly
MOD metrical three-phase AC network is given as: MVA is used.
""
60909-0 "
SU= 3I , where U is the nominal line-
volt ampere VA
S knk n
k
''
to-line voltage (item 159) and I the
k
subtransient short-circuit current (item 114).
144 60909-0 short-circuit power of a three- In practice mostly
" "
S = 3U I , where U is the nominal
kQ nQ kQ nQ
phase AC network feeder at the MVA Iis used.
"
connection point Q volt ampere VA
''
S
kQ line-to-line voltage and I the subtransient
kQ
short-circuit current at the connection point Q.
145 rated apparent power Examples for electrical equipment: S , S In practice mostly
rG rT
S volt ampere VA
r
(subscripts in subclause 6.3). kVA, MVA is used.
146 62428 transformation matrix
T transformation matrix for symmetrical
S
T one 1
components in the unnormalized form, (see
item 256).
147 121-11-27 voltage, electric tension
U is the general symbol, with no further
volt V
U indication: line-to-line voltage; line-to-line
tension.
148 195-05-11 (effective) touch voltage
U
volt V
B
60027-7 © IEC:2010                  – 13 –

Quantities Units
Item Unit, coherent
IEV and/or
Chief Reserve
number with the SI
IEC Name of quantity Remarks Remarks
symbol symbol
number
Name Symbol
149 direct voltage, direct tension Average value, for instance in case of a direct
current link.
U U volt V
d DC
For the qualifier DC, see IEC 60050,
151-15-02.
150 60909-3 earthing potential
U volt V
E
151 field voltage, excitation voltage Voltage at the field winding of a machine.
U U
volt V
F f
152 411-54-06 excitation system ceiling voltage
U U volt V
Fmax fmax
153 195-05-03 line-to-earth voltage i =1,2,3 in three-phase AC networks.
U U volt V
Li LiE
i, k = 1, 2, 3 with i ≠ k in three-phase AC
154 195-05-01 line-to-line voltage
U U volt V
LiLk Lik
141-03-06 networks
155 195-05-02 line-to-neutral voltage i =1, 2, 3 in three-phase AC systems with a
U volt V
LiN
neutral conductor (low-voltage networks)
156 421-09-01 highest voltage for equipment The highest r.m.s. line to-line voltage
U volt V
permanently admissible for equipment.
m
157 601-01-23 highest and lowest voltage of a IEV. Highest (lowest) voltage of a network.
U U
max Nmax
network
volt V
601-01-24
U U
min Nmin
158 601-01-32 neutral point or neutral conductor IEV: Neutral point displacement voltage.
U volt V
NE
to-earth voltage
159 442-01-04 nominal voltage of a network The nominal voltage of a network is always a
601-01-21 line-to-line voltage.
nominal voltage of a system U volt V

n
nominal voltage of a line
160 411-49-02 synchronous generated voltage Voltage, which would be generated in the
armature windings on open circuit, in the
U U absence of saturation, by the flux volt V

P p
corresponding to the excitation current for the
conditions under consideration.

– 14 –             60027-7 © IEC:2010

Quantities Units
Item Unit, coherent
IEV and/or
Chief Reserve
number with the SI
IEC Name of quantity Remarks Remarks
symbol symbol
number
Name Symbol
161 442-01-03 rated voltage Line-to-line-voltage of electrical equipment,
U
MOD generator, motor, transformer, reactor, etc., if volt V

r
necessary with an additional index from 6.3
162 411-54-08 rated field voltage See subclause 6.6.
U U volt V
rF fr
163 421-04-01 rated voltage of a winding See also item 128.
U , U volt V
rW rw
164 195-05-12 step voltage
U U volt V
S Step
165 131-12-22 source voltage, source tension
U U volt V
s q
'
166 411-49-13 direct-axis transient voltage
U
d
volt V
'
167 411-49-14 quadrature-axis transient voltage
U
q
volt V
"
168 411-49-11 direct-axis subtransient voltage
U
d
volt V
169 411-49-12 quadrature-axis subtransient
"
U volt V
q
voltage
170 411-50-07 direct-axis synchronous reactance
ohm
X Ω
d
171 411-50-08 quadrature-axis synchronous
X
ohm Ω
q
reactance
172 411-50-09 direct-axis transient reactance
'
ohm
Ω
X
d
173 411-50-10 quadrature-axis transient
'
ohm
X Ω
q
reactance
174 411-50-11 direct-axis subtransient reactance "
X is the effective reactance of a
d
60909-0
synchronous machine at the moment of three-
"
phase short circuit. For the calculation of ohm
X Ω
d
short-circuit currents the saturated value of
"
X shall be used.
d
60027-7 © IEC:2010                  – 15 –

Quantities Units
Item Unit, coherent
IEV and/or
Chief Reserve
number with the SI
IEC Name of quantity Remarks Remarks
symbol symbol
number
Name Symbol
175 411-50-12 quadrature-axis subtransient
"
X ohm Ω
q
reactance
176 mutual reactance
X ohm
Ω
m
177 self reactance
X ohm Ω
s
178 leakage reactance
X
X ohm Ω
l
σ
179 131-14-25 admittance matrix Names and symbols for the elements are siemens S
Y A
given in IEC 60027-2.
60027-2
180 admittance of the zero sequence ' siemens
' '
'
Y = G + jω C
S/m
Y 0 0 0
system of a line per length
per metre
181 admittance of the positive
' ' ' siemens
YG=+ jωC
'
1 11
sequence system of a line per S/m
Y
per metre
Length
182 admittance of the negative-
' ' '
siemens
' YG=+ jωC
sequence system of a line per 2 S/m

Y
per metre
length
183 131-14-24 impedance matrix Names and symbols for the elements are
ohm
Z Ω
given in IEC 60027-2.
184 60909-0 subtransient impedance of a
" " "
Z =+RXj ohm Ω
Z
G G Gd
generator
185 surge impedance of a line in the
' '
Z ohm
Ω
Z = Z/Y
W0
zero sequence system
W0 0 0
186 603-02-23 surge impedance of a line in the
''
Z Z ohm
Ω
Z = Z/Y
positive sequence system W1 W
W11 1
187 448-11-29 zero-sequence impedance of a ohm In practice often
' ''
'
Z =+RLjω
00 Ω/m
MOD line per length Z 0
Ω/km is used.
per metre
188 448-11-27 positive-sequence impedance of a ' ohm In practice often
' ''
Z
Z=+RLjω
11 Ω/m
MOD line per length 1
Ω/km is used.
per metre
– 16 –             60027-7 © IEC:2010

Quantities Units
Item Unit, coherent
IEV and/or
Chief Reserve
number with the SI
IEC Name of quantity Remarks Remarks
symbol symbol
number
Name Symbol
189 448-11-28 negative-sequence impedance of ' ohm In practice often
'
' '
Z
Z = R + jω L
Ω/m
2 2 2
MOD a line per length Ω/km is used.
per metre
190 admittance angle
α α = arctan(B/G) for Y = G + jB
radian rad
191 impedance angle
γ
γ = arctan(X/T) for Z = R + jX radian rad
ϑ
192 load angle
Load angle is the angle between U and
1G
δ
radian rad
P
U of a synchronous machine.
P
angle between U and U
i k δ δ = ϕ −ϕ radian rad
ϑ
ik ik ik ui uk
194 60909-3 earth penetration depth Earth penetration depth in case of infinite
length of line is δ =1,8514 / ωμ / ρ ,
E 0 E
where ω is the angular frequency, μ the
δ metre m
E
magnetic constant (IEC 60050-121,
121-11-14) and ρ the earth resistivity
E
(item 195).
195 60909-3 specific earth resistance
ρ ohm metre
Ωm
E
60027-7 © IEC:2010                  – 17 –

4 Letter symbols for space and time
Quantities Units
Item Unit, coherent
IEV and/or
Chief Reserve
number with the SI
IEC Name of quantity Remarks Remarks
symbol symbol
number
Name Symbol
196 diameter of a line conductor Apparent diameter of a stranded conductor,
D d
metre m
L L
see IEV 581-03-51
197 distance
Distance between two line conductors: d metre m
d a
LLik
198 466-05-13 line-to-earth clearance
d
metre m
LEmin
MOD
199 sag of a line-conductor of
f maximum value in case of a horizontal
max
an overhead line
f metre m
span between two towers (mid span sag), see
IEV 466-03-09 and its Figure 446-1.
200 702-01-07 eigenfrequency
f = ω / 2π, see item 219
e e
f hertz Hz
e
MOD
201 inertia constant
H =T/ 2 second s
H
m
202 effective height above ground of a
h = h – 0,7f
max max
line-conductor from an h h metre m

e
overhead line
203 line length overhead line or cable usually in km
L metre m
l
204 cross-section square usually in mm
Cross-section of a line-conductor q

L m
q A
metre
205 radius Radius of a line conductor
r
r metre m
L
206 60909-0 equivalent radius of a bundle
n−1
n
rn=⋅r⋅r
conductor
BLT
r metre m
B
where n is the number of sub-conductors; for
r see item 205; for r see item 207.
L T
– 18 –             60027-7 © IEC:2010

Quantities Units
Item Unit, coherent
IEV and/or
Chief Reserve
number with the SI
IEC Name of quantity Remarks Remarks
symbol symbol
number
Name Symbol
207 466-10-22 radius of the sub-conductors in a
twin bundle: ra= / 2 ;
T
bundle
466-10-23
triple bundle: ra= / 3 ;
T
466-10-24
r
metre m
T
quad bundle: ra= / 2 ;
T
where a is the distance between two sub-
conductors.
208 distance between the centres of
s
metre m
ik
gravity of the surfaces i and k
209 411-48-15 expected acceleration duration The duration that would be required to bring
MOD the rotating parts of a machine from rest to
T rated speed if the accelerating torque were second s

A
constant and equal to the quotient of rated
active power by rated angular velocity.
210 411-48-28 direct-axis transient short-circuit This applies to a synchronous machine.
'
second s
T
time constant d
211 411-48-27 direct-axis transient open-circuit This applies to a synchronous machine.
'
second s
T
time constant d0
212 411-48-30 direct-axis subtransient short- This applies to a synchronous machine.
"
second s
T
d
circuit time constant
213 411-48-29 direct-axis subtransient open- This applies to a synchronous machine.
"
second s
T
d0
circuit time constant
214 411-48-26 aperiodic time constant, This applies to a synchronous machine.
T T second s
a DC
direct current time constant
215 60909-0 short-circuit duration
T second s
k
216 electromechanical time constant
TH= 2
m
T
second s
m
where H is the inertia constant (see item
201).
217 411-48-35 quadrature-axis subtransient This applies to a synchronous machine.
"
second s
T
q
short-circuit time constant
60027-7 © IEC:2010                  – 19 –

Quantities Units
Item Unit, coherent
IEV and/or
Chief Reserve
number with the SI
IEC Name of quantity Remarks Remarks
symbol symbol
number
Name Symbol
218 60909-0 minimal time delay This is the shortest duration between the
beginning of the short circuit and the contact
t T second s
min k min
separation of the first pole to open the
switching device.
219 angular eigenfrequency
ω = 2π f
second to
e e
–1
ω the power
s
e
with f according to item 200. minus one
e
– 20 –             60027-7 © IEC:2010

5 Letter symbols for numerical values and ratios of quantities
Quantities Units
Item Unit, coherent
IEV and/or
Chief Reserve
number with the SI
IEC Name of quantity Remarks Remarks
symbol symbol
number
Name Symbol
220 refraction factor For travelling waves on lines, different for
b one 1
voltage and current.
221 60909-0 voltage factor Factor for the equivalent voltage at the short-
c one 1
circuit location.
222 damping factor
one 1
d
223 factor for the increase of a
R = F F R
~ =
SP
resistance by the proximity- F one 1

P
effect
224 factor for the increase of a
R = F F R
F ~ =
SP one 1
S
resistance by the skin-effect
225 60909-0 impedance correction factor For impedances of electrical equipment when
calculating short-circuit currents with the
K one 1
equivalent voltage source at the short-circuit
location according to IEC 60909-0.
226 overvoltage factor
b
ku= /(23U / );
o
u : subscript o, see item 277;
o
one 1
k
b
U : superscript b, see item 335
b
(instead of U sometimes U is used).
m
227 number of lines This applies to three-phase AC networks.
one 1
L
228 number of meshes This applies to three-phase AC networks.
one 1
M
229 60909-0 factor for the heat (thermal) effect See item 129
of the DC component in a short- m one 1
circuit current
230 number of nodes This applies to three-phase AC networks.
one 1
N
231 number of conductors per bundle This applies to high-voltage overhead lines in
n one 1
three-phase AC networks.
60027-7 © IEC:2010                  – 21 –

Quantities Units
Item Unit, coherent
IEV and/or
Chief Reserve
number with the SI
IEC Name of quantity Remarks Remarks
symbol symbol
number
Name Symbol
232 60909-0 factor for the heat (thermal) effect See item 129
of the AC component in a short- n one 1
circuit current
233 60027-1 number of pairs of poles This applies to synchronous and
one 1
p
asynchronous three-phase AC machines.
234 131-15-32 complex current reflection factor This applies to travelling waves on lines.
r one 1
I
235 131-15-33 complex voltage reflection factor This applies to travelling waves on lines.
r one 1
U
236 60909-3 reduction factor This applies to the case of interference.
r one 1
237 relative short-circuit resistance of This is the relative value in the case of a two-
a transformer winding three-phase AC transformer with
r one 1
T
reference values U/ 3 and I , see also
rT rT
item 243.
238 relative short-circuit reactance of This is the relative value in the case of a two-
a transformer winding three-phase AC transformer with
x one 1
T
reference values U/ 3 and I , see also
rT rT
item 244.
239 relative short-circuit impedance of This is the relative value in the case of a two-
a transformer winding three-phase AC transformer with
z one 1
T
reference values U/ 3 and I , see also
rT rT
item 242.
240 voltage ratio of a transformer See item 241. one 1
t
241 421-04-02 rated voltage ratio of a
tU=≥/U 1, where U is the
rT rTHV rTLV rTHV
transformer
436-01-15 rated voltage at the high-voltage side and
MOD
U the rated voltage at the low-voltage
rTLV
t
one 1
rT
442-01-03 side of the transformer.
MOD
The rated voltage ratio may be different
from the turns ratio (item 246).

– 22 –             60027-7 © IEC:2010

Quantities Units
Item Unit, coherent
IEV and/or
Chief Reserve
number with the SI
IEC Name of quantity Remarks Remarks
symbol symbol
number
Name Symbol
242 60909-0 related short-circuit voltage of a In the case of a two-winding transformer; in practice given in
transformer %
u (IEV 421-07-01) rated value with the
kr
u one 1
reference values: U/ 3 and I , see also

k
rT rT
item 239, the value of u is equal to the value
k
of z .
T
243 60909-0 resistive component of the related in practice given in
rated value: u , see also item 237, the value
Rr
u
short-circuit voltage of a one 1 %
R
of u is equal to the value of r
transformer R T
244 60909-0 reactive component of the related in practice given in
For rated value: u , see also item 238, the
Xr
short-circuit voltage of a u one 1 %

X
value of u is equal to the value of x

transformer X T
.
245 detuning coefficient This apllies to a resonant earthed neutral
network
one 1
v
(see IEV 195-04-09).
246 turns ratio
wW= /W one 1
w n
HV LV
247 195-05-14 earth fault factor
δ one 1
604-03-06
248 60909-0 factor for the calculation of the In case of a three-phase AC short circuit:
peak short-circuit current
i
p
κ = , where i is the peak short-
p
′′
2I one 1
κ
k
''
circuit current (item 118) and I is the
k
subtransient short-circuit current (item 114).
249 60909-0 factor for the calculation of the For one synchronous machine:
steady-state short-circuit current
I
kG
λ = , λ and λ , where I is the
max min rG
λ one 1
I
rG
rated current and I the steady-state short-
kG
circuit current of a generator.

60027-7 © IEC:2010                  – 23 –

Quantities Units
Item Unit, coherent
IEV and/or
Chief Reserve
number with the SI
IEC Name of quantity Remarks Remarks
symbol symbol
number
Name Symbol
250 60909-0 factor for the calculation of the
I
b
symmetrical short-circuit μ = , where I is the breaking current
b
"
breaking current I
k
μ
one 1
''
(item 105) and I the subtransient short-
k
circuit current (item 114).
– 24 – 60027-7 © IEC:2010
6 Subscripts and superscripts
6.1 Subscripts for natural quantities and components in three-phase AC systems

Item
Reserve
num- Meaning Chief symbol Example of use Remarks
symbol
ber
251 line L1, L2, L3 U , U , U

L1 L2 L3
conductors
252 windings I , I , I Windings of electrical equipment, for

W1, W2, W3
W1 W2 W3
instance the windings of a transformer
in delta-connection, see item 253.
253 windings 1W1, 1W2, 1W3 1, 2, 3 at the beginning of the symbol

I
1w1
defines the HV-, the MV- and the LV-
2W1, 2W2, 2W3
side of a transformer.
I1w2
HV-side
3W1, 3W2, 3W3
I1w3
1, 2, 3 at the end of the symbol defines
the three windings on each of the three
N
sides in case of a three-phase AC
transformer.
254 neutral N I Applicable in low-voltage networks.

N
conductor
255 source s q U , u See item 165.

s s
256 positive 1 (1) U , U Symmetrical components (IEC 62428)

1 (1)
sequence
⎡⎤11 1
system ⎡UU⎤⎡⎤
L1 1
⎢⎥
⎢⎥⎢2 ⎥
UU=⋅aa 1
⎢⎥
negative 2 (2) U , U L2 2
⎢⎥⎢⎥
2 (2)
⎢⎥
⎢⎥⎢2 ⎥
sequence UU
⎣ L3⎦⎣aa 1 0⎦
⎢⎥
⎣⎦
system
1 1
zero 0 (0) U , U
j2π/3
0 (0)
with a = = − + j 3
e
sequence
2 2
system
(1), (2), (0) used in IEC 60909-0.
dq0-components
257 direct axis d
258 quadrature q
axis
259 Clarke α
αβ 0 -components
component
260 Clarke
β
component
261 space phasor s Space phasor in a non-rotating frame of
uu=+ ju
αβ
s
reference.
262 space phasor r Space phasor in a rotating frame of
uu=+ ju
dq
r
reference.
263 active P
component
264 non-active Q
component
60027-7 © IEC:2010 – 25 –
6.2 Subscripts for operating conditions
Item
Reserve
num- Meaning Chief symbol Example of use Remarks
symbol
ber
265 general a . z Use lower case letters for any operating

condition.
266 actuated a Applicable to a protection device.
I
a
267 breaking b Breaking current of a circuit breaker. See
I
b
item 105.
268 earth fault e In networks with isolated neutral or
I
Ce
resonant earthed neutral. See item 107 and
see also item 125.
269 induced in Induced voltage.
U
in
In case o
...