ISO 1540:2006

INTERNATIONAL ISO
STANDARD 1540
Third edition
2006-02-15
Aerospace — Characteristics of aircraft
electrical systems
Aéronautique — Caractéristiques des systèmes électriques à bord des
aéronefs
Reference number
©
ISO 2006
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ii © ISO 2006 – All rights reserved

Contents Page
Foreword. iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions. 2
4 Requirements applicable to all systems .10
4.1 General. 10
4.2 On-aircraft power sources . 10
4.3 External power sources . 11
4.4 Source/distribution system coordination.11
4.5 Utilization equipment. 11
5 Constant frequency (CF) a.c. power system characteristics . 12
5.1 General characteristics . 12
5.2 Steady-state characteristics . 13
5.3 Transient characteristics . 13
6 Variable frequency (VF) a.c. power system characteristics. 15
6.1 General characteristics . 15
6.2 Consideration of CF power characteristics . 15
6.3 Steady-state characteristics . 15
6.4 Transient characteristics . 16
7 D.C. power system characteristics . 19
7.1 General characteristics . 19
7.2 Steady-state characteristics . 19
7.3 Transient characteristics . 20
8 Requirements allocation . 21
9 Utilization equipment restrictions. 21
9.1 General. 21
9.2 A.C. power utilization . 21
9.3 Power factor . 21
9.4 Load switching transients . 22
9.5 Inrush current. 22
9.6 Input current modulation . 22
9.7 Input current distortion . 22
9.8 Maximum input capacitance. 23
10 Power quality associated assumptions and background . 24
10.1 General. 24
10.2 Background to the document scope .25
10.3 A.C. power system assumptions . 25
10.4 A.C. source equipment assumptions .29
10.5 D.C. system assumptions . 31
10.6 D.C. engine starting power quality. 32
10.7 270 V d.c. input power. 32

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 1540 was prepared by Technical Committee ISO/TC 20, Aircraft and space vehicles, Subcommittee SC 1,
Aerospace electrical requirements.
This third edition cancels and replaces the second edition (ISO 1540:1984), which has been technically
revised.
iv © ISO 2006 – All rights reserved

Introduction
The purpose of this International Standard is to foster compatibility between the providers, distributors and
users of aircraft electrical power. This third edition takes into account several recent trends in aircraft electrical
system, including that towards increased nonlinear load content on aircraft. It defines design requirements for
electrical equipment that will be verified by the test requirements specified in ISO 7137.
Limits defined in this International Standard are based upon historical as well as near term projected
equipment characteristics, including recent trends towards increased nonlinear, electronic user equipment.
Since these limits are influenced by the overall combination of source, distribution and user equipment,
background to their integration sensitivities is also included herein. The intention is to provide system
integrator guidance, without restricting flexibility of means by which the specified interface characteristics are
achieved. This revision also addresses several power types not at present common on large transport aircraft,
such as variable frequency a.c., 230/400 V a.c. and 42 V d.c.
Also fundamental to the basis of these requirements is the assumption that cost-effective utilization equipment
needs to be usable on a wide range of new aircraft. This results in some penalties typically only realized on
large aircraft, e.g. those associated with longer distribution feeder voltage drops, being accepted for smaller
aircraft equipment. The realities of these situations and recent user equipment trends may likely be the reason
for differences between this International Standard and other historical standards.
INTERNATIONAL STANDARD ISO 1540:2006(E)

Aerospace — Characteristics of aircraft electrical systems
1 Scope
This International Standard specifies the characteristics of electrical power supplied to the terminals of
electrical utilization equipment installed in an aircraft. It is intended to support the interface definition for user
equipment designed to accept electrical power on a variety of new civil aircraft applications, such as those
certified via the Technical Standard Order (TSO) certification process. It might not be desirable for equipment
targeted to a single application or specific military application to follow this International Standard because of
the penalties associated with multi-application.
This document also attempts to provide background to the development of these requirements that may be
useful to those designing and/or integrating modern aircraft electrical systems. The delivered quality of this
electrical power is a result of the combined characteristics of the electrical power source, distribution and user
equipment. While only user equipment restrictions are specifically defined, background to key source and
distribution equipment interfaces are identified in order to support development of the overall system.
A wide variety of electrical supply types and distribution parameters have been considered, as may be found
on both small and large transport aircraft. Sources considered include physically rotating and static types,
provided either on-aircraft, or as part of the ground support equipment. Distribution voltages addressed are
⎯ nominal 14 V, 28 V and 42 V d.c.;
⎯ nominal 26 V a.c., 400 Hz, one-phase;
⎯ nominal 115/200 V rms and 230/400 V rms a.c., both one-phase and three-phase, at either a nominal
400 Hz constant frequency (CF), or over a variable frequency (VF) range which includes 400 Hz.
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.
ISO 6858, Aircraft — Ground support electrical supplies — General requirements
1)
ISO 7137:1995, Aircraft — Environmental conditions and test procedures for airborne equipment

1) Endorsement of EUROCAE ED-14C and RTCA/DO-160C.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
abnormal electrical system operation
aircraft operation where a malfunction or failure in the electrical system has taken place and the protective
devices of the system are operating to remove the malfunction or failure from the rest of the system before the
limits for abnormal power quality are exceeded
NOTE Once initiated, abnormal operation may continue for the remainder of a flight with the power quality delivered
to users exceeding normal operation limits, but staying within abnormal operation limits.
3.2
abnormal power quality limits
limits provided at user terminals during abnormal operation that take into account the operating tolerances of
the system protective devices and any inherently limiting characteristics of the system design
NOTE See also 3.30.
3.3
crest factor
absolute value of the ratio of the peak to the rms value of an a.c. waveform measured under steady-state
conditions
NOTE 1 It is unitless and the ratio for a true sine wave is equal to 2 .
NOTE 2 Written as | VV(pk) / (rms) | .
3.4
current modulation
difference between the maximum and minimum value of electrical current drawn during conditions of cyclic or
randomly repeating current variation
NOTE Measurable current modulation by user equipment can impact the quality and/or stability of the provided
electrical power.
3.5
distortion (current or voltage)
rms value of the a.c. waveform exclusive of the fundamental component in an a.c. system, or the rms value of
the alternating (ripple) component on the d.c. level in a d.c. system
NOTE a.c. system distortion can include harmonic and non-harmonic components. Harmonics are sinusoidal
distortion components which occur at integer multiples of the fundamental frequency. Interharmonics are distortion
components which occur at non-integer multiples of the fundamental frequency. These and all other elements of waveform
distortion are included in this general definition of distortion. (See also 3.23 and 3.25.)
3.6
displacement factor
〈a.c. user equipment〉 cosine of the angle (Φ) between the input current (provided at the fundamental
frequency) and the input voltage (provided at the fundamental frequency)
NOTE This value does not include the effect of distortion in the input current (and/or voltage) waveform, and it is
therefore not applied in this specification in favour of the more general power factor definition. (See also 3.35.)
2 © ISO 2006 – All rights reserved

3.7
distortion factor (current or voltage)
ratio of the distortion in a waveform to the rms value of the fundamental component of the waveform
NOTE 1 The distortion factor is typically expressed as a percentage:
()XX−
rms 1
df (per cent)=×100
X
where
X is the rms value of the complete (voltage or current) waveform;
rms
X is the rms value of the fundamental frequency component.
NOTE 2 In a d.c. system, this fundamental component is true d.c. (See also 3.5, 3.43.)
3.8
distortion spectrum
itemization of the amplitude of each frequency component found in the a.c. or d.c. distortion
NOTE 1 Its components may be harmonic or non-harmonic multiples of the fundamental frequency, some of which
result from amplitude or frequency modulation.
NOTE 2 Only components up to a frequency of 16 kHz (for 400 Hz, CF equipment) and 32 kHz (for VF equipment) are
addressed in this International Standard to clearly separate requirements related to electrical power quality from those
related to electromagnetic compatibility (EMC).
3.9
distribution system
collection of interconnection and circuit protection equipment between power sources and user equipment
NOTE See Figure 1.
3.10
drift
extremely slow variation in a random manner of a controlled parameter (such as frequency in a CF system)
inside of the specification limits from causes such as ageing of components or self-induced temperature
changes
3.11
drift rate
speed of variation due to drift of a controlled parameter
NOTE Drift rate is typically expressed in Hz/min or V/min, depending upon the parameter examined.
3.12
electric engine start operation
special case of normal electrical system operation where an extreme demand of electrical power is required to
support the starting of a main engine or the auxiliary power unit
NOTE 1 Normal voltage transient limits may be exceeded during this condition with only selected utilization equipment
required to operate throughout the event.
NOTE 2 Typical engine start times are between 15 s and 90 s.
3.13
electric power generating system
EPGS
combination of rotating and static electrical power sources and the devices which provide their control and
protection
3.14
electric power system
combination of electrical power sources, conversion equipment, control and protective devices and utilization
equipment connected via a distribution network
NOTE Also called simply ‘system’.
3.15
emergency electric system operation
electrical system condition during flight when the primary electric power system becomes unable to supply
sufficient or proper electrical power, thus requiring the use of independent and potentially limited source(s) to
power a reduced complement of distribution and utilization equipment selected to maintain safe flight and
personnel safety
3.16
emergency power source
generator, power conversion device (or a combination thereof not involving part of utilization equipment) or
battery installed to provide independent electrical power for essential purposes during conditions of electrical
emergency in flight
3.17
external power unit
ground power unit
GPU
rotating or static source (or combination thereof) supplied by the maintenance facility to source electrical
power demands while the aircraft is not in flight
NOTE It may be either a point-of-use or centrally located ground power electrical supply in land-based facilities, or a
shipboard power supply in marine applications.
3.18
frequency
reciprocal of the period of the a.c. waveform
NOTE 1 Frequency is measured in hertz (Hz).
NOTE 2 Steady-state frequency is the time average of the frequency over a period not to exceed one second.
Instantaneous frequency is the frequency of a single cycle.
3.19
frequency modulation
cyclic and/or random variation of instantaneous frequency about a mean frequency during steady-state
conditions
NOTE Amplitude of the frequency modulation is equal to the difference found between the maximum and minimum
frequency measured over a one minute interval.
3.20
frequency modulation rate
rate of change of frequency due to frequency modulation
NOTE Frequency modulation rate is measured in hertz per second (Hz/s).
3.21
fundamental frequency
frequency of the primary power producing component of a periodic waveform supplied by the generation
system (component of order 1 of the waveform’s Fourier series representation)
4 © ISO 2006 – All rights reserved

3.22
ground
point along a conductive structure or cable which serves as an essentially zero potential reference for a.c.
and/or d.c. voltages
3.23
harmonics
sinusoidal voltage or current components (distortion) of a periodic waveform which occur at a frequency that is
an integer multiple of the fundamental frequency
NOTE 1 Most nonlinear loads generate odd-numbered harmonics; for example, as a result of full wave rectification of
the input power.
NOTE 2 The frequencies at which these ‘characteristic harmonics’ are produced by a user with a diode-type input
rectifier are determined by the following equation:
f =×(1kq±)×f
H 1
where
H is the number of the harmonic;
k is an integer, beginning with 1;
q is an integer, representing the number of rectifier commutations per cycle;
f is the fundamental frequency.
NOTE 3 Half wave rectification produces even-numbered harmonics, which cause very undesirable results (e.g. d.c.
content) in the a.c. power system. Full wave rectification at the input of single-phase power users results in ‘triplen’
harmonics at odd multiples of three times the fundamental frequency. These are also very undesirable given the potential
quantity of single-phase users and the fact that these harmonics interact with the distribution system’s normally high (zero
sequence) impedance to this frequency. User distortion current requirements are therefore intentionally restrictive for even
and triplen harmonics. (See also 3.5, 3.39.)
3.24
impedance
complex electrical characteristic of a device or group of devices which relates the ratio of the phasor
steady-state voltage to the phasor steady-state current
3.25
individual frequency component of distortion
ratio of the rms value of the waveform distortion at one specific frequency to the rms value of the fundamental
component of the waveform
NOTE 1 See also 3.8.
NOTE 2 The individual frequency components of voltage distortion are expressed as
D=×100 (VV )
vn n 1
where
V is the rms value of an individual, non-fundamental frequency component;
n
V is the rms value of the fundamental frequency component.
3.26
linear load
user of electrical power whose total impedance is constant despite variations in applied voltage and whose
current spectrum matches that of the applied voltage
NOTE Conversely, nonlinear loads may have changing impedance with applied voltage and a different spectral
content from that of the applied voltage source.
3.27
load unbalance
difference between the highest phase power draw and lowest phase power draw in volt-amperes for a
three-phase a.c. power user
3.28
momentary power interruptions
short term power interruptions during which time the supplied voltage will decay at a rate dependent upon bus
and load characteristics, as is typical during transfer of power sources
3.29
normal electrical system operation
conditions which include all intended modes of aircraft ground and flight operation during which no electrical
system faults or malfunctions occur, except instances of propulsion engine or auxiliary power unit electric
starting
NOTE 1 It assumes proper functioning of all equipment within defined operating procedures and limits. Examples of
such operation are switching of utilization equipment loads, engine speed changes, source switching and synchronization,
and the intended paralleling of power sources.
NOTE 2 Normal operation also includes momentary power interruptions, transients and spikes. (See also 3.1.)
3.30
normal power quality limits
limits which should be maintained during periods of normal electrical system operation
NOTE See also 3.2.
3.31
per unit
PU
standardized quantity in a system where various parameters are quantified with respect to a base value
NOTE 1 The base value is generally the rated value.
NOTE 2 For power systems this is typically applied to the powers, voltages, currents, or impedances where ‘per unit’
numbers equal the actual parameter value divided by the base values.
EXAMPLE In a 115/200 V rms, 3-phase, 120 kVA system: 1 PU power is 120 kVA; 1 PU voltage is equal to 115 V
rms; 1 PU phase current is equal to 348 A rms; and 1 PU impedance is equal to 0,33 ohms. A three-phase load on this
system that consumes 52 A rms per phase would be considered as drawing 0,15 PU power.
3.32
phase voltage
phase-to-neutral voltages supplied to single-phase or three-phase utilization equipment
NOTE All a.c. voltage values defined in this International Standard are rms, line-to-neutral quantities unless
otherwise specified.
3.33
phase voltage displacement
maximum angular separation (about a nominal 120 degrees) between the zero voltage points of any two of
the three voltage waveforms in a three-phase a.c. system during steady-state conditions
3.34
phase voltage unbalance
maximum difference between rms phase voltage amplitudes during steady-state conditions
NOTE VV=−max. , , VV min.V , , VV
{ } { }
UNB AN BN CN AN BN CN
6 © ISO 2006 – All rights reserved

where
V , V , V are the phase voltage magnitudes.
AN BN CN
3.35
power factor
a.c. user equipment feature determined by the ratio of the real, or active, power consumed in watts to the
apparent power drawn in volt-amperes, with
NOTE 1
PFP= /S
where
P is the real power in watts;
S is the apparent power, product of rms voltage and current, in volt-amperes.
NOTE 2 This definition of power factor includes the effect of distortion in the input current (and/or voltage) waveform.
NOTE 3 When the fundamental current waveform drawn by a user electrically lags the fundamental voltage waveform
(as is typical in inductive loads), it is considered a ‘lagging’ power factor. Likewise if the current waveform electrically leads
the voltage waveform (as expected for capacitive loads), it is considered a ‘leading’ power factor. When the user only
draws real power (no reactive power) and its input current is exactly in phase with the supplied voltage, it is termed a
‘unity’ power factor load (PF= 1). (See also 3.6.)
3.36
primary power source
generator, usually driven by one of the aircraft propulsion engines, and any associated power conditioning
equipment (not forming part of the utilization equipment) installed to provide electrical power during all phases
of aircraft operation
3.37
ripple
cyclic variation about the mean level of the d.c. current or voltage value during steady-state electrical system
operation
NOTE Since it is not always a symmetrical quantity, the difference between upper and lower peak values is
measured instead of the mean value of voltage or current.
3.38
rms value (voltage or current)
value of voltage or current based upon the equivalence to the d.c. value that would yield the same power
transfer in a d.c. circuit
NOTE The rms voltage value can be computed as
T
Vv= ()t dt
rms
∫
T
where
T is the waveform time period;
v(t) is the instantaneous voltage at time t.
3.39
sequence impedances (/harmonics)
positive, negative and zero sequence impedances, determined from a mathematical analysis method termed
‘Symmetrical Components’ which breaks the quite difficult problem of analysing a three-phase unbalanced
system into a study of two balanced three-phase circuits and one zero-phased circuit
NOTE The practical application of these characteristic impedances allows for more complex power system analysis,
including the effects of harmonic currents to produce harmonic voltages. Positive and negative sequence impedances are
determined by the resistance and reactance of the generating power source and the distribution network. In a.c. electrical
systems, therefore, these impedances increase with increasing source (generator) frequency. For passive elements such
as distribution feeders, the positive and negative sequence impedances are identical; this is not true for electrical
machines.
Zero sequence impedances strongly relate to the impedance of the system to current flow through the power system
neutral. Therefore this impedance is heavily influenced by the application of a wired or structure return path, and for the
latter case, the exact three-phase wire bundle configuration and its distance from the return path structure. Unbalanced
currents and fault currents flow through this impedance.
While positive, negative and zero sequence impedances or currents are traditionally associated with particular harmonic
multiples, the harmonics present in a three-phase power system can also be characterized as having positive, negative
and zero sequence components. Positive sequence current harmonics consist of three phasors, equal in magnitude and
separated from each other by 120° phase displacement, with the same phase sequence as phasors representing the
fundamental bus current.
Negative sequence current harmonics also consist of three phasors, equal in magnitude and separated from each other by
120° phase displacement, but with a phase sequence which is opposite to that of phasors representing the fundamental
bus current. Whereas positive sequence harmonics provide for positive torque contribution to an a.c. bus fed synchronous
motor, negative sequence harmonics negate torque in a.c. bus fed synchronous motors.
Zero sequence harmonics consist of three phasors which are likewise equal in magnitude, but with identical phase angles,
and are therefore described as being ‘in phase’ with each other. Whereas balanced positive and negative sequence
harmonics do not result in any neutral conductor current, balanced zero sequence harmonics, such as those from
single-phase to neutral nonlinear loads, result in three times the harmonic current in the neutral conductor than is present
in any phase. The third harmonics of fundamental phase A, B and C currents, termed ‘triplen’ harmonics, have identical
phase angles and therefore act with a magnitude which is triple that of any one phasor. (See 3.25.)
3.40
spike
variation from the controlled steady state or transient level of a characteristic that occurs for an extremely
short duration (microseconds)
NOTE 1 Spikes generally produce a voltage peak and/or wave train, the characteristics of which are dependent on
relative impedances of the source, the line, and of the utilization equipment, as well as the manner in which the event
occurs.
NOTE 2 Typical voltage spikes result from the switching of inductive or capacitive load elements.
3.41
steady state
operating condition of the system when only negligible changes in electrical parameters appear
3.42
system stability
aspect of system dynamic compatibility associated with certain system performance criteria defined at the
power system interfaces
NOTE 1 For aircraft power systems the primary interface is the electrical bus.
NOTE 2 The key performance criteria are therefore associated with the maintenance of voltage and current values,
and spectral content thereof, at various points on that bus within the limits defined by this International Standard in the
presence of both internal and external stimuli.
8 © ISO 2006 – All rights reserved

3.43
total harmonic distortion (current or voltage)
ratio of the rms value of a waveform’s harmonics to the rms value of its fundamental component
NOTE 1 See also 3.7, 3.8.
NOTE 2 The total harmonic distortion may be defined by the following equation:
n
X
∑
n
THD (per cent)=×100
X
X
where
X is the fundamental value of current or voltage;
X is the nth harmonic value of current or voltage.
n
3.44
transient
momentary variation of a characteristic from its steady-state limits, and back to its steady-state limits, as a
result of a system disturbance
NOTE 1 Rapid load or engine speed changes followed by the conditioned response of the generating system, as well
as brief voltage variations or interruptions due to normal source or load switching are considered normal transients.
NOTE 2 Transients which exceed normal transient limits as a result of an abnormal disturbance and eventually return
to steady-state limits are defined as abnormal transients.
3.45
uninterruptible power
power (typically d.c.) delivered to essential and/or voltage transient sensitive users in such a manner that
normal power interruptions are either eliminated or reduced in severity and probability
3.46
utilization equipment
unit or functional group of units which receives electrical power on the aircraft
NOTE Sometimes referred to as ‘load equipment’.
3.47
utilization equipment rating
maximum power a user can be expected to continually consume over a time period which is not less than
200 ms
3.48
voltage modulation
〈a.c. voltage〉 cyclic and/or random variation of the a.c. peak voltage around a mean value during steady-state
conditions
NOTE 1 Voltage modulation amplitude is the difference between the maximum and minimum peak voltage values that
occur in a one second period during steady-state operating conditions. (See 3.27.)
NOTE 2 Frequency characteristics of voltage modulation are the components at individual frequencies that together
make up the voltage modulation envelope waveform.
NOTE 3 For d.c. power, see ripple (3.37).
3.49
voltage regulation
result of action by a voltage control mechanism and the source being controlled over the normal operating
range of the equipment
NOTE 1 d.c. voltages used herein are defined by the mean value measured between the positive terminal and ground.
NOTE 2 Steady-state d.c. voltages are the time average of the respective voltage values over a time period of between
0,2 s and 1 s.
4 Requirements applicable to all systems
4.1 General
Electrical power quality characteristics described in this document apply at the electrical input terminals of
utilization equipment. These characteristics include both normal and abnormal operating conditions of the
aircraft electrical power system during all phases of flight and ground operation. They have specifically been
examined to include realistic operating conditions for a large variety of aircraft.
All of the power system equipment should therefore be designed so that normal service maintenance will
ensure the retention of these specified characteristics throughout the full range of operational and
environmental conditions likely to be encountered in the aircraft, or support facility, in which they are installed.
In order to provide the necessary compatibility among electric power source, distribution and utilization
equipment to ensure these characteristics, constraints applicable to each of these, at the subsystem and
equipment level, are required.
General requirements found in this clause are intended for incorporation into source and utilization equipment
specifications. They relate to functional aspects of the power system interface. Detailed requirements for user
equipment, intended to be verified by test, are found in Clause 10. Detailed requirements for external and
on-board electrical power source equipment are found in related standards and specifications. Typical
subsystem level aspects, such as adequate control over the load balance, or the aggregate effects of
nonlinear load, are not easily addressed by an industry standard. It is the airframer’s and/or the subsystem
integrator’s responsibility to ensure that adequate measures are taken in the definition and application of
these subsystems such that the essential coordination occurs. Clause 11 includes applicable background in
this area.
Compatibility also involves topics that are outside of the scope of this document, including the topic of
electromagnetic compatibility (EMC). Specific aircraft electronic equipment requirements related to EMC and
other environmental effects are defined in ISO 7137.
4.2 On-aircraft power sources
4.2.1 Alternating current (a.c.) power sources
Primary a.c. power generation shall be three-phase, four-wire, wye-connected supplying a nominal voltage of
either 115/200 (line-to-neutral/line-to-line) V rms or 230/400 V rms and an A-B-C phase sequence
(see Figure 2). The nominal frequency for constant frequency sources shall be 400 Hz. Variable frequency
sources shall provide for a minimum generator frequency that is not less than 360 Hz. Where an auxiliary
single-phase supply is provided, it shall meet the line-to-neutral requirements stated herein.
The neutral point of each source of power is normally connected to vehicle structure, which shall then be
considered the fourth wire. If vehicle structure is used as the fourth wire, it may be preferable to make this
connection close to the point of power distribution in order to minimize the distortion of the supplied a.c.
voltage by single-phase rectifier type loads. In the case of aircraft with substantial composite structure, an
alternative grounding scheme that carries a neutral return wire to the utilization equipment may be preferred.
Performance of a.c. source equipment at its specific point of regulation (POR) shall be as specified in the
applicable specification for that source equipment.
10 © ISO 2006 – All rights reserved

4.2.2 Direct current (d.c.) power sources
Direct current power sources shall provide for a two-wire system having a nominal voltage of 14 V, 28 V or
42 V d.c. Performance of the d.c. source equipment at its specific point of regulation (POR) shall be as
specified in the applicable specification for that source equipment.
The negative of each power source is normally connected to vehicle structure, which shall then be considered
the second wire. If vehicle structure is used as the second wire, it is preferable, especially in the case of
aircraft with considerable composite structure, to make this connection close to the point of power distribution
to utilization equipment. An alternative grounding scheme that carries a neutral return wire through to the
utilization equipment may also be applied.
4.3 External power sources
External power sources shall conform to the requirements identified in Clause 3.2 in all areas regarding output
of power to the aircraft. Specific requirements for external power sources shall be as specified in ISO 6858. In
addition, external power sources may have environmental and/or safety requirements imposed upon them due
to local regulations and codes to which they must adhere.
4.4 Source/distribution system coordination
Necessary coordination between the design and control of the electrical power sources and the distribution
system, including all of the involved protective devices, shall be such as to ensure that the characteristics of
electrical power at the utilization equipment terminals are in accordance with this International Standard. In
addition, they shall specifically coordinate to ensure that the failure of any power source and its disconnection
from the system does not result in subsequent impaired performance or loss of the remaining power sources.
4.5 Utilization equipment
4.5.1 Equipment operation and performance
Utilization equipment shall maintain its specified performance when supplied with power having the ranges of
characteristics detailed herein and shall not degrade the power characteristics beyond their allowed limits.
This shall include potential failure modes of the utilization equipment.
When use is required of power having other characteristics, or closer tolerances than are specified herein, the
conversion to other characteristics or closer tolerances shall be accomplished as part of the utilization
equipment.
The individual specification for the utilization equipment shall state the degrees of degradation of performance,
if any, permitted in specific regions of normal, abnormal, emergency system or engine starting operation.
Utilization equipment shall not suffer damage or cause an unsafe condition and shall automatically resume
specified performance following the return to normal operating conditions from any of these regions of
operation.
In the case where equipment would require previous energizing to ensure its operation (pre-heating or storing),
its consumption in the waiting period shall be maintained at the minimum value.
4.5.2 Power type
Use of the primary power type generated shall be maximized, and where there are alternatives, due
consideration shall be given to the power type requiring the lightest weight distribution cable. Single-phase
supplied equipment should be connected between phase and neutral. All equipment for which power
consumption exceeds 500 VA should be supplied from a three-phase power source.
These requirements may be disregarded where
⎯ 28 V d.c. is the only supply available;
⎯ emergency operation is required; or
⎯ interrupt-free supplies are required (unless no break power transfer is available on the a.c. supply bus).
Utilization equipment shall preferably use only one power type (a.c. or d.c.). Equipment that uses multiple
power types shall provide its specified performance when subjected to simultaneous variations and singular
interruptions of each power input within the limits described in this International Standard.
The wiring of return currents shall be carried out as follows:
⎯ no point of equipment internal wiring shall be bonded to its casing; and
⎯ all connections to neutral or negative (d.c.) shall be separately brought out from the equipment before
being connected to the structure or ground.
The casing shall be bonded to the aircraft structure by an independent connection. The details of application
shall be in accordance with the applicable standards in the areas of interference elimination and bonding.
4.5.3 Power supply interruptions
The loss of power (a.c. or d.c.) or the loss of one or more phases of a.c. power to any utilization equipment
terminal, however repetitive, shall not result in an unsafe condition or damage to utilization equipment.
Equipment performance in these events shall be as defined in the applicable utilization equipment
specification for abnormal operation. Allowances for equipment re-initialization are expected in these
circumstances.
In the case of power supply interruptions, the voltage loss may be progressive; so-called transients of normal
operation may follow the recovery voltage.
Micro-interruptions or under voltages of variable duration may be seen as interruptions of supply by certain
components. In this case, equipment comprising digital circuits and/or memory or sequential circuits shall be
the subject of a self-test. (See ISO 7137.)
4.5.4 Power supply input polarity
User equipment is not required to operate, but shall not be damaged when the polarity of the input power it
receives is incorrect. Direct current or single phase a.c. user equipment shall accept reversal of power and
return inp
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