IEC 62310-3:2008

IEC 62310-3
Edition 1.0 2008-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Static transfer systems (STS) –
Part 3: Method for specifying performance and test requirements

Systèmes de transfert statique (STS) –
Partie 3: Méthode de spécification des performances et exigences d’essai

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IEC 62310-3
Edition 1.0 2008-06
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Static transfer systems (STS) –
Part 3: Method for specifying performance and test requirements

Systèmes de transfert statique (STS) –
Partie 3: Méthode de spécification des performances et exigences d’essai

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
XA
CODE PRIX
ICS 29.200; 29.240.30 ISBN 2-8318-9812-9

– 2 – 62310-3 © IEC:2008
CONTENTS
FOREWORD.4
1 Scope.6
2 Normative references .6
3 Terms and definitions .7
3.1 General definitions .7
3.2 Specified values .10
3.3 Input values.11
3.4 Output values.11
4 Performance requirements .12
4.1 Declared electrical characteristics .12
4.1.1 Performance classification.12
4.1.2 Main electrical characteristics.13
4.2 Normal service conditions .14
4.3 Transfer characteristics .14
4.3.1 Load voltage.14
4.3.2 Cross-current .15
4.4 Neutral management .15
4.4.1 Common neutral .15
4.4.2 Separately derived neutrals .15
4.4.3 Neutral current .15
4.4.4 Protective earth current .15
4.5 Performance requirements to be identified by the purchaser .16
5 STS tests .16
5.1 General .16
5.1.1 Type tests .16
5.1.2 Routine tests .16
5.1.3 Site tests .16
5.1.4 Testing schedule .16
5.2 Electrical tests.18
5.2.1 Insulation and dielectric.18
5.2.2 Light load and functional test.18
5.2.3 No load.19
5.2.4 Rated load.19
5.2.5 Transfer.19
5.2.6 Source tolerance .21
5.2.7 Overload and fault current .21
5.2.8 Operating losses .23
5.2.9 Backfeed .24
5.2.10 Abnormal operating conditions .25
5.3 Transportation and environment .26
5.3.1 General .26
5.3.2 Impact and shock .26
5.3.3 Free fall.26
5.3.4 Storage .27
5.3.5 Temperature and humidity .28
5.3.6 Acoustic noise .28

62310-3 © IEC:2008 – 3 –
Annex A (normative) Reference load.29
Annex B (normative) STS voltage sensing tolerance .33
Annex C (informative) Neutral management in static transfer systems .35
Annex D (normative) Neutral switching in static transfer systems – Additional
requirements.37
Annex E (informative) Purchaser specification guidelines .39
Annex F (informative) Output short circuit test procedure.43
Annex G (normative) Backfeed protection test .47
Annex H (normative) Minimum and maximum cross-sections of copper conductors
suitable for connection.48
Bibliography.49

Figure 1 – Complying load current flow - Common grounding of a.c. input sources -
common STS neutral .15
Figure 2 – Objectionable load current flow - Independent grounding of a.c. input
sources - common STS neutral .16
Figure 3 – STS cross current test.20
Figure 4 – Typical circuit for measuring conduction losses.24
Figure B.1 – Source tolerance classification 1.33
Figure B.2 – Source tolerance classification 2.34
Figure B.3 – Source tolerance classification 3.34
Figure B.4 – Source tolerance classification 4.34
Figure C.1 – Continuous STS neutral with common grounding of a.c. input sources
(complying load current flow) .35
Figure C.2 – Continuous STS neutral with independent grounding of a.c. input sources
+ isolation transformer (complying load current flow) .36
Figure C.3 – Switched STS with independent grounding of a.c. input sources
(complying steady state load current flow) .36
Figure D.1 – Load voltage variation upon neutral switch failure.37
Figure D.2 – Overlapping switched STS neutral .38
Figure F.1 – Test circuit for STS output withstand current.45

Table 1 – Transfer interruption classification “T” .13
Table 2 – Tests for STS performance characteristics .17
Table 3 – Short time withstand current.22
Table 4 – Free fall testing .27
Table E.1 – STS technical data sheet .41
Table F.1 – Values of power-factors and time-constants corresponding to test currents
and ratio n between peak and r.m.s. values of current .44
Table H.1 – Minimum and maximum cross-sections of copper conductors suitable for
connection (extract from IEC 60439-1).48

– 4 – 62310-3 © IEC:2008
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
STATIC TRANSFER SYSTEMS (STS) –

Part 3: Method for specifying performance and test requirements

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 provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with an IEC Publication.
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 62310-3 has been prepared by subcommittee 22H: Uninterruptible
power systems (UPS), of IEC technical committee 22: Power electronic systems and
equipment.
The text of this standard is based on the following documents:
FDIS Report on voting
22H/105/FDIS 22H/107/RVD
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.
A list of all parts of the IEC 62310 series, under the general title: Static transfer systems
(STS), can be found on the IEC website.

62310-3 © IEC:2008 – 5 –
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result 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.
– 6 – 62310-3 © IEC:2008
STATIC TRANSFER SYSTEMS (STS) –

Part 3: Method for specifying performance and test requirements

1 Scope
The IEC 62310 series of three standards applies to stand-alone operating a.c. static transfer
systems (STS) intended to ensure the continuity of load supply through controlled transfer,
with or without interruption of power, from two or more independent a.c. sources.
This series of standards includes information for the overall integration of the STS and its
accessories into the a.c. power network and includes requirements for the switching
elements, their control and protective elements, where applicable.
Part 1 of the series concerns general and safety requirements.
Part 2 of the series concerns electromagnetic compatibility (EMC) requirements.
This Part 3 of the series concerns methods for specifying performance and test requirements
including applicable safety tests referenced in standard IEC 62310-1 for general and safety
requirements.
This standard applies for single-phase, phase-phase and three-phase static transfers in a. c.
systems up to 1 000 V. It takes precedence over all aspects of generic performance
standards, and no additional performance testing is necessary.
The requirements have been selected so as to be consistent with compatibility levels for low-
frequency conducted disturbances and signalling in public low-voltage power supply systems
(see IEC 61000-2-2) as well as to ensure an adequate level of performance when the STS is
applied in diverse critical load situations. The requirements take into account the differing test
conditions necessary to encompass the range of physical sizes and power ratings of STS.
This standard applies to STS as a stand-alone product, whether presented as a unit or an
assembly of units. This standard does not apply to:
– devices for d.c. source switching;
– single source systems;
– transfer systems using only electromechanical switching devices with interruption of the
supply to the load during transfer and intended to be used in emergency power systems or
covered by IEC 60947-6-1;
– automatic switching devices integrated into UPS covered by the IEC 62040 series of UPS
product standards.
NOTE Additional or different requirements may apply to STS intended for use on board of vehicles including ships
and aircrafts, in emergency power systems subject to a particular regulation e.g. health care facilities, fire fighting
or emergency rescue, in tropical countries or where elevations are greater than 1 000 m.
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 60068 (all parts), Environmental testing

62310-3 © IEC:2008 – 7 –
IEC 60146-1-1, Semiconductor convertors – General requirements and line commutated
convertors – Part 1-1: Specifications of basic requirements
IEC 60439-1, Low-voltage switchgear and controlgear assemblies – Part 1: Type-tested and
partially type-tested assemblies
IEC 60529, Degrees of protection provided by enclosures (IP Code)
IEC 60947-6-1, Low-voltage switchgear and controlgear – Part 6-1: Multiple function
equipment – Transfer switching equipment
IEC 60950-1, Information technology equipment – Safety – Part 1: General requirements
IEC 61000-2-2, Electromagnetic compatibility (EMC) – Part 2-2: Environment – Compatibility
levels for low-frequency conducted disturbances and signalling in public low-voltage power
supply systems
IEC 62040-3, Uninterruptible power systems (UPS) – Part 3: Method of specifying the
performance and test requirements
IEC 62310-1, Static Transfer Systems (STS) – Part 1: General and safety requirements
IEC 62310-2, Static Transfer Systems (STS) – Part 2: Electromagnetic Compatibility (EMC)
requirements
ISO 7779, Measurement of airbone noise emitted by information technology and
telecommunications equipment
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1 General definitions
3.1.1
static transfer system (STS)
system that transfers a load, by static means, between a preferred source and an alternate
source
NOTE 1 The transfer may be automatic and/or manual.
NOTE 2 The transfer may be with or without interruption.
3.1.2
power pole or electronic power switch
in the context of this standard, an operative unit for electronic power switching comprising at
least one controlled electronic valve device
[IEV 551-13-01, modified]
3.1.3
primary circuit
internal circuit which is directly connected to the external supply source which supplies the
electric power to the load. It includes the primary windings of transformers, motors, other
loading devices and the means of connection to the supply source.

– 8 – 62310-3 © IEC:2008
3.1.4
input power
power derived either from the preferred or from the alternate source or from both and supplied
to the STS (and maintenance bypass if any)
3.1.5
SELV circuit
secondary circuit which is so designed and protected that under normal operating conditions
and single fault conditions, its voltages do not exceed a safe value
NOTE 1 The limit values of voltages under normal operating conditions and single fault conditions are specified in
2.2 of IEC 60950-1, see also Table 1A of IEC 60950-1.
NOTE 2 This definition of an SELV CIRCUIT differs from the term “SELV system” as used in IEC 61140.
3.1.6
preferred source
source used as normal power supply to the load, usually set by the operator
3.1.7
alternate source
source used as alternate power supply to the load when the preferred source fails or is out of
tolerance or is switched off for maintenance
3.1.8
normal mode of STS operation
condition where the load is supplied via the electronic (power) switches by either the
preferred source or by the alternate source
3.1.9
maintenance bypass
power path designed to allow isolation of an appropriate section or sections of an STS for
safety during maintenance and/or to maintain continuity of load power
3.1.10
transfer
act of altering the supply path to the load from one source to another
3.1.11
automatic transfer
transfer without human intervention as a result of the supplying source being outside specified
conditions
3.1.12
automatic retransfer
transfer without human intervention from an alternate to the preferred source once the
preferred source has returned to specified conditions
3.1.13
manual transfer
transfer that occurs as a result of local or remote operator intervention
3.1.14
normal transfer
transfer of load power between two sources while their voltage phase angle difference is
within a tolerance band as declared by the manufacturer
3.1.15
synchronous (or synchronised) transfer
transfer within a limited voltage phase angle difference specified by the user

62310-3 © IEC:2008 – 9 –
3.1.16
asynchronous (or non-synchronised) transfer
transfer of load power between two sources while their voltage phase angle difference, when
transfer occurs, is out of a tolerance band as declared by the manufacturer
3.1.17
transfer time
time interval between initiation of transfer and the instant when the output quantities have
been transferred
3.1.18
cross-current
current due to conduction from a phase of one source through electronic power switches to
the corresponding phase of another source
3.1.19
objectionable current
load current that, during normal operation and/or transfers, flows in paths other than those
intended and that contributes to any of the following:
– interference with the proper sensing and operation of ground-fault residual current devices
(RCDs)
– arcing of sufficient energy to ignite flammable materials
– electromagnetic emission in excess of levels prescribed in IEC 62310-2 for STSs
3.1.20
linear load
load where the current drawn from the supply is defined by the relationship:
I = U/Z
where
I is the load current;
U is the supply voltage;
Z is a constant impedance
3.1.21
non-linear load
load where the parameter Z (load impedance) is no longer a constant but is a variable
dependent on other parameters, such as voltage or time
3.1.22
active power
under periodic conditions, mean value, taken over one period T, of the instantaneous power p:
T
P = ⋅ p ⋅ dt
∫
T
[IEV 131-11-42]
NOTE 1 Under sinusoidal conditions, the active power is the real part of the complex power.
NOTE 2 The SI unit for active power is the watt.
NOTE 3 DC, fundamental and harmonic voltages contribute directly to the magnitude of the active power. Where
applicable, instruments used to measure active power should therefore present sufficient bandwidth and be
capable of measuring any significant non-symmetrical and harmonic power components.

– 10 – 62310-3 © IEC:2008
3.1.23
power factor, (λ)
ratio of the active power to the apparent power
P
λ =
S
[IEV 131-11-46, modified]
NOTE For the purpose of this standard, the load power factor is determined assuming an ideal sinusoidal supply
voltage, where the load is non-linear; the load power factor includes harmonic power components.
3.1.24
apparent power, (S)
product of the r.m.s. values of voltage and current at a port
[IEV 133-11-41, modified]
S = UxI
3.1.25
ambient temperature
temperature of the air or other medium where the equipment is to be used
[IEV 826-01-04]
3.2 Specified values
3.2.1
rated value
quantity value assigned, generally by a manufacturer, for a specified operating condition of a
component, device or equipment
[IEV 151-16-08, modified]
3.2.2
rating
set of rated values and operating conditions of a machine or a device or equipment
[IEV 151-16-11]
3.2.3
tolerance (band)
range of values of a quantity within specified limits
3.2.4
deviation
difference between the desired value and the actual value of a variable at a given instant
[IEV 351-12-15, modified]
3.2.5
transient
behaviour of a variable during transition between two steady states
[IEV 351-14-04]
3.2.6
rated voltage
input or output voltage (for three-phase supply, the phase-to-phase and phase-to-neutral
voltage when the STS supports neutral connection) as declared by the manufacturer

62310-3 © IEC:2008 – 11 –
3.2.7
voltage variation
difference between the r.m.s. voltage and the corresponding previously undisturbed r.m.s.
voltage
NOTE For the purposes of this standard, the term "variation" has the following meaning: the difference of the
value of a quantity before and after a change of an influence quantity.
3.2.8
rated current
r.m.s. input or output phase current of the equipment as declared by the manufacturer
3.2.9
rated frequency
operating frequency as declared by the manufacturer
3.2.10
total harmonic distortion (THD)
ratio in percent of the r.m.s. value of the harmonic content to the r.m.s. value of the
fundamental component of the alternating quantity
3.2.11
harmonic components
components of the harmonic content as expressed in terms of the order and r.m.s. values of
the Fourier series terms describing the periodic function
3.2.12
harmonic content
quantity obtained by subtracting from an alternating quantity its fundamental component
NOTE The harmonic content may be given as a time-function or as an r.m.s. value.
3.2.13
crest or peak factor
ratio of peak value to the r.m.s. value of a periodic quantity
NOTE The terms «crest factor» and «peak factor» have the same meaning.
3.3 Input values
3.3.1
input voltage tolerance
maximum variation of steady-state input voltage acceptable by the STS to function
3.3.2
input frequency tolerance
maximum variation of steady-state input frequency acceptable by the STS to function
3.4 Output values
3.4.1
critical load voltage protection limits
maximum variation of steady-state and of transient output voltage of the STS
3.4.2
conditional short-circuit current
prospective current that a STS can withstand when protected by a specified protective device

– 12 – 62310-3 © IEC:2008
3.4.3
output voltage tolerance
maximum variation of steady-state output voltage of the STS, generally adjusted to ensure
compliance with the critical load protection limits
3.4.4
overload capability
output current capability of the STS in excess of its stated continuous current over a given
time, with the output voltage remaining within its rated range
3.4.5
short time withstand current
current that a circuit or a switching device in a closed position can carry during a specified
short time and under prescribed conditions of use and behaviour
3.4.6
step load
instantaneous addition or removal of electrical loads to a power source
4 Performance requirements
4.1 Declared electrical characteristics
4.1.1 Performance classification
The manufacturer shall classify STSs complying with this standard in accordance with the
following coding.
XX YY B ST
where
XX characterises the management of fault current:
CB: STS capable of making and of breaking specified short-circuit currents and containing
integral overcurrent protection.
PC: STS capable of making and of withstanding specified short-circuit currents but not
intended for breaking short-circuit currents.
YY characterises the management of input neutrals:
00 – Neutral not supported
NC – Neutral common
NS – Neutral separation by switching
NI – Neutral separation by galvanic isolation
NOTE Galvanic neutral isolation can be achieved through the use of an isolation transformer.
B characterises the nature of the transfer:
B - Break before make (open transition) – no transient cross-conduction during transfer
M - Make before break (closed transition) – possible transient cross-conduction during
transfer
ST characterises the sensing and transfer features:
”S” - sensing tolerance prior to automatic transfer being initiated
Voltage in the conducting source, when beyond the specified sensing tolerance, shall
cause the STS to initiate an automatic transfer. Figures B.1, B.2, B.3 and B.4 of Annex B
define the over and undervoltage limits corresponding to sensing tolerances 1, 2, 3 and 4
respectively. The sensing tolerance (S) shall be agreed on between the manufacturer and
the purchaser in accordance with voltage limits deemed acceptable for the critical load -
see Annex E – Purchaser specification guidelines

62310-3 © IEC:2008 – 13 –
“T” - duration of any power interruption to the load once an automatic transfer is initiated
(see Table 1)
Table 1 – Transfer interruption classification “T”
T Interruption
ms
1 ≤ 0,1
2 ≤ 1
3 ≤ 10
4 ≤ 20
5 (reserved)
NOTE Transfer classification T = 5 is reserved for particular transfer characteristics if required and agreed upon
between the manufacturer/supplier and the purchaser.
Compliance with the STS sensing and transfer features is required when at least one input
source is within tolerance under conditions agreed between the purchaser and the
manufacturer. For guidance, refer to “Maximum voltage phase difference between sources to
enable synchronous transfer” in Table E.1. In the event of all preferred and alternate
source(s) being out of tolerance, the manufacturer shall specify the performance.
NOTE Phase to neutral voltage sensing is required in STS where a neutral connection is available to the load.
Example of STS performance classification: “PC NC B 23” characterises a STS that
– can make and withstand specified short-circuit currents but that is not intended for
breaking of short-circuit currents,
– presents a common input neutral terminal for all sources
– is break before make (open transition), causing no transient cross-conduction during a
transfer
– initiates a transfer automatically when the voltage of the conducting source is beyond
the limits for source tolerance classification 2 (see Annex B)
– may interrupt supply to the load for up to 10 ms during the transfer
4.1.2 Main electrical characteristics
The STS manufacturer shall specify the following characteristics in the application
documentation pertaining to a STS that complies with this standard.
4.1.2.1 Current
– Rated (phase r.m.s. value)
4.1.2.2 Voltage
– rated (phase-phase / phase–neutral as applicable, r.m.s. value)
– input tolerance (refer to 4.2a)
– input unbalance tolerance (for 3-phase STS, refer to 4.2.c)
4.1.2.3 Frequency
– rated
– tolerance (refer to 4.2.b)
4.1.2.4 Input / output wiring
– number of phases (1, 2 or 3) + N (if applicable)

– 14 – 62310-3 © IEC:2008
– pluggable or fixed wiring
4.1.2.5 Input sources
– number of input sources (2 or more)
– reference compatibility (common or separately derived, common or separately grounded)
– distribution compatibility (TNC, TNS, TT or IT, refer to annex V IEC 60950-1)
4.1.2.6 Power poles
– number of poles switched (1, 2, 3 or 4)
– switching sequence (open or closed transition – refer to 4.1.1)
– rated neutral current (percentage of rated phase current)
– state if the neutral is switched (refer to 4.1.1)
4.1.2.7 Overload
– declared (% of rated current and duration)
– load fault clearing capability (% of rated phase current and duration, refer to 5.2.7.2)
4.1.2.8 Fault (short-circuit) current
– management (PC or CB – refer 4.1.1)
– withstand capability, rated (kA and duration – refer to 5.2.7.3 )
– breaking capability, rated - CB management only (kA – refer to 5.2.7.3)
4.1.2.9 Operating losses at rated load (refer to 5.2.8)
4.2 Normal service conditions
Except where otherwise specified in this paragraph , the STS shall be compatible with
– levels for low-frequency conducted disturbances and signalling in public low-voltage power
supply systems as described in IEC 61000-2-2;
– linear loads;
– non-linear loads as specified in Clause A.5.
The STS shall be capable of operation when at least one source meets the following limits:
a) voltage variation ±10 % of nominal rated voltage, minimum;
b) frequency variation ±5 % of nominal rated frequency;
c) voltage unbalance of 5% for three-phase systems (see 2.5 of IEC 60146-1-1).
NOTE The conditions above represent compatibility requirements for proper operation of the STS and its control
circuits (as opposed to representing the actual operating limits required by the load − see Annex B).
4.3 Transfer characteristics
4.3.1 Load voltage
Transfers and retransfers in STS conforming to this standard, whether manual or automatic,
shall not contribute to an output voltage deviation in excess of the critical load voltage
protection limits as declared by the manufacturer in 4.1.1. Refer to 5.2.5.1 and 5.2.1.2 for test
guidance.
The manufacturer shall declare the maximum voltage phase difference permitted between
sources to comply with the STS transfer classification declared (see 4.1.1).

62310-3 © IEC:2008 – 15 –
4.3.2 Cross-current
Static transfer equipment shall not permit cross-current flow between input sources under any
normal operating condition except for inherent transient currents that may arise during
synchronised transfers through STS declared “closed transfer” or “make before break”.
Design of STS shall guard against current flow between sources arising as a result of a single
fault condition. Static transfer equipment declared by the manufacturer to be “closed transfer”
or “make before break” shall tolerate any inherent transient currents that may arise during
synchronised transfers without malfunctioning.
Refer to 5.2.5.3 for test guidance.
4.4 Neutral management
4.4.1 Common neutral
No neutral switching or particular management of the neutral reference is required where a.c.
input sources share a common neutral grounding point. Refer to Annex C for guidance.
4.4.2 Separately derived neutrals
A neutral management technique is required when separately derived a.c. input sources are
independently grounded and connected to a STS. Refer to Annex C for guidance.
4.4.3 Neutral current
The current carrying capacity of STS shall support the neutral current arising from any rated
combination of balanced and unbalanced linear load and of non-linear load as described in
Clause A.5.
4.4.4 Protective earth current
The STS shall not contribute to objectionable load current flow in the protective earth
conductor.
NOTE Current may flow in the protective earth conductor during transfers. This is permissible as long as the
current does not become objectionable (objectionable current is defined in 3.1).
Figure 1 is an example of a complying STS application wherein objectionable load current
cannot flow in the protective earth conductor because one a.c. source is referred to the
grounding potential of the other. The installer should verify the continuous effectiveness of the
neutral grounding path as applicable. In TN systems, this may impose additional conditions on
the upstream use of manual or automatic circuit-breakers that open the neutral.

AC sources Load
STS
IEC  788/08
Figure 1 – Complying load current flow –
Common grounding of a.c. input sources – Common STS neutral

– 16 – 62310-3 © IEC:2008
Figure 2 is an example of a non-complying STS application wherein objectionable load current
can flow in the protective earth because both a.c. sources are independently grounded.

AC sources
STS Load
Objectionable
current flow
IEC  789/08
Figure 2 – Objectionable load current flow –
Independent grounding of a.c. input sources – Common STS neutral
Compliance is checked by circuit analysis of the STS in the context of a.c. input power
distribution system classification (TN, TT, IT), the number of phases and of wires as
presented in Annex V of 60950-1.
4.5 Performance requirements to be identified by the purchaser
When applicable, and subject to an agreement between the manufacturer and the purchaser,
the purchaser shall identify alternative application related performance requirements and
deviation limits not specified in this Clause. Refer to Annex E for guidance.
5 STS tests
5.1 General
5.1.1 Type tests
Type tests are performed on a STS that represents a series of substantially identical
products. Type tests are intended to ensure that such identical products become compliant
with their full specification when produced under relevant quality standards and after having
passed the routine tests detailed in this Clause.
5.1.2 Routine tests
Routine and any optional tests are generally performed in the factory.
5.1.3 Site tests
Site tests are subject to an agreement between the purchaser and the manufacturer. If routine
tests are conducted on site, such tests shall use the available site load that shall not exceed
the rated continuous load of the complete STS site configuration.
5.1.4 Testing schedule
Tests shall be performed in accordance with Table 2.

62310-3 © IEC:2008 – 17 –
Table 2 – Tests for STS performance characteristics
Test description Routine test Type test Subclause
Insulation (to ground) 5.2.1a
x x
a)
5.2.1.b, c
Insulation (input to input and input to output) x
Light load 5.2.2
x x
Functional and Interconnection cable check 5.2.2
x x
Control device(s) 5.2.2.a
x x
Protective device(s) 5.2.2.b
x x
Auxiliary device(s) 5.2.2.c
x x
Supervisory, measuring, signalling device(s) 5.2.2.d
x x
Automatic transfer 5.2.2e
x x
Manual transfer x 5.2.2.f
x
No load x 5.2.3
x
Rated load x 5.2.4
x
Automatic transfer x 5.2.5.1
Manual transfer x 5.2.5.2
x
Cross-current 5.2.5.3
Source tolerance (voltage and frequency) x 5.2.6

Overload x 5.2.7.1
Overcurrent clearing capability x 5.2.7.2

x
Short-circuit withstand current 5.2.7.3

Operating losses x 5.2.8
Backfeed x 5.2.9
Abnormal operating conditions x 5.2.10

Transportation and environment x
5.3.1
Impact and shock x 5.3.2
Free fall x 5.3.3
Storage x 5.3.4
Temperature and humidity x 5.3.5

Acoustic noise x 5.3.6
Safety x See IEC 62310-1
Electromagnetic compatibility x See IEC 62310-2

a) The routine test exemption for input to input and input to output insulation applies only
when:
– manufacturing is subjected to a quality control programme that includes sample testing
to ensure reproducibility of the type-tested STS
NOTE
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