Directives & Standards
Technical Information
Safety
Safety
Safety Standards
There are different safety standards. They basically follow
the principles of double safety. The required measures depend on the application (e.g. hand-held, mobile or fixed installations) as well as on the user groups or use as per design intention.
Some important safety standards are:
IEC 60950, EN 60950, UL 1950, CAN/CSA C22.2 no. 95095: Safety of information technology equipment.
EN 41003: Particular safety requirements for equipment to
be connected to telecommunication networks. This standard is based on EN 60950.
IEC 60601-1, EN 60601-1, UL 2601: Medical electrical
equipment – General requirements for safety.
IEC 60601-1-1, EN 60601-1-1, UL 2601-1: Medical electrical equipment - General requirements for safety – Collateral standard: Safety requirements for medical electrical
systems.
UL 544, CSA 22.21: Medical and dental equipment.
IEC 61010-1, EN 61010-1: Safety requirements for electrical equipment for measurement, control and laboratory use
– General requirements.
Safety Terminology According to IEC 60950, EN 60950, UL 1950, CSA C22.2 No. 950
The following terms are used frequently. The definitions
given here are based on full definitions in IEC/EN 60950
including amendments 1-4 and EN 41003 but partially abbreviated to help the explanation.
Primary Circuit: An internal circuit which is directly connected to the external supply mains or other equivalent
source (such as a motor-generator set) which supplies the
electric power.
Secondary Circuit: A circuit which has no direct connection to primary power and derives its power from a transformer, converter or equivalent isolation device, or from a
battery.
Safety Extra-low Voltage (SELV) Circuit: A secondary circuit which is so designed and protected that, under normal
and single fault conditions, the voltage between any two
conductors and, for class I equipment, between any one
such conductor and the equipment protective earthing terminal, does not exceed a safe value (42.4 V AC peak or
60 V DC). Under single fault conditions the voltage is allowed to reach 71 V AC peak or 120 V DC for a maximum
time of 0.2 s. Note: Safety levels are specified in terms of
DC and AC voltages because the different types of voltages
have dangerous effects at different levels.
Telecommunications Network Voltage (TNV) Circuit:
A secondary circuit in the equipment to which the accessible area of contact is limited and that is so designed and
protected that, under normal and single fault conditions, the
voltages do not exceed specified limiting values. Voltage
limits after a single fault are given in Fig. 15 of IEC/EN
60950 (1500 V during 1 ms, 400 V during 0.2 s, 71 V AC
peak or 120 V DC continuously). TNV circuits are classified
as TNV-1, TNV-2 and TNV-3 circuits:
TNV-1 Circuit: A TNV circuit whose normal operating voltages do not exceed the limits for an SELV circuit value
(42.4 V AC peak or 60 V DC) under normal operating conditions and on which overvoltages from telecommunication
networks (up to 1500 V peak) are possible.
TNV-2 Circuit: A TNV circuit whose normal operating voltages exceed the limits for an SELV circuit, but do not exceed 71 V AC peak or 120 V DC under normal operating
conditions and which is not subject to overvoltages from
telecommunication networks.
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TNV-3 Circuit: A TNV circuit whose normal operating voltages exceed the limits for an SELV circuit, but do not exceed 71 V AC peak or 120 V DC under normal operating
conditions and on which overvoltages from telecommunication networks (up to 1500 V peak) are possible.
Extra Low Voltage (ELV) Circuit: A secondary circuit with
voltages between any two conductors of the ELV circuit,
and between any one such conductor and earth, not exceeding 42.4 V AC peak or 60 V DC, under normal operating conditions, which is separated from hazardous voltage
by at least basic insulation, and which neither meets all of
the requirements for an SELV circuit nor for a limited current circuit.
Hazardous (or Excessive) Voltage: A voltage exceeding
42.4 V AC peak or 60 V DC, existing in a circuit which does
not meet the requirements for either a limited current circuit
or a TNV circuit. (Note: The definitions of excessive and
hazardous voltages have been harmonised in the second
edition of IEC/EN 60950.)
Limited Current Circuit: A circuit which is so designed and
protected that, under both normal conditions and a likely
fault condition, the current which can be drawn is not hazardous.
Overvoltage Category: Assigns maximum expected transient voltages to nominal mains supply voltages. Examples
of preferred values of such transient voltages according to
IEC 60664-1 are: 500 V, 800 V, 1500 V, 2500 V, 4000 V. The
maximum expected transient voltage of an overvoltage category II circuit with a nominal voltage of >150 to ≤300 V is
2500 V. A secondary circuit derived from an overvoltage
category II primary circuit is considered to be subject to
overvoltage category I, i.e. in case of a primary voltage of
>150 to ≤300 V the secondary transient voltage is 1500 V.
Operational Insulation: Insulation needed for the correct
operation of the equipment. Note: Operational insulation by
definition does not protect against electric shock.
Basic Insulation: Insulation to provide basic protection
against electric shock.
Supplementary Insulation: Independent insulation applied in addition to basic insulation to ensure protection
against electric shock in the event of a failure of the basic
insulation.
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Directives & Standards
Technical Information
Double Insulation: Insulation comprising both basic insulation and supplementary insulation.
Reinforced Insulation: A single insulation system that provides a degree of protection against electric shock equivalent to double insulation.
Working Voltage: The highest voltage to which the insulation under consideration is, or can be, subjected when the
equipment is operating at its rated voltage under conditions
of normal use.
Creepage Distance: The shortest path between two conductive parts, or between a conductive part and the bounding surface of the equipment, measured along the surface
of the insulation.
Clearance: The shortest distance between two conductive
parts, or between a conductive part and the bounding surface of the equipment, measured through air.
Class I Equipment: Equipment where protection against
electric shock is achieved by using basic insulation and by
providing a means of connecting to the protective earthing
conductor in the building wiring those conductive parts that
are otherwise capable of assuming hazardous voltages if
the basic insulation fails. Note: Class I equipment may have
parts with double insulation or reinforced insulation, or
parts operating in SELV circuits. (Most of the Power-One
chassis mount and 19" AC-DC and DC-DC converters are
class I equipment.)
Class II Equipment: Equipment in which protection against
electric shock does not rely on basic insulation only, but in
which additional safety precautions, such as double
insulation or reinforced insulation, are provided, there being
no reliance on either protective earthing or installation conditions. (Power-One’s AC-DC and DC-DC converters of the
C/D/LMZ series are class II equipment.)
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Safety
Class III Equipment: Equipment in which protection against
electric shock relies upon supply from SELV circuits and in
which hazardous voltages are not generated. (E.g. PowerOne’s switching regulators PSA, driven by an SELV.)
TN-S Power System: A power distribution system having
separate neutral and protective earth conductors throughout the system.
TN-C Power System: A power distribution system in which
neutral and protective functions are combined in a single
conductor throughout the system.
TT Power System: A power distribution system having one
point directly earthed, the exposed conductive parts of the
installation being connected to earth independent of the
power system.
IT Power System: A power distribution system having no
direct connection to earth, the exposed conductive parts of
the electrical installation being earthed.
Service Personnel: Persons having appropriate technical
training and experience necessary to be aware of hazards
to which they are exposed in performing a task and of
measures to minimise the danger to themselves or other
persons. In the Power-One data sheets the term Installer is
used for this kind of persons. It includes design engineers
responsible to define safe equipment with Power-One
power supplies.
Operator (or User): Any person, other than service personnel.
Note: European and national deviations from IEC 60950
are given in annexes ZB to ZD of EN 60950, U.S. and Canadian deviations in annexes NAA to NAE of UL 1950/CSA
C22.2 No. 950.
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Directives & Standards
Technical Information
Safety
Principles of Safety from IEC 60950, EN 60950, UL 1950, CSA C22.2 No. 950
The following is a brief summary of the main safety principles and it is included to help the reader obtain a basic understanding. Anyone participating in, or responsible for,
equipment design must refer to the standards themselves.
Application of this standard is intended to prevent injury or
damage due to the following hazards:
• Electric shock
• Energy hazards
• Fire
• Mechanical and heat hazards
• Radiation Hazards
• Chemical Hazards.
Electric shock is due to current passing through the human body. Currents of the order of a milliampere can cause
a reaction in persons in good health and may cause indirect
danger due to involuntary reaction. Higher currents can
have more damaging effects. Voltages up to about 40 V AC
peak, or 60 V DC, are not generally regarded as dangerous
under dry conditions, but parts which have to be touched or
handled should be at earth potential or properly insulated.
Energy Hazards can be caused by arcing or ejection of
molten metal when adjacent poles of high current supplies
or high capacitance circuits are short-circuited. Protection
is made by separation, by shielding or by using safety interlocks.
Fire risks may result from overloads, component failure, insulation breakdown or loose connections. Fires should not
spread beyond the immediate vicinity of the source of the
fire or cause damage to the surroundings of the equipment.
Mechanical and heat hazards should be prevented by
providing adequate stability, by avoiding sharp edges and
points or by restricting access to dangerous parts.
Radiation Hazards encountered can be sonic, radio frequency, infra-red, high intensity visible and coherent light,
ionising, etc. Requirements are necessary to keep person
exposures to acceptable levels.
Chemical Hazards through contact with chemicals, their
vapours and fumes are required to be limited under normal
and abnormal conditions.
It is normal to provide two levels of protection for operators
to prevent electric shock.
Safety Concepts
It is the sole responsibility of the installer to observe the
applicable directives and standards when using power supplies. The following basic safety rules are among the ones
to be observed:
• All conductive parts of the equipment which are electrically connected with primary power have to be separated
from unearthed conductive accessible parts as well as
from a metal foil covering the accessible surface of a non
conductive enclosure by double or reinforced insulation.
• Between primary power and earthed metal parts basic
insulation is required.
Accessible safety extra low voltage (SELV) output
circuits
In order to give guidance to the installer, safety concept tables have been developed by Power-One for each family of
power supplies, based on the international standard IEC
60950. They can be found in the data sheets, chapter
Safety and Installation Instructions. The tables propose
safety measures to be taken if the output circuit of a power
supply is accessible, i.e. if it has to be an SELV circuit. They
describe many common situations, but other solutions and
other situations exist. In applications not falling under the
scope of IEC 60950 (Safety of information technology
equipment) other measures might be required, which, especially in medical applications, might be more onerous.
With the example of the IMR 40 DC-DC converters the
meaning and the use of these tables are explained:
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Directives & Standards
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Safety
Safety of operator accessible output circuit
If the output circuit of a DC-DC converter is operator accessible, it shall be an SELV circuit according to IEC/EN 60950
related safety standards
The following table shows some possible installation configurations, compliance with which causes the output circuit
of the DC-DC converter to be an SELV circuit according to
IEC/EN 60950 up to a configured output voltage (sum of
nominal voltages if in series or +/– configuration) of 30 V A.
However, it is the sole responsibility of the installer to assure the compliance with the relevant and applicable safety
regulations. More information is given in Technical Information: Safety.
Table 9: Safety concept leading to an SELV output circuit
Conditions Front end B
Supply
voltage
Minimum required grade
of isolation, to be provided
by the AC-DC front end,
including mains supplied
battery charger C
Mains
Basic
≤250 V AC
Double or reinforced
DC-DC converter
Result F
Maximum Minimum required safety
DC output status of the front end
voltage
output circuit D
from the
front end 1 D
Measures to achieve the
specified safety status of the
output circuit E
Safety status of
the DC-DC
converter output
circuit
≤60 V
Earthed SELV circuit 2
Operational insulation, provided by the DC-DC converter
SELV circuit
ELV circuit
Input fuse 3, output suppressor Earthed SELV
diodes 4 and earthed
circuit
output circuit 2
>60 V
Hazardous voltage
secondary circuit
≤60 V
SELV circuit
Operational insulation, provided by the DC-DC converter
>60 V
TNV-2 circuit
Earthed output circuit 2
Double or reinforced insu- Input fuse 3 and output
lated unearthed hazardous suppressor diodes 4
voltage secondary circuit 5
1
2
3
4
5
SELV circuit
Earthed SELV
SELV circuit
The front end output voltage should match the specified input voltage range of the DC-DC converter.
The earth connection has to be provided by the installer according to the relevant safety standard, e.g. IEC/EN 60950.
The installer shall provide an approved fuse (type with the lowest rating suitable for the application) in a non-earthed input conductor
directly at the input of the DC-DC converter (see fig. Schematic safety concept). For UL’s purpose, the fuse needs to be UL-listed.
See also Input Fuse.
Each suppressor diode should be dimensioned in such a way, that in the case of an insulation fault the diode is able to limit the output
voltage to SELV (<60 V) until the input fuse blows (see fig. Schematic safety concept).
Has to be insulated from earth by double or reinforced insulation according to the relevant safety standard, based on the maximum
output voltage from the front end.
Mains
~
10004
Fuse
~
AC-DC
front
end
Battery
Earth
connection
+
DC-DC
converter
Suppressor
diode
SELV
Earth
connection
–
Fig. 8
Schematic safety concept.
Use fuse, suppressor diode and earth connection as per
table Safety concept leading to an SELV output circuit.
A
Given by the definition of an SELV circuit, its voltage must stay below 60 V even in a single fault condition. Since in case
of a fault in the control circuit of this converter family the output voltage can rise by a maximum of 100% of the nominal
voltage, the maximum nominal voltage is 30 V between any two conductors of the SELV circuit.
B
Examples of front ends are: AC-DC converter, Transformer – rectifier – capacitor circuit, battery charger, battery. A battery without connection to a primary circuit is considered to be a secondary circuit.
C
It is the responsibility of the installer to provide the required grade of isolation of the front end.
D
It is the responsibility of the installer to provide the required safety status of the front end output circuit and the required
measures given in the footnotes.
E
If the input of a DC-DC converter is not connected to an SELV circuit and if the converter does not inherently provide a
higher grade of insulation than operational, the installer shall provide external measures specified in this column.
F
This column shows the resulting safety status of the output circuit for the case that all precautions of the preceding columns are taken.
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Directives & Standards
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Safety
Non Accessible Output Circuits at Hazardous Voltages
The insulation (clearance and creepage distances) within
the Power-One power supplies has been designed in order
to fulfil the SELV requirements up to the specified
configured output voltage if they are installed according to
the safety concept. If the installer proceeds differently or if
he does not maintain the specified output voltage limit (e.g.
by connecting several outputs in series, generating a
higher output voltage or by connecting an output terminal to
a non SELV circuit) the output circuit may no longer be an
SELV circuit and it has to be kept inaccessible in order to
avoid electric shock.
In such cases which not all could have been foreseen when
designing the power supplies, consideration shall be given
to the following:
When installation is not proceeded according to the
safety concept, overstressing of solid insulation could
occur between any of the following circuits: input,
case, output, output, auxiliary functions, depending on
the application.
If a solid insulation is continuously overstressed, partial discharge and tracking starts taking place, which can result in
a deterioration of the insulating material and – as a long
time effect – in a total breakdown of the insulation.
The specified electric strength test voltages do not
show the allowed external steady-state working voltage between circuits. With this test voltage the withstand
capability of the power supply against transient overvoltages is verified.
In the following guidance is given to how to avoid overstressing of solid insulation within Power-One power supplies which is due to an installation configuration other than
recommended in the table Safety concept.
Non SELV Configurations with Class I and Class II
Equipment
In class II equipment, power supplies designed to be connected to a primary circuit and to have an SELV output circuit (e.g. LMZ family) double or reinforced insulation is provided within the unit between input and output as well as
between input and case. Insulation between output and
case as well as between output and output is operational.
If the output of such a power supply does not need to be an
SELV circuit, only basic insulation between primary and
secondary is required. In such a case the working voltage
is allowed to be approx. twice as high as across reinforced
insulation. As a rule of thumb it can be said that the external
steady-state working voltage between input and output can
be twice the sum of the maximum input voltage and the
nominal output voltage of a power supply. If working with
higher external steady-state working voltages is required,
please consult the factory. See also table Inquiry form for
maximum steady state working voltage.
If power supplies with metal cases are used, consideration has to be given not to overstress input to case and output to case insulation:
Input to case insulation normally is basic and is designed based on the maximum input voltage. Therefore the
steady-state input to case working voltage has to be limited
to this value. Some power supply families allow a higher
external steady-state input to case working voltage. Please
consult the factory. See also table Inquiry form for maximum steady state working voltage.
If output to case insulation usually is operational, the
value of the acceptable external steady-state working voltage between output and case can change from one power
supply family to another depending on their constructional
design. For applications where the external output to case
voltage is higher than 60 V please consult the factory. See
also table Inquiry form for maximum steady state working
voltage.
Class I equipment power supplies which are designed to
be connected to a primary circuit provide minimum basic insulation between input and case and double or reinforced
insulation between input and output. Insulation between
output and case as well as between output and output is
operational.
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