IEC/TR 60664 2 1 Edition 2 0 2011 01 TECHNICAL REPORT RAPPORT TECHNIQUE Insulation coordination for equipment within low voltage systems – Part 2 1 Application guide – Explanation of the application o[.]
Basic principles
Insulation coordination implies the selection of the electric insulation characteristics of the equipment with regard to its application and in relation to its surroundings
Effective insulation coordination requires that equipment design considers the stresses from expected voltage levels and micro-environmental conditions throughout its anticipated lifespan.
With regard to voltage, due consideration shall be made to
The low-voltage supply system can experience various voltages, including working voltage (both RMS and peak), temporary overvoltage (peak), and impulse voltages (peak) Additionally, equipment within the system may generate voltages that could negatively impact other devices in the low-voltage supply network.
For steady-state voltage frequencies up to 30 kHz, IEC 60664-1 is adequate; however, for frequencies exceeding 30 kHz, it is essential to also consider IEC 60664-4 Additionally, the desired degree of continuity of service must be taken into account.
– the safety of persons and property, so that the probability of undesired incidents due to voltage stresses does not lead to an unacceptable risk of harm
Insulation coordination is essential for equipment linked to public low-voltage systems, but it is advisable to apply the same principles to all other low-voltage systems that are not connected to the public network In these instances, different overvoltage categories and temporary overvoltages may be relevant for the equipment involved.
Technical committees utilizing the IEC 60664 series must assess the maximum impulse voltage relevant to their specific applications, taking into account factors such as the source type, distribution, physical environment (indoor or outdoor), and cable length It is crucial to note that the impulse withstand voltage for non-mains systems is not solely determined by the voltage level Therefore, for certain applications, technical committees should consider a minimum impulse withstand voltage that is independent of the voltage.
Insulation coordination is relevant for specially protected areas, as outlined in IEC 60079 In these situations, specific additional requirements must be met, particularly concerning the overvoltage category and environmental conditions.
Coordination of overvoltage categories inside equipment
For equipment which is directly energized by the mains, the following coordination with respect to transient overvoltages originating from the mains is used:
– for circuits directly energized by the mains, the overvoltage category of the equipment is used for dimensioning;
Circuits powered from the secondary of an isolation transformer with an earthed secondary winding, or from a transformer with an earth screen between the primary and secondary, are not classified as directly energized by the mains Consequently, the applicable impulse withstand voltage is one step lower in the preferred series of rated impulse voltage as specified in IEC 60664-1:2007, section 4.2.3.
NOTE 1 A step can be considered within the numerals of the overvoltage categories or within the lines of Table F.1 in IEC 60664-1:2007
NOTE 2 The transfer ratio of the transformer is not taken into account for the choice of the overvoltage category
To ensure the proper functioning of circuits not directly powered by the mains but located within equipment, it is essential to test these circuits with a hybrid generator featuring a virtual impedance of 2 Ω when surge protective devices (SPDs) are employed to apply a lower overvoltage category.
The effectiveness of a surge protective device (SPD) is influenced by the series impedance in the associated circuit, making it essential to conduct tests of the SPD within that specific circuit.
Practical use of the IEC 60664 series for the dimensioning of clearances
General
All values specified in IEC 60664-1 and IEC 60664-5 are minimum requirements that must be upheld throughout the equipment's lifespan, factoring in manufacturing tolerances It is also essential to consider specific scenarios, such as the on-site assembly of large equipment, including wiring or protective conductive enclosures, which may require additional tolerances.
NOTE 1 When dimensioning clearances to accessible surfaces of insulating material, such surfaces are assumed to be covered by metal foil Further details can be specified by technical committees
According to IEC 60664-1, a voltage test is essential for clearances designed between case A and case B values to ensure no flashover occurs When conducting this test with impulse voltage on complete equipment, a low impedance generator, such as a hybrid generator with a virtual impedance of 2 Ω, may be necessary Additionally, it is crucial to measure the correct test voltage directly at the clearance.
NOTE 2 It is recommended to apply case A during design If not possible, impulse testing is necessary
NOTE 3 In practice, some design may exist that lie in between the situation described in case A and case B In this case, TCs should pay attention to 6.1.2.2.1.2 of IEC 60664-1:2007
NOTE 4 Case A is the most unfavourable case where the electrical field is absolutely inhomogeneous between a sharp needle and a plane surface Case B is the most favourable case where the electrical field is completely homogeneous between two plane surfaces This case can never be reached in a real design.
Practical use of Tables F.2 and F.7 of IEC 60664-1:2007 for the
Clearances are dimensioned to withstand the required impulse withstand voltage either:
- by requiring dimensions of not less than case A values; or
- by requiring verification by an impulse test (see 6.1.2.2.1.2 of IEC 60664-1:2007)
Clearances of basic and supplementary insulation are each dimensioned as specified in Table F.2 of IEC 60664-1:2007 corresponding to:
- the rated impulse voltage, according to 4.3.3.3 of IEC 60664-1:2007 or 4.3.3.4.1 of IEC 60664-1:2007; or
- the impulse withstand voltage requirements according to 4.3.3.4.2 of IEC 60664-1:2007
Clearances for reinforced insulation are determined according to Table F.2 of IEC 60664-1:2007, which corresponds to the rated impulse voltage These clearances are set one step higher in the preferred series of values outlined in section 4.2.3 of the same standard compared to those specified for basic insulation.
According to IEC 60664-1:2007, if the required impulse withstand voltage for basic insulation is not from the preferred series, reinforced insulation must be designed to endure 160% of this voltage.
NOTE 1 The rated impulse voltage specified in Table F.1 of IEC 60664-1:2007 depends on the appropriate overvoltage category Overvoltage category I is not applicable to any circuit directly energized by the mains
NOTE 2 In the case of d.c voltage, the rated impulse voltage can also be chosen from Table F.1 in IEC 60664- 1:2007 The overvoltage category can be chosen with the same rules used by TCs for a.c systems
For equipment connected to the supply mains, the required impulse withstand voltage is determined by the rated impulse voltage as specified in IEC 60664-1:2007, section 4.3.3.3 Clearances must be sized according to Table F.7a of IEC 60664-1:2007 to endure the peak value of steady-state voltage (d.c or 50/60 Hz), temporary overvoltage, or recurring peak voltage The sizing process involves comparing Table F.7 with Table F.2 of IEC 60664-1:2007, considering the pollution degree, and selecting the larger clearance.
NOTE 3 However, it is recommended to introduce a safety margin for the dimensioning according to Table F.7 of IEC 60664-1:2007 since this table provides a minimum dimensioning with respect to steady-state voltages
It is recommended that technical committees consider the consequences of a flashover in a d.c low-voltage system in order to decide whether it is necessary to introduce appropriate safety measures
NOTE 4 An equipment directly energized by the mains can be either a fixed equipment directly connected to the mains or a normally plugged equipment energized from the mains through a plug and socket-outlet
NOTE 5 It can be observed from the following example, applicable to most equipment used within an electrical installation directly connected to 230/400 V three-phase system, that the rated impulse voltage as specified in Table F.1 of IEC 60664-1:2007 is the highest overvoltage to be withstood by the equipment and leads to the appropriate dimensioning for clearances of basic insulation
A single-phase device rated at 250 V, connected directly to a 230 V mains supply, must endure a rated impulse voltage of 4 kV as specified in Table F.1 of IEC 60664-1:2007 Consequently, the required clearance is 3 mm, in accordance with Table F.2 Case A of the same standard.
The steady-state voltage and recurring peak voltages in this example are both 353 V, which is the peak voltage of the mains This value corresponds to a clearance of 0.013 mm, as specified in Table F.7 Case A of IEC 60664-1:2007.
According to IEC 60664-1:2007, section 5.3.3.2.3, short-term temporary overvoltages are defined as \$U_n + 1,200 \, \text{V}\$, resulting in a peak voltage of \$2,050 \, \text{kV}\$ This peak voltage corresponds to a clearance of \$1.27 \, \text{mm}\$, as specified in Table F.7 Case A of the same standard.
– The length of the clearance for the basic insulation is therefore dimensioned according to the rated impulse voltage
The degree of pollution has a limited impact on the sizing of clearances, as indicated in Table F.2 of IEC 60664-1:2007, which shows that distances remain consistent regardless of the pollution level above a certain minimum However, for smaller clearances, pollution factors like solid particles, dust, and condensation must be considered, as they can potentially bridge the air gap.
NOTE 6 More details regarding dimensioning of clearances for distances lower than 2 mm are given in IEC 60664-5 which takes into account humidity See Clause 7 of this application guide for examples
Reinforced insulation clearances are designed to handle steady-state voltages, recurring peak voltages, and temporary overvoltages, as outlined in Table F.7a of IEC 60664-1:2007, ensuring they can withstand 160% of the voltage required for basic insulation.
NOTE 7 It should be noted that while clearance for reinforced insulation is dimensioned with respect to 160 % of the temporary overvoltage for basic and supplementary insulation, the test voltage for verification of the clearance of the reinforced insulation is twice the voltage of the test voltage for verification of basic and supplementary insulation
4.3.2.2 Design for high altitude above 2 000 m
The dimensioning of clearances is essential for selecting an air distance that can endure the maximum peak voltage across the air gap between components at different voltages According to Paschen’s law, the ability of air to withstand maximum voltage is influenced by air pressure IEC 60664-1:2007 provides tables F.2 and F.7 for altitudes up to 2,000 m, while correction factors for elevations exceeding 2,000 m are detailed in Table A.2 of the same standard.
When applying correction factors for altitudes exceeding 2,000 m to determine clearances, the test voltage for the impulse voltage test is also adjusted Consequently, the test voltage is calculated using interpolation from Table A.2 of IEC 60664-1:2007, along with the formulas outlined in section 6.1.2.2.1.3 of the same standard.
Practical use of Tables 2 and 3 of IEC 60664-5:2007 for the
For clearances of 2 mm or less for basic insulation, IEC 60664-5 offers more precise dimensioning than IEC 60664-1 However, if such precision is unnecessary, IEC 60664-1 can be used as an alternative.
A flashover across a clearance is triggered by the peak value of the maximum voltage present It is essential to select this peak voltage based on the manufacturer's specifications under rated conditions For circuits powered directly by low-voltage mains, the necessary impulse voltage can be found in Table F.1 of IEC 60664-1:2007.
The choice of the pollution degree shall be made in accordance with the normal use of equipment within the macro-environment
Humidity significantly impacts pollution levels, as outlined in IEC 60664-5 For distances of 2 mm or less, humidity can increase conductivity and the risk of flashover This consideration is addressed in section 4.4.3 of the application guide, which details the calculation of creepage distances to prevent flashover, referencing Table 5 of IEC 60664-5:2007.
NOTE 1 A relationship between humidity levels and relative humidity of the micro-environment is given in Table 1 in IEC 60664-5:2007
The dimensioning of clearance for transient overvoltages is outlined in Table 2 of IEC 60664-5:2007 Notably, the minimum clearances for pollution degrees 2, 3, and 4 previously found in Table F.2 of IEC 60664-1:2007 have been removed Instead, this application guide emphasizes a more precise dimensioning approach as detailed in section 4.4.3, focusing on the potential flashover of the parallel creepage distance as specified in Table 5 of IEC 60664-5:2007.
When dimensioning clearances for steady-state voltages, manufacturers evaluate the maximum peak value of steady-state voltage, temporary overvoltage, or recurring peak voltage, and select the appropriate value from Table 3 in IEC 60664-5:2007.
NOTE 2 Considerations made on Case A and Case B for Table F.2 in 5.1.3 in IEC 60664-1:2007 and for Table 2 in IEC 60664-5:2007 also apply for this table
NOTE 3 However, it is recommended to introduce a safety margin for the dimensioning according to Table 3 of IEC 60664-5:2007 since this table provides a minimum dimensioning with respect to steady-state voltages
This value is compared to the value according to the procedure applicable in Table 2 of IEC 60664-5:2007.
Practical use of the IEC 60664 series for the dimensioning of creepage
General
IEC 60664-1 does not consider minimum insulation resistance in its dimensioning values Consequently, electronic equipment may necessitate larger dimensions or enhanced micro-environments at creepage distances for functional purposes For guidance on dimensioning related to minimum insulation resistance, refer to Table A.1 and Table A.2 in IEC 60664-5:2007.
For printed wiring materials used in pollution degrees 1 and 2, IEC 60664-1 allows for reduced creepage distances It is important to consider that components may lead to further reductions or alternative paths for creepage distances.
According to IEC 60664-5, the dimensioning of creepage distances for tracking and flashover along material surfaces for distances of 2 mm or less can result in reduced spacing requirements.
Practical use of Table F.4 of IEC 60664-1:2007 and Table 4 of
IEC 60664-5:2007 for the dimensioning of creepage distances
Dry pollution on a material's surface is typically non-conductive, but the presence of water alters this conductivity Increased conductivity facilitates the flow of tracking currents between live components or between live parts and the ground As the material dries, these tracking currents can break, leading to surface scintillation, which generates high temperatures (approximately 1,200 °C) that degrade the insulating material's surface This process is known as tracking.
NOTE It is obvious that pollution degree 4 cannot be used for the dimensioning of creepage distances since the surface is continuously conductive
Certain materials, like ceramic and glass, do not exhibit tracking due to the inability of scintillation to disrupt their surface chemical bonds Research indicates that materials with superior tracking performance tend to have similar rankings in the comparative tracking index (CTI) The CTI can be assessed using the method outlined in IEC 60112.
For practical reasons, IEC 60664-1 introduces four different material groups:
From the above explanation, Table F.4 of IEC 60664-1:2007 can be used as follows:
– first step: to choose the most appropriate pollution degree according to the normal use of the equipment;
– second step: to choose one insulating material and to allocate it to a material group based on its CTI;
The third step involves evaluating the maximum long-term root mean square (r.m.s) voltage across the creepage distance This maximum value may be the operational voltage or the highest rated voltage if multiple ratings exist for the equipment For direct current (d.c.) rated voltage, the corresponding equivalent rated r.m.s voltage is selected from Table F.4 of IEC 60664-1:2007.
– fourth step: to read the value given at the cross of the chosen column with the chosen line
At this stage, there are two cases to be considered Either the creepage distance is greater than the associated clearance or is smaller than the associated clearance
– If the creepage distance is greater than the associated clearance, no further test is needed;
If the creepage distance is less than the required clearance, and the electric field is between homogeneous and inhomogeneous conditions (as outlined in Case A and Case B of Tables F.2 and F.7 of IEC 60664-1:2007), the clearance must be tested according to section 6.1.2 of IEC 60664-1:2007 to ensure no flashover occurs In homogeneous conditions (Case B), the tables specify the minimum clearance necessary to withstand the given voltage, making it impossible to reduce the creepage distance below this clearance value However, since real-world electric fields are typically inhomogeneous but not as extreme as Case A, it is feasible that the actual conditions may allow the equipment to endure the maximum voltage stress This must be verified through an impulse voltage test.
Practical use of Table 5 in IEC 60664-5:2007 for dimensioning of
Humidity can lead to water adsorption on insulating materials, increasing the risk of flashover Insulating materials can be classified based on their ability to adsorb water, as outlined in Annex B of IEC 60664-5:2007 This standard categorizes materials into four distinct water adsorption groups (WAG).
The amount of water on material surfaces is influenced by the water activity (WAG) and humidity level (HL) As the humidity level rises and the insulating material's capacity to retain water increases, the risk of flashover along the creepage distance on the surface of the insulating material also escalates.
For HL1, the dimensioning of the clearances requirements according to Tables 2 and 3 of IEC 60664-5:2007 is applicable because the influence of water does not increase significantly the risk of flashover
Table 5 of IEC 60664-5:2007 outlines the creepage distance dimensions for HL2 and HL3 to prevent flashover, which occurs along surfaces in air This table is applicable for altitudes up to 2,000 meters above sea level, while for elevations exceeding 2,000 meters, the altitude correction factor specified in IEC 60664-1 should be applied.
The creepage distance, as defined in Tables 4 and 5 of IEC 60664-5:2007, must always be greater than or equal to the associated clearance under homogeneous field conditions In cases of inhomogeneous field conditions, a creepage distance that is less than the clearance specified in Table 2 of IEC 60664-5:2007 is permissible only for hazard levels HL1 and HL2.
Such dimensioning shall be verified with an impulse voltage test
NOTE In the case of d.c voltage, the peak value chosen in Table 5 of IEC 60664-5:2007 is the maximum d.c voltage across the creepage distance.
Practical use of IEC 60664-1:2007 for checking the dimensioning of
distances with regard to time under voltage stress
The creepage distances shown in Table F.4 of IEC 60664-1:2007 have been determined for insulation intended to be under continuous voltage stress for a long time
NOTE 1 Technical committees responsible for equipment in which insulation is under voltage stress for only a short time may consider allowing shorter creepage distances than those specified in Table F.4 of IEC 60664-1:2007
Creepage distances of basic and supplementary insulation are selected from Table F.4 of IEC 60664-1:2007 for:
– the rationalized voltages given in columns 2 and 3 of Table F.3a of IEC 60664-1:2007 and columns 2, 3 and 4 of Table F.3b of IEC 60664-1:2007, corresponding to the nominal voltage of the supply low-voltage mains;
– the rated insulation voltage according to 4.3.2.2.1 of IEC 60664-1:2007;
– the voltage specified in 4.3.2.2.2 of IEC 60664-1:2007
NOTE 2 For supplementary insulation, the pollution degree, insulating material, mechanical stresses and environmental conditions of use may be different from those for basic insulation
Creepage distances for reinforced insulation is twice the creepage distance for basic insulation from Table F.4 of IEC 60664-1:2007.
Practical use of IEC 60664-3:2003 for the reduction of micro-
conditions for the dimensioning of creepage distances
Dimensioning of spacings between conductors depends on environmental conditions Regarding tracking, the choice of the pollution degree is linked to macro-environmental conditions
The macro-environment significantly impacts the micro-environment at the surface of insulating materials In the absence of protective measures, the conditions of the micro-environment mirror those of the macro-environment.
Improving the micro-environmental conditions at the insulation surface can be achieved through the application of coating, potting, or moulding, as outlined in IEC 60664-3 This protective approach enhances the micro-environment, enabling a reduction in clearance and creepage distances.
NOTE 1 IEC 60664-3 deals mainly with evaluation and testing of the use of coating on PWBs The standard also covers evaluation and testing when protection is realized by means of potting or moulding In the latter case, technical committees should carefully consider the relevance of the verification and test procedures described in IEC 60664-3 Modifications to the verification and test procedures might be relevant to reflect the specific application
IEC 60664-3 outlines the requirements and testing procedures for two permanent protection methods that are applicable to various types of protected printed circuit boards, including the surfaces of inner layers in multi-layer boards, substrates, and similarly protected assemblies.
The two types of protection are as follows:
Type 1 protection improves the micro-environment of the parts under protection The dimensioning of clearances and creepage distances under protection follows the distance requirements of IEC 60664-1 or IEC 60664-5 for pollution degree 1 Between two conductive parts, it is a requirement that one or both conductive parts, together with all the spacings between them, are covered by this protection
Type 2 protection is considered to be similar to solid insulation Under this protection, the requirements for solid insulation specified in IEC 60664-1 are applicable and the spacings are not less than those specified in Table 1 of IEC 60664-3:2003 The requirements for clearances and creepage distances in IEC 60664-1 or IEC 60664-5 do not apply Between two conductive parts, it is a requirement that both conductive parts, together with all the spacings between them, are covered by this protection so that no air gap exists between the protective material, the conductive parts and the printed board
NOTE 2 Above 30 kHz, the additional requirements of IEC 60664-4 for solid insulation are applicable for Type 2 protection.
Practical use of the IEC 60664 series for the dimensioning of solid insulation
General
Solid insulation is often designed based on breakdown data provided by manufacturers However, it is important to consider that this data is derived under specific and favorable conditions.
– usually homogeneous field distribution has been provided;
– usually ambient room temperature has been applied during testing;
– usually short-time testing has been performed;
– in many cases d.c voltage has been used for testing
The general influence of the time of testing on the breakdown voltage is shown in Figure 1 The time scale applies to the power-frequency voltage
Key s seconds; d days; y years; t time; U voltage range 1 electrical breakdown range 2 breakdown caused by excessive heating range 3 breakdown caused by ageing (i e by partial discharges)
Figure 1 – Breakdown voltage of solid insulation depending upon the time of voltage stress
Data obtained from testing may significantly differ from the actual long-term withstand capability of insulation in real equipment Consequently, this information should not be directly applied for the dimensioning of solid insulation.
With regard to insulation coordination as described in IEC 60664-1, in general a design of solid insulation according to the thickness and the relevant breakdown field strength is only possible if:
– the field distribution is homogeneous and if no voids or air gaps are present within the insulation system (see IEC 60664-4 for high-frequency voltage stress); or
– the field strength is low enough so that no partial discharges occur.
Coordination of clearances and solid insulation
When both clearances and solid insulation are subjected to the same voltage, it is crucial to consider their differing properties during dimensioning Unlike solid insulation, clearances are self-restoring, which means their withstand capability should be lower This ensures that any breakdown occurs in the clearance before the solid insulation is compromised, thereby protecting the integrity of the insulation.
Practical information for checking the correct dimensioning of solid
Dimensioning solid insulation primarily relies on breakdown field strength data, which must be tailored to practical usage conditions, including long-term voltage stress and factors such as elevated ambient temperature, humidity, and mechanical stress While having access to this data is beneficial, establishing straightforward dimensioning rules is only feasible when the electric field distribution within the solid insulation is nearly homogeneous; otherwise, calculating the internal field strength becomes challenging.
E peak breakdown field strength of the solid insulation (peak value): 45 kV/mm; (specified by the manufacturer of the insulating material); d thickness of the solid insulation: 0,1 mm;
U peak maximum voltage stress (peak value): 4,5 kV
However, if air gaps are included within the solid insulation, this procedure can be greatly misleading in practice, see 4.5.3.4.2
The inhomogeneous voltage distribution within the insulation system leads to a reduced withstand capability of air compared to solid insulation.
To ensure the proper performance of solid insulation, it is essential to conduct appropriate testing as outlined in section 6.1.3 of IEC 60664-1:2007, especially when the breakdown field strength and field distribution within the insulation are unknown Additionally, correct conditioning in accordance with section 6.1.3.2 of IEC 60664-1:2007 is required for accurate results.
The applicable tests include the impulse voltage withstand test, as outlined in section 6.1.3.3 of IEC 60664-1:2007, which assesses the solid insulation's ability to endure the rated impulse voltage specified in section 5.3.3.2.2 of the same standard Additionally, the a.c voltage test, referenced in section 6.1.3.4 of IEC 60664-1:2007, evaluates the solid insulation's capacity to withstand the highest voltage value among specified criteria.
– short-term temporary overvoltage (see 5.3.3.2.3 of IEC 60664-1:2007);
– the recurring peak voltage (see 5.3.3.2.4 of IEC 60664-1:2007)
If the peak value of the a.c test voltage is equal to or higher than the rated impulse voltage, the a.c voltage test also covers the impulse voltage test
Solid insulation exhibits different withstand characteristics compared to clearances when the duration of stress is prolonged, leading to a significant decrease in withstand capability Consequently, the a.c voltage test, which is essential for verifying the withstand capability of solid insulation, cannot be substituted with an impulse voltage test Additionally, the partial discharge test, as outlined in IEC 60664-1:2007, is crucial to ensure that no partial discharges are present in the solid insulation.
– at the highest steady-state voltage;
– at the long-term temporary overvoltage (see 5.3.3.2.3 of IEC 60664-1:2007);
The recurring peak voltage is outlined in section 5.3.3.2.4 of IEC 60664-1:2007 Additionally, the high-frequency voltage test, as specified in section 6.1.3.7 of the same standard, is conducted to ensure there are no failures caused by dielectric heating, in accordance with section 5.3.3.2.5 of IEC 60664-1:2007.
For equipment linked to various low-voltage mains, specific test voltages apply, excluding the partial discharge test and the high-frequency voltage test for simplification purposes.
Table 1 – Examples for rated voltage 100 V and 230 V and overvoltage category II
To cover impulse voltage test For the highest of the voltages mentioned in 6.1.3.1 b) b of IEC 60664-1:2007
Rated voltage Time Rated voltage Time Rated voltage Time
The article discusses the testing of electrical equipment with five impulses of each polarity, ensuring a minimum interval of 1 second between impulses It highlights that the voltages considered include short-term temporary overvoltage, the highest steady-state voltage, and recurring peak voltage, with short-term temporary overvoltage typically being the most stringent requirement The peak values of these voltages correspond to the rated impulse voltage Additionally, the test duration may be shortened to 5 seconds if the short-term temporary overvoltage is the primary concern It is noted that the values in parentheses are specific to Japan, as referenced in footnote 5 of Table F.1 in IEC 60664-1:2007.
4.5.3.3 Series connection of clearances and solid insulation
Three cases can be distinguished
In the first case, the series connection of clearances and solid insulation is a consequence of the product design In this case, usually rather large clearances are addressed
The series connection of clearances and solid insulation arises from the specific design of the insulation system, such as incorporating multiple layers of thin sheet insulating material.
In the third case, the series connection of clearances and solid insulation is a result of the imperfect manufacturing of the solid insulation, including the interface to the conductive parts
In scenarios involving small air gaps or bubbles connected in series with solid insulation, it is essential to calculate the voltage distribution across the series-connected insulators based on their respective impedances.
In d.c voltage applications, the impedances are primarily influenced by insulation resistances Since the insulation resistance in air is almost infinite, the impedance of the air gap significantly exceeds that of solid insulators Consequently, the majority of the d.c voltage is effectively applied across the clearance.
In alternating current (a.c.) voltage systems, the impedances of series-connected insulators are primarily influenced by their capacitances At low frequencies, as outlined in IEC 60664-1, dielectric losses can often be neglected when calculating voltage distribution Consequently, the permittivity of solid insulators plays a crucial role in determining the voltage distribution.
For an easy calculation of the capacitive voltage distribution, those capacitances are considered as plate-to-plate capacitors with an homogeneous field distribution This situation is described in Figure 2:
Key d total distance d 1 clearance d 2 thickness of the solid insulation
Figure 2 – Series connection of clearance and solid insulation
C 1 and C 2 form a capacitive voltage divider according to Figure 3 and the applied a.c voltage
U 0 is divided according to Equations (2) and (3) in the voltages U 1 and U 2
C 2 capacitance of the solid insulation
U 2 voltage across the solid insulation
The capacitances C 1 and C 2 are given by Equations (4) and (5):
A is the area of the plate-to-plate capacitors C 1 and C 2; ε 0 is the permittivity of the air; ε r is the permittivity of the solid insulation
For the voltage division, the capacitance ratio as given in Equation (6) is relevant d r
The breakdown field strength of clearance (E₁) can be calculated using the a.c breakdown voltage and the corresponding clearance from Table A.1 of IEC 60664-1:2007 This application guide simplifies the examples based on homogeneous field conditions Additionally, the breakdown field strength of solid insulation (E₂) must be provided by the material's manufacturer.
Calculating voltage distribution accurately is complex, and the formulas provided serve only as approximations based on a homogeneous field distribution This approximation is quite accurate for small distances, approximately up to 0.1 mm, but becomes less suitable for larger distances.
4.5.3.3.2 Series connection of clearances and solid insulation by design
The following examples deal with the series connection of clearances and solid insulation as designed within equipment
4.5.3.3.3 Series connection of clearances and solid insulation by design for d.c voltage
In direct current (d.c.) voltage applications, the majority of the voltage is concentrated across the clearance, which is specifically engineered to endure this voltage If a flashover occurs in the clearance, the entire voltage is then transferred to the solid insulation.
Rule: In order to prevent any deterioration of the solid insulation in this situation, also the solid insulation is also designed to withstand the entire voltage
4.5.3.3.4 Series connection of clearances and solid insulation by design for a.c voltage
For a.c voltage, the voltage distribution is calculated according to the relevant capacitances
In the following example, the dimensions are assumed according to the most likely situation with a rather large clearance and a thin layer of solid insulation in series
Application of Equation (6) results in: C 1 = 0,0074 C 2
Application of Equation (2) results in: U 1 = 0,993 U 0
Application of Equation (3) results in: U 2 = 0,007 U 0
Practical use of the IEC 60664 series for designing functional insulation
General
The IEC 60664 series outlines the minimum clearances, creepage distances, and solid insulation requirements that also apply to functional insulation Additionally, for functional purposes, there may be extra requirements, such as those concerning minimum insulation resistance, as detailed in Annex A of IEC 60664-5:2007.
The withstand voltage requirements for functional insulation, however, can be different from those required for basic insulation.
Dimensioning and testing of functional isolation compared to basic
The principles for dimensioning of functional insulation are given in 4.1
To ensure proper clearance of functional insulation, the necessary withstand voltage must be determined based on the maximum impulse voltage, steady-state voltage, or recurring peak voltage as specified in IEC 60664-1:2007 This assessment should consider the rated conditions of the equipment, particularly the rated voltage and rated impulse voltage.
Creepage distances of functional insulation is dimensioned as specified in Table F.4 of IEC 60664-1:2007 corresponding to the working voltage across the creepage distance considered
When applying IEC 60664-5, the creepage distances for functional insulation are determined according to Table 4, which considers the working voltage related to tracking, and Table 5, which addresses the highest peak voltage to prevent flashover, using the greater of the two values.
When dimensioning with the working voltage, it is acceptable to interpolate values for intermediate voltages This process involves using linear interpolation, and the resulting values should be rounded to match the number of digits found in the Tables.
In equipment experiencing short-term voltage stress, the creepage distances for functional insulation can be dimensioned one voltage step lower than the specifications outlined in Table F.4 of IEC 60664-1:2007.
Testing of functional insulation follows the same procedures as specified in 6.1 of IEC 60664-1:2007 The test voltages, however, can be different from those required for basic insulation.
Practical use of the IEC 60664 series for dimensioning with respect to the
General influence of the frequency on withstand characteristics
According to IEC 60664-1, the impact of voltage frequency is addressed by the minimum values specified for frequencies up to 30 kHz However, for frequencies exceeding this range, it is essential to anticipate a decrease in the insulation's withstand capability, which must be considered during the dimensioning process.
For frequencies greater than 30 kHz and up to 10 MHz, IEC 60664-4 shall be applied together with IEC 60664-1 or IEC 60664-5.
Influence of the frequency on the withstand characteristics of
The withstand voltage capability as defined by IEC 60664-4 is affected solely by the frequency of periodic voltages For transient overvoltages, it is adequate to follow the guidelines set forth in IEC 60664-1 or IEC 60664-5 for proper dimensioning.
For frequencies above 30 kHz, as outlined in IEC 60664-4, the withstand voltage capability of clearances with homogeneous and nearly homogeneous field distributions may decrease by as much as 20% An approximately homogeneous field is defined for these frequencies when the radius of curvature of the conductive components is at least 20% of the clearance distance.
Dimensioning for nearly uniform field distribution is performed by considering 125% of the necessary withstand voltage clearance, as specified by the case A values in Table F.7 of IEC 60664-1:2007 or Table 3 of IEC 60664-5:2007 It is important to note that no withstand voltage testing is necessary.
When dimensioning for fields with approximately homogeneous distribution, as outlined in case A of IEC 60664-1:2007 or IEC 60664-5:2007, a withstand voltage test is necessary per section 6.1.2 of these standards Importantly, the minimum clearance must not be less than 125% of the required withstand voltage specified for case B in the same tables.
For frequencies above 30 kHz, an inhomogeneous electric field occurs when the radius of curvature of conductive components is less than 20% of the clearance This inhomogeneous field distribution can significantly reduce the withstand voltage capability of clearances The design for inhomogeneous field distribution is based on the required withstand voltage as specified in Table 1 of IEC 60664-4:2005, and no withstand voltage testing is necessary.
In high voltage conditions exceeding 1 kV, the dimensioning for inhomogeneous fields results in impractical distances Consequently, it is advisable to select a design that enhances field distribution, aiming for an approximately homogeneous field distribution.
Influence of frequency on the withstand characteristics of creepage
For voltages exceeding 30 kHz, it is essential to consider thermal effects alongside tracking when assessing the creepage distance's withstand capability The dimensioning process involves determining both the required root mean square (r.m.s.) withstand voltage based on Table F.4 of IEC 60664-1:2007 and the necessary peak withstand voltage as outlined in Table 2 of IEC 60664-4:2005 The peak withstand voltage represents the maximum periodic peak voltage across the creepage distance, and the greater of the two calculated distances should be applied.
According to Table 2 of IEC 60664-4:2005, interpolation for frequencies is permitted, with the values specifically applicable to pollution degree 1 For pollution degrees 2 and 3, creepage distances can be determined by applying multiplication factors of 1.2 and 1.4, respectively.
According to Table 2 of IEC 60664-4:2005, dimensioning is essential for insulating materials susceptible to thermal deterioration, such as epoxy resin used in printed wiring boards For insulating materials that are not affected by thermal effects and do not require tracking considerations, adhering to the clearance requirements outlined in section 4.7.2 is adequate.
Influence of frequency on the withstand characteristics of solid
When designing for frequencies above 30 kHz, it is crucial to consider the diminished withstand capability of solid insulation This reduction arises from two main factors: first, the increased heating of solid insulating materials due to dielectric losses, particularly in materials with a high loss factor like laminated paper; second, the accelerated deterioration from high-frequency partial discharges To ensure reliability, partial discharges must be avoided under normal operating conditions, and it is important to account for the decrease in partial discharge inception voltage as frequency increases.
Accurate dimensioning of solid insulation necessitates high-frequency voltage testing, which is challenging and demands specialized equipment To address this, IEC 60664-4 introduces a simplified method for dimensioning solid insulation based on distance requirements.
This simplified dimensioning method serves as an alternative to high-frequency testing as outlined in Clause 7 of IEC 60664-4:2005 It is applicable for voltage frequencies up to 10 MHz, provided that the field strength remains approximately uniform and does not exceed the limits specified in Equation (7) or Figure 4 Additionally, this method requires the absence of voids or air gaps within the solid insulation The electric field is deemed approximately uniform if the variations from the average field strength are within ±20%.
For solid insulation layers with a thickness of \(d_1 \geq 0.75 \, \text{mm}\), the maximum allowable electric field strength \(E\) must not exceed \(2 \, \text{kV/mm}\) In contrast, for thinner insulation layers where \(d_2 \leq 30 \, \mu\text{m}\), the peak electric field strength can be up to \(10 \, \text{kV/mm}\) For thicknesses where \(d_1 > d > d_2\), interpolation is performed using Equation (7) to determine the appropriate field strength for a specific thickness \(d\).
Figure 4 – Permissible field strength for dimensioning of solid insulation according to Equation (7)
For effective dimensioning of solid insulation, it is essential to achieve a nearly uniform field distribution without voids or air gaps If the field strength cannot be accurately calculated due to non-uniformity, if the peak value exceeds the limits set by Equation (7) or Figure 4, or if voids or air gaps are present, a withstand test or a partial discharge test with high-frequency voltage is necessary, especially for frequencies above 10 MHz The voltage withstand test is applicable for short-term stresses, while the partial discharge test is relevant for long-term stresses, as outlined in IEC 60664-1:2007, section 5.3.3.2.3.
IEC 60664-3 outlines two types of protection to enable smaller dimensioning, with type 2 protection resembling solid insulation Since IEC 60664-3 is derived from IEC 60664-1, its frequency scope is restricted to 30 kHz Consequently, for type 2 protection intended for frequencies exceeding 30 kHz, the additional requirements specified in IEC 60664-4 for solid insulation must be adhered to.
5 Four examples showing appropriate dimensioning of insulation within equipment
General
Four examples for dimensioning of clearances are shown in Figures 5a to 5d, each figure illustrating the most important factors influencing the dimensioning of clearances These are
IEC 228/11 the rated voltages, the steady-state withstand voltages, the impulse withstand voltages, the overvoltage category, the pollution degree and the type of insulation
Certain product standards lack specific clearance values for circuits powered by SELV The clearance requirements, illustrated in Figures 5a to 5d, are determined by the overvoltage category derived from the mains voltage, which is one impulse withstand voltage level lower following the transformer For additional details, refer to section 4.2.
IEC 60664-1:2007 defines four overvoltage categories in section 4.3.3.2, which are equivalent to the impulse withstand categories outlined in IEC 60364-4-44 The rated impulse voltage for equipment is determined based on the specified overvoltage category and the equipment's rated voltage, as detailed in Tables F.1 and F.2 of IEC 60664-1:2007.
There are four pollution degrees in the micro-environment given in 4.6.2 of IEC 60664-1:2007
This article illustrates the dimensioning of clearances using overvoltage category III and category II, along with pollution degree 2, as outlined in IEC 60664-1:2007, referencing Tables F.2 and F.7.
In Figure 5b, the following applies for class I equipment:
For non-earthed circuits that are not directly connected to the mains, such as the secondary part of a transformer, the basic insulation is designed to withstand an impulse voltage equivalent to that of circuits directly connected to the mains Additional information on reducing the impulse withstand voltage can be found in section 4.2 regarding the transformer.
In Figure 5c and 5d, the following applies for class II equipment:
In non-earthed circuits that are not directly connected to the mains, such as the secondary part of a transformer, the basic insulation is designed to withstand an impulse voltage that is one step lower than that of circuits directly connected to the mains Further details on the reduction of impulse withstand voltage related to the transformer can be found in section 4.2.
Information for basic protection and basic insulation is given in IEC 61140
Figure 5a – Example 1 – Simple illustration of insulation system containing functional, basic and reinforced/double insulation for a class I equipment
Examples for the dimensioning of clearances for class I equipment according
Protection against direct contact by means of conductive enclosure connected to protective earth (PE)
Figure 5b – Example 2 – Dimensioning of clearances for class I equipment, based on overvoltage category III
Table 2 – Example 2 – Dimensioning of clearances according to Table F.2 of IEC 60664-1:2007 (pollution degree 2) (see example 2 of Figure 5b)
Clearance mm a Functional 4 000 3,0 b Basic 4 000 3,0 c Functional 4 000 3,0 d Reinforced 6 000 5,5 e Basic 4 000 3,0 f Functional 800 b 0,2 b a Applicable to TN system b According to a voltage line to neutral of 50 V
Circuits equipped with surge protective devices can have their clearance for functional insulation (a and c) designed to be smaller than the values specified for case A in Table F.2 of IEC 60664-1:2007.
In this case, however, an impulse withstand test with the required impulse withstand voltage is necessary The impulse test generator shall have a low impedance of 2 Ω
The clearance for reinforced insulation is based on Table F.2, case A of IEC 60664-1:2007 choosing one impulse voltage level higher (preferred values)
OV cat III e PE f Functional insulation
Table 3 – Example 2 – Dimensioning of clearances according to Tables F.2 and F.7a of IEC 60664-1:2007, temporary overvoltages according to 5.3.3.2.3 of IEC 60664-1:2007
2 Insulation type Impulse withstand voltage
Temporary overvoltage (peak)/working voltage (peak) b
The dimensioning of insulation systems is guided by Table F.7 of IEC 60664-1:2007, which is compared to Table F.2 of the same standard, emphasizing the selection of larger clearances based on pollution degree When evaluating the insulation system against working voltage, recurring peak voltages are deemed negligible, focusing solely on the peak value of the sinusoidal mains voltage Dimensioning is primarily based on the peak value of temporary overvoltage, specifically considering a voltage line to neutral of 50 V It is important to note that while clearance for reinforced insulation is calculated at 160% of the temporary overvoltage, the verification test voltage for reinforced insulation is double that of basic and supplementary insulation.
5.3 Examples for the dimensioning of clearances for class II equipment according to IEC 60664-1
Protection against direct contact by means of non conductive enclosure (solid insulation) or clearance providing reinforced insulation
Figure 5c – Example 3 – Dimensioning of clearances (class II equipment)
Table 4 – Example 3 – Dimensioning of clearances according to Table F.2 of IEC 60664-1:2007 (pollution degree 2) (see example 3 of Figure 5c)
Example 3 Insulation type Impulse withstand voltage
The clearance values for various insulation types are as follows: Functional insulation has a clearance of 4,000 mm at 3.0 kV and 2,500 mm at 1.5 kV, while reinforced insulation has clearances of 6,000 mm at 5.5 kV and 4,000 mm at 3.0 kV The clearance is determined by the overvoltage category from the mains voltage (230 V a.c OV cat III) and is one impulse withstand voltage level lower after the transformer It is important to note that the actual d.c level after rectification does not affect the impulse withstand voltage used in the insulation system design For more details, refer to section 4.2.
Table 5 – Example 3 – Dimensioning of clearances according to Tables F.2 and F.7a of IEC 60664-1:2007, temporary overvoltages according to 5.3.3.2.3 of IEC 60664-1:2007
3 Insulation type Impulse withstand voltage
Temporary overvoltage (peak)/working voltage (peak) b
The dimensioning of insulation systems is guided by Table F.7 of IEC 60664-1:2007, which is compared to Table F.2 of the same standard, emphasizing the importance of pollution degree in selecting larger clearances When evaluating the insulation system against working voltage, recurring peak voltages are generally considered negligible, focusing solely on the peak value of the sinusoidal mains voltage The dimensioning process relies on the peak value of temporary overvoltage, with reinforced insulation requiring clearance dimensions based on 160% of this overvoltage Additionally, it is crucial to note that the verification test voltage for reinforced insulation clearance is double that of the test voltage for basic and supplementary insulation.
Examples for the dimensioning of clearances for class II equipment
Protection against direct contact by means of non conductive enclosure (solid insulation) or clearance providing reinforced insulation.
Figure 5d – Example 4 – Dimensioning of clearances (class II equipment)
Table 6 – Example 4 – Dimensioning of clearances according to Table 2 of IEC 60664-5:2007 (see example 4 on Figure 5d)
Example 4 Insulation type Impulse withstand voltage c
Clearance c mm a Functional 800 (1 500) 0,1 (0.5) b Basic 800 (1 500) 0,1 (0,5) c a Functional 500 (800) 0,04 (0,1) d 1 Reinforced 1 500 (2 500) 0,5 (1,5) d 2 b Reinforced 800 (1 500) 0,1 (0,5) f a Functional 500 (800) 0,04 (0,1) a The clearance is based on the overvoltage category determined from the mains voltage
The impulse withstand voltage level for reinforced insulation is determined based on the overvoltage category from the mains voltage of 100 V a.c OV cat II, which is one kV level lower after the transformer The actual d.c level post-rectification does not affect the design of the insulation system's impulse withstand voltage For specific values applicable to Japan, refer to footnote 5) of Table F.1 in IEC 60664-1:2007 For additional details, see section 4.2.
Table 7 – Example 4 – Dimensioning of clearances according to Tables 2 and 3 of IEC 60664-5:2007, temporary overvoltages according to 5.3.3.2.3 of IEC 60664-1:2007
4 Insulation type Impulse withstand voltage d
Temporary overvoltage (peak)/ working voltage
The dimensioning of insulation systems according to IEC 60664-5:2007 is compared with IEC 60664-1:2007, focusing on selecting the larger clearance values When evaluating the insulation system against working voltage, recurring peak voltages are considered negligible, with only the peak value of the sinusoidal mains voltage taken into account Interpolation from Table F.7a of IEC 60664-1:2007 is used for calculations, and values in parentheses are specific to Japan It is important to note that while clearance for reinforced insulation is based on 160% of the temporary overvoltage for basic and supplementary insulation, the verification test voltage for reinforced insulation is double that of basic and supplementary insulation.
6 Practical application of the IEC 60664 series with regards to particular questions
General
Clause 4 of this application guide outlines the requirements for clearance, creepage, and solid insulation dimensions for various types of insulation, including functional, basic, supplementary, double, and reinforced insulation, based on working voltage, recurring peak, temporary overvoltage, and transient overvoltage It also includes typical testing examples for clearance and solid insulation in specific applications, though these examples do not encompass all possible scenarios.
This clause does not address the testing of creepage distances, as such testing is generally unfeasible Instead, it focuses on evaluating the effective distance and the insulating material involved in the insulation under consideration.
Testing complete equipment in case of components bridging the basic insulation
To ensure compliance with IEC 60664-1:2007, the equipment must be prepared by disconnecting any components that bridge the basic insulation, including surge protective devices, as specified in section 6.1.4.1 Subsequently, the test is conducted according to section 6.1.4 of the same standard, adhering to the conditions or limitations outlined in the product standard.
It is essential to verify that components, which connect the basic insulation and were disconnected during the impulse voltage test, do not negatively affect the performance or safety of the basic insulation in the equipment's normal operation.
The disconnected components are reconnected, and the equipment undergoes testing following a specific procedure This includes an alternating current (a.c.) test designed to ensure that components bridging the basic insulation maintain safety standards against short-term temporary overvoltages.
The test voltage operates at a frequency of 50/60 Hz, with the r.m.s value for basic insulation equating to the short-term temporary overvoltage of 1,200 V plus the nominal voltage (U n) between line and neutral For details on the test duration, refer to section 6.1.3.4.1 of IEC 60664-1:2007.
NOTE 1 As an example, for an equipment having a rated voltage of U n = 250 V, the value of the a.c test voltage for basic insulation is 1 200 V + 250 V, thus the r.m.s test voltage is 1 450 V
NOTE 2 If it is not possible to perform the a.c test, a d.c test can be considered with a voltage value equal to or higher than the a.c peak voltage
NOTE 3 The short-circuit output current of the generator is not less than 200 mA For test voltages exceeding
For test voltages of 3 kV, the test equipment must have a rated power of at least 600 VA Additionally, the generator's tripping current should be adjusted to 100 mA, or to the maximum possible value for test voltages exceeding 6 kV.
Figure 6 – Arrangement for a.c (or d.c.) voltage test
The voltage is successively applied for the time given in 6.1.3.4.1 in IEC 60664-1:2007
The acceptance criteria require a visual inspection of the equipment, ensuring that no components bridging the basic insulation exhibit any visible alterations Additionally, the equipment must be connected to the mains following the manufacturer's instructions and should operate according to its intended purpose.
It is allowed to replace a fuse or similar protective device prior to connecting equipment to the mains Additionally, if the fuse protecting a surge arrester has blown, replacing the surge arrester is also permitted.
Testing complete equipment in case of components bridging the functional insulation
General
If the equipment incorporates components bridging the functional insulation between live parts connected to mains, the impulse test is applied as described in 6.3.2 and 6.3.3.
Verification of clearances and creepage distances
The proper sizing of insulation distances is initially evaluated without any components that may bridge the insulation The equipment is set up to ensure that any components bridging the insulation between live parts, such as surge protective devices, are disconnected.
6.1.4.1 of IEC 60664-1:2007 The test is then performed in accordance with 6.1.4 of IEC 60664-1:2007 within the condition or limitation given in the product standard
For this test, a low impedance generator may be necessary, and a hybrid generator with a virtual impedance of 2 Ω could be suitable It is essential to measure the correct test voltage directly at the insulation.
Verification of components bridging the insulation
To ensure the safe operation of components that were disconnected during the testing phase outlined in section 6.3.2, the components are reconnected Subsequently, the equipment undergoes a second round of testing under the same conditions.
NOTE 1 If the components bridging the functional insulation are used for EMC purposes only (e.g an SPD), it is allowed to use a generator having an internal impedance up to 500 maximum
The acceptance criteria require a visual inspection of the equipment, ensuring that no components bridging the functional insulation exhibit any visible alterations Following this, the equipment must be connected to the mains as per the manufacturer's instructions Ultimately, the equipment should operate according to its intended purpose.
NOTE 2 If the components bridging the functional insulation are used for EMC purposes only e.g an SPD, it is permitted to replace a fuse or a similar protective mean before connecting the equipment to the mains If a fuse protecting components disconnected during the test in 6.3.2 has blown, it is permitted to replace the components, too.
Dimensioning of insulation distances for parts of equipment which can have
General
General principles for the dimensioning of clearances for low-voltage equipment are given in 4.1
When dimensioning clearances between open contacts or moving parts in equipment, such as switch poles, it is essential to adhere to the same standards required for basic insulation.
NOTE For example, see 7.2.3.1 of IEC 60947-1:2007, 7.1.2 of IEC 60669-1:1998, or 4.5.1 and 4.5.2 of IEC 62019:1999.
Dimensioning for device associated with an equipment declared
When an auxiliary device is added to equipment deemed suitable for isolation, the minimum clearance requirements between moving parts of the auxiliary device must adhere to the basic insulation standards outlined in IEC 60664-1:2007 and IEC 61140:2001 Additionally, the technical committees take into account the minimum requirements for circuits dedicated to remote indication.
Dimensioning for device associated with an equipment not declared
Devices that are either added as auxiliary components to equipment not deemed suitable for isolation or are independent units lacking isolation suitability may have reduced requirements set by technical committees compared to basic insulation standards It is essential that such products are appropriately marked to reflect these specifications.
NOTE For example, see IEC 60669-1 or IEC 62019 ask for marking “m” on the device.
Testing with respect to high-frequency voltage stress
In principle, 4.7.1 to 4.7.4 of this application guide are also applicable to the frequencies of the voltage as specified in IEC 60664-4
It is important to note that the breakdown field strength values for both clearances and solid insulation are reduced due to the effects of frequency.
According to section 4.7.2 of this application guide, the frequency of the test voltage must match the frequency of the applied voltage The minimum duration for the power-frequency a.c voltage test is 60 seconds, although longer durations may be necessary for materials with high dielectric losses, as excessive heating from these losses can lead to failure.
For testing at high-frequency voltage, the capacitive loading caused by the test specimen is a very important factor The test equipment used shall allow a capacitive loading of at least
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Practical information in case of substitution an impulse withstand test by an
General
The impulse withstand voltage test is a crucial assessment designed to ensure that electrical clearances can endure specified transient overvoltages, thereby guaranteeing the reliability and safety of electrical systems Conducted in accordance with the IEC 60664-1:2007 standard, this test utilizes a 1.2/50 μs waveform, with voltage values specified in Table F.5, to simulate real-world transient overvoltage conditions and verify the adequacy of electrical clearances.
Impulse voltage tests require a minimum of three impulses for each polarity, with at least a 1-second interval between pulses, to account for the variability in test results.
Technical committees may specify an a.c or d.c voltage test for particular equipment as an alternative method to the impulse voltage test
Technical committees must recognize that while a.c and d.c voltage tests with the same peak value as the impulse test voltage in Table F.5 of IEC 60664-1:2007 can confirm the withstand capability of clearances, they may also impose greater stress on solid insulation due to the longer duration of voltage application This can lead to potential overload and damage to certain solid insulations Therefore, it is crucial for technical committees to take this into account when considering a.c or d.c voltage tests as alternatives to the impulse voltage test outlined in section 6.1.2.2.1 of IEC 60664-1:2007.
Characteristics of the a.c voltage substituted to an impulse
The characteristics are as follows:
– the waveshape of the sinusoidal power-frequency test voltage is substantially sinusoidal This requirement is fulfilled if the ratio between the peak value and the r.m.s value is 2 ± 3 %;
– the peak value is equal to the impulse test voltage of Table F.5 of IEC 60664-1:2007 and applied for three cycles of the a.c test voltage
NOTE It is not possible to reduce the peak voltage value of the a.c voltage test if the test duration is longer than three cycles.
Characteristics of the d.c voltage substituted to an impulse
The characteristics are as follows:
– the d.c test voltage is substantially free of ripple This requirement is fulfilled if the ratio between the peak values of the voltage and the average value is 1,0 ± 3 %;
– the average value of the d.c test voltage is equal to the impulse test voltage of Table F.5 of IEC 60664-1:2007 and applied three times for 10 ms in each polarity
7 Examples of a dimensioning worksheet (based on case A as described in IEC 60664-1:2007)