The safety factor Ks covers the differences between the standard test and service: assembly, manufacturing scatter, installation quality, ageing and other influences. If the product committee sets no other values, IEC 60071-2 recommends 1.15 for internal insulation and 1.05 for external. The test conversion factors bring the required voltage to the waveshape in which the level is standardized: switching to power frequency and lightning in range I, and power frequency to switching in range II.
Short answer
The safety factor Ks and the test conversion factors take the coordination voltage to a voltage that can actually be tested. Ks covers the differences between the test and the real life of the insulation. If the product committee sets no other value, IEC 60071-2 recommends Ks = 1.15 for internal insulation and 1.05 for external insulation. The conversion factors bring a required voltage to the waveshape of the standard test. They are used only in the direction the standard indicates. This guide follows clauses 4.3 and 5.1 to 5.2 of the 1996 edition and the definitions of IEC 60071-1:2006.
Where they fit in the coordination
Insulation coordination advances in steps. The coordination withstand voltage becomes the required withstand voltage (Urw) by applying the atmospheric correction and the safety factor. The required withstand voltage is then compared with the standard values to choose the insulation level.
- Coordination voltage. Result of the deterministic or statistical method
- Atmospheric correction. External insulation only, for example Ka for altitude
- Safety factor Ks. Applied after the atmospheric correction
- Required withstand voltage. The one that must be demonstrated by test
- Conversion factor. If the standard test has a different waveshape
- Rated withstand voltage. The next standard value, equal or higher
IEC 60071-1 defines Ks as a global factor. It is applied to the coordination voltage, after the atmospheric correction. It covers the differences between the conditions in service over the equipment’s life and those of the standard test.
What the safety factor Ks covers
The standard builds it from five effects:
- differences in the assembly of the equipment,
- scatter in product quality,
- quality of installation,
- ageing of the insulation over its expected life,
- other unknown influences.
The weight of each effect changes with the type of equipment. IEC 60071-2 details how each one is treated:
| Effect | How the standard treats it |
|---|---|
| Ageing | External insulation is not assumed to be subject to ageing, except those containing organic materials, which require study, especially outdoors. In internal insulation it can be significant and is covered by Ks. |
| Production and assembly scatter | In fully factory-assembled equipment it is negligible. In equipment assembled on site, the actual withstand voltage may be lower than required and is covered by Ks. |
| Inaccuracy of the withstand voltage | In external insulation, the differences between the test arrangement and the service arrangement, and the influence of the laboratory. In internal insulation, the statistical uncertainty of the three-impulse test, which is commonly used because 100 % probability of withstand is assumed. |
Recommended values
When the apparatus committees specify no others, the standard recommends:
| Insulation | Recommended Ks |
|---|---|
| Internal | 1.15 |
| External | 1.05 |
For range II GIS, the standard warns that higher safety factors may apply and that in that case on-site tests can be considered. If the effects cannot be evaluated one by one, IEC 60071-1 indicates adopting a global factor derived from experience.
The test conversion factors
The insulation level is standardized in certain waveshapes: in range I (up to 245 kV), short-duration power frequency and lightning impulse; in range II (above 245 kV), switching impulse and lightning impulse. A required voltage of another class is therefore converted before being compared. A single standard value can thus demonstrate several required voltages.
The standard defines them as the factor by which the required withstand voltage of one class is multiplied when the waveshape of the chosen test is that of another class. They were obtained from existing results to give a conservative value and must be used only in the direction indicated. They apply if the product committee has no suitable factors.
Range I. They convert the required switching voltage (Urw, in kV) to short-duration power frequency and to lightning impulse. They apply to phase-to-earth voltage and to the sum of the phase-to-phase and longitudinal components:
| Insulation | Power frequency | Lightning impulse |
|---|---|---|
| Air clearances and clean insulators, dry, phase-to-earth | 0.6 + Urw / 8,500 | 1.05 + Urw / 6,000 |
| Same, phase-to-phase | 0.6 + Urw / 12,700 | 1.05 + Urw / 9,000 |
| Clean insulators, wet (rain) | 0.6 | 1.3 |
| GIS | 0.7 | 1.25 |
| Liquid-immersed insulation | 0.5 | 1.10 |
| Solid insulation | 0.5 | 1.00 |
The power-frequency factors already include the division by the square root of two to go from peak value to r.m.s. value.
Range II. They convert the required short-duration power-frequency voltage to switching impulse, and also apply to longitudinal insulation:
| Insulation | Switching impulse |
|---|---|
| Air clearances and clean insulators, dry | 1.4 |
| Clean insulators, wet (rain) | 1.7 |
| GIS | 1.6 |
| Liquid-immersed insulation | 2.3 |
| Solid insulation | 2.0 |
These include the square-root-of-two factor to go from r.m.s. to peak.
An illustrative example
An air clearance, dry, phase-to-earth, in range I, with a required switching voltage of 300 kV. The power-frequency factor is 0.6 + 300/8,500, about 0.635, giving roughly 191 kV r.m.s. The lightning impulse factor is 1.05 + 300/6,000 = 1.10, giving 330 kV. A standard value equal to or higher than each result would be chosen. This illustrates the operation; it is not project data.
What the level selection must cover
In range I, the standard power-frequency or lightning impulse voltage must cover the required switching voltages phase-to-earth, phase-to-phase and longitudinal. In range II, the standard switching impulse voltage must cover two things. One is the required short-duration power-frequency voltage. The other is the continuous power-frequency voltage, if no product committee sets another value. For the long-duration power-frequency test, IEC 60071-1 leaves the definition to the product committee.
What not to do
- Do not use Ks = 1.15 or 1.05 if the product committee sets another value: the ones in this guide are for when it does not.
- Do not apply the conversion factors in reverse or between classes other than those in the tables.
- Do not add the altitude correction and the safety factor as if they were the same concept: Ka corrects for the atmosphere and Ks for the other differences.
- Do not consider site-assembled insulation covered by the factory value: its scatter is greater.
To go further, see Deterministic insulation coordination method, Withstand voltage tests: types and acceptance criteria, Self-restoring and non-self-restoring insulation and Lightning and switching impulse: standard waveshapes.
When to call us
Verifying that the selected switchgear meets the required withstand voltage at your site means knowing its factors and its actual level. At Tiza Group we carry out that verification in engineering design and studies, remotely and worldwide. Write to us and we will send you a quote.
Related guides
- Insulation coordination under IEC 60071: the steps
- Altitude correction of external insulation: Ka and m
- Deterministic insulation coordination method
- Insulation coordination in substations above 245 kV
- Self-restoring and non-self-restoring insulation
References
- IEC 60071-2:1996, Insulation co-ordination, Part 2: Application guide, clauses 4.1, 4.3, 5.1 and 5.2, tables 2 and 3.
- IEC 60071-1:2006, Insulation co-ordination, Part 1: Definitions, principles and rules, definitions 3.30 to 3.32 and clauses 5.4 and 5.5.
Frequently asked questions
What values does IEC 60071-2 recommend for the safety factor Ks?
If the apparatus committees specify no others, it recommends Ks = 1.15 for internal insulation and Ks = 1.05 for external insulation (clause 4.3.4). For range II GIS, higher values may apply, and in that case on-site tests can be considered.
What is a test conversion factor?
It is the factor by which the required withstand voltage of one overvoltage class is multiplied when the standard waveshape of the chosen test is that of another class (IEC 60071-1, definition 3.32). It is used to verify more than one required voltage with a single standard test.
Can the conversion factors be used in either direction?
No. The standard derived them from existing results to give a conservative value of the rated level, so they must be used only in the specified direction: switching to power frequency and lightning in range I, and power frequency to switching in range II.