Insulation Coordination in Switchboards: Uimp, Clearance and Creepage
insulation coordination: Insulation coordination in a switchboard means selecting the air clearance between live parts and the creepage distance across insulating surfaces according to the expected impulse voltage (Uimp) and the pollution degree. Minimum clearances are around 3 mm for 4 kV, 5.5 mm for 6 kV and 8 mm for 8 kV Uimp.
Contents
- What is insulation coordination and why does it belong to panel design?
- How are rated impulse withstand voltage and overvoltage category chosen?
- What is the difference between clearance and creepage distance?
- How do pollution degree and material group affect the design?
- Practical rules and verification in assembly design
- Key takeaways
- Frequently asked questions
- Related articles
A switchboard can have the correct short-circuit withstand rating and IP code and still fail on the first lightning-borne surge if the distances between live parts are too small. Insulation coordination removes that risk from chance: the expected overvoltage is defined first, then clearances, creepage distances and solid insulation properties are chosen to match that voltage and the surrounding conditions. This article looks at Uimp, overvoltage category, pollution degree and material group from the point of view of assembly design.
What is insulation coordination and why does it belong to panel design?
Insulation coordination is the process of matching the insulating properties of equipment to the overvoltages expected in the supply system and to the micro-environment in which the equipment operates. In low-voltage assembly design this means selecting three things together: the air clearance between conductive parts, the creepage distance measured along an insulating surface, and the withstand capability of the solid insulation used. IEC 61439-1 defines minimum values for all three by reference to IEC 60664-1 and requires the assembly manufacturer to verify compliance. Verification is achieved either by measurement and calculation or by type test, and in both cases the record becomes part of the technical file.
How are rated impulse withstand voltage and overvoltage category chosen?
Uimp is the peak voltage that equipment can withstand without damage when subjected to a 1.2/50 µs impulse, and it is declared on the assembly rating plate. Its value follows the overvoltage category that corresponds to the position of the assembly in the network: the metering and main incoming zone (category IV) sees the highest stress, fixed installation distribution (category III) one step lower, and appliance level (category II) lower still. On a 400/230 V system typical declarations are 6 kV or 8 kV for main distribution boards, 4 kV for sub-distribution and 2.5 kV for control circuits. Fitting surge protective devices limits the expected impulse but does not by itself lower the declared Uimp; an adequate margin must remain between the protection level Up and the equipment withstand.
What is the difference between clearance and creepage distance?
Clearance is the shortest distance through air between two conductive parts and it governs withstand against impulse voltage, which is why it is tied directly to Uimp. Creepage distance is the shortest path along the surface of insulating material and it addresses degradation caused by surface contamination, humidity and tracking under continuous operating voltage, which is why it is tied to pollution degree and material group. In practice clearance is managed through busbar spacing, terminal design and barrier walls, while creepage is managed through insulator geometry, surface ribs and coatings over conductors. The two are not interchangeable: an arrangement with adequate clearance can still be deficient in creepage in a polluted environment.
How do pollution degree and material group affect the design?
| Pollution degree | Micro-environment | Typical assembly equivalent |
|---|---|---|
| 1 | No pollution, or only dry non-conductive dust | Sealed electronic compartments |
| 2 | Non-conductive pollution only, occasional condensation | Indoor boards in IP54 or better enclosures |
| 3 | Conductive pollution, or dust made conductive by condensation | Ventilated boards in dusty production areas |
| 4 | Persistent conductive pollution, conductive dust or rain | Outdoor and heavy industry duty, extra measures required |
Pollution degree describes the micro-environment inside the enclosure, not the room where the enclosure stands. A well sealed enclosure of IP54 or better can maintain pollution degree 2 conditions inside even in an industrial plant, which noticeably shortens the required creepage distance. Material group classifies the insulating material by its comparative tracking index (CTI) and produces different minimum distances at the same voltage. Enclosure sealing, filtration and humidity management are therefore design decisions with a direct cost impact on insulation distances.
Practical rules and verification in assembly design
The most common field problems are distances squeezed between busbar supports, cable lugs added later that eat into the clearance, and bare conductor sections placed too close to a barrier. Verification measurement must be taken at the least favourable position, with real assembly tolerances and conductor movement taken into account; for flexible connections the displacement under short-circuit forces is also considered. Above 2000 m altitude clearances are increased by an altitude correction factor, while creepage distances are unaffected. Any design change, such as increasing a busbar cross-section, calls for the distances to be checked again.
Key takeaways: insulation coordination
- Uimp follows the position of the board — typically 6–8 kV at the main incoming point, 4 kV for sub-distribution and 2.5 kV for control circuits.
- Clearance resists impulse, creepage resists pollution — they are not interchangeable and are verified separately.
- Pollution degree describes the inside of the enclosure — a good IP rating shortens the required creepage and saves space.
- Measure at the least favourable point — account for tolerances, cable lugs and displacement under short-circuit forces.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
What is the difference between Uimp and Ui?
Ui is the rated insulation voltage and is the reference for continuous operating voltage and creepage dimensioning. Uimp expresses withstand against short-duration impulse voltage and governs clearances. An assembly is declared with both values and each requires its own verification.
I installed an SPD, may I reduce clearances?
No. A surge protective device limits the expected impulse, but the declared Uimp of the assembly and the associated minimum distances remain unchanged. The correct approach is to keep an adequate margin between the protection level Up and the equipment Uimp.
How does altitude affect insulation distances?
Above 2000 m air density falls and dielectric withstand drops, so clearances are increased by an altitude correction factor. Creepage distances are not affected by altitude; separate derating applies to temperature and current-carrying capacity.
Related articles
- What is the IEC 61439 Standard? Type Testing and Conformity in Low Voltage Switchgear.
- IK Impact Rating Explained: IK07, IK08 and IK10 Compared
- Form 1, 2b, 3b and 4b Internal Separation Explained
- All technical articles
In Devpan assemblies insulation coordination is calculated at the design stage, from busbar layout to terminal selection, and recorded by measurement during routine verification. Contact our engineering team for a solution matched to the overvoltage category and environmental conditions of your project. Related Devpan solution: electrical panel design services.
