4 Determine Clearances and Creepage Distance

This section demystifies clearances and creepage distance based on IEC 62477-1:2022 .

1. Clearances

   Clearances shall be determined for functional insulation, basic insulation or supplementary insulation directly from Figure 4-

   The impulse withstand voltage for reinforced insulation is one step higher for that of basic insulation. For example, if the impulse withstand voltage is 6000V for basic insulation, then it should be 8000V for reinforced insulation.

   Note that it shows the minimum clearances in air up to 2000m above sea level. For altitude above 2000m applications, it needs to consider the altitude correction factor discussed in section 3.6.

   Figure 4-1 Clearances Determination Table
2. Creepage Distances

   Creepage distances for functional insulation, basic insulation and supplementary insulation shall be dimensioned according to Figure 4-2.

   Creepage distances for reinforced insulation shall be twice the distances required for basic insulation. For example, if the creepage distance is 8mm for basic insulation, then it should be 16mm for reinforced insulation.

   Note that in all cases, the associated creepage distance is not permitted to be smaller than the required clearance (i.e. CPG always ≥ CLR).

   Note that interpolation for working voltage is allowed. For example, 1500V is not listed here, but we can use interpolation between 1250V and 1600V to determine the creepage distance (interpolation between 6.3mm and 8mm, pollution degree 2, CTI I).

   Figure 4-2 Creepage Distances Determination Table
3. Simplified Flowchart to Determine CLR and CPG

   Referring to IEC 62477-1: 2022, guidelines for determination of clearance and creepage distances are very complex because it needs to consider all scenarios. For most high-voltage applications, such as solar inverter and ESS, we can simplify them, as shown in Figure 4-3 and Figure 4-4.

   Figure 4-3 Simplified Flowchart to Determine CLR
   Figure 4-4 Simplified Flowchart to Determine CPG
4. CLR and CPG Determination Example

Figure 4-5 shows step-by-step clearance determination example for 1500VDC solar inverter or ESS.

Figure 4-6 shows step-by-step creepage distances determination example. In step 4, the interpolation is calculated as, (8 - 6.3) / (1600 - 1250) × (1500 - 1250) + 6.3 = 7.514mm.

Table 4-1 summarizes the results in conditions of 1500VDC, OVC II, CTI-I, pollution degree 2.

In some cases, the isolation design may be difficult to satisfy the required impulse withstand voltage under the corresponding OVC level. A practical way is to reduce the impulse withstand voltage a step lower, such as from 8000V to 6000V (OVC II to OVC I) by adding SPD to lower the isolation design difficulties. More examples can refer to Annex I (informative) Examples of overvoltage category reduction in IEC 62477-1:2022 .

Table 4-2 summarizes the results of reducing OVC II to OVC I. Now we can see that 8.14mm clearance and creepage distance is required for Basic insulation at 5000m altitude. Now this is supported in TI latest digital isolator ISO64xx, as shown in Figure 4-7.