To better assimilate the process described previously, this chapter introduces a detailed example of how to define the insulation distances according to Figure 2-1 step by step.

Step 1: Requirement collection. An industrial motor drive system is listed with the following specifications:

• Class I device (In a metallic cabinet with chassis connect to earth)
• 3-phase power source with nominal 220 Vac and 380 Vac (wye power source), OVC III
• Industrial pollution degree 2 micro-environment
• Isolation required by end-user (Human Machine Interface - HMI)
• Hot side MCU control
• Operating altitude < 2000 m

Figure 2-2 shows the block diagram of the system.

Step 2: Voltage Block definition. This step specifies the voltage blocks in the electrical schematic. The electrical circuit without internal galvanic isolation in which the voltage between two conductors cannot be above 50 Vac and 120 Vdc for clearance.

Figure 2-3 shows all the voltage blocks defined based on the block diagram of step 1.

Step 3: determine the insulation type and OVC level. The insulation type includes basic, functional and reinforced insulation. The OVC level includes I, II, III and IV. Table 2-1 shows the insulation type and OVC level between each voltage blocks.

Step 4: determine the working voltage (WV). The rated working voltage for clearance (CL) is the peak value and is referred to table B of IEC60664-1 while the working voltage for creepage (CR) is the RMS value. The table B of IEC60664-1 consists of 50 V, 100 V, 150 V, 300 V, 600 V, and 1000 V these 6 levels working voltages. Table 2-2 shows the working voltage for both clearance and creepage between each voltage blocks. The working voltage value is done by simulation and calculation.

Step 5 and 6: Clearance and creepage distance definition.

Clearance distance: Table F.1 of IEC60664-1 defines the rated impulse voltage according to each working voltage and OVC level while table F.2 defines the clearances to withstand transient over-voltages. Clearances shall be dimensioned to withstand the required impulse withstand voltage according table F.1 and F.2. With respect to impulse voltages, clearances of reinforced insulation shall be dimensioned as specified in Table F.2 corresponding to the rated impulse voltage but one step higher in the preferred series of values than that specified for basic insulation.

For 220-V or 380-V systems, the impulse voltage is 1500 V for OVC1 with functional isolation, 4000 V for OVCIII with basic isolation and 6000 V for reinforced insulation. Then the corresponding clearance of pollution degree 2 is 0.5 mm for 1500 V with functional isolation, 3 mm for 4000 V with basic isolation and 5.5 mm for 6000 V with reinforced isolation.

Creepage distance: Table F.4 of IEC60664-1 defines the creepage distances for functional, basic insulation to avoid failure according to different pollution degrees and material group of printed circuit board. Creepage distance for reinforced insulation shall be twice the creepage distance for basic insulation from table F.4. IEC61800-5-1 also defines that when the creepage distance is less than clearance determined by impulse voltage, then it shall be increased to that clearance.

Table 2-3 shows clearance and creepage distance between each voltage blocks.

Finally, select the maximum value between clearance and creepage as the insulation distance, then put the values into PCB design tool as the constrain rules as Table 2-4 shows.

Since the requirement is defined, the operating altitude is below 2000 meters. When the product or circuit board requires higher altitude than 2000 m, be sure to consider altitude correction factors. Table A.2 of IEC60664-1 defines the multiplication factor for air clearance. For example, 4000-m altitude correction factor is 1.29. Clearance distance for 2000 m is 3 mm, then clearance for 4000 m is 3 mm × 1.29 = 3.87 mm.