5 Methods to Change Creepage Distances Requirements

The definition of creepage distance is the shortest distance between two conductive materials (pins) measured along the insulating surface. It means that the creepage distance can be changed if it's not enough in some situations.

Figure 5-1 shows the PCB cutout method to increase the creepage distance. For example, if using CTI IIIa or CTI IIIb PCB to replace CTI I or CTI II PCB for saving cost, the PCB creepage distance may be not enough and PCB cutout is needed to increase the creepage distance.

Note that PCB cutout can only increase PCB creepage distance but can't increase the isolated devices or components creepage distance.

Figure 5-2 illustrates the PCB conformal coating solution for lowering the pollution degree of PCB, which enables reduced creepage distance requirements.

This does not eliminate air gaps between conductive materials (pins), meaning the insulation form remains air-based. Under such protection, the clearance and creepage distance criteria for Pollution Degree 1 specified in Figure 4-1 and Figure 4-2 are applicable. Notably, only the pollution degree can be downgraded, while clearance values cannot be reduced. Furthermore, only Type 1 coating that meets the conditions for pollution degree downgrade can reduce the required creepage distance in accordance with pollution degree 1. Poor coating quality such as pinholes, air bubbles or incomplete coverage results in invalid safety compliance, and the original pollution degree shall still be adopted for calculation.

When air insulation design fails to meet specified clearance and creepage distance limits, the potting method shown in Figure 5-3 can be adopted to achieve solid insulation.

Potting is Type 2 protection (as defined in IEC 60664-3:2016) which replaces air insulation completely with solid insulation materials. Accordingly, the conventional clearance and creepage distance requirements stated in Figure 4-1 and Figure 4-2 are no longer applicable.

In general, potting delivers higher insulation performance than conformal coating, yet it involves additional design efforts, complicated manufacturing procedures and significantly higher costs. Besides, solid insulation compliance must be fully validated via official certification tests such as AC/DC withstand voltage test, impulse withstand voltage test, partial discharge test and thermal cycling test by authoritative bodies such as VDE and TUV, rather than relying solely on theoretical calculations.

For this reason, designers tend to select isolators and components that inherently meet insulation specifications instead of adopting potting solutions, unless no alternative design scheme is available.