Creepage distance is, simply, solid insulation surface distance between two conductors. Creepage distance is not insulation. On the other hand, clearance is insulation – air insulation.
Electrical insulation is always comprised of solid material and gas (usually air) in parallel or in series. See Figure 1. All product constructions necessarily employ both solid and air insulations. Every solid insulation has an associated air insulation (e.g., for connection to the conductors). And, every air insulation has an associated solid insulation (e.g., to support the conductors). An example of such construction is the C14 appliance inlet described in IEC 60320-1. See Figure 2. Conductors must emerge from solid insulation into air insulation for interconnection, and conductors utilizing air insulation must be supported by solid insulation.
Electric strength is the principal parameter of insulation. Whether solid or air insulation, the distance through the insulation is the principal factor determining the electric strength of the insulation. The greater the through-insulation distance, the greater the insulation electric strength.
For the functional operation of insulation, the electric strength of both solid insulation and air insulation must be more than the voltage that is through or across the insulation. The electric strength rating of typical solid insulation material is 10 kV/mm or greater. The electric strength of air insulation is about 1.5 kV/mm, but can be more or less, depending on the parameters affecting electric strength of air. See Table 1.
| Parameter | Solid insulation | Air insulation |
| Normal voltage | x | x |
| Transient voltage (if any) | x | x |
| Voltage duration | x | |
| Distance through | x | x |
| Electric field shape | x | x |
| Mechanical strength | x | |
| Temperature | x | x |
| Humidity; moisture | x | x |
| Air density | x | |
| Air pressure | x | |
| Surface characteristics | x | |
| Surface contamination | x |
Table 1: Parameters affecting electric strength of insulation
Where conductors emerge from solid insulation (e.g., an appliance coupler) or lie on solid insulation (e.g., printed wiring) or equivalent, the conductors at the surface of solid insulation are insulated from each other by both air insulation and solid insulation in parallel. Since air has lower electric strength than solid insulation, the distance between conductors along the surface of the solid insulation must be at least the air (clearance) dimension. See IEC 60664-1, 5.2.2.6.
However, the surface of solid insulation can be contaminated by deposition of unknown materials, which comprise a third material in parallel with the air and solid insulations. These materials may compromise the electric strength of the surface of solid insulation. If the contamination has a lower electric strength than the air insulation, the contamination is likely to break down in micro-arcs. The micro-arcs heat the solid insulation and may cause pyrolysis of the solid insulation surface and thus, over time, reduce the electric strength of the solid insulation surface between the conductors.
This deterioration is a long-term phenomenon. As such, the functional voltage, rms or DC, between conductors is the major cause of the surface-damaging micro-arcs. As a practical matter, at distribution system voltages, 100-120 or 220-230-240, solid insulation surface deterioration is not likely because air is not likely to ionize and arc at these voltages. (According to Paschen, air does not break down at voltages below 327 peak or DC, regardless of distance between conductors.) While transient voltages can also cause micro-arcs, transient voltages are very infrequent and therefore do little damage to the surface. See IEC60664-1, 5.2.2.2.
Solid insulation surface distance (creepage distance) requirements only apply to solid insulation surfaces that are subject to pollution. According to IEC 60664-1, 4.6.1:
“Means may be provided to reduce pollution at the insulation under consideration by effective use of enclosures, encapsulation or hermetic sealing.”
To me, solid insulation surfaces that are not subject to pollution are not subject to creepage distance requirements. Others (and some certification organizations) may say that all solid insulation surfaces must comply with creepage distance requirements.
In Summary
The surface of solid insulation between conductors (creepage distance):
- must not have a voltage between conductors that exceeds the Paschen air no-breakdown value,[1] or
- must be a solid insulation material whose surface is not pyrolytic, e.g., glass, ceramic, or other inorganic material, or
- must be protected (or oriented) against deposition of contaminants,[2] or
- must have sufficient (creepage) distance to minimize the likelihood of voltage-caused insulation deterioration due to:
- surface characteristics of the solid insulation, and
- surface contaminants on the solid insulation.
The three parameters for determining solid insulation surface (creepage) distance between conductors are:
- voltage between conductors across the solid insulation surface;
- comparative tracking index (CTI) of the solid insulation surface; and
- pollution (material group) deposition on the surface environment.
Using these three parameters, a table in various standards specifies the minimum (creepage) distance across the surface of solid insulation, but not less than the distance requirement for air insulation (clearance).
[1] May not be accepted for safety certification.
[2] May not be accepted for safety certification.
