6.2. The following
are the major criteria for determining the creepage distances:
- Rated voltage (see 4.
Deterioration increases during the time which currentflows. Under these conditions many the materials
may exhibit two, even all three the characteristics stated.
4.1. The
different properties the surface the insulating materials and
contamination from the surroundings promote this process.6.] does not exist. The
behaviour the insulating material under various contaminants and
voltages extremely complex.
A method classification for insulating materials [. flashover can result complete connection made..2 Creepage Distances
Creepage paths for surface currents can form the surface insulation
materials.6.4)
The creepage distance the shortest distance along the surface insulation
material between two conductive parts.6)
- Insulation group
- Degree pollution (see 4.1 Insulation material groups
Extract from IEC 60664-1
Comparative tracking index (CTI)
Insulating materials can roughly characterized according the damage
they suffer from concentrated release energy during surges when a
surface leakage current interrupted due drying the contaminated
surface.4. direct
164 ____________________________________________________________
.. The following behaviour insulation materials the presence of
surges can occur:
- decomposition the insulating material;
- the wearing away insulating material action electrical
discharges (electrical erosion);
- the progressive formation conductive paths which are produced on
the surface solid insulating material due the combined effects of
electric stress and electrolytic contamination the surface (tracking)
NOTE: Tracking erosion will occur when
- liquidfilm carrying the surface leakage current breaks, and
- the applied voltage sufficient break down the small gap formed
when thefilm breaks, and
- the current sufficient provide sufficient energy locally thermally
decompose the insulating material beneath the contamination
