CTI, comparative tracking index, is found by dripping ammonium chloride solution between two electrodes on a sample and recording the highest voltage at which it survives 50 drops without forming a conductive track. Materials fall into groups: I (600 V and above), II (400 to below 600 V), IIIa (175 to below 400 V) and IIIb (100 to below 175 V). A better group permits shorter creepage distances, so CTI drives the choice of plastics and laminates.
Standard FR-4 lands in group IIIa, if you want group I creepage figures, buy a high-CTI laminate.
Also heard as
- comparative tracking index
Related terms
Tracking
Conductive carbon trail that leakage arcs through dirt and moisture gradually burn across an insulating surface.
Creepage distance
Shortest route over an insulating surface from one conductive part to another, the dimension that resists tracking.
Pollution degree
Grade from 1 to 4 describing how much conductive dirt and moisture can settle on insulation in a location.