Cut Sheet
Tech Brief
CTI: The Comparative
Tracking Index Test
The Comparative Tracking Index
(CTI) is the maximum voltage,
measured in volts, at which a
material withstands 50 drops
of contaminated water without
tracking. Tracking is dened as the
formation of conductive paths
due to electrical stress, humidity,
and contamination.
The CTI test provides an
accelerated simulation of
conditions of surface discharges
and possible resulting tracking
and failure (typically a “short”)
in equipment using insulating
materials. This test also provides
a means to compare insulating
materials performances under wet
and contaminated conditions.
CTI requirements became
important to manufacturers after
the publication of the Underwriters
Laboratories document UL 1950
in March 1992. UL 1950 is the UL
Standard for Safety titled
“Information Technology
Equipment including Electrical
Business Equipment.” Section
2.9 of this document outlines
requirements for Creepage
Distance, which is the shortest
path between two conductive parts
measured along the surface of
the insulation.
Depending on the CTI of the insulating material used, the minimum
creepage distance required will vary. The higher the CTI value, the lower the
minimum creepage distance required. In practice, the higher the CTI of the
insulating material used, the closer two conductive parts can be. The result
is often a smaller part, increasingly desirable in technology and
industry today. These values would be of particular interest to design
engineers who must comply with UL requirements.
The original test method referred to in UL 1950 was IEC Publication 112. This
method requires a 3-mm thick sample*, subjected to a voltage, while one
drop of test electrolyte solution (0.1% ammonium chloride) falls every 30
seconds onto the sample surface. The test continues until either 50 drops
fall or tracking occurs. Failure results if tracking occurs before 50 drops fall,
generating enough amps to trip the tester’s built-in over-current relay. If the
*In order to meet the 3-mm thickness requirement, the electrical tapes must be carefully stacked
one layer upon another until the thickness is achieved.