Cut Sheet

sample burns the test is inconclusive
and should be repeated using a thicker
sample. The maximum test voltage is
600 volts; if failure occurs voltage is
typically decreased by 25 volts and the
test repeated until a voltage is reached
that the sample can pass.
The passing voltage must be repeated
for a total of ve test sites on the
sample. The sample must, in addition,
also pass 100 drops at 25 volts less than
the original passing voltage.
When all conditions are met, a
Material Group is determined for the
insulating material based upon the
passing voltage for the 50-drop test.
Both the backing and adhesive
sides of tapes are tested. Prior to
2008, the lower of the two voltage
levels determined the Material
Group assigned.
Material Groups are identied in UL
1950, Section 2.9:
(a) Material Group I 600 </= CTI
(b) Material Group II 400 </= CTI < 600
(c) Material Group IIIa 175 </= CTI < 400
(d) Material Group IIIb 100 </= CTI < 175
UL revised the requirements and references for Material Group
Classications after 2008, eliminating the Material Group IIIb rating.
The current notes regarding CTI Material Groups state the following in
the Online Certications Directory:
(a) Material Group I May be marked “Comparative Tracking Index (CTI) equal to
or greater than 600V, PLC=0, UL840 Material Group I, when
tested to IEC60112 on both sides of tape.
(b) Material Group II May be marked “Comparative Tracking Index (CTI) ) equal to
or greater than 400V but less than 600V, PLC=1, UL840
Material Group II, when tested to IEC60112 on both sides
of tape.
(c) Material Group IIIa May be marked “Comparative Tracking Index (CTI) equal to
or greater than 175V but less than 400V, PLC=2 or 3, UL840
Material Group IIIa, when tested to IEC60112 on both sides
of tape.
(d) Material Group IIIa May be marked “Comparative Tracking Index (CTI) 325(275)
on Adhesive side, UL840, Material Group IIIa and/or CTI 225
on Film Side, Material Group IIIa or equivalent when tested to
IEC60112, 4th Edition (2003).
UL Standard 840 Standard for Insulation Coordination Including Clearances
and Creepage Distances for Electrical Equipment. UL 746A is
referenced in UL840.
UL Standard 746A Standard for Polymeric Materials - Short Term Property
Evaluations.
The 3M Electrical Markets Division (EMD) Laboratory tested and
determined a CTI value for more than 50 Electrical Insulating Tapes from
3M. Underwriters Laboratory has tested 37 insulating and conductive tapes
from 3M to verify results. CTI testing at UL is optional.
UL added Material Groups assigned to the tapes tested at UL to the UL
Online Certications Directory. To see these references, go to
www.ul.com, then click on Online Certications Directory, and enter
E17385 into the search cell for UL File Number. Click on the link provided
there to nd a list of all EMD UL-Recognized tapes; CTI values are
indicated by subscripts (a, b, c, or d). Of the tapes tested, 3M qualied
34 for Material Group I rating, and UL qualied 24 of those, verifying
3M procedure and method.