How to Read a CTI and Tracking Test Report

A bare CTI number can be read wrong. “CTI 400” looks like a clean result until you learn it was run with the aggressive contaminant, or that the 100-drop figure sat 50 V lower, or that erosion ate 3 mm off the specimen while it passed. All of that lives in the report — and a compliant report is required to carry it.

So there are two skills here: knowing what a valid tracking-test report must contain, and knowing how to decode the result string once you have it. This page covers both, for the IEC 60112 drop test and the IEC 60587 inclined plane test. For the tests themselves, start at the comparative tracking index pillar.

What an IEC 60112 report must contain

The drop test specifies the same reporting skeleton whether you ran a PTI proof test or a full CTI determination. Eight items:

1. Material identification and conditioning. What was tested, and how it was conditioned beforehand.
2. Thickness and number of layers. Including how many pieces were stacked to reach the thickness.
3. Surface nature, where the original surface was not tested. Any cleaning process, any machining such as grinding, any coating on the tested face.
4. Surface state before testing. Scratches, blemishes, impurities, and the like.
5. Cleaning procedure for the electrodes and dropper.
6. Air flow rate, if the test was not run in an essentially draught-free space.
7. Electrode orientation relative to any known feature of the material.
8. The result, in the notation below.

None of these is filler. Items 2 through 4 tell you whether the specimen was comparable to others; item 7 tells you whether the weak direction was tested; item 6 flags a condition that can shift the number. A report missing them is a number without its context.

What an IEC 60587 report must contain

The inclined plane test has its own report list, shaped by its two-method, two-criterion structure:

• Material type and designation.
• Specimen details — fabrication, dimensions, cleaning procedure and solvent used, any surface treatment, and pre-conditioning.
• Orientation of a composite specimen relative to the electrodes — machine direction, cross-machine, bias, and so on.
• Test method, test voltage, and endpoint criterion applied — Method 1 or 2, and criterion A or B.
• Results for each specimen.
• Classification of the material.
• Maximum erosion depth, carried into the classification.

The surface-treatment line matters more here than on the drop test, because 60587 permits grinding a specimen to make it wet — and that grinding has to be on the record.

Decoding an IEC 60112 result

The result string packs up to four pieces of information. Read them in order.

The base value. CTI 175 or Pass PTI 175 — the tracking index in volts, or a proof pass/fail at a specified voltage. Higher is better.

The M suffix. CTI 175 M means the more aggressive Solution B contaminant was used instead of the default Solution A. A value with M and one without are not comparable.

The parenthetical. CTI 400(350) means the 50-drop ceiling was 400 V but the 100-drop ceiling was only 350 V. The parenthetical appears only when the 100-drop figure falls more than 25 V below the 50-drop figure — and when it does, it is the number that matters for prolonged wetting.

The erosion depth. A trailing number after a dash is the maximum erosion depth in millimeters. CTI 275 – 1,2 is a 275 V result that eroded 1.2 mm.

Stack them and you get the full grammar:

Result	Meaning
CTI 175	175 V, Solution A
CTI 175 M	175 V, Solution B
CTI 400(350) M	50-drop 400 V, 100-drop 350 V, Solution B
CTI 275 – 1,2	275 V, erosion 1.2 mm
CTI 400(350) M – 3,4	All of the above combined
Pass PTI 250 – 3	Proof pass at 250 V, erosion 3 mm
Fail PTI 175 M	Proof fail at 175 V, Solution B

Three more entries are reported in words rather than in the code, and each changes how you read a result:

• A hole through the specimen is reported with its depth, and the result still stands.
• Flaming that prevented erosion measurement is reported instead of an erosion number.
• Tests invalidated by air arcs are reported as invalid — not as a pass at the last surviving voltage.

Decoding an IEC 60587 result

The inclined plane test reports a class, not a voltage on its own.

Method 1, constant voltage: Class 1A x or Class 1B x, where x is the highest preferred voltage — 2.5, 3.5, or 4.5 kV — the material withstood for six hours. The letter A or B is the endpoint criterion used: A is the current criterion, B is the track-length criterion. Class 1A 0 means the material failed even at 2.5 kV.

Method 2, stepwise voltage: Class 2A x, where x is the highest one-hour voltage step the material withstood.

Erosion is appended the same way as on the drop test — a maximum depth of 0.5 mm on a 3.5 kV class 1A result reads 1 A 3,5 – 0,5.

The fields people skip

If you read only the base number, four things will eventually catch you. Each is in the report for a reason.

• M changes the ranking. Solution B defeats hydrophobic surfaces, so materials that lean on surface wettability drop under it. Always check for the suffix.
• The parenthetical is the durability number. A wide gap between the 50-drop and 100-drop figures means the material holds up to a short exposure and degrades under a sustained one.
• Erosion is a separate verdict. A good tracking value with heavy erosion is not a good material. See tracking vs erosion.
• Invalid is not pass. An air-arc-invalidated test tells you nothing about the material’s tracking resistance. It is not a quiet pass.

And the base CTI itself is only the start of the compliance chain — it maps to an IEC material group or a UL PLC before it means anything for creepage design. That mapping, and the reason an IEC and an ASTM number may not match, is covered in IEC 60112 vs ASTM D3638 vs UL 746A.

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