A datasheet quotes “CTI 400V, PLC 1, Material Group II” and an engineer new to the part assumes those are three separate properties. They are not. They are one measurement — surface tracking resistance — expressed in three different bookkeeping systems, because three different bodies standardized the same drop test.
Knowing which system a number came from matters, because they do not all count the same way, and two of them can disagree on the number even when the test is nominally identical. Here is how IEC 60112, ASTM D3638, and UL 746A relate, and where they part company.
The underlying test is the one covered in the comparative tracking index pillar: platinum electrodes 4 mm apart, drops of 0.1% ammonium chloride every 30 seconds, up to 600 V, five specimens. All three standards run that test. What differs is the paperwork wrapped around the result.
Table of Contents
IEC 60112 — the reference method
IEC 60112 is the base document. It defines the apparatus, the electrolytes, the failure criteria, and the two outputs — CTI and PTI. It is what the other two are measured against.
Its rating system, though, lives in a different standard. IEC 60112 produces a raw CTI number; IEC 60664-1 turns that number into a material group used for creepage design:
| Material group | CTI (V) |
|---|---|
| I | ≥ 600 |
| II | 400 – 599 |
| IIIa | 175 – 399 |
| IIIb | 100 – 174 |
So in the IEC world, the chain is: run 60112, get a CTI, look up the material group in 60664-1, use the group to set creepage. The group — not the raw number — is what most IEC-based equipment standards actually ask for.
ASTM D3638 — the North American method
ASTM D3638 is the current North American test method for CTI, and it is active. It runs the same basic procedure: platinum electrodes, ammonium chloride drops, the 600 V ceiling, tree-like carbonaceous tracks propagating between the electrodes until they bridge and short.
D3638 declares itself technically equivalent to IEC 60112 — and then, in its own scope, adds a warning worth pinning up. The current fourth-edition IEC 60112 can yield CTI values that differ significantly from D3638.
That is the trap. “Technically equivalent” describes the method, not the numbers. The two standards changed on different schedules, and the failure criteria in the current IEC edition are not identical to the ones D3638 references. So a material’s IEC 60112 CTI and its ASTM D3638 CTI are close cousins, not the same figure. Do not assume a number measured under one satisfies a requirement written against the other without checking which standard the requirement means.
UL 746A — the PLC banding
UL 746A is UL’s material-characterization standard, and it takes the same CTI result and buckets it into a Performance Level Category (PLC). The PLC exists for one reason: to stop people reading false precision into a single test number. A material that tests at 248 V and one that tests at 251 V are, for practical purposes, the same material, and banding them keeps a 3 V measurement wobble from looking like a real difference.
| PLC | CTI (V) |
|---|---|
| 0 | ≥ 600 |
| 1 | 400 – 599 |
| 2 | 250 – 399 |
| 3 | 175 – 249 |
| 4 | 100 – 174 |
| 5 | < 100 |
Lower PLC is better — PLC 0 is the top. That inversion catches people used to reading CTI, where higher is better.
The crosswalk — and where it doesn’t line up
Put the three side by side and most of it maps cleanly. One band does not.
| CTI (V) | IEC material group | UL PLC |
|---|---|---|
| ≥ 600 | I | 0 |
| 400 – 599 | II | 1 |
| 250 – 399 | IIIa | 2 |
| 175 – 249 | IIIa | 3 |
| 100 – 174 | IIIb | 4 |
| < 100 | — | 5 |
Look at the 175–399 span. IEC calls all of it Group IIIa. UL splits it into two categories, PLC 2 and PLC 3. So a single IEC material group corresponds to two UL performance levels, and you cannot back-calculate a precise PLC from “Group IIIa” alone — you need the underlying CTI number.
That is the practical reason to keep the raw CTI on the datasheet even when a group or a PLC is quoted. The banded ratings lose information that the other system needs.
Reading a datasheet line
Back to “CTI 400V, PLC 1, Material Group II.” Decoded:
- CTI 400V — the raw measured tracking index. This is the number everything else derives from.
- Material Group II — the IEC 60664-1 bucket for 400–599 V, used to look up creepage distance.
- PLC 1 — the UL band for 400–599 V, same range, different label.
All three agree here because 400 V sits cleanly inside one band in every system. The three “properties” are one property. What you check next depends on your compliance path: an IEC-based design uses the material group; a UL-based one uses the PLC; and if a requirement cites ASTM D3638 specifically, confirm the number was measured to D3638 rather than assumed equivalent to an IEC figure.
Two more things travel across all three systems unchanged:
- The M suffix. A CTI followed by M means the more aggressive Solution B contaminant was used. That marking carries regardless of which standard reported the value.
- CTI versus PTI. The proof tracking index — the pass/fail check at one specified voltage — is defined the same way across the standards. CTI is the maximum; PTI is a proof point.
FAQ
Is ASTM D3638 the same as IEC 60112? The test method is technically equivalent — same electrodes, contaminant, and 600 V ceiling. But ASTM D3638 warns that the current IEC 60112 fourth edition can produce significantly different CTI numbers, so the methods match while the values may not. Treat them as equivalent procedures, not interchangeable results.
What is the difference between material group and PLC? Both come from the same CTI value. Material group is the IEC 60664-1 system (I, II, IIIa, IIIb) used for creepage design. PLC is the UL 746A Performance Level Category (0 through 5) used to avoid reading false precision into the number. They mostly align, but IEC Group IIIa spans two UL categories, PLC 2 and PLC 3.
Why is a lower PLC better but a higher CTI better? They count in opposite directions. CTI is the actual voltage, so more is better. PLC is a rank where 0 is the top band (CTI ≥ 600) and 5 is the bottom. A high-CTI material earns a low PLC.
Which standard should I design to? Whichever your product standard cites. IEC-based equipment standards call for a material group derived via IEC 60112 and 60664-1. North American work typically uses the UL 746A PLC. If a requirement names ASTM D3638, verify the value was measured to it.
Can I convert an IEC CTI to an ASTM CTI? Not with a formula. The methods are equivalent but the current editions can diverge on the number. If you need a value valid under a specific standard, use a value measured to that standard rather than converting.
Does UL 746A use a different test than IEC 60112? The tracking test itself is the same drop method. UL 746A is the characterization standard that reports the result as a PLC band, alongside other material properties. The measurement is shared; the reporting differs.