CTI tells you what a material can do. PTI tells you whether the part in front of you still does it.
The two come out of the same machine, run the same drops of the same electrolyte across the same platinum electrodes. The difference is the question. CTI hunts for a ceiling. The proof tracking index checks a floor: five specimens, one voltage you named in advance, 50 drops, pass or fail.
That makes PTI the acceptance test. Incoming inspection on a resin lot. Quality control on a molded run. Confirming a supplier’s new batch behaves like the batch you qualified. This page is about running it and reading it — for the full mechanism of how tracking damages a surface, see the comparative tracking index pillar.
Table of Contents
The voltage is not yours to invent
The single most important thing about a PTI test happens before any drop falls: choosing the voltage. Get this wrong and a clean pass means nothing.
The voltage is not arbitrary and it is not a round number someone liked. It traces back through the design.
Work the chain backwards. Your product has a working voltage and sits in an environment with a pollution degree. The equipment standard uses those, plus the material group, to set a minimum creepage distance. The material group came from a CTI value. So when a product standard calls for “PTI 250,” it is saying: this part must behave, at 250 V, the way a material at the bottom of its assigned group behaves — and it wants that reconfirmed on production parts, not just on the qualification plaque.
That is why PTI exists as a separate test. Qualification proved the material. PTI keeps proving the parts, cheaply, one voltage at a time, without re-running a full CTI search on every batch.
The specified voltage is always an integer multiple of 25 V. If your incoming spec does not state one, the test cannot be run — go back to the product standard or the drawing and find where the requirement is set.
Running it
The apparatus and setup are identical to a CTI test. Nothing about the hardware changes.
- Two platinum electrodes, 4 mm apart, 1 N each on the surface.
- Solution A unless the spec calls for the more aggressive Solution B — in which case the result carries an M.
- Drops every 30 s, about 20 mg each.
- Five specimens, conditioned 24 h at 23 °C and 50 % RH, tested at 23 °C ambient.
Then you fix the voltage at the specified value and run each specimen to 50 drops.
The recommended number of specimens is five, though a product committee can name a different number. All of them have to survive.
What “survive” means
A specimen passes when the full test period elapses — at least 25 s past the fiftieth drop — with no tracking failure and no persistent flame.
- Tracking failure: current across the surface hits 0.5 A and holds for 2 s. The over-current device trips.
- Persistent flame: the specimen burns for more than 2 s. Brief flashes are allowed; a material that never flames is preferred.
Both are failures at the specified voltage. One specimen failing fails the lot, because the requirement is that the required number of specimens withstand the period.
The air-arc exception
Here is where PTI and CTI diverge in a way that catches people.
Sometimes the over-current device trips because the arc jumped between the electrode tips through the air, not across the specimen surface. The material did nothing. In a CTI determination this invalidates the test — you clean up and repeat.
In a PTI test, an air-arc trip does not count as a tracking failure. The standard says so directly. The part is not condemned by an event that had nothing to do with its surface.
In practice you still resolve it — clean the electrodes, take a fresh site, run it again to get a clean pass or a real failure. But the distinction matters when you are writing the acceptance decision. An air arc is not evidence the part is bad.
Reading the result
PTI results are written as a pass or fail against the voltage, with a few modifiers.
| Reported as | Meaning |
|---|---|
| Pass PTI 175 | Passed at 175 V, Solution A |
| Fail PTI 175 M | Failed at 175 V, Solution B |
| Pass PTI 250 – 3 | Passed at 250 V, max erosion depth 3 mm |
| Fail PTI 250 – 3 | Failed at 250 V, erosion 3 mm noted |
Read the number as a threshold, not a ceiling. “Pass PTI 250” means the part cleared 250 V. It says nothing about 275 V, because nobody tested 275 V — that is the whole economy of the method. If you need the ceiling, you need a CTI determination.
A few things always get recorded alongside the verdict:
- Erosion, when the spec requires it. Specimens that passed are cleaned and measured with a depth gauge; report the deepest of the five. Under 1 mm is reported as < 1 mm. A part can pass the tracking criterion and still be eaten away — a “Pass PTI 250 – 3” is a warning, not a clean bill.
- Holes. If a scintillation burns through the specimen, the result still stands, but the hole and its depth (the specimen thickness) get reported.
- Flaming that prevents erosion measurement. If the specimen burned too badly to gauge, say so.
- Invalid tests from air arcs. Report them as invalid rather than passing them off as a result.
PTI or CTI — which test do you actually want
| PTI | CTI | |
|---|---|---|
| You already have a target voltage | Yes — this is the test | No |
| You need to characterize an unknown material | No | Yes |
| Batch acceptance / incoming QC | Yes | Overkill |
| Choosing a material for a new design | No | Yes |
| Fast and cheap | Yes | Slower — it is a search |
| Output | Pass/fail at one voltage | Maximum withstand voltage |
The rule of thumb: if someone handed you a voltage, run PTI. If you have to find the voltage, run CTI. A shop doing incoming inspection almost never needs CTI — the design work that produced the voltage is already done, and re-deriving it every batch wastes specimens.
Reporting checklist
The report carries the same identifying information as a CTI report, because the apparatus and conditions are the same. At minimum:
- Material identification and any conditioning applied.
- Specimen thickness and number of stacked layers used to reach it.
- Surface condition before test — and if the original surface was not tested, the cleaning, machining or coating that changed it.
- Cleaning procedure for electrodes and dropper.
- Air flow rate, if the test was not run in a draught-free space.
- Electrode orientation relative to any grain or flow direction in the material.
- The pass/fail result at the specified voltage, with the M suffix if Solution B was used, and erosion depth if required.
FAQ
What is the proof tracking index?
The proof voltage, in volts, at which five specimens survive 50 drops of standard electrolyte without a tracking failure and without a persistent flame. It is a pass/fail acceptance test at one specified voltage.
How is PTI different from CTI?
PTI checks whether a material passes at a single voltage you specify in advance. CTI searches for the maximum voltage a material can pass. PTI is for acceptance and quality control; CTI is for characterizing and comparing materials.
Where does the PTI test voltage come from?
From the product or material standard, which sets it based on the working voltage, pollution degree, and required material group of the application. It is always an integer multiple of 25 V. If no voltage is specified, the test cannot be run.
Does an air arc fail the part?
No. In a PTI test, an over-current trip caused by an air arc between the electrodes does not count as a tracking failure. Clean the apparatus and repeat at the same voltage to get a valid result.
How many specimens fail the lot?
One. The requirement is that all specimens — normally five — withstand the test period. A single tracking failure or persistent flame at the specified voltage is a fail.
What does “Pass PTI 250 – 3” mean?
The material passed the tracking test at 250 V, but eroded to a maximum depth of 3 mm. It met the tracking criterion while losing significant material, which is worth flagging even though the verdict is a pass.
Can PTI tell me a material’s CTI?
Not directly. A pass at 250 V confirms the material reaches at least that threshold, but the actual ceiling could be higher and was never tested. To get the ceiling, run a CTI determination.
Which electrolyte should I use?
Solution A by default. Use Solution B only if the spec calls for it — it is more conductive and wets hydrophobic surfaces, and any result obtained with it carries an M suffix. Do not compare M and non-M results as if they were the same test.