The drop test tops out. IEC 60112 discriminates between poor, moderate and good tracking resistance for indoor equipment under moist conditions, and for that job it works. But it runs at hundreds of volts on 20 mm specimens for under an hour, and its own scope says the results do not qualify a material for outdoor service.
Outdoor insulation lives a harder life. Insulators, bushings, cable terminations and enclosures sit in rain, salt fog, industrial fallout and UV, at voltages the drop test never reaches. To rank materials for that, you need a harder test. IEC 60587 is it — the inclined plane test.
What makes it severe is worth stating up front, because it is not what people assume. The contaminant is not much nastier than the drop test’s. Both use roughly 0.1% ammonium chloride and land near the same conductivity, about 0.25 S/m. The severity comes from everything else: kilovolts instead of volts, contaminant flowing continuously instead of falling in drops, six hours instead of minutes, and a 45° geometry that keeps the surface wet and draining the whole time.
For the underlying failure mechanism — how a contaminated film builds a conductive track, and how tracking differs from erosion — see the comparative tracking index pillar and the tracking vs erosion page. This page is about the inclined plane method itself.
Table of Contents
The setup that makes it harsh
Picture a flat slab of material clamped at 45°, tested face down on the underside. Two stainless steel electrodes sit on that face, 50 mm apart. Contaminant feeds continuously into a filter-paper pad under the top electrode and runs down the face between the electrodes under gravity. Voltage is applied. Scintillations fire at the lower electrode, and the material either tracks, erodes, or survives.
Every one of those choices is a step up in severity from the drop test.
| IEC 60112 drop test | IEC 60587 inclined plane | |
|---|---|---|
| Voltage | 100–600 V | up to 6 kV |
| Electrodes | Platinum, 4 mm apart | Stainless steel 302, 50 mm apart |
| Contaminant delivery | Drops every 30 s | Continuous flow |
| Geometry | Flat, horizontal | 45° incline, face down |
| Duration | Under 1 h | Up to 6 h per voltage |
| Specimen | ≥ 20 × 20 mm, ≥ 3 mm thick | ≥ 50 × 120 mm, 6 mm preferred |
| Use | Indoor screening, creepage groups | Severe / outdoor conditions |
The stainless electrodes matter. The drop test uses platinum precisely because it is inert and leaves the material as the only variable. The inclined plane test uses stainless steel type 302 — the alloy real hardware is made from — because it is simulating service, not isolating a material property. New electrodes go on each test, well deburred on the edges facing the stressed area.
The 45° angle and the continuous feed are the other half of it. Contaminant runs down the face in a steady film instead of pooling in drops. The surface never gets the chance to fully dry and recover between wettings the way it does under a 30-second drop cycle. That sustained wet path is what lets damage propagate at voltages and over durations the drop test cannot reach.
The contaminant and the flow
The contaminant is distilled or deionized water with about 0.1% analytical-grade ammonium chloride, plus a small amount — (0.02 ± 0.002)% — of a nonionic wetting agent, octylphenoxypolyethoxy-ethanol. Target conductivity is around 0.25 S/m at 23 °C, checked before every test series. It cannot be more than four weeks old.
Wettability is not optional here — it is a stated prerequisite. If the contaminant beads up instead of wetting the surface, the test does not work. Two fixes are allowed: lightly abrade the surface with a fine aluminium-oxide or zirconia-alumina abrasive under water until it wets, or temporarily raise the flow rate until the surface is properly wetted before the voltage goes on. Any grinding gets noted in the report.
Flow rate is not a free choice either. It scales with voltage, along with the series resistor, straight from the parameter table.
Test parameters
| Test voltage (kV) | Preferred for Method 1 (kV) | Contaminant flow (ml/min) | Series resistor (kΩ) |
|---|---|---|---|
| 1.0 – 1.75 | – | 0.075 | 1 |
| 2.0 – 2.75 | 2.5 | 0.15 | 10 |
| 3.0 – 3.75 | 3.5 | 0.30 | 22 |
| 4.0 – 4.75 | 4.5 | 0.60 | 33 |
| 5.0 – 6.0 | – | 0.90 | 33 |
The three preferred voltages — 2.5, 3.5 and 4.5 kV — are the ones that produce the standard class ratings. The power supply is 45–65 Hz, near-sinusoidal, with a 200 W series resistor per specimen and an over-current relay that trips when 60 mA has persisted for a few seconds. If one supply feeds several specimens, each needs its own breaker so one failure does not drop the voltage on the rest.
Two endpoints, two methods
Before running anything, you pick an endpoint criterion and a method. They are independent choices.
Two ways to call the endpoint:
- Criterion A — current (preferred). The test ends when current through the specimen exceeds 60 mA and the over-current device trips. This is the automatable one — it lets a rig run several specimens at once without someone watching each.
- Criterion B — track length. The test ends when the track reaches a reference mark 25 mm from the lower electrode. This needs constant visual supervision and manual control.
Under either criterion, two events are automatic failures: the specimen igniting, and a hole forming through it from erosion — whether the hole shows up during the test or only after the eroded material is cleaned away.
Two ways to apply voltage:
- Method 1 — constant voltage. Pick a voltage, hold it for six hours. This is the common one, since it needs less continual watching.
- Method 2 — stepwise increased voltage. Start at a multiple of 250 V low enough that failure will not come before the third step. Hold one hour, step up 250 V, hold another hour, and so on until the specimen fails by criterion A. Flow rate and series resistor climb with the voltage.
Note one change in the current edition: criterion B does not apply to Method 2. Track length as an endpoint was removed there because it does not work with a stepping voltage.
Passing, and the class notation
For Method 1, the pass rule uses two sets of specimens.
Test five. If none fails at the voltage, it passes. If one of the five fails, test a second set of five — and if only one of all ten fails, it still passes. More than one failure out of ten is a fail at that voltage.
The classification is the highest preferred voltage the material passes for six hours, written with the criterion letter:
| Class | Meaning |
|---|---|
| 1A 4.5 | Passed 6 h at 4.5 kV, criterion A |
| 1A 3.5 | Passed 6 h at 3.5 kV, criterion A |
| 1A 2.5 | Passed 6 h at 2.5 kV, criterion A |
| 1A 0 | More than one of ten failed at 2.5 kV in under 6 h |
| 1B 4.5 … 1B 0 | Same ladder, criterion B |
Method 2 produces a different label. The withstand voltage is the highest step that all five specimens survive for one hour without hitting criterion A, without igniting, and without holing. It is written 2A x, where x is that voltage in kilovolts.
Erosion depth gets appended to the class when it is measured. Failed specimens and those that survived the full run are dismantled, and the erosion depth is gauged after cleaning off decomposed material — carefully, without removing sound material underneath. A maximum erosion depth of 0.5 mm on a 3.5 kV class 1A result reads 1 A 3,5 – 0,5.
Reading a result
Put the pieces together and the notation tells you the whole test at a glance.
- The number is the voltage in kV — a 4.5 material is tougher than a 2.5 material by a wide margin.
- The letter A or B tells you which endpoint was used. A is current-based and automatable; B is track-length and manual. They are not the same test and a 1A result is not interchangeable with a 1B result.
- The 2A prefix means the stepwise method, not constant voltage — a different pass condition.
- A trailing number is erosion depth in mm. As with the drop test, a good tracking class with heavy erosion is a warning, not a clean pass.
And the same caution the standard puts on the drop test applies in reverse here: the inclined plane method may rank materials in a different order than the drop test does. A material that scores well on CTI can place poorly on the inclined plane, and the other way around. They are answering different questions about different service conditions. Use CTI to set creepage groups for indoor equipment; use the inclined plane class to rank materials for severe and outdoor duty.
FAQ
What is the inclined plane test?
A test to IEC 60587 that ranks insulating materials for severe or outdoor conditions. A specimen is mounted at 45°, contaminant flows continuously down its face between two electrodes, and kilovolt-level AC voltage drives scintillations that either track, erode, or are withstood. The result is a class rating.
How is IEC 60587 different from the CTI test?
The CTI drop test to IEC 60112 runs at hundreds of volts, drips contaminant onto a flat horizontal specimen, uses inert platinum electrodes, and lasts under an hour — it screens indoor materials. The inclined plane test runs at kilovolts, flows contaminant down a 45° face, uses stainless steel electrodes, and runs up to six hours — it qualifies materials for severe and outdoor service.
What do Class 1A 3.5 and 1A 4.5 mean?
The number is the highest preferred voltage, in kV, the material withstood for six hours under the constant-voltage method. The letter A means the current endpoint (criterion A) was used. A 4.5 material passed at 4.5 kV; a 3.5 material passed at 3.5 kV but not 4.5.
What is the difference between criterion A and criterion B?
Criterion A ends the test on over-current — 60 mA tripping the over-current device — and can be automated across several specimens. Criterion B ends it when the track reaches a mark 25 mm from the lower electrode, which requires continuous visual supervision. Criterion B does not apply to the stepwise method.
Why stainless steel electrodes instead of platinum?
Because the inclined plane test simulates service rather than isolating a pure material property. Stainless steel type 302 is representative of real hardware. The drop test uses platinum specifically to keep the electrode inert and the material the only variable.
Does a good CTI mean a good inclined plane result?
Not necessarily. The two methods can rank materials in different orders because they test different conditions. A material strong on CTI can perform poorly on the inclined plane, and vice versa.
How is erosion reported in the class?
As a maximum depth in millimeters appended to the class — for example, 1 A 3,5 – 0,5 for a 0.5 mm maximum erosion depth. A perforation through the specimen counts as a failure regardless.