ASTM D495 says it plainly in its own referenced-documents clause: IEC 61621 is technically equivalent to D495 and was built directly from it. That sentence gets quoted a lot, usually to justify treating a number from one as a number from the other.
It does not mean that. “Technically equivalent” describes the method. It does not make the two documents interchangeable, and the same clause that says it goes on to name the first difference.
Table of Contents
Same experiment
Before the differences, the common ground, because it is most of the test.
Both calibrate the open-circuit operating voltage to 12,500 V from a transformer rated 15 kV open circuit and 60 mA short circuit. Both use a 15 kΩ suppressing resistor and 1.2 to 1.5 H of air-core inductance to kill parasitic high frequency. Both run seven 60-second steps to a maximum of 420 s, interrupted arc at 10 mA for the first three and then continuous at 10, 20, 30 and 40 mA. Both set the electrode gap at 6.35 mm nominal. Both grind the tungsten tip at 30° to the rod axis and inclined the rods 35° from horizontal, 110° between the axes. Both press each electrode down at about half a newton. Both call for at least five specimens. Both describe the same four failure families and both warn that materials cluster their failures just after a stage change.
The result, in both, is a number of seconds.
Where they diverge
Electrodes: two systems or one
This is the difference D495 names itself.
D495 describes two electrode systems. Stainless steel strip electrodes are cut from 0.15 mm sheet into 12.7 × 25.4 mm strips, bent to roughly 160°, and set corner-down on the specimen at 45° to the line joining the corners. Tungsten rod electrodes are the 2.4 mm rods with the ground elliptical face.
The choice is not free. D495 makes the stainless steel strip the preferred technique for materials in the poor-to-moderate band, meaning arc resistance up to 180 s, because it reduces the variability that rod electrodes show on those materials. Above 180 s the tungsten rod is recommended instead, because the strip corners erode noticeably over that long an exposure. D495 states outright that results from the rod system may differ from results from the strip system.
IEC 61621 has only the tungsten rod.
So for a material that fails somewhere under 180 s, ASTM’s preferred configuration is one that IEC does not contain. Two labs following their own standards correctly will produce numbers from different electrode systems, and the standard that defines both says those may not agree.
Specimen orientation
D495 defines normal orientation, with the electrodes on the upper surface of the specimen, and inverted orientation, with them on the under surface. Inverted is the more severe condition. It reduces data scatter on some materials and increases it on others, particularly those that evolve a lot of gas during the test. D495 requires you to report which orientation you used.
IEC 61621 does not define orientation at all. Its electrode assembly applies the arc to the top surface, and there is no inverted variant.
This is the difference most likely to bite you in practice, because it survives the obvious check. Both labs used tungsten rods, both ran seven steps, both reported seconds — and one of them ran the specimen upside down.
Conditioning
IEC 61621 conditions specimens for at least 24 hours at 23 °C ±2 °C and 50 % ±5 % relative humidity, standard atmosphere B.
D495 sends you to Practice D6054, Procedure A, and permits omitting conditioning entirely if you can show it makes no difference for the material. Where humid-atmosphere effects matter, it points to Procedure C instead. Its own note suggests that warming specimens in a 50 °C oven for about half an hour is adequate for most materials.
Those are not the same starting condition, and moisture affects tracking.
Electrode cleaning
IEC 61621 cleans after every single test: solvent, then deionised water, then dry with a lint-free tissue.
D495 cleans the tungsten rods only when decomposition products build up excessively, and treats the continuous 40 mA burn-off as the most effective method when they do. For the stainless steel strips there is no cleaning at all — you use two fresh corners for each test and replace the strips after four tests.
Both re-sharpen on the same criterion: no burrs or rough edges at 15× magnification. D495 adds a quantitative trigger, sharpening once the tip edge has rounded to about 0.08 mm.
What you report
| IEC 61621 | ASTM D495 | |
|---|---|---|
| Central statistic | Central value | Median |
| Spread | Minimum and maximum | Minimum only |
| Electrode system | not applicable, one system | Required |
| Orientation | not applicable | Required, normal or inverted |
| Contaminant | — | Kind and amount, if any |
| Fabrication | — | Conditions of fabrication |
Both require the material identity, the thickness, the conditioning and any special observations such as burning or softening.
Specimen dimensions
IEC 61621 works in millimetres: 3 mm +0.4/−0 thickness, 6 mm minimum from the specimen edge, 12 mm from a previously tested area.
D495 works in inch-pound units, which it states are the standard for the method: 0.125 ± 0.010 in (3.17 ± 0.25 mm), 1/4 in (6.4 mm) from the edge, 1/2 in (12.7 mm) from a previously tested area.
Close, deliberately, but not identical, and the thickness tolerance is meaningfully different — IEC allows nothing under 3 mm, D495 allows down to 2.92 mm.
Arc geometry check
Both tell you to watch the early arcs and confirm they lie flat against the surface. The threshold differs slightly: D495 sets the acceptable lift at not more than 1/16 in (1.6 mm) at the middle of the arc, IEC flags roughly 2 mm.
Supply
D495 is written around a 115 V, 60 Hz primary. IEC 61621 specifies line frequency anywhere from 48 Hz to 62 Hz and an autotransformer suitable for the local line voltage, which is what makes it workable on 50 Hz supplies.
The appendix IEC does not have
D495 carries a non-mandatory appendix for a volts-after-tracking test, which determines the surface breakdown voltage ratio remaining after a specimen has failed the arc resistance test. It compares breakdown voltage across the damaged track against undamaged areas of the same specimen.
IEC 61621 has no equivalent. If a specification calls for a surface breakdown voltage ratio, it is calling for D495 whether it says so or not.
What to do about it
Test to the standard your specification names. If a datasheet, product standard or customer document calls out one of them, that is the method, and a number produced under the other one is supporting evidence rather than a compliant result.
When you quote or receive an arc resistance value, ask for four things alongside it: the standard and edition, the electrode system, the orientation, and the specimen thickness. A bare number in seconds is close to meaningless. This is a comparative screening test, and the comparison only holds inside a fixed configuration.
Do not build an equivalence claim on the phrase “technically equivalent” without checking editions. This page is based on D495-99 against IEC 61621:1997. D495 has been revised since, most recently as D495-22, and a second edition of IEC 61621 is in development under IEC TC 112. Verify against whichever editions you are actually working to.
FAQ
Can I submit a D495 result where IEC 61621 is specified? Only if the requester accepts it. If your D495 test used tungsten rods in normal orientation you are close to the IEC configuration, but conditioning and cleaning still differ. If it used stainless steel strip electrodes, the configuration does not exist in IEC 61621 at all.
Which electrode system should I use? Under D495, the material decides. Below roughly 180 s, use the stainless steel strip. Above it, use tungsten rod. If you do not know where the material will land, run a screening specimen first. Under IEC 61621 there is no choice.
Why does the electrode system change the result? Rod electrodes show more scatter on materials with poor to moderate arc resistance. Strip corners erode over long exposures. Each system has a range where it behaves, and outside that range it adds variability rather than measuring it.
Is the electrode gap 6.0 mm or 6.35 mm? 6.35 mm nominal in both standards. You will see 6.0 mm quoted in some equipment literature; it does not come from either method as written. Tolerances differ slightly, ±0.08 mm in D495 and ±0.1 mm in IEC 61621.
What thickness does D495 need? 0.125 ± 0.010 in, which is 3.17 ± 0.25 mm. Figures around 10 mm circulate online and are not in the standard. Thin material is tested by clamping sheets together to build up close to the standard thickness.
Do both standards handle a burning specimen the same way? No. IEC 61621 counts burning as failure when it continues between arc interruptions. D495 takes the view that when smoke and particulates obscure the arc you may not be able to identify an end point at all, and that reporting failure by burning is preferable to reporting an arc resistance value you cannot defend.
Neither standard gives a pass mark. So what is a good number? Neither has one, and D495 goes further, saying it generally should not be used in material specifications. Use the number to rank candidates, catch a formulation change, or hold a supplier to a baseline. Not to qualify a material on its own.