Dielectric withstand voltage is the ability of a material or electrical component to withstand a defined electrical voltage without a breakdown or electrical conduction occurring inside the insulating material. On a finished part, appliance, motor or cable it is stated as a test voltage in volts or kilovolts and verified with a high-voltage test. On the material, the same quantity related to thickness in kV/mm is called dielectric strength. Test voltages lie far above the operating voltage: a household appliance on a 230 V mains must withstand 1,000 V for 1 s in the routine test, 2,500 V with reinforced insulation (EN 60335-1, Annex A), a motor winding 2 U + 1,000 V, at least 1,500 V, for 1 min (IEC 60034-1), and a control cable rated 300/500 V withstands 4,000 V (Lapp, ÖLFLEX CLASSIC 110 data sheet). This article shows which test voltage each standard requires, how the test runs, where the line to dielectric strength is and how you derive thickness and number of layers of the insulation materials for your part from it.
Withstand voltage and dielectric strength: the part withstands kilovolts, the material kilovolts per millimetre
Withstand voltage belongs to the finished part with its geometry, edges, air gaps and creepage paths. Dielectric strength belongs to the material and is measured on a specimen between standardised electrodes to IEC 60243. A polyester film data sheet shows that the two numbers cannot be converted linearly: Hostaphan RN withstands 12 kV at 50 µm and 45 kV at 350 µm (Mitsubishi, data sheet via Deffner and Johann). That is 240 kV/mm for the thin film and about 129 kV/mm for the thick one. The kV/mm value falls with thickness, yet the test voltage of the part still rises.
| Quantity | Refers to | Unit | Where it comes from |
|---|---|---|---|
| Withstand voltage | part, appliance, winding, cable | V or kV as a test voltage with a test duration | product standard, component data sheet, high-voltage test |
| Dielectric strength | material, specimen of defined thickness | kV/mm | IEC 60243, material data sheet, falls with increasing thickness |
| Breakdown voltage | the event on a specific gap | kV | measured up to breakdown, never reached in service |
| Dielectric strength of air | gas, depends on gap, pressure, humidity | kV/mm, about 3 kV/mm at 1 mm | Paschen’s law, basis of clearances |
| Insulation resistance | part or installation | MΩ at 500 V DC | insulation measurement, detects moisture and contamination |
So the test voltage is in the standard for your product, the dielectric strength is in the material data sheet, and between them lie thickness, geometry and a safety factor.
Test voltage versus operating voltage: the 2 U + 1,000 V rule
The test voltage is not a value a part is meant to withstand permanently. In one minute or one second it proves that enough margin remains between operating voltage and breakdown for overvoltages, moisture and ageing. The oldest rule of thumb comes from the machine standard IEC 60034-1: twice the rated voltage plus 1,000 V, at least 1,500 V. For a 230 V appliance that gives 1,460 V, rounded to 1,500 V in practice (Sourcetronic). One manufacturer of cleaning equipment therefore tests all class I appliances uniformly at 1,500 V AC and all class II appliances at 2,500 V AC, although EN 50106 only requires 1,000 V for class I (Kärcher, factory standard KN 050.046).
Three voltages must be kept apart. The rated voltage is on the nameplate. The working voltage is the highest rms value permanently applied to an insulation in service (EN 61010-031, cited by SAB Kabel), and in the equipment standards it determines the test voltage. The rated impulse voltage is the transient overvoltage from the mains that equipment must survive according to its overvoltage category, on a 230 V mains 2.5 kV in category II and 4 kV in category III (DIN EN 60664-1, Weidmüller).
An objection from the shop floor: the insulation measurement at 500 V DC is faster and harmless, is it not enough? No. It measures resistance in megohms and finds moisture and contamination. A thin spot, a burr under the film or an air gap that only ignites at 1,500 V is found only by the high-voltage test, because it stresses the insulation with the voltage that appears in a fault (Weetech, Schleich). The two tests belong together, insulation measurement first. Its limits are covered in the article on insulation resistance.
Test voltages by standard: household appliances, motors, switchgear, cables, capacitors
Every product standard sets its own test voltage, test duration and trip current, and the test equipment manufacturers explicitly refer to them (Schleich). The table summarises the values that matter most for insulating parts. All apply to AC at 50 or 60 Hz unless stated otherwise.
| Standard | Applies to | Test voltage | Duration |
|---|---|---|---|
| IEC 60335-1, clause 16.3 (type test) | household appliances, 150 to 250 V | basic insulation 1,250 V, supplementary 1,750 V, reinforced 3,000 V | 1 min |
| IEC 60335-1, Annex A (routine test) | every appliance in production, above 150 V | basic insulation 1,000 V, double or reinforced 2,500 V, class III 400 V | 1 s |
| IEC 60034-1 | motor and generator windings below 6 kV | 2 U + 1,000 V, at least 1,500 V | 1 min |
| IEC 61439-1 (routine verification) | switchgear assemblies, Ui 300 to 690 V | 1,890 V | 1 s |
| EN 60204-1 | electrical equipment of machines | 1,000 V | 1 s |
| EN 61800-5-1 | frequency inverters, example AMK iDT5 | 2,120 V DC, trip at 2 mA | 1 s |
| cable standard, data sheet | control cable 300/500 V (Lapp ÖLFLEX CLASSIC 110) | 4,000 V | manufacturer’s test |
| IEC 60502-2, DIN VDE 0276-620 | medium-voltage cables after installation | VLF 0.1 Hz up to 2 U0 | 15 to 60 min |
| IEC 60384 | film capacitors MKS, MKP, FKP | 1.6 UN to 2 UN | 2 s |
Sources: EN 60335-1, OVE guide 2024, Hensel, Kärcher KN 050.046, AMK, Lapp, Sprecher Automation, WIMA. Two patterns run through all standards. The type test lasts one minute and is higher, the routine test on every unit lasts one second and is lower, because it must not pre-damage the insulation. And reinforced insulation is tested at roughly twice the voltage of basic insulation, 2,500 instead of 1,000 V in the routine test. For insulating parts in white goods this column is the one that counts, because slot liners, separators and covers almost always sit between live and accessible parts. Appliances we supply for the household appliance industry go through exactly this one-second test after assembly.
For windings an additional rule applies: the full test voltage is applied only once to the new machine. A repeat test runs at 80 % (Kärcher), partially rewound windings are tested at 75 % (OVE guide). Every test costs service life. How the winding insulation passes this test with margin is covered in the article on stator insulation, the voltage classes of cables in the article on cable insulation.
Testing withstand voltage: how a high-voltage test runs
The procedure is similar in all standards; the figures come from the OVE guide for machine windings and the Kärcher factory standard for appliances.
- Measure insulation resistance, usually at 500 V DC, and for windings also the polarisation index to IEC 60034-27-4. If the value is below the minimum, the part is dried, not tested.
- Connect the specimen: short all live conductors (L1 to L3 and N) and test them against the protective earth and accessible parts. Accessible insulating surfaces are covered with metal foil (EN 60335-1, 16.3). Switch the appliance on, otherwise half the insulation remains untested.
- Raise the voltage: start at no more than half the test voltage, increase at 1 to 2 kV/s or in steps of 5 % to the final value, with the rise from half to full value taking at least 10 s (OVE). In routine testing the ramp of the tester does this.
- Hold: 1 min in the type test, 1 s in the routine test. During this time no breakdown and no flashover may occur.
- Evaluate: the tester monitors the current. If it exceeds the trip current, 5 mA for household appliances, up to 30 mA for appliances with Y capacitors (Kärcher), 2 mA for inverters (AMK), the part has failed.
- Discharge: after a DC test, discharge the specimen in a controlled way, for windings through a series resistor for four times the test duration (OVE). Record peak value, duration and date.
The standard chooses the type of voltage. AC at mains frequency is the default, because it stresses the insulation as service does. DC is permitted where the standard allows it, then at 1.5 times the AC test voltage (Schleich, QS Control), at a factor of 1.414 for household appliances (Kärcher) and with the equivalence factor 1.7 for machines from 6 kV (OVE). It gets by with small testers but leaves space charges in XLPE cables, which is why medium-voltage cables are now tested at 0.1 Hz (KMFO). The impulse test with the standard 1.2/50 µs impulse, three times per polarity, verifies the rated impulse voltage (EN 60335-1, Table 6) and belongs to the type test. For the test station DIN EN 50191 applies: if the source delivers more than 3 mA AC or 12 mA DC, the station needs positive protection against contact or two-hand operation (BGI 891), which is why line testers limit the short-circuit current to 3 mA (EA Electronic).
Withstand voltage of air: clearances and creepage distances to IEC 60664-1
Under standard conditions air withstands about 3 kV per millimetre (Wikipedia, Durchschlagfestigkeit, gives 1 to 3 kV/mm at 1 mm). For design this value counts only indirectly. A clearance is dimensioned by the rated impulse voltage of its overvoltage category; the figures are in DIN EN 60664-1 and were compiled for the 230/400 V mains by Weidmüller.
| Overvoltage category on the 230/400 V mains | Typical equipment | Rated impulse voltage | Minimum clearance, inhomogeneous field | Minimum clearance, homogeneous field |
|---|---|---|---|---|
| I | equipment on plug-in power supplies | 1.5 kV | 0.5 mm | 0.3 mm |
| II | household appliances, PCs, plug-connected equipment | 2.5 kV | 1.5 mm | 0.6 mm |
| III | fixed installation, industrial machines, motors | 4 kV | 3.0 mm | 1.2 mm |
| IV | meters, service entrance | 6 kV | 5.5 mm | 2.0 mm |
By the 3 kV/mm rule the 3.0 mm for category III would withstand 9 kV. The standard requires them for a 4 kV impulse because moisture, dust, altitude and every burr at the edge weaken the gap; above 2,000 m a correction factor is added (Cotronic). The creepage distance along an insulating surface is longer and depends on the material: at 250 V and pollution degree 2 it is 1.25 mm for material group I, 1.8 mm for group II and 2.5 mm for group III (Weidmüller), and at pollution degree 3 in category III already 5.0 mm next to 3.0 mm of clearance (Cotronic). A phase separator replaces the clearance with solid insulation, and the withstand voltage of the part is decided at its edge, where the creepage path runs around the cut. That is why equipment standards prescribe overhangs. The material side of creepage is covered in the article on the creepage path.
Selecting films and insulating parts for a defined test voltage
The product standard supplies the test voltage, the data sheet the breakdown voltage at the thickness, the design the number of layers. EN 60335-1 requires solid insulation to be 1 mm thick as supplementary and 2 mm as reinforced insulation. A film part does not need this thickness if the supplementary insulation consists of at least two layers, each of which passes the test to 16.3, and the reinforced insulation of at least three layers, of which any two together pass the test; natural mica is excluded (EN 60335-1, clause 29.3). For a 230 V appliance every layer of a supplementary insulation must therefore withstand 1,750 V and any two layers of a reinforced insulation 3,000 V. We recommend writing this test voltage together with the number of layers on the drawing of the insulating part, in addition to the thickness.
The data sheets give the answer if you read them at the right thickness. Hostaphan RN withstands 12 kV at 50 µm, 19 kV at 100 µm, 30 kV at 190 µm and 35 kV at 250 µm, measured to DIN 40634 at 23 °C and 50 Hz; at 150 °C the value of the 190 µm film drops from 30 to 26 kV (Mitsubishi). Nomex 410 reaches 32 kV/mm at 0.25 mm, about 8 kV per layer, and DuPont recommends no more than 1.6 kV/mm in continuous service (DuPont, Nomex 410 data sheet, values explicitly not design values). A 3,000 V test is covered four times over on paper by a single 50 µm layer, but the layer rule, the temperature inside the appliance and the edge decide whether one layer is enough. Which thickness follows from stiffness and slot is covered in the article on polyester film, the aramid papers in the article on Nomex.
- Take test voltage and test duration from the standard of the end product, not from the material standard.
- Read the breakdown voltage of the data sheet at the delivered thickness and derate it for the temperature inside the appliance; never extrapolate the kV/mm value.
- Set the number of layers by the type of insulation: two layers for supplementary, three for reinforced insulation, if the thickness stays below 1 or 2 mm.
- Plan overhangs and treat the edge as an inspection feature.
GOBA die-cuts insulating parts from polyester film, Nomex, laminates and pressboard in thicknesses from 0.023 to 3.0 mm and up to 1,000 x 2,000 mm, and slits roll stock in widths from 5 to 1,600 mm with ±0.1 mm. For films below 36 µm the cut edge decides the withstand voltage of the finished part. Our pages on polyester films and insulation and molded parts list the materials and thicknesses we process. Send us the test voltage of your standard, the type of insulation and the drawing, and we will propose material, thickness and number of layers. The other characteristic values of an insulation are classified in the article on insulation properties, the overview of the materials is in the article on insulating material.
