Capacitor insulation is the electrical separation in and around a capacitor: the dielectric between the two electrodes that holds the charge, and the insulation of winding, can and terminals from chassis, circuit board and neighbouring parts. How well the dielectric insulates is stated in the data sheet as insulation resistance in MΩ·µF. A polypropylene film capacitor typically reaches 100,000 MΩ·µF at 25 °C and a dissipation factor of 0.05 percent at 1 kHz, a tantalum electrolytic 100 MΩ·µF and 8 percent (WIMA, main catalogue 2026). This page compares the dielectrics, works through insulation resistance, field strength and derating with examples, and shows how the capacitor is insulated from its surroundings. For that outer layer we produce insulating parts to drawing, such as washers, caps and spacers made of polyester film, aramid paper or pressboard.
Capacitor insulation has two levels: the dielectric and the external insulation
The dielectric separates the positive and negative electrode. It sets capacitance, rated voltage, losses and self-discharge. The external insulation separates the whole capacitor from its surroundings: a potted plastic case for a film capacitor, a sleeve around the aluminium can for an electrolytic, a housing with bushings for a power capacitor. The two levels are often confused in practice, with consequences. The rated voltage of an electrolytic says nothing about the voltage its sleeve withstands against the chassis. Most data sheets describe the dielectric over several pages and the external insulation in one sentence.
Dielectrics compared: film, paper, ceramic, mica and oxide
The material between the electrodes determines how much capacitance fits into a volume (permittivity εr and the thinnest layer that can be made), which field strength it tolerates and up to which temperature it works. The table summarises the values from the Wikipedia overviews Capacitor types and Kunststoff-Folienkondensator, the temperatures for ceramic and tantalum come from the WIMA catalogue. 1 V/µm equals 1 kV/mm.
| Dielectric | εr at 1 kHz | Dielectric strength, of which used in the component | Thinnest layer | Upper temperature | Typical type |
|---|---|---|---|---|---|
| Polypropylene (PP) | 2.2 | 650 V/µm, 450 V/µm | 1.9 µm | 105 °C | MKP, AC, pulse, DC link |
| Polyester (PET) | 3.3 | 580 V/µm, 280 V/µm | 0.7 µm | 125 to 150 °C | MKT and MKS, DC |
| Polyethylene naphthalate (PEN) | 3.0 | 500 V/µm, 300 V/µm | 0.9 µm | 150 °C | SMD film capacitor |
| Polyphenylene sulphide (PPS) | 3.0 | 470 V/µm, 220 V/µm | 1.2 µm | 150 °C | SMD film capacitor, temperature stable |
| PTFE | 2.0 | around 450 V/µm (value uncertain), 250 V/µm | 5.5 µm | 200 °C | Special type for military, geophysics and audio |
| Paper | 3.5 to 5.5 | 60 V/µm, oil impregnated around 100 V/µm | 5 to 10 µm | 85 °C (MP types) | MP, interference suppression, power capacitors |
| Mica | 5 to 8 | 118 V/µm | 4 to 50 µm | not stated | Mica capacitor for transmitters and standards |
| Ceramic class 1 (NP0, C0G) | 12 to 40 | below 100 V/µm, value uncertain | 1 µm | 125 °C | MLCC for high frequency and filters |
| Ceramic class 2 (X7R) | 200 to 14,000 | below 35 V/µm | 0.5 µm | 125 °C | MLCC for decoupling and smoothing |
| Aluminium oxide | 9.6 | 710 V/µm | below 0.01 µm (6.3 V) to 0.8 µm (450 V) | 85 to 125 °C by series | Aluminium electrolytic |
| Tantalum pentoxide | 26 | 625 V/µm | below 0.01 µm (6.3 V) to 0.08 µm (40 V) | 125 °C | Tantalum electrolytic |
Oxide and ceramic pack the most capacitance into a small space, because their layers are only nanometres to a few micrometres thick or their permittivity is very high. The plastic films win on losses, insulation resistance and stability. Among film capacitors, PP with around 50 percent and PET with around 40 percent share almost the entire market. Styroflex capacitors with a dielectric of polystyrene film had even lower losses, but stopped at 85 °C and have been largely displaced since around 1990.
Our guidance for selection: PP wherever significant current flows through the capacitor, that is with AC, pulses, snubbers and in the DC link of inverters. PET for DC and tight space, because at the same film thickness it delivers 1.5 times the capacitance and can be made as thin as 0.7 µm. PPS and PEN when a film capacitor has to survive 150 °C as an SMD part. Class 1 ceramic for resonant circuits and high frequency. Electrolytics when a lot of capacitance is needed in a small space, and then with dedicated external insulation, as described further down. PP film as a material, including its temperature limit, is covered in the article on polypropylene films.
Calculating capacitance and field strength in a parallel-plate capacitor
Electrically, a film winding is a rolled-up parallel-plate capacitor. Two formulas are enough for a first design: C = ε0 · εr · A / d for capacitance and E = U / d for the field strength in the dielectric. A is the electrode area, d the thickness of the dielectric and ε0 the vacuum permittivity of 8.854 × 10⁻¹² As/(Vm). The capacitance formula is an approximation that holds better the smaller the spacing is relative to the area. With films a few micrometres thick this is always the case. A worked example with 1 m² of electrode area:
- PP film, 3 µm thick: C = 8.854 × 10⁻¹² × 2.2 × 1 m² / 0.000003 m = 6.5 µF
- PET film, also 3 µm thick: 9.7 µF on the same area
- 600 V across the 3 µm film: E = 600 V / 3 µm = 200 V/µm
The 200 V/µm is just under a third of the 650 V/µm that PP film withstands as a material, and less than half of the 450 V/µm that manufacturers use at most in the component. That margin is intended. It covers defects in the film, heat and ageing over many years. For the designer this means: halving the film thickness doubles the capacitance and halves the voltage reserve. The derivation and further field formulas are in the article on electric field strength.
Capacitor insulation resistance is stated in MΩ·µF
A real dielectric lets a small leakage current through under DC voltage. Insulation resistance is the ratio of the applied DC voltage to this leakage current after a specified time (TDK, definition of terms). In a larger capacitor made of the same material the resistance falls, because more area conducts, and the capacitance rises by the same factor. The product of the two therefore stays nearly constant for a given material. This product is the time constant τ = R · C. In megohms times microfarads it gives seconds directly: 1 MΩ·µF = 1 s. After τ has elapsed, a charged capacitor without load has dropped to 37 percent of its voltage.
| Type | R · C at 25 °C | R · C at 85 °C |
|---|---|---|
| PP film capacitor | 100,000 MΩ·µF | 10,000 MΩ·µF |
| PET film capacitor | 10,000 MΩ·µF | 1,000 MΩ·µF |
| PPS film capacitor | 10,000 MΩ·µF | 1,000 MΩ·µF |
| Ceramic NP0 | 10,000 MΩ·µF | 1,000 MΩ·µF |
| Ceramic X7R | 1,000 MΩ·µF | 500 MΩ·µF |
| Tantalum electrolytic | 100 MΩ·µF | 10 MΩ·µF |
The guide values come from the comparison table in the WIMA main catalogue 2026. Two things stand out: PP insulates ten times better than PET, and from 25 to 85 °C almost every type loses an order of magnitude. For ceramic, a NASA analysis cites the military requirement: above 100,000 MΩ or 1,000 MΩ·µF at 25 °C, 10,000 MΩ or 100 MΩ·µF at 125 °C, whichever is lower. Murata requires at least 500 MΩ·µF for MLCCs above 0.047 µF (Teverovsky, NASA NEPP 2013). Aluminium electrolytics have no insulation resistance in the data sheet. Their data sheets give the leakage current in µA, measured by TDK after 5 minutes at 20 °C.
Film capacitor data sheets work with two limits. The WIMA MKP 4 series requires at least 100,000 MΩ at 20 °C up to 0.33 µF and at least 30,000 s above that, measured at 100 V after one minute. For a 10 µF capacitor of this series that means 30,000 s divided by 10 µF gives at least 3,000 MΩ, or 3 GΩ. Even at this limit, charged and disconnected, it still holds 37 percent of its voltage after a good eight hours. A 10 µF tantalum electrolytic with 100 s comes to 10 MΩ and drops to the same remainder in under two minutes. A megohm value without capacitance and temperature is therefore not comparable.
How to measure the insulation resistance of a capacitor:
- Discharge the capacitor completely through a resistor.
- Connect an insulation tester with adjustable DC voltage. WIMA measures 63 V types at 50 V and types rated 100 V and above at 100 V, so at most at rated voltage.
- Let the charging current decay and read the value after one minute. In the first seconds the tester mainly shows charging current and the reading is still rising.
- Note the temperature, because at 85 °C one order of magnitude less is normal.
- Multiply the resistance in MΩ by the capacitance in µF and compare with the data sheet.
- Discharge again after measuring. Otherwise dielectric absorption builds voltage back up at the terminals.
Limits for installations and machines under DIN VDE 0100-600 are a separate subject and are covered in the article on insulation resistance.
Dissipation factor tan δ: why polypropylene is the standard in power capacitors
The dissipation factor is the ratio of the resistive to the reactive part of the impedance. It tells you which share of the reactive power handled by the capacitor turns into heat. For the films it rises with frequency, for PET from 0.8 to 3 percent, while NP0 ceramic stays constant.
| Type | tan δ at 1 kHz | at 10 kHz | at 100 kHz |
|---|---|---|---|
| PP film | 0.05 % | 0.08 % | 0.25 % |
| PPS film | 0.2 % | 0.25 % | 0.5 % |
| PET film | 0.8 % | 1.5 % | 3.0 % |
| Ceramic NP0 | 0.10 % | 0.10 % | 0.10 % |
| Ceramic X7R | 2.5 % | not stated | not stated |
| Tantalum electrolytic | 8 % | not stated | not stated |
Source: WIMA main catalogue 2026, typical values. At 100 kHz PET loses twelve times as much as PP. Loss power grows with voltage, frequency, capacitance and tan δ, so this factor sets self-heating in the DC link and the snubber. A 10 µF capacitor with 20 V ripple at 10 kHz handles 251 var of reactive power. With the WIMA values from the table that becomes around 3.8 W of heat with PET and around 0.2 W with PP. The calculation behind it is in the article on dielectric losses. An MKP capacitor handles around 100 A RMS per mF, an electrolytic around 20 A per mF, according to an article by the distributor Hy-Line (all-electronics.de). PP’s weak point remains heat. Film makers are therefore working on more heat-resistant polyolefins, such as Tervakoski with a capacitor film made from the Borealis material Stelora.
Capacitor voltage rating: rated voltage, test voltage and derating
Dielectric strength in V/µm is a material value. The voltage rating in volts is a component value, and a capacitor has a chain of limits that all sit below the material value:
- The rated voltage UN may be applied continuously up to the rated temperature (TDK). Above it, derating applies.
- With WIMA MKP 4 the permissible DC voltage falls by 1.35 percent per kelvin from 85 °C, with AC already from 75 °C. The PET series MKS 2 loses 1.25 percent per kelvin.
- The test voltage of the MKS and MKP series is 1.6 UN for 2 seconds, 2 UN for pulse capacitors (WIMA).
- Interference suppression capacitors to IEC 60384-14 must withstand surge voltages, class X1 4 kV and class X2 2.5 kV.
- Power electronic capacitors to IEC 61071 may briefly see 1.1 to 1.5 UN, limited in duration and frequency.
A derating example: an MKP 4 capacitor rated 400 V is to run at 100 °C. That is 15 K above 85 °C, so 15 × 1.35 = 20.25 percent reduction. Around 320 V remain permissible. If you need 400 V continuously at 100 °C, choose the next step in the series: 630 V rated, still around 500 V at 100 °C. How test voltages are set in other standards is shown in the article on dielectric withstand voltage, material values in kV/mm are listed under electrical breakdown strength.
Self-healing: why film capacitors fail open
In metallised film capacitors the electrode is a vapour-deposited metal layer of 0.02 to 0.05 µm. If the film breaks down at a defect, according to WIMA the breakdown takes around 10⁻⁸ seconds. Temperatures of around 6,000 K vaporise the metal layer around the breakdown channel, the spot is isolated, and the capacitor keeps working. Several thousand such healing events usually cost well under 1 percent of capacitance. Manufacturers already use the effect in production and burn out defects with high voltage before the film is wound. In large DC link capacitors, for wind turbines for example, segmented metallisation divides the electrode into fields whose narrow bridges burn out like fuses. Capacitors with thick metal foil electrodes do not self-heal.
This gives the typical failure mode for each type (WIMA, TDK, Wikipedia):
- PP, PET and PPS film capacitors go high-resistance, capacitance drops gradually.
- Ceramic capacitors fail short, MLCCs often after a crack.
- Tantalum electrolytics also fail short.
- Aluminium electrolytics build up overpressure that is released through a vent in the can.
Where a short circuit becomes dangerous, directly on the mains or in a high-voltage DC link for example, the failure mode is therefore a selection criterion with the same weight as capacitance and voltage.
External insulation: case, sleeve, cap and insulating washer
Film capacitors usually sit in a case made of plastic, ceramic, paper laminate or metal, with the winding potted and hermetically sealed. WIMA uses a flame-retardant plastic case to UL 94 V-0. Because the winding has an outer and an inner electrode, manufacturers mark the outer one, which lies closest to the case wall, with a line or bar. Power capacitors fill the winding inside the can with insulating gas, polyurethane resin or vegetable oil. Their high-voltage terminals pass through ribbed ceramic that extends the creepage distance.
In an aluminium electrolytic the can itself conducts. In axial designs it is connected directly to the cathode, in radial designs it is linked to it through the electrolyte with an undefined resistance (Wikipedia, Aluminium-Elektrolytkondensator). The data sheet of the TDK series B43611 is explicit: aluminium can with PET sleeve, no insulating washer at the can bottom, negative pole not insulated from the can. To insulate the can from the circuit board, you order the version with PVC sleeve and an additional PET insulation cap on the terminal side. The unconnected mounting pins go on insulated pads or pads at negative potential.
The obvious objection is: the electrolytic has a plastic sleeve. The sleeve does insulate, but the voltage it withstands is rarely in the data sheet, and TDK warns against damaging it when mounting with clamps. The lid is usually bare. The sleeve only counts as insulation if the data sheet states a test. Exxelia does this for the Snapsic 125 series: 2,000 V at 50 Hz for one minute between terminals and mounting hardware, 100 MΩ insulation resistance at 20 °C. Without such a statement, treat the can as live at cathode potential and provide a dedicated insulating part.
| Insulating part | Where it sits | Typical material |
|---|---|---|
| Insulating washer | under the can bottom, against chassis or circuit board | PET film, sold for example at 0.125 mm; at higher temperature aramid paper or polyimide |
| Insulation cap | on lid and terminal side | PET film |
| Sleeve or heat-shrink tube | on the can wall | PET or PVC, as shrink film |
| Spacer or barrier | between cans in capacitor banks and modules | PET, aramid paper, pressboard, NMN laminate |
| Potted case | around the winding of a film capacitor | flame-retardant plastic to UL 94 V-0 |
For round cans with a close-fitting cover, see insulating sleeving. For flat parts such as washers, caps and barriers a die-cut part is the usual route: hole, outer contour and tabs are made in one stroke.
What GOBA supplies for capacitor insulation
Capacitor manufacturers buy the dielectric itself as metallised film directly from the film maker, PET from 0.7 µm and PP from 1.9 µm. Our range starts at 7 µm, so we are the wrong partner for these thin films. For everything that surrounds and holds the capacitor, we supply:
- Slit rolls of polyester film, including for wound capacitors, from 5 mm width with ±0.1 mm tolerance, up to 1,600 mm, on cores with 25, 55, 76 or 152 mm inner diameter
- Insulating washers, caps, spacers and barriers as die-cut parts from 0.023 to 3.0 mm material thickness, up to 1,000 x 2,000 mm part size
- Materials from polyester film (Hostaphan, Mylar) through Kapton and Nomex to pressboard, NMN and DMD laminate and mica
- Quantities from 1 piece, delivery within two weeks at most when the material is in stock
We slit the film rolls through contract slitting, and the polyester films for them are presented on our page on polyester films. Washers, caps and barriers are made through stamping and forming. For an enquiry we need the drawing, quantity and thickness, plus operating voltage and temperature at the installation point. Our technical consulting will work out with you which material suits your test voltage.
GOBA Takeaway
Capacitor insulation can be judged by two numbers: R · C in MΩ·µF for the dielectric and a tested voltage for the external insulation. The first is in every good data sheet, the second is often missing. If it is missing, an insulating part belongs in the design before the prototype breaks down on the test bench. For an overview of all materials we convert, see our page on insulation materials.
