IEC 61620 calculator: tan δ and conductivity of insulating liquids
Calculate tan δ, conductivity, resistivity and permittivity of insulating oils from G and C to IEC 61620: square wave, test cell and repeatability.
Tan δ, conductivity and permittivity of insulating liquids to IEC 61620
Every calculation in the IEC 61620 method: the dissipation factor from the measured conductance and capacitance, the currents of the square-wave method, conversion between tan δ, conductivity and resistivity, the test cell, and the cleanliness, repeatability and reproducibility criteria.
Result
Indicative result. The calculation runs in your browser and is not sent to any server. This website may contain errors or omissions: before making any decision about an installation, the information must be checked and validated by a qualified engineer. Legal notice and terms of use
What this calculator does
It solves every calculation in IEC 61620:1998 (EN 61620), Insulating liquids – Determination of the dielectric dissipation factor by measurement of the conductance and capacitance – Test method. It has five tabs:
- G and C result: tan δ, cell resistance, relative permittivity, conductivity and resistivity, with a check of the measuring ranges.
- Square wave: capacitance and conductance from the measured currents, with checks on the generator, the field in the cell and the ion transit time.
- tan δ ↔ σ ↔ ρ: conversion between the three quantities and from one frequency to another.
- Cell: empty capacitance and constant of a coaxial or parallel-plate cell, with the minimum electrode spacing.
- Precision: repeatability, reproducibility, cell drift at two minutes and temperature tolerance.
Everything runs in your browser and no data is sent anywhere.
The method in the standard
Instead of an AC bridge, the standard applies a low-amplitude, low-frequency square-wave voltage to the cell. During the voltage rise the current is mainly capacitive; on the steady part only conduction current flows, and it is measured before ions build up at the electrodes. That gives the two cell quantities:
C = IC / (dV/dt) G = IR / V
and the dissipation factor at whatever frequency is wanted, usually 50 or 60 Hz:
tan δ = G / (C ω), with ω = 2πf
If the liquid’s relative permittivity at the measuring temperature is known, the conductivity is σ = ε0 εr G / C, and since εr = C / C0, the empty-cell capacitance is enough: σ = ε0 G / C0. Resistivity is the inverse, ρ = 1/σ.
What the standard considers suitable
| Parameter | Value | Clause |
|---|---|---|
| Square-wave amplitude | 10 V to 100 V | 5.3 |
| Square-wave frequency | 0.1 Hz to 1 Hz | 5.3 |
| Rise time | 1 ms to 100 ms | 5.3 |
| Ripple | < 1% | 5.3 |
| Measurable conductance (example) | 2 × 10⁻¹⁴ S to 2 × 10⁻⁶ S, error < 2% | 5.4 |
| Measurable capacitance | 10 pF to 1,000 pF, uncertainty < 1% | 5.4 |
| tan δ range | 10⁻⁶ to 1, up to 200 under particular conditions | 1 |
| Electrode spacing | typically 4 mm, never below 1 mm | 5.1 |
| Cell temperature | ±1 °C of the set value | 5.2 and 8.3 |
| Permissible drift at 2 min | 2% | 8.1.3 and 8.1.4 |
The calculator warns when a value falls outside these margins. Annex C adds two physical checks that are also made: the field in the cell should stay below 1 kV/cm so that neither field-enhanced dissociation nor ion injection distorts the result, and each half-wave should be shorter than the transit time of an ion between the electrodes, t = L² / (kV), with the mobility estimated from Stokes’ law, k = e / (6πηa).
Worked example
It is the same one the standard gives in clause 5.4. Oil with εr = 2 in a cell of 40 pF empty (80 pF filled), with a measured conductance of 2 × 10⁻¹⁴ S:
- tan δ = 2 × 10⁻¹⁴ / (80 × 10⁻¹² × 2π × 50) = 0.8 × 10⁻⁶ at 50 Hz
- σ = 8.854 × 10⁻¹² × 2 × 10⁻¹⁴ / 40 × 10⁻¹² = 0.0044 pS/m
- ρ = 2.3 × 10¹⁴ Ω·m
With a ±30 V, 0.5 Hz generator and a 10 ms rise time, the slope is 6,000 V/s: that cell draws a capacitive current of 480 nA and a conduction current of just 0.6 pA. With 4 mm between electrodes the field is 0.075 kV/cm and an ion takes about 500 s to cross, against a 1 s half-wave: it is an equilibrium measurement.
Cleanliness and precision
With a highly insulating liquid the result depends more on the cell than on the liquid itself. The standard sets an exhaustive cleaning procedure (Annex A, the reference in a dispute) and a simplified one for cells dedicated to a single type of liquid (Annex B), plus a criterion for knowing the cell is clean: σ or tan δ must not drift by more than 2% two minutes after filling.
Two results are acceptable if |A − B| < α · Min(A, B):
| New liquid | Used liquid | |
|---|---|---|
| Repeatability r (one laboratory) | α = 0.2 | α = 0.1 |
| Reproducibility R (two laboratories) | α = 0.35 | α = 0.20 |
For reference, in interlaboratory tests on new mineral oil with tan δ = 2 × 10⁻⁶, the standard reports r = 0.4 × 10⁻⁶ and R = 1.2 × 10⁻⁶.
What this calculator does not do
It does not measure anything: it starts from the instrument readings. It does not correct tan δ or σ for temperature, because the standard gives no correction law: it says they can change by up to 5% per degree depending on the liquid, which is why it requires the cell to be held within ±1 °C. The conversion between tan δ and conductivity only holds if the liquid has no appreciable dipolar losses at that frequency. The coaxial cell formula is basic electrostatics — the standard only writes the plane case — and neither formula accounts for fringing. And it sets no acceptance limits for the oil: those come from the specification and maintenance standards for each liquid.
What to measure it with
The TAND220A meter applies exactly this method: a ±30 V square wave at 0.5 Hz, a stainless-steel coaxial cell of about 60 pF empty and conductivity from 0.01 pS/m. For the classic IEC 60247 bridge method, with tan δ and resistivity at elevated temperature, there is the HZJD-2Z tester. If you also want to know how much water the oil holds, use the moisture in oil calculator.
Frequently asked questions
- What is the difference between IEC 61620 and IEC 60247?
- IEC 60247 measures tan δ with an AC bridge and DC resistivity with fields of up to 250 V/mm for one minute. IEC 61620 measures conductance and capacitance at the same time with a low-voltage, low-frequency square wave, without pushing the liquid out of thermodynamic equilibrium. That is why it reaches tan δ values of 10⁻⁶ at power frequency and works for highly insulating liquids even at ambient temperature. It does not replace IEC 60247: it complements it.
- Why do tan δ and conductivity carry the same information?
- Because in a liquid without dipolar losses, which is the case for almost every electrical liquid, the equivalent circuit is a capacitance with a resistance in parallel, and tan δ = σ / (ε0 εr ω). Knowing the permittivity, one can be calculated from the other at any frequency. A tan δ of 10⁻⁶ at 50 Hz in a liquid with εr = 2 is about 0.006 pS/m.
- When are two tan δ results acceptable?
- When their difference is smaller than α times the lower of the two. Within one laboratory (repeatability), α is 0.2 for new liquid and 0.1 for used; between two laboratories (reproducibility), 0.35 and 0.20. These criteria apply to measurements at ambient temperature.
- How do I know the cell is clean enough?
- With a clean cell and a constant temperature, conductivity does not change with time. The standard accepts a drift of up to 2% two minutes after filling the cell; the first reading then stands. If it drifts more, the cell is cleaned again, a second sample is measured and the lower value is recorded.