Choosing an Insulation Tester: Which Test Voltage Is Right?
An insulation tester checks the insulation resistance of electrical installations with a defined DC voltage. This guide explains how to pick the test voltage under DIN VDE 0100-600, the required minimum values in MΩ, the right measuring range and the CAT safety category.
View insulation testersWhat does an insulation tester measure?
An insulation tester applies a defined DC voltage between the live conductors and earth and measures the resulting leakage current. From voltage and current it calculates the insulation resistance in megohms (MΩ). The higher the value, the better the insulation separates live parts from touchable surfaces.
The measurement reveals weak spots that a simple continuity tester cannot see: damp cables, porous sheathing or creepage currents across dirty terminals. Under DIN VDE 0100‑600 the insulation resistance test is a mandatory part of the initial verification of an electrical installation.
From the continuity tester to the installation tester - the essentials at a glance.
Read the guideWhich test voltage suits which installation?
The test voltage follows the nominal voltage of the installation. DIN VDE 0100‑600 (Table 6A) defines three levels: 250 V DC for SELV and PELV circuits, 500 V DC for common installations up to 500 V nominal, and 1000 V DC for installations above 500 V. Each level has a minimum resistance that must not be undercut.
For a standard 230/400 V domestic installation the 500 V level therefore applies, and the resistance must be at least 1 MΩ. A sound installation with intact insulation reads far higher, often in the hundreds of MΩ or into the GΩ range. Photovoltaic systems are partly tested at 1000 V under DIN VDE 0126, so a PV-capable meter should offer that level.
What matters for measuring range and safety?
Beyond the test voltages, the measuring range, safety category and feature set decide suitability. The range should reach at least several hundred MΩ, ideally into the GΩ range, to resolve sound installations cleanly. The measuring category must match the point of use.
- Test voltages: at least 250 V and 500 V, plus 1000 V for PV and industrial work.
- Measuring category: CAT III 600 V for distribution boards, CAT IV 600 V for the supply side before the meter.
- Short-circuit current of at least 1 mA so the test voltage stays stable on capacitive loads.
- Automatic discharge of the load capacitance after the test to protect against residual voltage.
- Live-voltage warning so no measurement is taken on an energised circuit.
- A combined installation tester if low-ohm, loop and RCD tests per VDE 0100‑600 are also needed.
Frequently asked questions
Which test voltage applies to a 230/400 V installation?
500 V DC under DIN VDE 0100‑600. The insulation resistance must be at least 1 MΩ; sound installations read considerably higher.
What does the 1 MΩ minimum mean?
At 500 V test voltage, 1 MΩ is the lower limit. If it is undercut the insulation is deemed defective and the installation must not be put into service.
Is a pure insulation tester enough for installation testing?
A full initial verification under VDE 0100‑600 also needs low-ohm, loop impedance and RCD tests. An installation tester combines these functions in one instrument.
Why CAT IV rather than CAT III?
CAT IV covers the supply side before the meter with higher transient energy. Anyone measuring at the service entrance or main distribution is safer with CAT IV 600 V.
Looking for the right insulation tester?
We stock insulation and installation testers with 250 to 1000 V test voltage, CAT IV safety and test reporting - suitable for tests to DIN VDE 0100-600.
Standard-compliant
Test voltages per DIN VDE 0100-600.
Measured safely
Instruments in measuring category CAT III and CAT IV.
Precise readings
Measuring ranges reaching into the GΩ region.
Expert advice
Our specialists help you pick the right instrument.