A good insulation resistance tester is the instrument that tells you whether the insulation around a conductor is doing its job, before a breakdown puts a person or a plant at risk. It applies a known DC test voltage and measures the resistance in megohms or gigohms, so you can catch damp, damaged or ageing insulation early. This guide explains what these testers do, how they take their readings, what to look for when you buy one, and five Fluke models that earn their place in an electrician's kit.
What an insulation tester does, and why it matters
An insulation resistance tester applies a high DC voltage, typically between 250 V and 1,000 V for general work, across insulation and measures how much current leaks through it. High resistance means healthy insulation; a low or falling reading means the insulation is breaking down. That single check underpins safe electrical work, because failing insulation is what leads to earth faults, electric shock and fires.
Insulation testing is not optional on Irish installations. Verifying insulation resistance is a required part of the initial and periodic inspection of an installation under the ETCI National Rules and IEC 60364. You will use the same instrument for commissioning new circuits, for periodic testing of existing ones, and for fault-finding when something trips and you need to know why.
How insulation testers actually measure
When you apply the test voltage, the current you see is made up of three parts, and understanding them explains why some tests are timed. The first is the capacitive charging current, which is large at the instant you press test and then decays as the insulation charges up like a capacitor. The second is the absorption current, which decays more slowly as the dielectric polarises. The third is the leakage current, the steady current that actually flows through and across the insulation, and the one that reflects its true condition.
Because the first two currents fade with time, a reading taken after a few seconds can look worse than the insulation really is. That is why better testers offer timed tests. The polarisation index (PI) compares the reading at ten minutes with the reading at one minute, and the dielectric absorption ratio (DAR) compares sixty seconds with thirty. A healthy winding or cable gives a rising resistance and a PI comfortably above one; a flat or falling trend points to moisture or contamination.
What to look for when choosing an insulation tester
The right tester depends on what you work on. Beyond the headline resistance range, these are the features that matter day to day.
- Test voltages. 250 V, 500 V and 1,000 V cover most installation and appliance work. Motors, cables and switchgear at higher voltages need a 2,500 V, 5 kV or 10 kV instrument.
- Maximum resistance range. Good insulation reads in the gigohms, so a tester that tops out too low will not show you the headroom you have.
- Timed PI and DAR tests. Essential for motors, generators and long cable runs where trend matters more than a single number.
- Safety rating. Match the CAT rating to where you work; CAT III or CAT IV is the practical minimum on fixed installations.
- Data storage and download. If you test at volume, on-board memory and a link to software saves you re-typing results into reports.
The 5 best Fluke insulation testers
Powerpoint Engineering is an authorised Fluke distributor in Ireland, and these five models cover the range from everyday installation work up to high-voltage plant. The table sums them up, and the notes explain who each one suits.
|
Model |
Test voltages |
Best for |
|
Fluke 1507 |
50, 100, 250, 500, 1,000 V |
General insulation testing, compact and fast |
|
Fluke 1587 FC |
250, 500, 1,000 V |
Two-in-one insulation tester and true RMS multimeter |
|
Fluke 1577 |
500, 1,000 V |
Simple insulation multimeter for troubleshooting |
|
Fluke 1555 FC |
Up to 10 kV |
High-voltage motors, cables and switchgear |
|
Fluke 1663 FC |
250, 500, 1,000 V (multifunction) |
Full installation testing to IEC 60364 |
Fluke 1507
The 1507 is the go-to compact insulation tester for most electricians. It offers five test voltages up to 1,000 V, reads insulation up to the gigohm range, and includes PI and DAR timed tests. It is light, quick and rugged, which is why it lives in so many toolbags.
Fluke 1587 FC
The 1587 FC combines a full true RMS digital multimeter with a 1,000 V insulation tester in a single handheld. For anyone who does not want to carry two instruments, it is the practical choice, and Fluke Connect lets you log and share results from your phone.
Fluke 1577
The 1577 is the simpler insulation multimeter, offering 500 V and 1,000 V insulation testing alongside voltage, continuity and resistance. It suits straightforward troubleshooting where you do not need the extra ranges of the 1587 FC.
Fluke 1555 FC
When you move onto high-voltage motors, long cables and switchgear, you need more than 1,000 V. The 1555 FC delivers test voltages up to 10 kV and is built for utility, industrial and commissioning work where insulation is stressed at higher levels.
Fluke 1663 FC
If your work is whole-installation testing rather than a single insulation check, a multifunction installation tester like the 1663 FC bundles insulation resistance with earth loop, RCD and continuity testing to IEC 60364. One instrument covers the full periodic inspection.
Keep it calibrated
An insulation tester is only trustworthy if its output voltage and its readings are accurate, and both drift over time. Annual calibration against traceable standards keeps your certificates defensible and your fault-finding honest. Powerpoint Engineering runs an in-house calibration and repair laboratory that calibrates Fluke and other brands. It is also worth comparing your insulation tester against the other core instruments in a working kit, such as the best digital multimeters and the best earth ground testers.
FAQs
What test voltage should I use for a 230 V or 400 V circuit?
For low-voltage installations up to 500 V, the standard insulation test voltage is 500 V DC. For circuits operating between 500 V and 1,000 V, use 1,000 V DC. Sensitive electronic equipment should be disconnected first, as the test voltage can damage it.
What is a good insulation resistance reading?
For a low-voltage installation, IEC 60364 sets a minimum of 1 megohm, but a healthy circuit usually reads far higher, often in the hundreds of megohms or into the gigohms. It is the trend over time, not just the single figure, that tells you whether insulation is degrading.
Do I need a 10 kV insulation tester?
Only if you work on high-voltage assets such as MV cables, large motors, generators or switchgear. For general installation and appliance work, a 1,000 V tester like the Fluke 1507 or 1587 FC is enough.
The best insulation tester is the one matched to the work you do, from a compact 1,000 V unit for daily installation testing to a 10 kV instrument for high-voltage plant. If you would like help choosing the right model, or you need your current tester calibrated, speak to our team about the Fluke range and our calibration service.