Insulation Tester vs Megger: How to Choose Test Voltage
Insulation tester, megohmmeter, Megger: one instrument, three names
An insulation resistance tester and a megohmmeter are the same instrument. "Megger" is a brand name that became the everyday word for it. In Chinese the old hand-cranked version is still called a "shaking meter" (摇表). Whatever the label, the job is the same: put a known DC voltage across a piece of insulation and read how much resistance it shows, in megohms (MΩ) or gigohms (GΩ).
Vicimeter builds this function in three forms. The VC60B+ is a dedicated insulation resistance tester. The VC1508 and VC4060 series are insulation multimeters that add a full multimeter, and the VC4060C is a motor checker that pairs insulation with low DC resistance and inductance. All of them are listed under insulation testers.
How insulation resistance testing works
Good insulation lets almost no current through. The tester applies a stabilized DC voltage, typically 250V to 1000V, between a conductor and earth (or between two conductors), measures the small leakage current, and calculates R = V / I. A motor winding reading 500MΩ at 500V is passing about 1μA.
The reason you cannot do this with a normal ohmmeter is voltage. A multimeter's resistance range drives a few volts at most. Moisture, carbon tracking, cracked enamel and contamination often only start conducting when they are stressed at something close to the working voltage. A cable can read "open" on a multimeter and still fail at 400V.
The current that flows has three parts: a capacitive charging current that dies away in seconds, an absorption current that decays over minutes as the insulation polarizes, and a steady leakage current through and across the insulation. That decay is what PI and DAR measure.
Two safety rules apply to every test. Isolate and lock out the circuit first, and check it is dead (the VC60B+ gives an audible and visual high-voltage alarm, and the VC1508 models show a high-voltage symbol when more than 30V is present). After the test, let the instrument discharge the circuit before you touch it, because long cables and large windings store charge.
Choosing the test voltage: 50V to 2500V
Test at or somewhat above the equipment's rated voltage, never far above it. Common practice:
- 50V and 100V: telecom wiring, electronics, sensors and low-voltage control circuits that a higher voltage could damage.
- 250V: SELV and PELV circuits (IEC 60364-6 uses 250V DC for these), and 120V class equipment.
- 500V: building wiring and equipment up to 500V, including 230V and 400V installations (IEC 60364-6 uses 500V DC here).
- 1000V: circuits above 500V, 690V motors, and NEV high-voltage battery, motor and inverter systems.
- 2500V: windings and cables rated 1000V to 2500V. IEEE 43 suggests 500V to 1000V DC for windings rated below 1000V and 1000V to 2500V DC for windings rated 1000V to 2500V.
The test voltage sets the upper limit of what you can read. On the VC1508PD the 2500V step reads to 50GΩ, while 1000V reads to 20GΩ. On the VC60B+ the 250V and 500V steps read to 200MΩ and the 1000V step to 2GΩ.
PI and DAR in brief
A single spot reading depends heavily on temperature and humidity. Timed ratios cancel much of that out, because both readings are taken on the same insulation minutes apart.
- Polarization index (PI) = resistance at 10 minutes / resistance at 1 minute. Dry, clean insulation keeps polarizing, so resistance keeps rising and PI comes out well above 1. IEEE 43 recommends a minimum PI of 2.0 for class B, F and H windings.
- Dielectric absorption ratio (DAR) = resistance at 60 seconds / resistance at 30 seconds. It is the quick version of PI. A ratio close to 1 means the reading did not rise, which usually points to moisture or contamination.
PI and DAR matter most on motors, generators, transformers and long cables, where trends from one maintenance visit to the next tell you more than a single number. The VC1508PC, VC1508PD, VC4060PC and VC4060PD show PI/DAR and let you select the measurement time.
Which Vicimeter model fits
| Model | Test voltages | Max reading | PI/DAR | Other functions | Fits |
|---|---|---|---|---|---|
| VC60B+ | 250V, 500V, 1000V | 2GΩ at 1000V | No | Resistance to 2kΩ, DCV, ACV; 6×AA or 9V DC adapter; CAT. II 600V | Electricians who want a dedicated megohmmeter for wiring, sockets, motors and cables |
| VC1508PC | 50V, 100V, 250V, 500V, 1000V | 20GΩ at 1000V | Yes | Remote control test lead, True RMS, 6000 counts; CAT.III 600V | Mixed work that includes low-voltage electronics and control wiring at 50V |
| VC1508PD | 100V, 250V, 500V, 1000V, 2500V | 50GΩ at 2500V | Yes | Remote control test lead, True RMS, 6000 counts; CAT.III 600V | Motor, transformer and cable maintenance that needs 2500V and timed tests |
| VC1508AT | 250V, 500V, 1000V | 1000V step | No | Remote test lead, AC/DC current to 20A, temperature | NEV and plant troubleshooting where current and temperature are read on the same meter |
| VC1508B+ | 500V, 1000V | 1000V step | No | Remote test lead, inductance 6mH to 100H, current to 20A | Winding checks where inductance matters |
| VC4060PD / VC4060PC | PD: 100V to 2500V; PC: 50V to 1000V | PD 50GΩ, PC 20GΩ | Yes | True RMS, NCV, 6000 counts | Same insulation ranges as the VC1508PD/PC, standard test leads |
| VC4060AT | 250V, 500V, 1000V | 20GΩ at 1000V | No | Current to 20A, temperature, duty cycle, 6000 counts | NEV repair shops wanting one meter for insulation and electrical readings |
| VC4060C | 500V, 1000V | 10GΩ at 1000V | No | DC resistance 200mΩ to 200Ω, inductance to 99.99H | Motor checks: winding balance, inductance and insulation in one tool |
A simple way to decide: if you only test insulation, start with the VC60B+. If you need 2500V or PI/DAR, go to the VC1508PD or VC4060PD. If insulation is one test among many on NEV or industrial jobs, the AT and B+ models save carrying a second meter. The VC1508 versions add a remote control test lead, so you can hold both probes on the circuit and start the test from the lead. The VC4060A and VC4060B test at 1000V only, which suits shops that always test at that voltage.
FAQ
What is the difference between a Megger and an insulation tester?
There is no technical difference. "Megger" is a trade name widely used as a generic word for an insulation resistance tester, also called a megohmmeter. Compare instruments on test voltages, maximum resistance reading, PI/DAR and safety rating, not on the name.
Hipot tester vs megohmmeter: which one do I need?
A megohmmeter measures how much resistance the insulation has at a voltage near its working level, and is used for routine maintenance without harming the insulation. A hipot (dielectric withstand) tester applies a much higher AC or DC voltage, often several kilovolts, and gives a pass or fail result based on leakage current. Hipot testing is mainly a production and acceptance test. Field maintenance teams normally need the megohmmeter.
Can a multimeter or ohmmeter replace a megohmmeter?
No. A multimeter or ohmmeter measures resistance with a few volts, which does not stress insulation the way service voltage does, so many faults stay hidden. A multimeter's top resistance range is also limited (for example 200MΩ on the VC9802A+), while insulation is often judged in the hundreds of megohms to gigohms.
How long should an insulation test take?
A spot reading is usually taken after 60 seconds, once the charging current has settled. DAR needs 60 seconds, and PI needs 10 minutes. Large motors and long cables take longer to charge, so wait until the reading is stable before you record it.