Electronic Insulation Tester: 4 Megohmmeters Compared

As an Amazon Associate we earn from qualifying purchases. Details

An electronic insulation tester measures how well the insulation on a wire, winding or component resists current. Most of these are megohmmeters, which apply a high DC voltage and read resistance in megohms. This page covers four testers whose listings name an insulation or megohm function. Two electronic load testers in the same search are for batteries and chargers, so they are left out.

An insulation test puts a high voltage across the equipment under test, so the safety notes come first. Never connect a lead to live equipment, and confirm the listed test voltage, measuring range and battery needs before ordering. Where a listing leaves any of these out, verify it on the product page first.

Quick answer

Choose an electronic insulation tester by test voltage, measuring range and what you will test. The three M500-style units list a 500 volt test and a range up to 1000 megohms, which suits motors, compressors and control circuits. The digital unit lists 1000 volts and a 1999 megohm range, which suits higher-voltage work. Check each listing for battery type and whether the batteries come with the tester. None of the listings states a CAT rating, so do not use any of them on household mains or on a live panel. Insulation work on equipment that is energized is for trained people only.

Safety first

Work on mains circuits can kill. The tester’s CAT rating must suit the circuit, and a tester whose listing gives no CAT rating should not be used on household mains. Panel and service work belongs to a licensed electrician.

An insulation test applies a high DC voltage. Disconnect the equipment from its supply, discharge any stored energy as the manual says, and keep your hands off the probes while the test runs. Megohmmeters provide high DC voltages, typically from 500 V up to 5 kV, and some reach 15 kV, so the voltage itself is a hazard.

Follow the tester’s manual and your local electrical rules. OSHA lists contact with power lines, missing grounding and misuse of cords among frequent causes of electrical injury, and it recommends insulation, guarding, grounding and safe work practices.

What matters for an electronic insulation tester

Test voltage

The test voltage sets how much stress the insulation sees during the test. The listings in this set give 500 volts for the M500-style units and 1000 volts for the digital unit. Pick the voltage that matches your equipment’s rating and your local standard, and do not guess a voltage the listing does not state.

Measuring range

Each listing states a range in megohms. One reads from 20 to 1000 megohms, another reads up to 1000 megohms, and the digital unit reads to 1999 megohms. A range shows you what the tester can display, but it does not tell you whether a reading is acceptable. Acceptable insulation resistance depends on the standard you follow, and it is typically 1 to 10 megohms in some references.

Why insulation is tested

Insulating components must be tested for insulation strength at commissioning and during maintenance of high-voltage equipment. An insulation test shows whether the insulation has broken down or is starting to. The test is a check of condition at one moment, so repeat it on the same schedule, under the same conditions, to see a trend.

Leads and cords

The test leads take the most wear. OSHA’s rule for flexible cords says they should be protected from sharp corners, projections, doorways and other pinch points, and supported so that they are not damaged. Apply the same care to a tester lead, and replace any lead whose insulation is cracked before you run a test.

Batteries and power

Several M500-style listings require 2 C batteries, and say the batteries are not included. Another says they are included. Check the battery line before you order, and carry a spare set on site so the test does not stop halfway.

Indicators and readout

LED indicators and a zone scale give a quick judgment, while a digital display gives a number. A zone scale helps a beginner read the result without a calculation. A digital display helps when you need to write the reading down. Choose the readout that suits your records.

Picks

The cards below show each tester’s stated voltage, range, batteries, pros, cons and a skip note. The order does not rank them.

How to choose

Use these rules in order.

First, match the test voltage to the equipment. If the equipment is rated for a 500 volt test, choose a 500 volt tester. If your equipment needs a higher test voltage, choose the digital unit with a 1000 volt listing, and confirm the voltage with the maker.

Second, match the range to what you expect to read. A tester that reads to 1000 megohms covers most motor and compressor checks in these listings. A tester with a 1999 megohm display gives more headroom.

Third, decide between a zone scale and a number. Choose the zone scale for a fast pass or fail feel. Choose the number if you log readings over time.

Fourth, check the batteries and the included set before you buy. A tester that needs 2 C batteries that are not included is not ready to use on arrival.

Fifth, plan the record before you test. Write the date, the equipment, the test voltage, the reading and the tester’s serial or model number on each log line, so later readings compare fairly with earlier ones.

Sixth, skip any tester whose listing gives a voltage that conflicts with its other fields. One listing says 500 VAC in its bullets while its title says 500 volts. Megohmmeters are normally DC testers, so confirm the rating with the seller before you use it.

The tester reads to 1000 megohms, so the reading sits inside the range. 1000 megohms minus 800 megohms = 200 megohms of headroom, which means the top of the range is not near the reading. A second test reads 20 megohms. One listing gives its range as 20 to 1000 megohms, so that reading sits at the bottom edge of the range. Whether 20 megohms is acceptable depends on the standard you follow, not on the tester, and a reading at the edge of a range deserves a second look. Record each reading, the date, the voltage and the equipment, and check the standard before you decide.

Worked example: the 500 volt and 1000 volt tests

The M500-style units list a 500 volt test, and the digital unit lists 1000 volts. Dividing 1000 V by 500 V gives 2, so the digital unit’s test voltage is twice the 500 volt test. Its 1999 megohm display is about twice the 1000 megohm top of the M500-style range, so the two differ on both voltage and range.

Mistakes to avoid

Testing live equipment is the first mistake. Disconnect the supply, and confirm it is off before you connect any lead.

Touching the probes during a test is the second. The test voltage is high, and the probes stay live until the tester discharges.

Reading the range as a pass mark is the third. A range shows what the tester can display, not whether the insulation is good.

Ignoring the battery line is the fourth. A tester with no batteries on arrival cannot run the test.

Using a tester with a conflicting voltage field is the fifth. Ask the seller to confirm the voltage in writing, or choose a listing that is clear.

Skipping the isolation check is the sixth. Confirm the circuit is de-energized and that nothing is connected at the far end before you start.

Assuming a CAT rating is present is the last. None of these listings states one, so do not use them on mains without a rating that suits the circuit.

FAQ

What is an electronic insulation tester?

It is a meter that applies a test voltage and reads the resistance of an insulator in megohms. The picks here are megohmmeters, which are the common form of insulation testing tool.

Can I use one of these on household wiring?

Not from this guide. None of the listings states a CAT rating, and household mains needs a tester whose rating suits the circuit. For a basic voltage check, see the best voltage tester for home use.

Why is the test voltage high?

Insulation has to be tested at a voltage high enough to show weak spots. Megohmmeters provide high DC voltages, typically from 500 V up to 5 kV. Treat the probes as live during the test.

What does a megohm reading mean?

It is a measure of how well an insulator resists current at the test voltage. Acceptable readings depend on the standard you follow, so check the standard and the equipment’s documentation.

Do I need a tester with a digital display?

Not always. A zone scale with LED indicators is quick to read. A digital display gives a number you can log, which helps when you compare readings over time.

Can I test a cable that is still connected at the far end?

Disconnect the cable from the equipment and from the supply before you test. A test voltage applied to one end can reach equipment connected at the other end, so isolate the circuit fully. Follow the manual’s steps, and confirm the isolation before you connect a lead.

How do I keep the test leads in good shape?

Store the leads in the case, coil them loosely, and keep them away from heat and oil. Check the insulation along each lead before a test. A lead with a cut or a crack is a hazard, and it can give a false reading as well.

Should I buy a higher test voltage?

Only if the equipment’s rating or the standard you follow calls for it. A higher test voltage applies more stress to the insulation, so it is not an upgrade for its own sake. The listings in this set give 500 volts and 1000 volts. Check the equipment’s rating before you choose.

What does a zone scale tell me?

The zone scale on one listing shows condition zones next to the resistance scale. The listing does not define the zone boundaries, so read the scale in the manual and follow the standard for your equipment.

Are electronic load testers the same as insulation testers?

No. The electronic load testers in the same search set are for batteries and chargers, and they are a different product type. This guide does not include them.

How often should I test insulation?

The standards that apply to your equipment set the schedule. Insulating components are tested at commissioning and during maintenance of high-voltage equipment, so follow the maintenance plan for your equipment.

Pick notes

Compressor and motor work

The first tester lists 500 volts and up to 1000 megohms for HVAC, compressor and motor checks. It names a soft case and a 2-year limited warranty. Choose it for refrigeration and motor work where a 500 volt test suits the equipment.

Refrigeration and appliance work

The second tester lists 20 to 1000 megohms at 500 volts, with a zone scale and LED indicators. It names refrigeration compressors, motors and panel boards as uses. Choose it for appliance work where the zone scale is enough.

Compatible replacement

The third tester is listed as compatible with a named megohmmeter model. Its bullets give 500 VAC while its title gives 500 volts. Confirm the voltage with the seller before you use it on any equipment.

Digital tester at a higher voltage

The fourth tester lists 1000 volts and a range to 1999 megohms, with an LCD display. Its listing does not state a CAT rating. Choose it when you need a higher test voltage and a number on the screen.

The example above shows the arithmetic for one motor test. Use your own numbers, and keep the record with the date, voltage and equipment.

Bottom line

Match the test voltage and the range to the equipment, and check the battery line before you buy. Read the range as a display limit, not a pass mark. Disconnect the supply before any lead touches equipment, and do not use any of these testers on household mains or a live panel. For a broader look at insulation testers, see the best insulation resistance tester. For a digital option to compare, see the best digital insulation tester.

Sources

• Wikipedia, megohmmeter: megohmmeters measure the resistance of insulators, provide high DC voltages, and are used on insulation during commissioning and maintenance.
• OSHA, electrical safety: frequent causes of electrical injury include contact with power lines and misuse of cords, with insulation, grounding and safe work practices as controls.