Best insulation resistance tester for HVAC motors and electrical fault finding

A clamp meter can tell you what a motor draws. An insulation tester helps you investigate where current can leak to ground. Put the right one in the service bag: a dedicated tester for repeat checks, a combined multimeter for a smaller kit, or a straightforward display meter for readings you record on the job.

Quick picks: Fluke 1507 for dedicated insulation work; Fluke 1587 FC for a multimeter, insulation tests and phone records; Klein ET600 for four test-voltage choices and direct field readings.

Insulation testers compared

Read the range at the test voltage you will use. A headline 10 GΩ range doesn’t mean the tester reaches 10 GΩ at 500 V.

Pick	DC test voltages	Range ceiling at 500 V / 1000 V	Timed tests and records	Live-circuit protection / rating
Fluke 1507	50, 100, 250, 500, 1000 V	500 MΩ / 10 GΩ	Automatic PI/DAR; Compare function; remote test probe	Insulation test inhibited above 30 V; automatic discharge; CAT IV 600 V
Fluke 1587 FC	50, 100, 250, 500, 1000 V	500 MΩ / 2 GΩ	Native PI/DAR; Fluke Connect Measurements app captures readings and graphs	Insulation test inhibited above 30 V; automatic discharge; CAT IV 600 V / CAT III 1000 V
Klein ET600	125, 250, 500, 1000 V	4000 MΩ / 4000 MΩ	Lock mode for timed PI/DAR work; record readings manually	External-voltage alarm and icon; CAT IV 600 V / CAT III 1000 V

The ranges come from the 1507 manual, 1587 FC manual and ET600 manual. PI and DAR compare readings over time; use the equipment maker’s test method and interpretation criteria. Klein documents lock mode for that timed work, rather than an automatic ratio calculation.

Best dedicated tester: Fluke 1507

Why you’ll like it: The 1507 keeps insulation testing front and center. Automatic polarization index and dielectric absorption ratio tests handle the timed readings, while the remote probe lets you start a test at the probe. Set the Compare function to the equipment’s specified limit for repeat checks.

Five voltage settings make it flexible across equipment with different approved test procedures. The live-circuit detector inhibits an insulation test above 30 V, and automatic discharge follows the test. Those features and its CAT IV 600 V rating are on the 1507 product page.

Good to know: Its 10 GΩ ceiling is at 1000 V; at 500 V, the manual’s range tops out at 500 MΩ. Choose the test voltage for the motor, not to chase a bigger display number.

Best meter with app records: Fluke 1587 FC

Why you’ll like it: Carry one instrument for true-RMS multimeter work and insulation tests, then capture the reading on your phone. This is the pick for a tech who wants the test result ready for the service record, alongside voltage and temperature checks.

The 1587 FC has its own PI/DAR control. Fluke Connect Measurements adds reading capture and timed-test graphs; the separate Fluke Connect Assets service is sold separately. The 1587 FC product page identifies the FC model as part 4691215, and the datasheet lists CAT IV 600 V / CAT III 1000 V.

Good to know: Match 1587 FC / 4691215, not the older non-FC 1587 or a bundle with an added clamp. Its insulation range reaches 2 GΩ at 1000 V and 500 MΩ at 500 V.

Best straightforward field tester: Klein ET600

Why you’ll like it: The ET600 gives you four insulation voltages, a backlit display and a lock function for hands-free timed readings. It also handles true-RMS AC/DC voltage, resistance and continuity, making it a useful service-bag instrument when you prefer to put readings directly into the job notes.

Its insulation settings are 125, 250, 500 and 1000 V DC, with a maximum range of 4000 MΩ. Klein lists CAT IV 600 V / CAT III 1000 V and an external-voltage warning alarm and icon on the ET600 product page.

Good to know: Use lock mode for the timed test and record the readings. Don’t buy it expecting the Flukes’ automatic PI/DAR workflow or phone graphs.

A clamp meter’s MΩ range is a different test

Keep your clamp meter for current draw and routine electrical checks. A selectable-voltage insulation tester applies the specified DC test voltage to assess insulation leakage. An ordinary resistance range does a different job.

For example, the Fieldpiece SC680 manual lists a resistance range up to 50 MΩ with typical open-circuit voltage of -1.1 V DC (-3.2 V on its 100 Ω range). That isn’t a 250, 500 or 1000 V insulation test. Our HVAC clamp-meter guide covers that part of the kit.

Test a compressor to ground: the service-call checklist

De-energize and lock out all power, prove the circuit dead, and discharge capacitors before disconnecting or testing the motor. Copeland’s compressor bulletin sets out the electrical isolation precautions.

Use the compressor maker’s procedure for the model in front of you. Fluke’s application note illustrates a conventional hermetic compressor with common-connected windings; ECM and inverter equipment need the manufacturer’s isolation and test procedure.

1. Isolate the compressor. Remove external wiring and terminal bars as directed. Disconnect solid-state controls from the insulation-test circuit; high test voltage can damage them.
2. Prepare the test points. Clean and dry the terminals, and use a clean case-ground point. Record terminal temperature; Fluke calls for it to be above the ambient dew point.
3. Use the correct winding arrangement. Fluke shunts the common-connected compressor terminals where possible and tests them to the case. For separately isolatable windings, its note gives a different sequence: test one winding with the other two grounded, then repeat. Follow the arrangement for your compressor.
4. Select the approved voltage and duration. Fluke’s illustrated procedure uses 500 V DC for 60 seconds. Use the equipment maker’s specified settings for the actual unit. Never insulation-test or operate a compressor under vacuum.
5. Finish the test and discharge. Keep the leads connected until the circuit has discharged and the meter shows near-zero voltage. Record resistance, voltage, test duration and terminal temperature before restoring the manufacturer’s wiring arrangement.

The Fluke compressor application note supplies that sequence; its motor-testing guidance explains control isolation, and the Klein manual covers waiting for discharge with the leads connected.

Make the repair call from the whole record

Use the compressor maker’s criteria and a comparable reading history. Copeland explains that refrigerant, oil, terminal contamination and connected external components can affect a hermetic compressor’s megohm reading. Its evaluation bulletin recommends recording values over time rather than condemning a compressor from one reading.

Keep test voltage, duration and temperature with the result so the next tech can make a useful comparison. A meter’s Compare threshold is one you configure to the right criteria, not a universal compressor-replacement rule.

Save a reading the next tech can use

Put the unit ID, model/serial, test points, voltage, duration, terminal temperature and resistance in the job record. Add a photo of the display or an exported graph/report. A bare “passed” leaves the next service call starting from scratch.

In Jobber, add the text and file to the job or visit through mobile notes and attachments. In Housecall Pro, use the job’s Attachments control for the photo or PDF, as shown in its mobile attachment help. Save the file from your meter workflow, then attach it to the job.

Build those fields into a reusable checklist with our Jobber vs Housecall Pro job-forms comparison, and keep the unit identity straight with the equipment-records comparison. For the rest of a furnace call, see our combustion-analyzer picks and digital-manometer guide.