Fluke 1577 Insulation Test

Fluke 1577 Insulation Test

ByKen

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The person typing “fluke 1577 insulation test” wants to know how to run an insulation resistance test with this meter and how to read the result. The Fluke 1577 is a 2-in-1 unit: it works as a true-RMS multimeter and as an insulation tester. For an insulation test, you turn the rotary switch to the INSULATION position, choose a test voltage, connect the probes to a de-energized circuit, press the test button, and read the resistance in megohms or gigohms. The manual states that insulation tests should only be performed on dead circuits, so the meter’s lockout and the circuit’s isolation both matter.

Quick answer

  • Test only dead circuits. The manual says insulation tests “should only be performed on dead circuits.”
  • Check the fuse first. The manual tells you to check the fuse before testing.
  • Voltage choices: 500 V (default) and 1000 V on the 1577, according to the manual and the product page.
  • The lockout: if the meter sees more than 30 V ac or dc, the test is inhibited.
  • Range: the product page lists an insulation test range of 0.1 MΩ to 600 MΩ.

What the 1577 is

The Fluke 1577 product page describes it as an insulation multimeter that combines two tools, and says it is “ideally suited” for work on “motors, generators, cables, or switch gear.”[1] The same page lists true RMS, a backlit display, auto power off, and a CAT III 1000V and CAT IV 600V measurement category.[1] The manual covers the 1587 FC, 1587 and 1577 together, so some pages describe features that apply to the 1587 family rather than the 1577 alone.[2]

Before you test

Two checks come before any insulation reading. The first is the circuit. The manual states that “insulation tests should only be performed on dead circuits.”[2] Confirm the circuit is de-energized by the method your site requires, and do not rely on the meter alone. The second is the meter. The manual says to “check the fuse before testing” and points to a fuse test later in the manual.[2]

The manual also describes a battery load check that starts when you move the switch to INSULATION. If the battery fails, the display shows “B” and “bat,” and the insulation test cannot run until the batteries are replaced.[2]

The step-by-step test

The manual’s procedure, reduced to the main steps, is as follows.[2]

  1. Insert the test probes in the input terminals marked for insulation testing.
  2. Turn the rotary knob to the INSULATION position. The battery load check runs when the switch moves to this position.
  3. Press the r button to select the test voltage. On the 1577 the choices are 500 V (default) and 1000 V.[2][1]
  4. Connect the probes to the circuit under test. The meter checks automatically whether the circuit is energized.
  5. Press and hold the s button. The meter outputs the high voltage and measures insulation resistance. The primary display shows the resistance in megohms or gigohms, and the high-voltage symbol appears while the test runs.
  6. Keep the probes on the test points and release the s button. The manual says the circuit under test then discharges through the meter, and the reading stays on the display until a new test starts, the function or range changes, or more than 30 V is detected.[2]

The steps above are a summary. Read the manual’s full procedure for your unit before using it on a job.

The 30 V lockout

The meter watches for voltage before it runs a test. The manual says that when the meter detects 30 V or more, the high-voltage symbol and a reading of “>30 V” appear, and “the test is inhibited.” It then says to “disconnect the Meter and remove power before proceeding.”[2] The Fluke product page describes the same feature as “live circuit detection prevents insulation test if voltage > 30 V is detected.”[1] A lockout is a warning, not a replacement for confirming the circuit is dead.

Choosing 500 V or 1000 V

The manual states that the source voltage rating for an insulation test is “500 (default) and 1000 V ranges available on the 1577.”[2] The product page lists test voltages of 500 V and 1000 V, with maximum resistance of 600 MΩ and a minimum insulation test value of 0.1 MΩ.[1] A higher test voltage is not automatically better. The right voltage depends on the equipment, its rating and the procedure your site follows, so check the requirements for the equipment you are testing.

Reading the result

The Wikipedia megohmmeter article defines the instrument as “a special type of ohmmeter used to measure the electrical resistance of insulators,” and says that such meters “provide high DC voltages (typically in ranges from 500 V to 5 kV, some are up to 15 kV).”[3] The 1577 sits at the low end of that range with its 500 V and 1000 V options. A reading is a resistance value at the test voltage, and it changes with the voltage, temperature and the condition of the insulation. The sources do not give acceptance limits for a specific piece of equipment, so use the limit in your equipment’s standard or the manufacturer’s data.

When the resistance is higher than the display can show, the manual says the meter displays a “Q” symbol and the maximum resistance for the range.[2] Note that symbol as a “more than” reading, not an exact value.

Polarization index and DAR

The manual also covers the polarization index (PI) and dielectric absorption ratio (DAR). It defines PI as “the ratio of the 10-minute insulation resistance to the 1 minute insulation resistance,” and DAR as “the ratio of the 1-minute insulation resistance to the 30 second insulation resistance.”[2] Both tests take minutes rather than seconds, and the manual recommends test clips because of the time required.[2]

If you are new to the test, the basic overview of insulation testing is in our guide to using a megger meter. For a comparison of insulation resistance testers, see our insulation resistance tester roundup.

Safety

An insulation test applies high voltage to the circuit under test, and the manual limits the test to dead circuits.[2] The manual also warns that you should “disconnect circuit power and discharge all high-voltage capacitors” before measuring resistance in a different mode.[2] Read the full safety information in the manual before using the meter. Work on energized equipment belongs to qualified personnel and to the procedures your site requires.

How we chose

This is an explainer, so it does not pick products. We used the manufacturer’s product page for the 1577’s features and ratings, the Fluke users manual for the procedure and warnings, and the Wikipedia megohmmeter article for the general definition of the instrument. The manual is shared across three Fluke models, so each step is taken from the text that covers the 1577 or the shared controls.

What we could not verify

  • The product page’s Q&A section includes a customer answer that gives a maximum reading of about 660 MΩ for the 1577, which differs from the 600 MΩ figure on the specification table. We used the specification table and did not confirm the difference, so check the manual for your unit.
  • The manual is shared with the 1587 FC and 1587, so a few controls and displays described in it may differ on a 1577. Check your unit’s labels.
  • We did not find acceptance limits for specific equipment in the sources we opened. Use the limit in the standard or the equipment manufacturer’s data.

FAQ

Can I run an insulation test on a live circuit with the 1577?

No. The manual says insulation tests should only be performed on dead circuits, and the meter inhibits the test when it detects more than 30 V.[2] Confirm the circuit is de-energized by your site’s method.

What does the “>30 V” symbol mean?

It warns that voltage of 30 V or more is present, and the insulation test is inhibited. The manual says to disconnect the meter and remove power before continuing.[2]

What test voltages does the 1577 offer?

The manual and the product page both list 500 V (default) and 1000 V for the 1577.[1][2]

What is the range for an insulation test?

The product page lists 0.1 MΩ to 600 MΩ for the insulation test.[1] A customer answer on the same page gives a different maximum, which we could not confirm.

Why does the reading stay on the display after I release the button?

The manual says the circuit discharges through the meter after you release the button, and the reading stays on the display until a new test starts, the function or range changes, or more than 30 V is detected.[2]

Is the 1577 the same as the 1587?

The manual covers both models, and the 1577 has 500 V and 1000 V test options while the 1587 also offers 50 V, 100 V and 250 V.[2] Check your model’s specifications before you buy or test.

Bottom line

Run the Fluke 1577 insulation test only on a dead circuit, check the fuse first, choose 500 V or 1000 V for the equipment, and treat the 30 V lockout as a warning. Read the reading as a resistance at a specific voltage, and compare it with the limit in the equipment’s standard.

Sources

  1. [1] Fluke 1577 Insulation Multimeter, Fluke product page. https://www.fluke.com/en-us/product/electrical-testing/insulation-testers/fluke-1577 (opened 2026-10-29)
  2. [2] 1587 FC/1587/1577 Insulation Multimeter Users Manual, Fluke. https://media.fluke.com/1db33f5b-028c-4fbb-adfc-b10a000beb6b_original%20file.pdf (opened 2026-10-29)
  3. [3] Megohmmeter, Wikipedia. https://en.wikipedia.org/wiki/Megohmmeter (opened 2026-10-29)

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