Okay, Rob, you're right to be concerned. What you're describing is absolutely not a proper megger test and the results are essentially meaningless. Here's a breakdown of how to do it correctly, along with explanations, considerations, and references where possible. We'll focus on getting valid data and catching potential insulation problems.

Understanding the Goal of a Megger Test (Insulation Resistance Test)

The core purpose of a megger test (insulation resistance test) is to verify the integrity of the insulation surrounding the conductors. You're applying a high DC voltage and measuring the leakage current through the insulation. High resistance = low leakage = good insulation. Low resistance = high leakage = insulation problem (damage, contamination, moisture, etc.). A "good" megger test isolates the insulation you're testing so you can get an accurate reading.

Here's the correct procedure, step-by-step



1. Safety First! De-energize and Lockout/Tagout

This is paramount.

Absolutely confirm that the cables are de-energized at both ends before you start. Use a proper voltage tester to verify zero voltage. Lockout/Tagout procedures must be followed to prevent accidental re-energization. Personal Protective Equipment (PPE)

Wear appropriate PPE for the environment (gloves, safety glasses, etc.). While the test voltage is DC, it's still high voltage and needs to be respected.

2. Preparation Cable Identification

Clearly identify each cable. Label them at both ends (A and B). Document your labeling scheme. This is critical for accurate testing and troubleshooting later. Draw a diagram showing the cables, terminations and points where the test is performed.

Cleaning and Preparation

Ensure the exposed insulation at each end of the cables (at both locations A and B) is clean and dry. Wipe down the insulation with a clean, dry cloth to remove any dirt, dust, or moisture. Contamination on the surface of the insulation can create a surface leakage path, giving you a false low reading.

Isolation

This is

critical. The cables must not be connected to anything at either end. They should be completely isolated.3. Megger Connections (Crucial!) Choosing the Test Voltage

This is important. The Fluke 1587 FC offers multiple test voltages. Industry standard practice is to use 500V DC or 1000V DC depending on the cable rating. Generally, for cables rated 600V or less, 500V DC is sufficient. For cables rated higher than 600V, 1000V DC is often used.

Consult the cable manufacturer's specifications for the recommended test voltage. If you cannot find the cable manufacturer's specifications, consult with an experienced engineer. Using too high of a voltage can damage the insulation. Connections



Line (L) Lead

Connect the Fluke 1587 FC's

red (positive) lead to one of the conductors you want to test. For example, connect it to the end of the Green wire at Location A. Earth (E) Lead (Guard Terminal, if Available)

Connect the Fluke 1587 FC's

black (negative) lead to a known, reliable ground source. This is often the grounding electrode system of the building. This is the crucial step your worker was missing! Connect the clip to the grounded metal of an appropriate item that is a part of the electrical system or its ground conductor. DO NOT rely on building steel for ground. If the Fluke 1587 FC has a Guard terminal, use it. Connect the guard terminal to an uninsulated metal part near the insulation you are testing to shunt away surface leakage. This will allow a more accurate reading. If your model lacks a guard terminal, this step is unnecessary. Testing Each Conductor to Ground

1. Connect the Fluke 1587 FC according to the procedure above, but make sure that the red test lead (Line) is connected to the Green wire.
2. Set the megger to the appropriate test voltage (500V DC or 1000V DC).3. Press and hold the "Test" button.

Do not tap it!4. Observe the reading. Let the test run for at least 60 seconds. The reading will typically increase over time as the insulation becomes polarized.5. Record the reading after 60 seconds. Also note if the reading is stable or continuing to rise slowly.
6. Repeat this process for each of the other conductors (Brown, Yellow, Orange), keeping the black lead connected to ground. Testing Conductor-to-Conductor

1. Move the black test lead to either the Brown, Yellow, or Orange wire.
2. Connect the red test lead (Line) to the Green wire.3. Set the megger to the appropriate test voltage (500V DC or 1000V DC).
4. Press and hold the "Test" button.

Do not tap it!5. Observe the reading. Let the test run for at least 60 seconds. The reading will typically increase over time as the insulation becomes polarized.6. Record the reading after 60 seconds. Also note if the reading is stable or continuing to rise slowly.
7. Repeat for each of the remaining conductors.4. Interpreting Results Minimum Acceptable Resistance

There is no single magic number. It depends on the cable voltage rating, the type of insulation, and the applicable standards. However, a general rule of thumb:

At 500V DC test voltage

1 Megohm or greater is generally considered acceptable for low-voltage cables.

At 1000V DC test voltage

2 Megohms or greater is generally considered acceptable for low-voltage cables.

Trends Are Important

Pay attention to trends. If the resistance is initially high but then starts to drop significantly during the test, that's a red flag. It could indicate insulation breakdown under voltage stress.

Consistency

Compare the readings between similar cables. Inconsistent readings between identical cables under the same conditions can indicate a problem with the cable showing the lower reading.

Polarization Index (PI) and Dielectric Absorption Ratio (DAR)

These are more advanced measurements that can provide additional insight into the insulation condition. The Fluke 1587 FC may calculate these values.

PI

Resistance at 10 minutes divided by resistance at 1 minute. A PI > 2 is generally considered good.

DAR

Resistance at 60 seconds divided by resistance at 30 seconds. A DAR > 1.4 is generally considered good. Lower PI and DAR values may indicate contamination or moisture in the insulation.

5. Documentation Record EVERYTHING

Record the date, time, cable identification, test voltage, temperature, humidity, meter model, and resistance readings for each test. Keep a log of your tests.

Create a Test Report

Compile your data into a clear and concise test report.

Addressing Your Specific Concerns "Tapping" the Wires

This is completely wrong. A megger test requires sustained application of voltage. "Tapping" is essentially useless.

Testing Against a "Known Good" Ground

Absolutely essential! You are correct. Without a reliable ground reference, you're not measuring insulation resistance, you're just measuring…something. The ground lead completes the test circuit and allows the megger to measure the current leakage through the insulation.

Cable Not Landed

Correct. The cables need to be unlanded/disconnected/isolated on both ends before running the test.

Where to Find Standards and Best Practices NETA (InterNational Electrical Testing Association)

NETA standards are widely regarded as the gold standard for electrical testing. Look at the NETA Acceptance Testing Specifications (ATS) and NETA Maintenance Testing Specifications (MTS) for detailed procedures and acceptance criteria for insulation resistance testing. These documents are not free; you'll need to purchase them from NETA.

IEEE (Institute of Electrical and Electronics Engineers)

IEEE 43 Recommended Practice for Testing Insulation Resistance of Rotating Machinery While focused on rotating machinery, it contains fundamental principles applicable to insulation resistance testing in general.

Manufacturer's Specifications

As mentioned earlier, the cable manufacturer's specifications are the best source of information for the cable type.

Key Takeaways for You1. Stop the current practice immediately. It's providing false information.2. Emphasize safety. Lockout/Tagout is non-negotiable.3. Follow the step-by-step procedure outlined above.4. Use a reliable ground reference.5. Test each conductor individually to ground and conductor to conductor.6. Run the test for at least 60 seconds and record the readings.7. Document everything.

By following these guidelines, you will get reliable, repeatable, and actionable results from your insulation resistance tests. Good luck, and feel free to ask if you have more questions! Flag for review