Megger Passes But Fault Persists: A Misleading-Reading Decision Tree

Why this matters

An insulation-resistance test reads several hundred megohms, the megger "passes," and the tech crosses insulation breakdown off the list. The circuit keeps faulting anyway. A passing megger reading is one of the most over-trusted results in the trade. It proves the insulation does not leak at the meter's test voltage under static, dry, unloaded conditions. It does not prove the conductor is free of a series fault, a voltage-dependent breakdown that only flashes over at line peak, a tracking path that needs moisture or heat to appear, or a high-resistance termination that megging cannot see at all. Walking away on a passing megger leaves the customer with the same fault.

Why the good reading misleads

A megohmmeter applies a DC test voltage, commonly 250 V, 500 V, or 1000 V, between a conductor and ground or between conductors, and measures leakage current. It is excellent at finding a wet, carbonized, or broken-down insulation path that leaks at that DC level. It is blind to several real faults:

• Series faults. A loose lug, an oxidized splice, or a broken strand bundle has high resistance in series with the load, not a leakage path to ground. The megger sees the conductor as open or as a normal high resistance and passes.
• Voltage-dependent breakdown. Some insulation faults conduct only above the megger's test voltage or only at the 170 V peak of a 120 V AC sine wave. A 500 V DC megger may still read clean if the breakdown threshold is higher under the test conditions, or if the fault needs the AC waveform and load current to ignite.
• Moisture- or heat-triggered tracking. A carbon track that is dry at test time reads good and conducts only after humidity rises or the conductor heats under load.
• Arcing faults. Series and parallel arcing is intermittent and waveform-dependent; a static DC test does not reproduce it.

The core trap is the same single-point, no-load problem: the megger tested insulation integrity under one condition, not the conductor's behavior under operating voltage, current, temperature, and moisture.

Symptom presentation

• Insulation resistance reads well above the acceptance threshold, yet the breaker, GFCI, or AFCI keeps tripping.
• Fault is intermittent and tracks with weather, load, or temperature.
• Burn marks, tracking, or odor at a location the megger declared healthy.
• The circuit runs at no load but faults the instant it is loaded.

Quick checks

• Repeat the megger at the highest appropriate test voltage for the system and insulation rating; a fault hidden at 250 V may show at 1000 V on a 600 V-rated conductor. Do not exceed the insulation rating.
• Add a polarization-index or step-voltage test. A megohm value that collapses as test voltage rises, or a poor PI ratio, exposes a marginal insulation that a single spot reading missed.
• If the megger still passes, switch tools. Run a loaded voltage-drop test to find series faults and energize the circuit under controlled conditions to reproduce arcing or moisture-dependent tracking.

Isolation tree

1. Megger passes, circuit faults under load, breaker trips on overload. Suspect a series high-resistance fault generating heat, not insulation breakdown. Loaded voltage-drop test isolates the bad connection.

2. Megger passes at low test voltage, fails or drops at higher test voltage. Voltage-dependent insulation breakdown. The conductor or splice needs replacement even though the first reading looked fine.

3. Megger passes dry, fault returns when wet. Moisture-triggered tracking. Re-megger after wetting the suspect location, or test during the conditions that produce the fault. Outdoor and below-grade runs are prime suspects.

4. Megger passes, AFCI trips, no leakage to ground. Series or parallel arcing that DC testing cannot reproduce. Isolate by branch under energized conditions and inspect terminations for arc evidence.

5. Megger passes, GFCI trips under load. Leakage that appears only with the neutral carrying current; a downstream N-G contact or a load with operating leakage. Lift and re-test the load neutral.

Confirming diagnosis

The confirming test is whichever one reproduces the fault under operating conditions. For a series fault, a loaded voltage-drop test pinpoints the bad joint and the megger result becomes irrelevant. For voltage-dependent breakdown, a step-voltage or higher-voltage IR test that shows collapsing resistance confirms the insulation is failing. For moisture-dependent tracking, re-megging under wet conditions drops the reading and confirms the path. For arcing, energized isolation by branch plus visual arc evidence at a termination confirms it. In every case, after the repair, a clean loaded test and a stable circuit under full duty are the real acceptance criteria, not a single passing spot reading.

Remediation

• Replace conductors or splices that fail a higher-voltage or step-voltage IR test.
• Re-terminate or replace high-resistance connections found by loaded voltage-drop testing.
• Correct moisture intrusion at outdoor and below-grade runs; reseal or reroute as needed.
• Re-test under operating load and, where applicable, repeat the IR test under the conditions that originally produced the fault to confirm the repair holds.

References

• NFPA 70, National Electrical Code, Article 110.14 (terminations) and 250.4 (fault-current path).
• NETA MTS, Standard for Maintenance Testing Specifications (insulation-resistance and step-voltage test methods and acceptance).
• IEEE 43, Recommended Practice for Testing Insulation Resistance (polarization index, voltage dependence).
• NFPA 70E, Standard for Electrical Safety in the Workplace (energized testing justification and PPE).