Myth: If It Passed the Megger Test, the Cable Is Fine

“We meggered it. 500 meg-ohms. The cable is fine.”

This statement is heard on every construction site, in every maintenance shutdown report, after every cable fault investigation. It reflects a fundamental misunderstanding of what insulation resistance testing actually measures — and, more importantly, what it does not.

An insulation resistance test applies a DC voltage (typically 500V or 1000V for LV cables, 2.5kV or 5kV for MV cables) and measures the resulting leakage current. The ratio gives insulation resistance in megohms.

This test detects:

• Gross insulation damage — cuts, crushing, burn-through
• Water saturation — fully flooded cable sections
• Contaminated terminations — dirt, salt, moisture on exposed insulation surfaces
• Manufacturing defects — voids, inclusions, thin spots (if severe)

In other words, it finds catastrophic damage. If a cable has a hole in the insulation or is sitting in a flooded pit, the megger will find it. This is genuinely useful for commissioning tests and post-installation verification.

1. Partial Discharge

Partial discharge (PD) is localised electrical breakdown within a void or defect in the insulation, without complete bridging. PD requires AC voltage above the inception voltage — typically 1.5–3× the cable’s rated voltage. A DC megger test at 500V or 1000V does not excite PD in a cable rated for 600V or 3.3kV. The defect is invisible. But under normal AC operating voltage, PD progressively erodes the insulation from inside until catastrophic failure occurs — typically in 2–10 years.

2. Water Treeing

Water trees are microscopic tree-shaped structures that grow in XLPE insulation in the presence of moisture and electric field stress. They develop over 5–20 years and progressively reduce the insulation’s dielectric strength. A cable with advanced water treeing can show perfectly acceptable insulation resistance on a megger test — the DC resistance of the tree structure is extremely high. But under AC stress, the capacitive coupling through the water tree creates a low-impedance path that eventually converts to an electrical tree and fails in milliseconds.

3. Thermal Aging

Insulation that has been chronically overheated (e.g., from sustained overloading or inadequate derating) undergoes polymer degradation — chain scission, oxidation, loss of plasticiser. The insulation becomes brittle and its dielectric strength decreases, but its volume resistivity (what the megger measures) may remain acceptable until very late in the degradation process. A thermally aged cable can pass a megger test at 500 MΩ and fail the next day under a voltage transient that healthy insulation would easily withstand.

4. High-Resistance Joints

A poorly made joint or termination can have a connection resistance of 10–100 mΩ (compared to <1 mΩ for a good joint). Under load, this creates localised heating that degrades the surrounding insulation over time. A megger test measures insulation resistance between conductors or conductor-to-earth — not the resistance of the conductor path itself. A joint that will overheat and fail at 200A passes the megger test without comment.

5. Mechanical Damage to Individual Strands

A cable with 30% of its strands severed (from an installation nick, compression, or rodent damage) still shows excellent insulation resistance — the insulation is intact. But the remaining strands carry 143% of their design current density, causing localised heating, accelerated aging, and eventual failure. Only a resistance test (comparing measured resistance against the manufacturer’s published value for the conductor size and length) or a detailed visual inspection can detect this.

IEEE 400-2012 describes a hierarchy of cable testing methods, from least to most diagnostic:

For critical cables (MV feeders, life-safety circuits, process-critical supplies), a megger test alone is insufficient for condition assessment. Tan delta and partial discharge testing provide actual diagnostic information about remaining insulation life.