Overview A Megger (megohmmeter or insulation resistance tester) applies a high DC test voltage to measure insulation resistance between conductors and ground (or between conductors). In ALC airfield lighting, this detects insulation breakdown in series circuits. Poor insulation leads to leakage current, voltage stress on regulators, dim/out lights, arcing, or sudden failures—especially in wet, icy, or high-voltage environments.

Regular Megger testing (weekly/monthly per FAA AC 150/5340-26C recommendations) prevents downtime, identifies degrading trends (e.g., 10-20% annual drop expected), and supports proactive repairs. Low readings (<1 MΩ often indicate imminent failure; aim for >50 MΩ on shorter/new circuits, >30 MΩ on longer ones—base on your facility's history).

FAA Circular 150_5340_26a states:
"The resistance value inevitably declines over the service life of the circuit; a 10-20 percent decline per year may be considered normal."

 

Always prioritize safety:

• De-energize and lockout/tagout (LOTO) the circuit/regulator before testing.
• Confirm zero voltage with a non-contact tester or multimeter.
• Discharge stored capacitance after testing (short leads or use discharge function).
• Wear PPE: insulated gloves (rated for test voltage), safety glasses, arc-flash clothing if needed.
• Use a Megger rated CAT III/IV for airfield use; test leads must be high-voltage rated.
• Never test live circuits—high DC voltage (500V–5000V) can shock or damage equipment.
• For high-voltage series circuits (up to 5kV operating), use appropriate test voltage (e.g., 1000V–2500V DC) and follow FAA AC 150/5340-26C.

 

General Setup

1. Power off the Megger and inspect for damage.
2. Connect leads: Red (positive/high) to the conductor/circuit under test; Black (negative/ground) to earth/ground point (e.g., vault ground bus, conduit, or stake).
3. Select test voltage: Start with 500V or 1000V DC for most airfield cables/transformers (higher like 2500V for longer circuits if specified).
4. Many modern Meggers auto-range and display in MΩ/GΩ.

 

Step-by-Step Instructions

Prepare the Circuit

• • LOTO the constant current regulator (CCR) and confirm de-energized.
  • Disconnect loads if needed (e.g., isolation transformers at ends or sections) for isolated testing.
  • Full circuit: Test from vault S1 cut-out or regulator output to ground.
  • Sectional troubleshooting: Isolate segments to pinpoint faults.

 

Perform Insulation Resistance Test (Primary Function)

• • Set Megger to insulation resistance mode (often labeled MΩ or with voltage settings).
  • Select test voltage (e.g., 1000V DC per FAA common practice for airfield cables).
  • Connect: Red lead to one leg of the series circuit (or all paralleled if testing group); Black to ground.
  • Press/hold TEST (or LOCK for timed test).
  • Run for at least 1–2 minutes (spot reading) or longer for polarization index (PI) if advanced (ratio of 1-min to 10-min reading >2 is good).
  • Read value: High MΩ/GΩ = good insulation; dropping/low = leakage (e.g., moisture, cracks, bad splices). Tip: Test in wet/poor conditions to reveal hidden faults (ice melts or water ingress lowers readings). Record trends over time in Salesforce Cases.

 

Continuity / Low Resistance Check (Many Meggers Include This)

• • Set dial to continuity or low Ω (often with buzzer).
  • Short probes together: Should beep and show near 0Ω.
  • Test across known good sections (e.g., cable ends or transformer primaries/secondaries).
  • Beep/low Ω = good continuity; no beep/high Ω = open/break. Tip: Use after insulation test to rule out opens vs. insulation issues.

 

Interpreting Results (Per FAA AC 150/5340-26C)

• No universal "good" value—depends on circuit length, age, environment.
• Suggested minimums for series circuits below:

Circuit Length in Feet	Suggested Minimum Resistance to Ground in Megohms
10,000 or less	50
10,000 to 20,000	40
20,000 or more	30

 

• <1 MΩ: High risk of rapid failure—prioritize repair (e.g., replace cable section).
• Drastic drops or fluctuations: Investigate (moisture, damage).
• Improving after high-step run: Moisture driven out—monitor.
• Document: Date, conditions (wet/dry), voltage used, reading, circuit ID.

Common Issues

• Low/zero reading: Ground fault—check for water in manholes, cracked insulation, bad transformers/splices.
• Erratic/increasing: Capacitive charging—wait for stabilization.
• No reading/OL: Open circuit or disconnected lead.