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Clamp Meter Inrush Current Measurement — Diagnosing Start-Up Faults

2026年06月08日

Nuisance trips at start-up are an inrush problem. Here is how to capture and interpret inrush current.

The Problem Inrush Mode Solves

A breaker that trips only at start-up, a contactor that chatters, a soft-starter that faults intermittently — these are almost always inrush problems. Ordinary current measurement averages too slowly to catch the brief surge. Clamp meter inrush current measurement uses a dedicated capture mode that freezes the initial surge so you can see what the breaker sees.

1. What Inrush Current Is

When a motor, transformer, or capacitive load energises, it briefly draws far more current than its running value — 6 to 12 times for motors, even higher for transformers. This surge lasts tens to hundreds of milliseconds, long enough to trip a marginal breaker but too short for a normal meter to display.

2. How Inrush Mode Works

  1. Arm the inrush function before the load starts.
  2. The meter watches for current to cross a threshold.
  3. It captures and holds the peak over a fixed window (typically 100 ms).
  4. The held value is the inrush figure to compare against breaker curves.

3. Interpreting the Reading

  • Compare measured inrush against the breaker or fuse let-through curve.
  • Motor inrush above 8× running current suggests a stiff supply or wrong starter.
  • Transformer inrush depends on the point-on-wave of energisation — repeat several times.
  • Rising inrush over time can indicate a developing winding fault.

4. Practical Field Technique

  • Clamp around a single conductor, not the whole cable bundle.
  • Use true-RMS for VFD or electronically-started loads.
  • Capture several starts — inrush varies start to start.
  • Log the values for trend comparison.

5. When Inrush Points Elsewhere

If inrush is normal but the breaker still trips, the problem may be a marginal breaker, a loose connection heating up, or a thermal-magnetic mismatch. A clamp meter plus a thermal check narrows it down quickly.

EK Recommendation

The EK-G645 1200 A AC/DC clamp meter includes a dedicated inrush capture mode with true-RMS accuracy and a 52 mm jaw that fits substation bus and motor cables. Talk to EK service teams for distributor pricing.