Fluke - Why True-rms? Non-linear loads need a true-rms current clamp for accurate readings

Our thanks to Fluke Corp for allowing us to reprint the following article.


Troubleshooting the electrical service feeding adjustable speed motor loads can be difficult if you don't have the right tools. New solid state motor drives and heating controls often conduct non-sinusoidal (distorted) current. In other words, the current occurs in short pulses rather than the smooth sine wave drawn by a standard induction motor. The current wave shape can have a drastic effect on a current clamp reading.

Basically, there are two types of current clamps commonly available: "average responding" and "truerms". The average responding units are widely used and are usually lower cost. They give correct readings for linear loads such as standard induction motors, resistance heaters, and incandescent lights. But when loads are non-linear, containing semiconductors, the average responding meters typically read low. Worst case nonlinear loads include small adjustable speed drives (5 hp or less) connected line to line across two phases of a 480 V, three-phase system, solid state heater controls connected single phase to 240 V, or computers connected to 120 V. When troubleshooting a branch circuit that suffers from circuit breaker tripping (or fuse blowing), the cause of the trouble can usually be separated into one of three categories:

  • Too much current.
  • Too much heat in the electrical enclosure.
  • Faulty circuit breaker (or fuse).

Your first instinct will probably be to measure the current with a current clamp while the load is on. If the current is within the circuit rating, you may be tempted to replace the circuit breaker...

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