Amprobe ACDC-620T Clamp-On-Multimeter - Manual (Page 52)

CURRENT HARMONICS THEORY
True-RMS current is very important because it directly relates to the amount of
heat dissipated in wiring, transformers, and loads. Most clamp-on meters
already in the field measure average current, not true RMS current, even if
this average value is displayed on a scale calibrated in RMS. These average-
sensing meters are accurate only for sinusoidal signals.
All current signals are distorted in some way. The most common is harmonic
distortion caused by non-linear loads such as office machines, medical equip-
ment, personal computers, or speed controls for motors. Harmonic distortion
causes significant currents at frequencies that are odd multiples of the power
line frequency. Harmonic current can cause a substantial load on the neutral
wires of wye-connected power distribution systems.
In most countries, 50Hz or 60Hz power distribution systems include 3-phase
delta primary - wye secondary transformers. The secondary generally pro-
vides 120V AC from phase to neutral, and 208V AC from phase to phase.
Historically, balancing the loads on each phase was a big headache for the
electrical system designer.
Typically, the vector addition of the phase currents in the transformers' neutral
wire is zero or quite low in a wellbalanced system. Typical devices that pres-
ent linear loading include incandescent lighting and small motors. The result is
essentially a sine wave current in each phase and a low neutral current at a
frequency of 5OHz: or 60Hz.
Devices such as TV sets, fluorescent lighting, video machines, and microwave
ovens are now commonly drawing power line current for only a fraction of
each cycle so that they cause non-linear loading and subsequent
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