Ieee std 1159-1995, ieee recommended Practice for Monitoring Electric Power Quality

8.4.2.2 Analysis tips
As various loads are turned on, both instantaneous voltages and currents are affected due to OhmÕs Law and
system impedance. This impact will be both in magnitude (a voltage drop), and in terms of waveshape. For
example, if the load is a PC or other electronic load that draws current in large pulses, then the voltage drop
will occur at the peak of the voltage waveform. This causes the peak to be ßattened somewhat, a condition
known as ßat-topping.
In systems that have neutral-to-ground bonds, a great deal of information is available from the neutral-to-
ground voltage and waveshape. OhmÕs Law predicts that the neutral-to-ground voltage is proportional to the
current in the neutral conductor. The voltage at low frequencies with zero ground conductor current is
directly proportional to the neutral current. Consequently, the neutral-to-ground waveshapes and voltages
can allow conclusions about the current through the neutral. 
This can be especially useful in examining ÒdedicatedÓ single-phase circuits that are intended to operate
electronic loads exclusively. If the neutral-to-ground voltage waveshape shows a large sine wave component,
as opposed to the typical pulsed current drawn by electronic loads, there is a non-electronic load sharing the
dedicated circuit.
It can also be useful in determining the cause of a low-voltage situation at a load. If the neutral-to-ground
voltage on a 120 V circuit is less than a few volts, it implies that the voltage drop across the neutral is low, so
presumably the drop across the line conductor is low as well. On the other hand, if the neutral-to-ground
voltage is more than a few volts, the voltage drop across the neutral is high, so it is likely that the distribution
wiring and connectors are undersized for the load.
Figure 19ÑWaveform graph illustrating ßat-topping due to switch-mode power supplies

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IEEE
Std 1159-1995
IEEE RECOMMENDED PRACTICE FOR
54
Conventional wisdom says that the current in a shared neutral conductor of a balanced three-phase system
(i.e., a three-phase system in which the current in each phase is equal) is zero, so the neutral conductor can
have a smaller cross-section than the phase conductors. When dealing with single-phase electronic loads,
especially ones with switching power supplies, this conventional wisdom is faulty. 
Because electronic loads tend to draw all of their current in pulses near the peak of the sine wave, the har-
monic currents in each phase fail to cancel even in a perfectly balanced system, and the neutral current can
be as much or greater than the current in each phase conductor. The current waveshape may be roughly sinu-
soidal, but at 180 Hz, it is often referred as the third harmonic neutral current. For single-phase electronic
loads sharing a common neutral between phases, the neutral conductor should have twice the cross-sectional
area of each of the phase conductors.
Keep in mind also that not only is the neutral-ground rms voltage proportional to the neutral current, but also
its waveshape. So if the neutral current is 180 Hz, so will be the neutral-ground voltage. Since the neutral
current can be very high, this neutral-ground voltage may also become excessive. Also, remember that a
change in neutral-ground voltage can occur due to a change in impedance (e.g., loose connection) in the neu-
tral circuit, or due to abnormal current in the ground circuit.