Industrial communication via fieldbus-transceiver systems
often requires long transmission lines. Designers, unaware
of the large ground-potential differences (GPDs) between
remote bus locations, either rely on the local earth ground
as a reliable signal return path or directly connect remote
grounds to each other—thus creating noisy ground loops.
In both cases the integrity of the transmission signal is
compromised, which can lead to system lockup and, at
worst, destroy the bus transceivers.
To make designers aware of these design pitfalls, this
article explains where GPDs originate in the electrical
installation, how ground loops are created unintentionally,
and how isolation circumvents both conditions to yield a
robust data-transmission system.
The link between the direct-current (DC) ground of a local
electronic circuit and the earth reference potential of the
mains is usually provided by the local power supply
Figure 1. Simplified block diagram of an SMPS
converting the line voltage into the required DC output.
Figure 1 shows a simplified block diagram of a low-cost
switched-mode power supply (SMPS) typically used in
personal computers, laser printers, and other equipment.
Here the DC ground of the SMPS output is referenced to
the protective earth (PE) conductor of the mains via the
SMPS chassis. This direct link, therefore, acts as a sense
conductor, establishing the PE voltage as the local DC
Figure1: Simplified block diagram of an SMPS
(Click on image to enlarge)
Linear and nonlinear loads
Large office and industrial buildings operate a vast number
of nonlinear loads such as PCs, laser printers, solid-state
heater controls, fluorescent tubes, uninterruptible power
supplies, and variable-speed drives. In comparison to linear
loads such as incandescent lamps, whose phase currents
maintain a sinusoidal waveform, nonlinear loads distort
phase currents, introducing large harmonic content.
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