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ctllp05044Mistakes.qxd 4/18/2005 11:29 AM Page 5

N ... AND HOW TO AVOID THEM 5

FIGURE 2. POOR MOUNTING

interference can cause random
fluctuations in the sensors’ output
and may even damage the sensors
themselves.
As the name implies, ground
loops most often occur when instru-
ments and their cables are ground-
ed improperly or not at all.
Interestingly, the best way to isolate
Even the orientation of an instrument can affect its performance. Here, the sensor a plant’s instruments from ground
is enclosed in a housing designed to dissipate the heat it generates. The fins must loop currents is to connect them
be mounted vertically to allow warm air to escape. together at one master grounding
point.
ice. Over time, outdoor instruments ings, it’s absolutely critical to estab- If that’s not possible, a grid of
can fail slowly unless enclosed in lish electrical isolation. The most grounding points must be spread
appropriate housings. common electrical problems due to throughout the plant, making sure
But even the housings them- poor installation are ground loops. that all points on the grid are at
selves can cause problems for the Ground loops occur when an the same electrical potential.
enclosed instruments, particularly extraneous current flows through Insecure connections and inade-
temperature sensors. If an RTD or the instrumentation wiring between quate wires can cause a voltage
thermocouple is mounted on the two points that are supposed to be imbalance in the grid and ground
same piece of metal that supports at the same voltage, but aren’t (see loops between the instruments
the housing, the housing will work Figure 3). The resulting electrical connected to it.
like a heat sink when the ambient
temperature drops low enough. It FIGURE 3. POOR GROUNDING
will tend to draw heat out of the
sensor and artificially lower its
reading. The heat-sink effect will } Data signals
to I/O panel
also tend to reduce the benefits of Instruments must be
any internal heat that has been grounded to provide
applied to prevent an instrument a reference voltage
from freezing. } Instruments for the data signals
Conversely, if a housing is they generate.
equipped with fins intended to Relying on earth
draw heat out of the enclosed sen- ground is risky since
sor during warm weather, the fins not all of the earth
must be mounted vertically. shares the same
}
Otherwise, the warm air around the Earth grounds at electrical potential.
fins will not be able to rise away unequal voltages The resulting currents
from the housing (see Figure 2). will interfere with the
sensors’ signals.
G Grroouunndd llooooppss
While it’s generally a good practice
to insulate a sensor from the ther- Ground loop currents
modynamic effects of its surround-