Section 7. Installation
7.3.1 ESD Protection
Related Topics:
• ESD Protection (p. 97)
• Lightening Protection (p. 98)
ESD (electrostatic discharge) can originate from several sources, the most
common and destructive being lightning strikes. Primary lightning strikes hit the
CR800 or sensors directly. Secondary strikes induce a high voltage in power
lines or sensor wires.
The primary devices for protection against ESD are gas-discharge tubes (GDT).
All critical inputs and outputs on the CR800 are protected with GDTs or transient
voltage suppression diodes. GDTs fire at 150 V to allow current to be diverted to
the earth ground lug. To be effective, the earth ground lug must be properly
connected to earth (chassis) ground. As shown in figure Schematic of Grounds
(p.
98),
signal grounds and power grounds have independent paths to the earth-ground
lug.
Communication ports are another path for transients. You should provide
communication paths, such as telephone or short-haul modem lines, with spark-
gap protection. Spark-gap protection is usually an option with these products, so
request it when ordering. Spark gaps must be connected to either the earth ground
lug, the enclosure ground, or to the earth (chassis) ground.
A good earth (chassis) ground will minimize damage to the datalogger and
sensors by providing a low-resistance path around the system to a point of low
potential. Campbell Scientific recommends that all dataloggers be earth (chassis)
grounded. All components of the system (dataloggers, sensors, external power
supplies, mounts, housings, etc.) should be referenced to one common earth
(chassis) ground.
In the field, at a minimum, a proper earth ground will consist of a five foot
copper-sheathed grounding rod driven into the earth and connected to the large
brass ground lug on the wiring panel with a 14 AWG wire. In low-conductive
substrates, such as sand, very dry soil, ice, or rock, a single ground rod will
probably not provide an adequate earth ground. For these situations, search for
published literature on lightning protection or contact a qualified lightning-
protection consultant.
In vehicle applications, the earth ground lug should be firmly attached to the
vehicle chassis with 12 AWG wire or larger.
In laboratory applications, locating a stable earth ground is challenging, but still
necessary. In older buildings, new Vac receptacles on older Vac wiring may
indicate that a safety ground exists when, in fact, the socket is not grounded. If a
safety ground does exist, good practice dictates the verification that it carries no
current. If the integrity of the Vac power ground is in doubt, also ground the
system through the building plumbing, or use another verified connection to earth
ground.
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