Campbell Scientific CR3000 CR3000 Micrologger - Page 92

Section 7. Installation PortSet() is a measurement task instruction. Use it when powering analog input sensors that need to be powered just prior to measurement. A 12-Vdc switching circuit, designed to be driven by a digital I/O port, is available from Campbell Scientific and is listed in the appendix Relay Drivers (p. 565). Note The SW12V terminal supply is unregulated and can supply up to 900 mA at 20°C. See table Current Source and Sink Limits (p. 89). A resettable polymeric fuse protects against over-current. Reset is accomplished by removing the load or turning off SW12V for several seconds. 7.5 Grounding Grounding the CR3000 with its peripheral devices and sensors is critical in all applications. Proper grounding will ensure maximum ESD (electrostatic discharge) protection and measurement accuracy. 7.5.1 ESD Protection ESD (electrostatic discharge) can originate from several sources, the most common, and most destructive, being primary and secondary lightning strikes. Primary lightning strikes hit the datalogger or sensors directly. Secondary strikes induce a 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 CR3000 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. 93), power ground and signal grounds have independent paths to the ground lug. Nine-pin serial ports are another path for transients. Communications paths, such as telephone or short-haul modem lines, should be provided with spark-gap protection at installation. Spark-gap protection is usually an option with these products, so it should always be requested when ordering. Spark gaps for these devices 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 6- to 8-foot copper-sheathed grounding rod driven into the earth and connected to the CR3000 Ground Lug with a 12-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. 92