Campbell Scientific CR10 CR10 Measurement and Control - Page 198

SECTION 14. INSTALLATION AND MAINTENANCE 14.7 GROUNDING 14.7.1 PROTECTION FROM LIGHTNING Primary lightning strikes are those where lightning hits the datalogger or sensors directly. Secondary strikes occur when the lightning strikes somewhere near the system and induces a voltage in the wires. The purpose of an earth ground is to minimize damage to the system by providing a low resistance path around the system to a point of low potential. Campbell Scientific recommends that all dataloggers in use in the field be earth grounded. All components of the system (datalogger, sensors, external power supplies, mounts, housings, etc.) should be referenced to one common earth ground. Every terminal on the Wiring Panel, with the exception of ground (G) and analog ground (AG)terminals are spark gapped. The spark gaps willfire at 150 V and the current will be diverted to ground. As shown in Figure 14.7-1, the power ground and analog ground are independent lines until joined inside the CR10. The fuse shown in Figure 14.7-1 (located on the underside of the Wiring Panel) is a 30 AWG wire, equivalent to a conventional 5 Amp fuse. It will blow if a sufficient transient comes in on the G or AG lines. at which time the current is SENSOR SHIELDS,+ REFERENCE FOR PULSE COUNTERS OR BINARY INPUTS RETURN FOR POWER TO EX1ERNAL DEVICES. directed away from the CR10 through the diodes. The fuse may be replaced by soldering another 30 AWG wire to the soldering pads provided. A modem/phone line connected to the Wiring Panel provides another path for transients to enter and damage the CR10. Campbell Scientific's DC112 phone modem has spark gaps on the phone lines. A 12 AWG wire should be run from the modem ground terminal to the earth ground. Additional protection is provided by the ground (Pin 2) of the 9 pin Serial l/O which is tied to power ground on the Wiring Panel. The transient protection designed into Campbell Scientific's equipment is meaningless if a good system earth ground is not provided. lt is the users responsibility to provide this earth ground. In laboratory applications, locating a stable earth ground is not always obvious. ln older buildings, new cover plates on old AC sockets may indicate that a safety ground exists when in fact the socket is not grounded. lf a safety ground does exist, it is good practice to verify that it carries no current. lf the integrity of the AC power ground cannot be verified, it is better to ground the system to a massive metalobject such as a steelwater pipe. WIRING PANEL +12V SINGLE ENDED REFERENCES AND EXCITATION RETURNS so ewc VREJ EQUIVALENT TO 5 AMP FUSE EXCITATION PORT EXCITATION VOLTAGE EXCITATION CONTROL EARTH GROUND FfGURE 14.7-1. Wiring PanelGrounding Diagram and Excitation Control 14-6