Campbell CR800 Specifications download pdf (Page 59)

Section 1. Installation and Maintenance

1-7

CR800

Panel

+12V

FIGURE 1.6-1. Connecting CR800 to Vehicle Power Supply

1.7 CR800 Grounding

Grounding the CR800 and its peripheral devices and sensors is critical in all

applications. Proper grounding will ensure the maximum ESD (electrostatic

discharge) protection and higher measurement accuracy.

1.7.1 ESD Protection

An ESD (electrostatic discharge) can originate from several sources.

However, the most common, and by far potentially the most destructive, are

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 CR800 are protected with GDTs

or transient voltage suppression diodes. The 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. The power

ground and signal ground are independent lines until joined inside the CR800.

The 9-pin serial I/O ports on the CR800 are another path for transients to enter

and damage the CR800. Communications devices such a telephone or short-

haul modem lines should have spark gap protection. Spark gap protection is

often an option with these products, so it should always be requested when

ordering. The spark gaps for these devices must be connected to either the

CR800 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

CR800 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, consult the literature

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Campbell CR800 Specifications Page 59