Emerson Process Management Series 9110, CI-9110, 9110-00A Transients and Interference

the equipment. Wiring from input signal circuits and power circuits should be separated as much as possible to minimize noise and transient pickup. Power and signal leads should be run in separate conduit to minimize inductive pickup.

oTerminal Lugs. The use of crimp-type terminal lugs as connections for screw terminals should be avoided. Terminal lugs, in many industrial climates, can be affected by hidden corrosion. It is preferable to tin the wire end with solder and loop it around the terminal screw. The screw should be tightened sufficiently to hold the lead in place but not excessively so that the lead is sheared or the screw is stripped. Equipment furnished with compression-type terminals include an opening for inserting tinned ends.

TRANSIENTS AND INTERFERENCE

The extensive use of low-power integrated circuitry in modern electronic equipment requires proper grounding techniques to insure reliable system operation. The following checklist will help identify some critical areas:

1.All instrumentation devices at the site should be checked so that no potential greater than the standoff voltage can exist within or between devices.

2.To minimize outside signal interference and provide equipment protection from lightning or transients, the earth ground at the site must be tested to insure that its impedance measures less than 10 ohms at 7 MHz. This qualification is essential since a transient potential or an interference signal at the instrument site can vary over the entire electromagnetic spectrum from DC to several hundred MHz.

Note that transients can be produced through natural phenomena and man-made conditions. Natural transients may result from lightning (7-14 MHz), static (many frequencies), and wind (DC charge and static). Man-made transients can result from defective light bulbs or electrical appliances, sudden electrical load shifts, inductive load surges, arcing contacts and poor AC power connections.

3.If radio frequency (RF) interference is present at the input of an instrument, observe if it has a consistent or irregular pattern. Constant interference can come from commercial radio stations, while irregular interference can come from private stations. Although shielding and grounding will eliminate or minimize most cases of RF interference, obstinate cases may require attenuation filters.

RF interference can also be caused by power companies that apply modulated RF signals to power lines to communicate data. Other RF noise sources include digital clocks, computers, relay contacts, motors transformers, switches, arc welders, etc.

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