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Supplement S1400 / 5
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.
o Terminal 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.