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Watlow Series 93 Appendix ■ A.1
Appendix
Noise and Installation Guidelines
For wiring guidelines, refer to the IEEE Standard No. 518-1982, available
from IEEE, Inc. 345 East 47th Street, New York, NY 10017.
Noise Sources
• Switches and relay contacts operating inductive loads such as motors,
coils, solenoids, and relays, etc.
• Thyristors or other semiconductor devices which are not zero crossover-
fired (randomly-fired or phase angle-fired devices).
• All welding machinery and heavy current carrying conductors.
• Fluorescent and neon lights.
Decreasing Noise Sensitivity
• Physical separation and wire routing must be given careful consideration
in planning the system layout. For example, ac power supply lines should
be bundled together and physically kept separate from input signal lines
(sensor lines). A 305-mm (12-inch) minimum separation is usually effec-
tive. Keep all switched output signal lines (high power level) separate from
input signal lines (sensor lines). Cross other wiring at 90° angles whenev-
er crossing lines is unavoidable.
• Look at the system layout; identify and locate electrical noise sources such
as solenoids, relay contacts, motors, etc. Route the wire bundles and cables
as far away as possible from these noise sources. Don't mount relays or
switching devices close to a microprocessor controller. Don't have phase
angle-fired devices in the same electrical enclosure or on the same power
line with the controller.
• Shielded cables should be used for all low power signal lines to protect
them from magnetic and electrostatic coupling of noise. Some simple
pointers are:
◊ Whenever possible, run low-level signal lines unbroken from signal
source to the controller circuit.
◊ Connect the shield to the controller circuit common at the controller end
only. Never leave the shield unconnected at both ends. Never connect
both shield ends to a common or ground.
◊ Maintain shield continuity at daisy chain connection points by recon-
necting the broken shield.
◊ Assume no electrostatic shielding when using the shield as a signal
return. If you must do this, use triaxial cable (electrostatically shielded
coaxial cable).
A
Appendix