Appendix 1 28 Noise Sources; Decreasing Noise Sensitivity - Watlow 945 Series User Manual

Noise Guidelines
28
WATLOW Series 945 User's Manual
Appendix 1
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, A.C. power supply lines should be
bundled together and physically kept separate from input signal lines (sensor
lines). A 12" (305 mm) minimum separation is usually effective. Keep all
switched output signal lines (high power level) separate from input signal
lines (sensor lines). Cross other wiring at 90° angles whenever crossing 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 control. Don't have phase angle-
fired devices in the same electrical enclosure or on the same power line with
the control.
• Shielded cables should be used for all low power signal lines to protect from
magnetic and electrostatic coupling of noise. Some simple pointers are:
◊ Run low level signal lines unbroken from signal source to the control circuit.
◊ Connect a shield to the control circuit common at the control end only.
Never leave shields unconnected at both ends or connect both shield ends
to a common ground.
◊ Maintain shield continuity at daisy chain connection points by reconnecting
the broken shield.
◊ Assume no electrostatic shielding when using the shield as a signal return.
If you must, use triaxial cable (electrostatically shielded coaxial cable).
Appendix