Watlow Series 942 - Manual (Page 50)

Appendix
50
WATLOW Series 942 User's Manual
Wiring Guide
• Another important practice is to 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 enclo-
sure 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:
◊
Whenever possible, run low level signal lines unbroken from signal source
to the control circuit.
◊
Connect the shield to the control circuit common at the control 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 reconnecting
the broken shield.
◊
Assume no electrostatic shielding when using the shield as a signal return.
If you must, use triaxed cable (electrostatically shielded coaxial cable).
• Twisted pair wire should be used any time control circuit signals must travel
over two feet, or when they are bundled in parallel with other wires.
• The size or gauge of wire should be selected by calculating the maximum
circuit current and choosing the gauge meeting that requirement. Using
greatly larger wire sizes than required generally will increase the likelihood of
electrostatic (capacitance) coupling of noise.
• Ground loops must be eliminated in the entire control system. There are
obvious loops which can be spotted by studying the "as-built" wiring diagram.
There are also the not-so-obvious ground loops that result from connecting
internal circuit commons in the manufacturer's equipment. An example is a
control circuit designed to work with a grounded sensor input.
• Do not daisy chain A.C. power (or return) lines, or output signal (or return)
lines to multiple control circuits. Use a direct line from the power source to
each input requiring A.C. power. Avoid paralleling L1 (power lead) and L2
(return lead) to load power solenoids, contactors, and control circuits. If an
application uses L1 (power lead)to switch a load, L2 (return lead) has the
same switched signal and could couple unwanted noise into a control circuit.
• Grounding the chassis of each piece of equipment in the system is very
important. Here is a simple practice that works best. 1) Connect each indi-
vidual equipment to the over-all chassis immediately adjacent to that piece.
2) Tie all major chassis ground terminals together with one lead (usually
green wire) tied to ground at one point. Don't connect ground to the control
case if the control is in a grounded enclosure (preventing ground loops).
• Do not confuse chassis grounds (safety ground) with control circuit commons
or with A.C. supply L2 (return or neutral line). Each return system wiring
must be separate. Absolutely never use chassis ground (safety) as a con-
ductor to return circuit current.