Watlow Series 94 - Manual (Page 26)

A . 2 ■ A p p e n d i x W a t l o w S e r i e s 9 4
Appendix
• Use twisted pair wire any time control circuit signals must travel over two
feet, or when you bundle them in parallel with other wires.
• Select the size or gauge of wire by calculating the maximum circuit current
and choosing the gauge meeting that requirement. Using greatly larger
wire sizes than required generally increases the likelihood of electrostatic
(capacitance) coupling of noise.
• Eliminate ground loops in the entire control system. You can spot the obvi-
ous loops by studying the “as-built” wiring diagram. There are also not-so-
obvious ground loops resulting from connecting internal circuit commons
in the manufacturer’s equipment.
• Do not daisy chain ac 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 ac 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 cir-
cuit.
• Tie all 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 com-
mons or with ac supply L2 (return or neutral line). Each return system
wiring must be separate. Absolutely never use chassis ground (safety) as a
conductor to return circuit current.
Eliminating Noise
• Use “snubbers” (QUENCHARC™ P/N: 0804-0147-0000) to filter out noise
generated by relays, relay contacts, solenoids, motors, etc. A snubber is a
simple filter device using a 0.1µf, 600 volt, non-polarized capacitor in
series with a 100
Ω
, 1/2 watt resistor. The device can be used on ac or dc
circuits to effectively dampen noise at its source. Refer to output wiring in
Chapter Two for proper Quencharc installation.
• The ultimate protection is an “uninterruptable” power supply. This “sens-
es” the ac power line; when the line fluctuates, a battery powered 60Hz
inverted circuit takes over, supplying power within one-half to one cycle of
the ac line; very expensive.