Watlow Series 1500 - Manual (Page 30)

Installation-Wiring
Installation - Wiring, Chapter 3 30 WATLOW Series 1500 User's Manual
keep the line "clean."
Do keep low power control wires physically separated as far as possible from line
voltage wires. Also keep all controller wiring separate from other nearby wiring.
Physical separation is extremely effective. A 12 inch minimum separation is
usually effective.
Do use common mode, differential mode or a combination of the two filters wherever
power may have electrical interferences.
Do cross other wiring at 90
°
angles whenever crossing lines is unavoidable.
Do have a computer ground line that is separate from all other ground lines. This
computer ground line should ideally terminate at the ground rod where the
electrical service is grounded.
Don't connect computer ground to safety ground or any other ground points in the
electrical system, except at the ground rod.
Don't mount relays or switching devices close to a microprocessor control.
Don't run wires carrying line voltage with signal wires (sensor, communications or
other low power lines) going to the control.
Don't use conduit for computer ground.
Don't have phase angle-fired devices in the same electrical enclosure or on the
same power line with the control.
Don't connect ground to the control case if the control is mounted in grounded
enclosure (prevent ground loops.)
Don't fasten common mode line filters or filters with metal cases to metal that is at
ground potential. This prevents ground loops and maintains filter effectiveness.
How to Check for Ground Loops
To check for ground loops, disconnect the ground wire at the ground termination.
Measure the resistance from the wire to the point where it was connected. The
ohmmeter should read a high ohm value. If you have a low ohm value across this
gap, that means there is at least one ground loop present in your system.
Or check for continuity; your reading should be "open". If you do find continuity, you
must now begin looking for the ground loops. Begin disconnecting grounds in the
system one at a time, checking for continuity after each disconnection. When
continuity reads "open" you have eliminated the ground loop(s). Also as you recon-
nect grounds, keep making the continuity test. It is possible to reconnect a ground
loop.