Watlow Series 920 Microprocessor-Based Ramping Control - Manual (Page 21)

Definitions
Ground Loop - A condition created when two or more paths for electricity are
created in a ground line, or when one or more paths are created in a shield.
Earth Ground - The starting point for safety and computer grounds. It is usually
a copper rod driven into the earth.
Safety Ground - A ground line run with electrical power wiring to protect personnel.
Computer Ground - A ground line for the ground connections to computers
or microprocessor-based systems. This line is isolated from safety ground.
Common Mode Line Filter - A device to filter noise signals present on both
power line legs with respect to ground.
Differential Mode Line Filter - A device to filter noise signals present
between the two power lines themselves.
The D
OS
and Don’ts of Clean Input Power
Do
keep line filters as close to the control as possible to minimize the area for
interference pick up.
Do use twisted pair wire and possibly shielded wire from line filters to the control to
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 (304.8mm) 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 separate from all other ground lines. This computer
ground line should 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 a 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 Install and Wire, Chapter 4 WATLOW Series 920 User's Manual 21