Emerson Process Management ControlWave EFM 3808 - Manual (Page 285)

Section 5 - Wiring Techniques Page 5-1 S1400CW
Section 5 - Wiring Techniques
5.1 OVERVIEW
This section provides information pertaining to good wiring practices. Installation of Power
and “Measurement & Control” wiring is discussed. Information on obscure problems,
circulating ground and power loops, bad relays, etc. is presented. Good wire preparation
and connection techniques along with problems to avoid are discussed.
5.2 INSTRUMENT WIRING
Each of the rules listed below is briefly discussed; the emphasis herein is placed on the
avoidance of problems as well as equipment safety.
Rule 1 - Never utilize common returns.
Rule 2 - Use twisted shielded pairs (with overall insulation) on all Signal/Control circuits.
Rule 3 - Ground cable shields at one end only.
Rule 4 - Use known good earth grounds (Rod, Bed, System) and test them periodically,
Rule 5 - Earth connections must utilize smoothly dressed large wire.
Rule 6 - Perform all work neatly and professionally.
Rule 7 - Route high power conductors away from signal wiring according to NEC Rules.
Rule 8 - Use appropriately sized wires as required by the load.
Rule 9 - Use lightning arresters and surge protectors.
Rule 10 - Make sure all wiring connections are secure.
5.2.1 Common Returns
Use of common returns on I/O wiring is one of the most common causes of obscure and
difficult to troubleshoot control signal problems. Since all wires and connections have
distributed resistance, inductance and capacitance, the chances of a achieving a balanced
system when common returns are present is very remote. Balanced systems (or circuits) are
only achieved when all currents and voltages developed in association with each of the
common returns are equal. In a balanced system (or circuit) there are no noise or
measurment errors introduced due to by “sneak circuits.”
The illustration of Figure 5-1 shows the difference between testing an I/O circuit that is
discrete and has no sneak circuits and one that utilizes common returns. Common sense
tells us that it is tough to mix up connections to a twisted shielded pair (with overall vinyl
covering) to every end device. Do yourself a favor; to make start up easier, DON’T USE
COMMON RETURNS!