The Delta V manual suggests using an arc suppression device across a contact if using their discrete input modules. Can someone explain why they suggest to connect it across the contact rather than the plc input itself?

 

By applying the surge suppression device directly across the source of the inductive surge, the damping factor of the suppressor is maximized, as the resistance in the circuit is minimized. The transient overshoot seen by the Delta V hardware will therefore be minimized also.

 

Arc suppression with Metal Oxide Varistor and RC snubber is recommended by the vendor to protect devices being switched with line voltage. Typically vendor has already applied similar protection at the relay and specification is to protect load equipment, only. Recommendation is to confirm arc suppression available at control and load equipment and apply additional suppression where needed.

 

Responding to Anonymous' Tue, Feb 25, 5:21pm query:

Protection is considered in order to minimize radiated and conducted "noise" when the contact in question controls an inductive load! The most severe condition occurs when the current to the load is interrupted.

Protection, however, it is more of an "art" than a science. It can be done by placing components such as resistors, capacitors, diodes, zeners, varistors, etc, across the load, the contact, or both. In some cases it's equally effective at either location. Normally, trial and error is required. Solution is easier if the "L" and "R" of the load is known! But, empirical solutions are also available.

The major disadvantage of placing it across the load is that it will adversely affect the release time of the load! Conversely, placing it across the contact can have serious consequences depending on the "failure-mode" of the components, for example, if the capacitor of an
R-C circuit were to short.

The question to ask Delta V folks is, why is protection needed at all? Are the discrete input modules to be treated as inductive loads? Or,
they may be recommending such protection because the line length between contact and module are so long that their inherent "inductive" characteristics are a problem!

Regards,
Phil Corso, PE
Boca Raton, FL
[[email protected]] ([email protected]) {[email protected]}

 

By using a suppression device at the source of spark limits transients going in to other wires.
And the best place to eliminate a problem is definitely at the source.

Regards,
Sekar