Interface between MCC and control system


Thread Starter


I need to know the best way to exchange descrete signals (contacts) between MCC (motor control center) and control system I/O. The dry (voltage free) contacts from MCC powered by control system`s power supply or wet (with voltage on it) contacts powered by MCC power supply? Which one is better? Any idea?

Matthew Hyatt

Dry, lower voltage to work with and overall cheaper cost, plus you don't bring another source of control power into your enclosure. Of course this is not to say dry is better, both have their + and - aspects, your choice. Usually, with dry you will not need interposing relays.

I think in both cases it is more preferable to use the dry (voltage free) contacts.

You may need to add suitable relays that the power from either MCC or controller can energize to close or open the relay contacts as the case may be upon arrival of signal and subsequently interfaces to the MCC or the controller.

Contact me at [email protected] for a possible sketch.


Alan Hartwell

Either method will work fine, but they both have different considerations.

In your "wet" case, if you have individual control transformers in your starters, you need to have isolated inputs in your I/O system if you run two or more separate sources of power to an I/O module. This is necessary to comply with electrical codes. Sharing a module between different MCC control transformers would create the code violation of multiple grounding points on the neutral of the MCC control transformers.

If you have only a single control supply for the entire MCC then the above does not apply. Inputs modules with common terminals could be used, and this case would be the simplest and cheapest.

In your "dry" case, ideally you should route the external I/O power through an auxiliary contact on the MCC bucket's disconnect switch. This removes all power inside the MCC enclosure when it is being worked on. An alternative allowed in the code is to have a sign on the starter stating EXTERNAL POWER SOURCE.
Dear Fred,

Its always better to use the dry contacts with the constrol system power supply instead of the wet ones of MCC power supply. This is what i have been doing and what i have seen in most ind. applications.