ECN Forum Forums General Topics of the Trade General Discussion Area Dry Contacts

A dry contact is a reference to a device contact that does not supply voltage. basically a switch. I think the definition reads along the lines of "One through which no DIRECT current flows.
Hope this is helpful.

http://www.leachintl2.com/english/english2/vol6/properties/how4.htm
http://www.leachintl2.com/english/english2/vol6/properties/00047.html

These two links discuss dry contacts.

The following reply from ggardiner tends to
make a little sense to me:

"A dry contact is a reference to a device contact that does not supply voltage. basically a switch. I think the definition reads along the lines of " One through which no direct current flows".

Could one example be the n/o or n/c contacts on some motor starters etc???

Ahem...throat clearing...

The IEEE Standard Dictionary of Electrical and Electronics Terms Centennial Edition states:

dry contact. One through which no direct current flows.

wet contact (telephone switching systems). A contact through which direct current flows. Note: The term has significance because of the healing action of direct current flowing through contacts.

Al

And so when 'dry contacts' are specified, is that a type of relay or a circumstance of the installation?

I would relate "dry" contacts to the auxiliary contacts (NO & NC) that you can add to a contactor that operate when the contactor operates, but does not provide current from the device it's attached to. It acts as only a switch for another separate circuit. A pressure switch is a great example of this. The action that does the switching is mechanical, but unless an electrical circuit is connected through the "dry" contacts, there is no current flow when the switch closes. I can't think off-hand when an auxiliary contact on a contactor is "wet"

I checked Google, and found many references to electronic items available with "dry or Hg wet (or wetted) contacts." Hg is the symbol for mercury.
The wet contacts are capable of carrying higher current than the dry contacts, and the contact surface (mercury) "replenishes itself" so there is not a problem with arcing and pitting.

and I won't sit in the corner all day!

In control and instrumentation systems the terms dry and wetted contacts describe where the power (voltage) source is electrically located.

In a dry contact the source is in a different area than the contact. In fact the load voltage may even be from a different electrical system than the contact control voltage, such as in relay circuits. Stand alone (isolated) contacts are always "dry".

In a wetted circuit the source is located in the same electrical area as the contact. This is similar to a time clock where the contacts share the same hot conductor as the motor. Solid state (transistor) devices are additional examples. Grouped contacts that share a common hot connection point are always "wetted".

JBD,
yes well.....
that's how i understood it, especially from the alarm guys point of view.

Certs is a breath mint! No, Certs is a candy mint!...Wait a minute. You're both right.

I think possibly all of these explanations have been correct.
Steve & JBD's "auxiliary contact" certainly makes sense for the application here. (Example= full voltage motor starter. When an aux. contact is used to control another device it would be dry. When used as a holding contact it would be wet)
Neither AL nor IEEE publish balderdash, so the DC explanation must apply.
My Stepdad taught me about dry/wet contacts as I explained them in about 1965. My answers are verifiable also, and still apply.
Anybody agree?