I've often heard fire alarm technicians use the term "dry contacts". At the risk of posting a dumb question I ask what are dry contacts.

Didn't use saline before you put them in your eyes?

Sorry, couldn't resist...

Dunno... An askeral (grease, noalox) to help prevent arcing?

Got me...

Frank,
they are a set of N.O. or N.C. contacts that are not energized, therrfor the term 'dry'

yaknow, if someone published a book of 'trade slang' i'd buy it

Sparky,

Got something close;

Dictionary of Comstruction Terms

Bill

I understood they were very low resistance contacts-like gold plated. I have run across
that requirement for alarm circuits in a/c units before. An official ruling would be helpful.

"Dry Contacts" refer to a set of electrical contacts that switch such low voltage/current that there is no "self-cleaning" effect caused by arcing/sparking during operation.

Contacts used in such applications should be made of precious metals (gold, rhodium, platinum, etc.) to ensure reliable operation, especially in a corrosive atmosphere where an insulating film might build up.

Well, let's see. A mercury switch is a "wet contact", so...
A "dry contact" is the one all of us are accustomed to seeing, with contact points.
The wet contacts don't wear out like the dry ones, and unless I'm wrong (as usual), are capable of carrying higher current for their relative size.
Scott 35, are you around?

I would say any set of "field contacts", such as those that are part of a pressure, or flow switch assembly that do not contain an internal power source.

The terms dry and wetted have nothing to do with the physical construction of the contacts.

A dry contact is a contact that receives power from a source. Examples include:
Thermostats, Pushbuttons, Doorbell buttons, relay contacts, door switches, and even standard light switches.

A wetted contact is a contact that provides power to a load. Examples include:
Time clock switches, Commercial float/pressure switches, and most low voltage solid state controls.

I'm missing the boat here in a big way.
Not to be sarcastic or disrespectful but:
All contacts receive power from a source.
No contact makes its own power.
All contacts supply power to a load.
The examples given by JBD could be reversed, and it would still seem to make just as much sense to me...none.
(I'm looking around the house for my dunce cap)

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?

Scott, i'm still
I think i've been terminologized......

How's this:

Dry Contact: has external power source...

Wet contact: has an internal power source...

Wetted contact: Mercury switch...

Does that make sense?