Wet & Dry Contact

The dry and wet contact terminology is often used when talking about an interface to some sort of control system (which might be a computer, a plc, or something else).

If dry you have the opportunity to use a dry contact, it means that it is not supplied with any voltage from the device you wish to connect to. Your device supplies the voltage to the dry contact, and thus you can choose the voltage to use (as long as it is within certain limits).

In this context, a wet contact is one that is supplied with a voltage by the device which you wish to connect to. Thus, your device has to make use of the voltage supplied by that device--you can't choose just any voltage--when that wet contact closes, a certain voltage will be supplied to your device. Thus, your device must be set up to work properly with that voltage.

Of course, your system could supply a contact to that other system, and then the description above becomes reversed--your system's contact is wet or dry, and the other system either gets to use a voltage of their designer's choice (if you supply a dry contact), or is forced to use the voltage of your choice (if you supply a wet contact).

Hmm, I hope that is reasonably clear.

The terms wet and dry contact are also used for other purposes, some of them almost obsolete. In the good old days, some relays or switches used liquid mercury as part of the switching device. At that time, a device that used mercury was known as a wetted contact, and one that didn't was a dry contact.