In this article, we provide clarifications on two terms often encountered when using relays for switching.
There are two terms about using relays for switching that you may have encountered. One term is "dry" and the other is "wet." Where these two words come from is beyond my understanding, but rest assured that those two words do NOT refer to the following:
Please therefore consider a clarification of terms. If you have a relay which has its switching contacts closed and in consequence of the actions of some other nearby goodies, those contacts carry some level of current that may rise from zero up to some value, may go back to zero again and so forth while the contacts themselves remain closed, that is NOT switching. It is "carrying" and that is not the topic at hand. If however, those nearby goodies which are bringing about a current flow through the relay contacts and those contacts are opened and closed in some way so that they interrupt that current and then allow current to flow again, that is "switching" and that IS the topic at hand. "Dry" switching means that the goodie current being switched is "negligible", that the current being interrupted and resumed is negligibly small. What constitutes negligibility may be open to debate or even to dispute, but one relay company I came across defined dry switching for their products as applying for contact currents of no greater than 1 mA. "Wet" switching, by the obvious reasoning, means that the goodie current is substantial where that word might mean an ampere or more, perhaps a tenth of an ampere or more, perhaps 10 mA or more , but you get the idea. Again, this may be open to debate and dispute as well. The choice of contact materials makes the difference between contacts that are suitable for wet switching service or for dry switching service. Relays rated for wet service will have contact materials that can withstand arcing when their contacts open up or when they bounce a bit when closing. Not only can they withstand that arcing, they can actually depend on that arcing as a contact cleaning mechanism. In fact, a relay that is rated for wet service that gets put into dry service can have its contacts develop surface contamination that may over the course of time interfere with low resistance contact closure. If that happens, you have an operational failure mechanism. We don't want that. Conversely, relays rated for dry service will maintain their low resistance contact closure capability as long as they never get called upon to do wet switching. Not even once! Gold is often used as the contact material and that material is set up to remain clean without having to depend on arcing. Reed relays for example, actually have their dry rated contacts sealed up inside of a glass tube filled with inert gas. Thus, if you have a dry switching rated relay that you use for wet switching, that relay may (or may not) suffice for wet switching purposes, but it will no longer be suitable for dry switching service afterward. If you try to use that relay again for dry switching, you will be setting yourself up for an operational failure. This raises a point about equipment maintenance and repair. If you have a plug-in relay whose contacts are rated for both wet or dry switching service and then in the course some equipment maintenance or repair, you allow this to happen:
Contact number 2 having first served for wet switching may very well fail if moved over to the dry circuit, its suitability for dry switching having been lost. Be sure that a relay like this is kept in its original orientation for the entire service life of the product.