There's another, different but related definition that I'd like to offer. The term is usually used to refer to small-signal situations. If the current and voltage are too low, they are not going to reliably punch through oxides on the contacts. In that case, if the relay is already provided, you need to provide enough voltage and current to ensure good contact (to "wet" the contacts). In the opposite situation, where you know you are going to be switching very small currents, you need a relay with gold-plated contacts(typically). Those small-signal relays are often referred to as having a "dry" rating.
Note that these are mutually exclusive situations: one relay cannot do both jobs. The first time a gold-plated relay has to make or break sufficiently high current, the gold burns off and you no longer have a relay capable of reliably switching very small currents. Another way to think of this is that relay contacts have both maximum and minimum ratings. Dry contacts can also refer to a situation where contacts are provided but no voltage or current is provided - the user connects the contacts to whatever he intends to use them for, within the ratings of the relay. Typically this will be for things like alarm/annunciation circuits. This could well be a misuse of the term. Jim Eichner, P.Eng. Compliance Engineering Manager Xantrex Technology Inc. phone: (604) 422-2546 fax: (604) 420-1591 e-mail: [email protected] web: www.xantrex.com Confidentiality Notice: This email message, including any attachments, is for the sole use of the intended recipient(s) and may contain confidential and privileged information. Any unauthorized review, use, disclosure or distribution is prohibited. If you are not the intended recipient, please contact the sender by reply e-mail and destroy all copies of the original message. From: Chris Maxwell [mailto:[email protected]] Sent: Monday, February 23, 2004 10:49 AM To: Cereceres, David; [email protected] Subject: RE: dry circuit questions David, Dry contact relays are named because of what they don't have. Many high current relay designs feature contacts that are wetted with Mercury in order to prevent damage from arcing during make/break cycles. Dry contact relays have no such protection. As such, they are usually rated for lower currents than wet contact relays. Dry contact relays are prone to having their contacts weld together if they are used in situations where there is either: excessive in-rush, excessive steady state current and/or excessive inductance in the load (causing showering arcs during opening). For either type of relay, you must read the specification sheet thoroughly and understand the switching curves (many spec sheets provide a curve which shows the maximum switched current versus voltage for the relay). Be sure that your application is within the relay's specs. Otherwise, you will end up with welded contacts. Some types of dry contact relays have extra wide contact gaps in order to extinguish the arcing and prevent damage and/or welding without the use of wet contacts. American Zettler used to have some good information in their catalogs which explained this in better detail. Chris Maxwell Design Engineer Nettest From: [email protected] [mailto:[email protected]] On Behalf Of Cereceres, David Sent: Monday, February 23, 2004 1:26 PM To: '[email protected]' Subject: dry circuit questions Hello Group, I was forwarded the following question from a fellow engineer. I was wondering if anyone knows of any reference material pertaining to Dry Circuits and what constitutes a Dry Circuit. "When you see a reference to either 'dry contact' or 'dry circuit' as far a relay goes, what does this mean the relay is designed to handle?" As always, your guidance is always appreciated. Regards, David Cereceres Product Safety Engineer Pelco Phone:559-292-1981 ext.3493 Fax: 559-294-3707 mailto:[email protected] <mailto:[email protected]> http://www.pelco.com/ <http://www.pelco.com/> This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ieee-pses.org/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ieee-pses.org/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
