Chris, Think it is not a good idea to recommend Mercury wetted relays as mercury is already forbitten in many countries or will be forbitten in the near future.
Telecom or Signal relays with gold platted contacts which are well protected >from environmental impacts are able to handle dry circuit loads reliable. So far I have experience up to 250 million operations. As Chris mentioned it might be critical when you have to handle both dry circuits and high loads at the same time. In order to cover this situation we measure the contact reistance during all our electrical endurance tests - independent on the load on the contatcts - with a dry circuit signal (30mV/10mA). This covers the mixed load situation, when high loads and low loads are applied on the same set of contacts. Werner Dr. Werner Johler Technoloy Manager Tel.: +41 1 782 9151 FAX: +41 1 782 9000 Email: [email protected] Von: Chris Maxwell [mailto:[email protected]] Gesendet: Montag, 23. Februar 2004 19:49 An: Cereceres, David; [email protected] Betreff: 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
