In a message dated 5/29/01, Dan Roman writes: > I am looking for a relay that can be used without restriction for a fail > over function between CO trunks and station sets in a PBX type application. > The fail over relays switch select CO trunks over to individual phones > should the PBX fail or lose power. The coil to contact insulation must > protect SELV from TNV-3 and the contact to contact insulation must protect > TNV-2 (which is not isolated from SELV) from TNV-3.
Hi Dan: Sounds like what you want is a relay that provides "supplementary insulation for a working voltage of 250 volts" between open contacts. I've started down that path before, but have usually found another solution before I had to actually source such a relay. Back in 1993 I looked at the Omron G5H2 relay, which provided supplementary insulation between open contacts. The internal design of this relay was an engineering marvel. The relay footprint was 12 mm by 27 mm, so it was larger than most telecom relays, and it was more expensive. I have heard that this relay may have been discontinued, but it was certainly a nice part if you had to have supplementary insulation between open contacts. In practice, I have found that the requirement for supplementary insulation between open contacts can usually be waived by exercising certain clauses in EN 60950. In particular, the exemptions involving a permanent connection to earth ground are very useful for some types of equipment, including PBXs. You might look into this approach some more before you go crazy trying to source a special relay. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 http://www.randolph-telecom.com
