Laryn, The simple, and most practical solution, is to use a 3PDT AC relay to switch between normal and emergency sources. Feed the coil of the relay from your normal source, and wire the hot, neutral, and ground from the normal source to the NO contacts, and the hot, neutral, and ground wires from the emergency source to the NC contacts. The common (swinger) leads go to your repeater. This way, the relay normally stays energized, and the now-closed NO contacts feed your repeater. When the normal buss fails, the relay relaxes and closes the NC contact to connect your repeater to the emergency buss. Voila!
It is important to switch all three power leads, not just the hot lead, because neutrals from separate sources cannot be made common- especially when a separately-derived power source is involved- and the equipment grounds must guide any fault current back to the source via the most direct route, which is alongside the respective hot conductor. You might discuss this issue with the hospital electrician, since the critical buss may already be supplied from the normal buss during normal operation. If that's true, the proposed relay scheme is redundant. But, before you do anything, find out what caused that tripped circuit breaker- there may be a dangerous condition that must be fixed. 73, Eric Lemmon WB6FLY -----Original Message----- From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Laryn Lohman Sent: Sunday, July 13, 2008 5:18 PM To: Repeater-Builder@yahoogroups.com Subject: [Repeater-Builder] Switching a Repeater Betwen AC Sources We have two repeaters, plus an IRLP computer, on one emergency-fed circuit at a hospital. There are normally no problems with this. During a recent storm, the AC panel circuit breaker tripped, taking everything down in the middle of our Skywarn net. There are two receptacles near our equipment. One is normal power, the other is the red Critical Power receptacle. What problems would anyone see if we would feed everything from the normal power circuit, and if it would ever trip off, switch to the red receptacle. That way, if lightning trips the normal circuit, we would instantly feed our equipment from the red receptacle. This sounds so simple, and I'm inclined to build such a setup, but am I missing something obvious that could cause problems? Any better ideas? Laryn K8TVZ
