> Lew Hodgett wrote: > > >From a safety stand point, latching relays are a disaster waiting to > happen. >>>>>>>>>>>>>SNIP <<<<<<<<<<<<<<< > > Why exclude latching relays? SAFETY. > > If you loose power, the only safe way is to reset every thing to the off > position and start over. > > A relay with a basic holding circuit accomplishes this by providing > under voltage protection. REPLY Why indeed? Lew you bring up an very contemporary topic. Also a hotly debated issue. <VBG>
It used to be devices were designed to FAIL SAFE. Especially in motor controls etc. Thomas Kobelt of Kobelt Engineering gave a presentation to the Vancouver chapter of Marine Engineers. He mentioned that IMO had begun a shift to a new and very different concept. called FAIL in MODE. It was obvious from the audience reaction they had widely divergent opinions on the subject. What is the difference you might well ask? Fail safe neans if a failure occurs the device reverts to a safe configuration. An example of that is a spring loaded relay that pulls the contacts open when power fails. When motion control is involved this is probably the best and safest approach. However with the advent of digital control we have seen a number of situations where a momentary failure in a system might cause the controls to change from the mode they were currently in. As a result the momentary failuer caused the digital controls to CHANGE whatever they were controlling to some other state. This could be a problem in some cases. Take autopilot and engine throttle controls as examples. Suppose there is a momentary failure in the power supply to the device. In "fail safe" the devices revert to zero. This could mean the engines went from 3/4 throttle to no throttle. Or in gear to out of gear. In the case of the autopilot if the helm was turned over slightly to one side to compensate for wind and current, and then the power failed, the rudder would be returned to midships. This may or may not be a good thing depending on outside factors of wind and waves. FAIL in MODE on the other hand means the controls would stop in whatever position they were in immediately before the power failure. When power is resumed the controls woudl be exactly where they had been left. . What I had mentioned to Norm was using latching relays for cabin LIGHT CIRCUITS. Lights are seldom toggled on or off very often. And when they are ON or OFF you really do not want an additional drain to keep the relay coil energized.in that state. It could also be argued that a FAIL in MODE is better than a FAIL SAFE whereby th lights go out and stay off after the power is resumed. Having worked in automated machine control myself; I fully agree that when talking about factory process and movement control then a FAIL SAFE method is the only way. The last thing you want to happen is for a conveyor belt or pump or press to start up when the power is suddenly and unexpectedly restored. Except for the start surge if all the lights are left turned on, there is little harm in letting them resume after the power operation. On the other hand, configuring the lights to drop out and stay off even when the power resumed (such a a momentary outage from for example a reclosure in the utulity power during a storm) is not good either. Which just goes to show that it becomes problematic when amateurs begin to delve into sophisticated electrical systems as a DIY project. Norm lives off grid and relies partly on solar panels for his electrical power. To me it doesn't make sense to waste electrical energy on a relay coil that may stay in one mode for many hours. For such light circuits, a latching or bi-stable relay makes sense. For motor controls, no way! I fully agree that they pose an unacceptable safety risk. Let the debate rage on. <VBG> Arild _______________________________________________ Liveaboard mailing list [email protected] To adjust your membership settings over the web http://www.liveaboardnow.org/mailman/listinfo/liveaboard To subscribe send an email to [EMAIL PROTECTED] To unsubscribe send an email to [EMAIL PROTECTED] The archives are at http://www.liveaboardnow.org/pipermail/liveaboard/ To search the archives http://www.mail-archive.com/[email protected] The Mailman Users Guide can be found here http://www.gnu.org/software/mailman/mailman-member/index.html
