Hi Richard: The original question was in regard of rating markings for a product with two power inputs. I really didn't want to expand the discussion to ALL of the safety issues related to multiple power inputs -- because all of the other issues had not been raised by the original question. > ...for an equipment with two power > cords (etc.) there should be no hazard > when the equipment is plugged into both > supplies. Agreed. > ...the unplugged supply cord must not > be live... Agreed. > ...if the equipment consumes so much > juice that the load must be shared > between two cords then... Agreed that equipment that consumes too much power can split that power among two sources. > ...then there had better be a way for > the equipment to detect that only one > cord is connected so that it reduces > the load to a level that can be safely > carried by that cord... Agreed. A less expensive way to do this (and not require fail-safe detection) is for each input to supply independent circuits within the product. > So now we come to when we can use > instructions as a safety mechanism. Agreed that instructions can be used as a safety mechanism. > One could say that the purpose of a > rating plate is to ensure that the > requirements of the load are known > so that they can be matched to the > capabilities of the supply (and any > supply cord not fitted as part of > the equipment). This raises the question as to the SAFETY purpose of product ratings. I agree that, if the cord is not fitted as part of the equipment, the purpose of the ratings is to provide data for the selection a suitable supply cord. However, this requires the user to be cognizant of the relationship between equipment ratings and power cord characteristics. To me, this places an undue burden on the user as compared to a simple statement as to a suitable cord or cords. If a cord is supplied with the product, then the plug at the end of the cord detemines the supply to which the product can be connected. If a cord is supplied with the product AND the plug fits into the socket we have at hand, we NEVER (well, hardly ever) check the product ratings before plugging the cord into the socket. I take a different point of view of the safety purpose of ratings. I suggest that the SAFETY purpose of ratings is to determine that the primary circuit component ratings equal or exceed the marked input ratings. > For equipment with redundant > supplies the full load could be > taken by either cord and the rating > information and selection of cord > must reflect this. Agreed. > For equipment having two cords > because the load is too excessive > for one supply then it is necessary > to determine the highest power down > each cord and then mark that value > for each cord. Agreed. I attempted to say this in one of my previous messages. > Now, it may be that the maxima for > one cord does not occur at the same > operating condition as that for the > second cord. Indeed, and a likely scenario. > If the manufacturer wants to give > accurate information regarding the > maximum power consumed by the > equipment then this should be done > in the instructions or on an enlarged > rating label. Of course. There is no restriction as to what data can be included in the ratings label. ;-) Best wishes for the New Year, Rich 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
