Hi Rich, I did contemplate including some comments about regular users of the equipment, but chose not to for reasons of simplicity. Well now I can say what I was going to say...
Equipment should be designed so that it can be operated safely by staff who typically have little or no knowledge of hazards. Hence, for an equipment with two power cords (etc.) there should be no hazard when the equipment is plugged into both supplies. Equally, the equipment must be safe for such persons under conditions of foreseeable misuse. Therefore, the unplugged supply cord must not be live, e.g. because it is wired directly in parallel with the other supply cord. Likewise, if the equipment consumes so much juice that the load must be shared between two cords 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 (unless the cord is rated for the full output of the socket and you are relying on the building's protective over-current devices to trip). So now we come to when we can use instructions as a safety mechanism. For users it is normally not permitted to rely only on warnings, though there are some cases when such warnings are permitted (e.g. when the hazard is inherent in the function of the equipment, like an electric saw). However, service persons are expected to take greater care and follow instructions more assiduously: as such, I seem to recall that a warning is considered to be adequate by IEC 60950-1 (though I haven't looked up the relevant clauses). Of course, many companies would still want to protect service personnel and so they would fit covers/guards/etc. which would prevent accidental contact with hazards without perhaps going to the lengths required to protect operators. 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). 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. 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. Now, it may be that the maxima for one cord does not occur at the same operating condition as that for the second cord. However, this is irrelevant when it comes to matching the supplies to the load. 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. Happy New Year, Richard Hughes Rich Nute wrote: > > > > > Hello Richard: > > > > A significant safety issue is that, whether the equipment needs > more than one > > connection to fully function or not, equipment with more than one > connection > > to a supply can appear to be 'dead' when it isn't. [In fact, the > same can > > also be true of certain equipment types that have internal > batteries that cut-in > > when the external supply is turned off or disconnected.] > > Unplugging an item of equipment is generally considered to be a > good way of > > making it safe to maintain etc. - so an equipment may give the > appearance of > > being safe because its supply lead is laying next to the > equipment. However, if > > it has two power cords, then it may appear to be 'dead' when in > fact there > > are hazards inside. > > As I followed this thread, the original discussion > was in regard of rating markings for a main input > and for a backup input. > > Mr. Bacher suggested a warning was necessary, to > which I disagreed. The basis of my disagreement > was that of safety of the operator/user, not the > serviceman. > > You suggest that a warning is necessary for the > serviceman; from this, I take it that you hold that > a warning is not necessary for the operator? > > I would not agree to a hard rule that said that a > serviceman warning is always necessary for a product > with multiple power inputs. If the multiple power > inputs are adjacent or otherwise reasonably close > to each other, physically, then I do not feel that > a serviceman warning would be necessary. If the > multiple power inputs are remote from each other > such that it was not obvious from the physical > construction that there are two or more power inputs, > then I agree that a serviceman warning is warranted. > > > Following the above logic it will be seen to be irrelevant what > the reason > > for the dual supply cords is. In the case of equipment that needs > two supply > > connections to function fully, of course the equipment will not > operate. > > In regard of rating markings... > > 1) If the second input is for backup, then the > rating markings for the second input would be > exactly the same as for the main input. > > 2) If the second input is for functional purposes, > i.e., additional power, then the second input > rating markings would be based on THAT input, > and would be independent of the first input. > > > 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 > 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
