Joe, So you want to know more about the Nordic deviations regarding EN 60950-1, well sit yourself down and get a mug of coffee as I explain the reasons behind these deviations…
In Scandinavia (we are told in the IEC by representatives from the countries mentioned in the deviations) it is common to find socket outlets that have an earth connection that is in fact not connected. Now, it has also been discussed that other countries have this situation too, but they have chosen not to adopt the same deviations as the Scandinavian countries [if you consider that this applies in your country and think that something should be done about it, then please contact your national committee]. Most equipment only has Basic Insulation between the Primary Circuit and Protective Earth and also has Y capacitors connected across this Basic Insulation. Now, you may think that this situation is questionable from a safety perspective, but it is referred to as a Class 0 installation. In the vicinity of the equipment everything is (supposed to be) earth free and so if you were to bridge between a such a part and a part that is connected to the mains by Y caps (i.e. anything connected to the Protective Earth connection of most ICT equipment) then no current (or only a small, depending on the insulation resistance) would flow and so you would not get an electric shock. Now consider a telecommunications system. It can, and is, touched by network engineers far removed from the customer’s premises. These engineers can be earthed and so it is important that they are effectively protected from parts that are only separated from the Primary Circuit by Basic Insulation. Following the classic approach, it should now be easy to see where the original requirement for Supplementary Insulation came from and why this was for a Primary Circuit and not a Secondary Circuit. Given that the ac mains supply has a tolerance of 10% and the peak of the ac mains is 1.414 times the rms value, we have a combined magnification factor of 1.5554, or 1.6 to be a bit on the safe side. A device, whether it be a gas tube or a semiconductor device, rated 300Vdc will break down when the mains is applied to it and so it will not protect the network engineer in the situation as described above. Of course, all devices have a tolerance and that is why the standard defines the MINIMUM value: it is for the manufacturer to choose a rated value and tolerance that will not break down when the mains is applied to it. I hope that I have explained why the insulation is for a Primary Circuit rather than for a Secondary Circuit (as could be expected because a TNV Circuit is a Secondary Circuit). Regards, Richard Hughes Safety Answers Limited www.safetyanswers.ltd.uk 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
