Ron Pickard commented (questioned) ** "Also, an interesting paragraph exists in 5.2.2. It states "Equipment designed for multiple (redundant) power supplies shall be tested with only one supply connected." How would one interpret this? Only one supply connected and then the other, and then add the currents?" **
I will not specifically comment on Ron's question but will pose another. One of the problems we have today is that many products have multiple power supplies that are not always considered redundant. IEC TC74 changed the IEC60950 3rd Edition text to ?clarify? this requirement. All the IEC60950 3rd Ed. based standards now read: Equipment which is designed for connection to multiple power sources, only one of which is required at a time (e.g. for backup) shall be tested with only one source connected. Equipment requiring power simultaneously from two or more power sources shall be tested with all power sources connected. Perhaps that in itself helps Ron but... this text was debated prior to release because it is still not clear in its intent for some applications. There are at least 2 prevailing opinions (perhaps there are more). Multiple AC connections are common with the use of smaller power supplies in larger quantity within a products that might have 1 power supply providing backup for several "required" power supplies (2+1 or 3+1 protection schemes). Many designs included AC filtering at each power supply and overall product leakage currents (touch and protective earth currents as they are now known) are perhaps increasing along with the number of mains connections. Now we approach this from at least 2 ways .... Opinion 1 - You should test the system with all the "required" power supplies collectively by testing the loop from all the AC mains to any accessible collective earthing connections. Effect = Sum the touch/PE currents from each required source and the sum of all required sources shall not exceed the limits. Opinion 2 - With more than one power supply required to be connected, then the product has multiple "required" earthing connections. Test for the touch/PE currents from each power supply with the other supply connected to the mains (including its earthing connection). Any measurable touch/protective earth connection would flow through the 2nd earthing path. Effect = The product's chassis is earthed through the second required power supply, the measurable touch/PE currents will be negligible and the test quite easy to pass regardless of the limits. So... Is the requirement intended to capture and limit the increasing leakage currents as the number of power supplies per product grows. OR is the requirement intended to give credit (and latitude) to a product that has required multiple earthing connections? It will be interesting to see where this goes as the 3rd Edition standard's use becomes more widespread. Bob Griffin Compaq Computer Corp. [email protected] -----Original Message----- From: David Gelfand [mailto:[email protected]] Sent: Monday, 22 January, 2001 12:25 PM To: [email protected] Subject: Re: Looking at leakage current specs As I understand it, a truly redundant power supply system means that only one supply is connected and working at a given time. In this case you can measure the leakage current from only one supply. Our system uses two power supplies that share the load, if one dies, the other can maintain the full load. In this case the leakage current is measured with both supplies connected. I think the way to approach this is, what happens if there is no protective earth? In test 5.2.3 and 5.2.4, there is no permanent earth connection to the EUT. Any current leaking to the chassis can be shunted to ground through someone touching it. This current must be less than 3.5 mA. My CSA rep accepts the permanent ground connection, but management does not like the warning. We are also working with the manufacturer to reduce the leakage current on each supply. Best regards, David. ----- Original Message ----- From: "Ron Pickard" <[email protected]> To: "Gary McInturff" <[email protected]> Cc: <[email protected]> Sent: Friday, January 19, 2001 3:42 PM Subject: RE: Looking at leakage current specs Gary, >If this equipment has a chassis bonding connection like you find in >telephone line up equipment, would then it fall under the class B pluggable. >I can still remove power cords, but have a chassis to frame bonding wire in >parallel to that. (Yes, I know most of that stuff is DC but there is a small >number that is not). I hope to work with the power supply folks to reduce >the problem through filter design if I can, but I think this is an >interesting questions. What do you think? Pluggable Type B is a term which only refers to an industrial type power plug. Having a permanently connected earthing connection may provide the relief from the excessive leakage currents you're experiencing. Such a connection might be described in 6.3.3.2, 6.3.4.1 and 6.3.4.3.2 (yeah I know, these are telco references), but, they adequately describe the earthing conditions and requirements. As an aside (well, sort of) I could never understand why such a protective earthing connection is described for the protection of a telecom circuit, and not mentioned at all for the protection (leakage current, etc.) of an AC mains circuit. Maybe someone more knowledgeable than I can explain that. At any rate, pass this by your friendly NRTL for their interpretation of doing this. Also, an interesting paragraph exists in 5.2.2. It states "Equipment designed for multiple (redundant) power supplies shall be tested with only one supply connected." How would one interpret this? Only one supply connected and then the other, and then add the currents? Again, maybe someone more knowledgeable than I can explain this. Just trying to stir the pot. Best regards, Ron Pickard [email protected] ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: [email protected] Michael Garretson: [email protected] For policy questions, send mail to: Richard Nute: [email protected] ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: [email protected] Michael Garretson: [email protected] For policy questions, send mail to: Richard Nute: [email protected] ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: [email protected] Michael Garretson: [email protected] For policy questions, send mail to: Richard Nute: [email protected]
