Hi Chris,
As usual, Rich is correct that removing more then one ground connection would be an improper single fault test. However, Rich's comment... >A single-fault test is with one ground open (a meaningless >test when there is a second ground in place)... Is not quite accurate, the test is only meaningless if you have already verified that both grounds are fully contiguous throughout the equipment. I have once or twice ran across multiple ground connections in larger IT equipment where one of the grounds did not have continuity throughout the equipment. In that case, removing the good ground does expose you to a higher leakage current, as the second ground is poorly constructed and not functioning. For that reason, it is crucial that your verify ground performance for each ground, throughout the equipment, prior to single fault testing. Best Regards, Frank W. ---[From the computer of...]----------------------------------------- Mr. Frank West Sr. Engineer TUV Rheinland 7853 SW Cirrus Dr. Beaverton, OR. 97008 T 503-469-8880 Ext 205 F 503-469-8881 fw...@us.tuv.com Rich Nute <ri...@sdd.hp.com> To: chris.maxw...@nettest.com Sent by: cc: emc-p...@majordomo.ieee.org owner-emc-pstc@majordom Subject: Re: Secondary Grounding o.ieee.org 06/01/2001 01:55 PM Please respond to Rich Nute Hi Chris: Consider a product with two, independent protective grounding/earthing connections. This may be by means of two power cords (as is done for uptime reliablity by employing parallel power supplies) or by means of one power cord and a separate ground connection (as, for example, by mounting in a grounded rack). > Safety standards call for single fault testing. For Class I equipment, one > of the single fault test conditions is removal of the ground connection. Agreed. The disconnection of one ground is a single- fault condition. > I'm curious how most test labs would reconcile the two statements above. My > guess is that they would interpret removal of ground to mean removal of all > ground connections. So putting on an extra ground wire wouldn't help. It > would just make the safety engineer disconnect another wire to perform the > test. I don't agree. The requirement is that of a single- fault condition. If normal operation employs redundant grounding, then a single-fault condition is that of failure of one ground connection. > "What if the product is used in a building or environment with an unreliable > ground?" or "How can you garantee that the product's ground potential will > always be equal to the potential of the floor where the user is standing" If the ground within the building installation is subject to failure, then the fault is that of the building installation, not of the product. So, it would be nonsense to require a product single-fault "no-ground" test on that basis. A faulty ground in the building installation allows cumulation of leakage currents from all equipment to be available on each and every grounded equipment, a truly dangerous situation because the cumulative leakage current could be in the hundreds of milliamperes! (Ironically, the equipment with a faulty ground would be the only safe equipment in such a situation!) I was recently invited to comment on the subject of single-fault testing requirements for products with multiple power cords. My argument was based on the idea that any product with multiple power cords is "professional" equipment where the advantage of such equipment is only achieved by connecting to multiple power sources. So, this is normal operation. A single-fault test is with one ground open (a meaningless test when there is a second ground in place). I recommended a leakage current test with one power cord connected which would simulate the situation where the redundancy was not used, and there was a fault in the grounding system. Best regards, Rich ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: pstc_ad...@garretson.org Dave Heald davehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.rcic.com/ click on "Virtual Conference Hall," ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: pstc_ad...@garretson.org Dave Heald davehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.rcic.com/ click on "Virtual Conference Hall,"