Doug, 60601-1 is not the easiest standard to interpret, but I think that in this instance you are interpreting it incorrectly. Patient leakage current is that current which flows between an Applied Part and Earth via the patient. The accessible parts of the frame are not considered as Applied Parts. It is acceptable for protection of the patient from electric shock risk arising >from contact with an accessible part to be provided by earthing. As you rightly point out, the potential of the accessible part when the earth impedance is low will be negligible. You must consider single fault conditions, so it would be wise to include interruption of the earth conductor in this case. However, perhaps you could have one earth connection via the power cord and one separate connection to a permanent earth conductor, thus providing a redundant protective earth. This is my opinion only, and I stand to be corrected by anybody more familiar with this standard than I.
Best regards Neil R. Barker CEng MIEE FSEE MIEEE Manager Compliance Engineering e2v technologies (uk) ltd 106 Waterhouse Lane Chelmsford Essex CM1 2QU UK Tel: (+44) 1245 453616 Fax: (+44) 1245 453410 Mob: (+44) 7801 723735 From: Doug Beckwith [mailto:[email protected]] Sent: 11 October 2005 16:33 To: EMC-PSTC Subject: 60601-1 and patient leakage current Hi All, Question time again, this time concerning patient leakage current and permanently connected Class 1 equipment with a permanently connected earth. I have a product that exceeds the patient leakage current between the mains and the accessible metal parts of the frame (a motorised bed). The accessible metal part is pemanently connected to PE. The measured leakage current from the power supply exceeds the patient leakage requirements (0.1mA). I searched 60601-1 and I cannot find any exception that allows for increased leakge current limits. If you have a metal part that is connected to earth with a resistance of <0.1 ohm, the potential on the accessible part shoul closely approximate 0V. I can understand the leakage requirement if the part is not earthed, but there should be an exception if the part is at earth potential. Help, I have a feeling I am missing something. All assistance gratefully received. Regards Doug - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
