Mark. As described, the situation to me sounds no worse than any other earthed connection. The only hazard would seem to be as a return for hazardous voltages - exactly the same situation as the sheet metal enclosure that surrounds it. In my experience sometimes the "approver's" representative makes a snap judgement when confronted with something unfamiliar. It then becomes difficult for them to back down without losing face. I would suggest gently trying to convince the representative of the logic of your position.
Scott Lacey -----Original Message----- From: owner-emc-p...@majordomo.ieee.org [mailto:owner-emc-p...@majordomo.ieee.org]On Behalf Of Mark Haynes Sent: Thursday, October 18, 2001 12:44 PM To: IEEE EMC-PSTC Discussion List (E-mail) Cc: Peter Deneault (E-mail); Tom Brenner; Doug Harris Subject: Motor Drive Grounding Scheme Greetings All, I work for a product safety consulting firm/test lab/agent. I am currently having some difficulties with UL relating to one of our customer's products. The grounding scheme of the products has become a barrier which has halted the planned testing. In our opinion, the engineering rationale behind this position is not very strong or clear. The products are small open-type stepper and servo motor drives (rated up to 6 A) which are intended to be used within another enclosure. They are powered by an 18 - 74 V dc external source which is supplied by the user. The drive output is a DC pulse width modulated waveform. UL 508C and UL 840 are the standards being used. The main issue is the fact that the DC - (common) input supply lead is connected internally to the input ground (PE) lead. UL has referenced UL 508C requirements (not really applicable to these particular products since we have agreed to use UL 840 for spacings) that indicate that spacings are required within the product between these two leads. This implies that this grounding scheme cannot be used. The manufacturer has indicated that the drives will not operate properly without this grounding connection. The product designers made this connection internally to prevent the common from floating above/below ground potential and for EMI purposes. The customer and my company do not agree with UL's position. One possible hazard UL stated was that the heatsink was connected to the grounded DC - (common) internal bus and could be electrically "live". Since the heatsink is referenced to ground potential, it is not "live" during normal operation. If an internal fault does occur, the circuitry is designed such that the fault current will flow through the ground, as intended. In addition, a "hint" of possibly increasing the risk of shock was also mentioned. No one involved has been able to identify any real safety issues resulting from this grounding scheme. In order to address all potential safety hazards, we have recommended that testing be conducted to confirm compliance with the intent of the standards. This would hopefully show that the products are "safe" and that all foreseeable safety hazards (during normal and fault conditions) have been identified and minimized/eliminated. After weeks of research and discussions, we have not been able to convince UL that this grounding scheme should be allowed and that we should proceed with the testing. Does anyone know of any similar UL certified motor drives (or other similar DC powered products) that employ this grounding scheme? The closest examples we could find were AC powered products with DC ground referenced secondary circuits. However, this is not the same since a transformer usually provides the required isolation. Does anyone have any comments/information on this grounding scheme that might be helpful in building a stronger case either way? The closest thing we could reference was a grounded DC distribution system in the 1999 National Electrical Code (NEC - NFPA 70). Please respond at your earliest convenience. Thanks in advance for your assistance, Mark A. Haynes Senior Product Safety Engineer D.L.S. Conformity Assessment, Inc. 1250 Peterson Drive Wheeling, IL 60090-6454 (847) 537-6400 (Ext. 157) Fax (847) 537-6488 mhay...@dlsemc.com ------------------------------------------- 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: No longer online until our new server is brought online and the old messages are imported into the new server. ------------------------------------------- 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: No longer online until our new server is brought online and the old messages are imported into the new server.
