Ron,
I'm sure you'll get lots of responses on this one. Here is what I believe/know. For cord connected equipment that connects to wall outlets protected with 20 A overcurrent protection devices, 25 Amps is 125% of rated current. 25A X 0.1Ohms=2.5V, safe even in a damp environment. The NEC requires selecting an overcurrent protection device at 125% of continuous load (210.20 A of 2002 NEC). The rationale is that if an internal conductor shorts to the grounded enclosure the overcurrent protection device will interrupt the supply and no damage will occur to the conductors or the enclosure due to I squared X R heating effects. This 25 A value for a test current is a conservative way to ensure that the voltage developed across the bonding impedance is of a low value. MIL-STD 882, System Safety, requires testing bonding impedance at 125% of the rating of the overcurrent protection device and the calculated impedance must be less than 0.1 ohm. I suspect that the MIL-STD "boilerplate" worked its way into some industry/UL standards. Best regards, Daniel E. Teninty, P.E. Managing Partner DTEC Associates LLC (509) 443-0215 (509) 443-0181 fax http://www.dtec-associates.com Streamlining The Compliance Process While Advancing New Products To Market This e-mail message may contain privileged or confidential information. If you are not the intended recipient, you may not disclose, use, disseminate, distribute, copy or rely upon this message or attachment in any way. If you received this e-mail message in error, please return by forwarding the message and its attachments to the sender. -----Original Message----- From: [email protected] [mailto:[email protected]]On Behalf Of [email protected] Sent: Wednesday, July 24, 2002 1:58 PM To: [email protected] Subject: Bonding Impedance Hello all, I recent issue has come up for me regarding why some safety standards specify 100 milliohms as the acceptable impedance for a protective earth ground bonding test. I am curious as to what the rationale is for this specific value and why 25 amps is chosen as the default test current. Regards, +=================================================================+ |Ronald R. Wellman |Voice : 408-345-8229 | |Agilent Technologies |FAX : 408-553-2412 | |5301 Stevens Creek Blvd., |E-Mail: [email protected]| |Mailstop 54L-BB |WWW : http://www.agilent.com | |Santa Clara, California 95052 USA| | +=================================================================+ ------------------------------------------- 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: [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://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list" ------------------------------------------- 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: [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://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
