Creepage distance has nothing to do with current within the context of the product safety standards. Creepage distance is the distance across the surface of an insulating material. The specific Creepage distance for any product safety application is based on: 1) Working Voltage, 2) Installation Category - defines where on the power grid the product gets power = this defines the maximum anticipated overvoltage that the product will be subjected from upstream switching surges and other transients that may. 3) Pollution degree - the amount of potentially conductive contaminants that could reduce the creepage distance. 4) The insulation material's resistance to tracking - designated by the CTI rating of the insulating material.
Creepage distances are specified such that, if there is a breakdown due to a short term transient, we want the breakdown to occur through the clearance distance (air) rather than across the creepage distance (insulating surface). The rationale being that once the transient that caused the breakdown subsides, the clearance is replaced by new air = no permanent damage. However, if a breakdown occurs across a Creepage distance, it leaves a permanent carbon path which thereby reduces the voltage required for the next breakdown. And so on and so on until a fire hazard (heat in the carbon path) or shock hazard occurs (complete breakdown). This helps to explain why clearance distances are also based on Air Pressure (altitude) since it directly related to the insulating properties of air. Again, if there is a breakdown, we want the air (clearance) to break down before the insulation (creepage). I hope this helps. Bill Bisenius bi...@productsafet.com Educated Design & Development, Inc. (ED&D) 2200 Gateway Centre Blvd. Suite 215 Morrisville, NC 27560 919-469-9434 www.productsafet.com -----Original Message----- From: owner-emc-p...@majordomo.ieee.org [mailto:owner-emc-p...@majordomo.ieee.org] On Behalf Of David Sproul Sent: Friday, October 11, 2002 8:06 AM To: Ted Rook Cc: EMC-PSTC Subject: RE: David Sproul...UL creepage limits ;~) Ted, thank you for your response. I do not claim to be an expert, but I cannot accept that creepage has anything to to with the current flowing in a circuit. Surely it is the voltage across the material and the CTI of that material which determines the likelihood of tracking across the material to take place. As for your car battery melting story, cars must be wired differently in the US than in the UK, because I have connected negative to negative and positive to positive on many occaisions, and never had anything anymore exciting happen than the second car starts. Best regards, David Sproul. -----Original Message----- From: owner-emc-p...@majordomo.ieee.org [mailto:owner-emc-p...@majordomo.ieee.org]On Behalf Of Ted Rook Sent: 09 October 2002 15:28 To: < Subject: David Sproul...UL creepage limits ;~) This is because when you double the voltage the power is proportional to a quarter of the current squared. In America the 120V power is at lower voltage but the current is twice as much and so the creepage is twice as well. Very high voltage circuits hardly creep at all whereas low voltages creep the most. That is why you should never join the two negative terminals when you jump start a car, the car battery charging circuits have so much creepage they can melt the battery. I though everybody knew that........... ------------------------------------------- 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: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com 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://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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com 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://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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com 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://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
