Kurt, be aware that IF the insulation between all parts of the hinged door and any uninsulated part operating at hazardous voltage will fully meet the requirements for reinforced (or double) insulation (hipot, creepage distance, clearance, insulation thickness etc.) grounding the hinged door is unnecessary. The hinged door can be considered to have class II protection (against electric shock) even though it is part of a class I appliance. Refer to the standard for the details. Rgds, Lou
Lou Aiken 27109 Palmetto Drive Orange Beach, AL 36561 USA tel +1 334 981 6786 fax +1 334 981 3054 mobile +1 334 979 4648 ----- Original Message ----- From: Andrews, Kurt To: EMC-PSTC Sent: Wednesday, May 16, 2001 1:17 PM Subject: Protective Bonding-UL 60950 (replacement for UL 1950) Hi group, We are designing a new piece of class I equipment to be evaluated under UL 60950 (replaced UL 1950). The equipment has a metal enclosure. It has a metal access panel that is hinged at the bottom with a piano type hinge. It is secured at the top by two screws. This panel is hinged so that the customer's service personnel can have access to a PC board for configuration purposes. There are hazardous (AC mains) voltages behind this panel. We are concerned that this panel may not make a reliable earth connection to the rest of the metal enclosure, especially if the customer fails to fully tighten the two screws. We are planning on adding a wire from this panel to a side panel of the enclosure to reliably ground the panel to the rest of the enclosure. This wire will be on the inside of the unit. According to UL 60950 this wire would be considered a Protective Bonding Conductor. We are planning on using a stranded 14 AWG wire with ring terminals on both ends for this Protective Bonding Conductor. It would be fastened to the two panels via threaded studs mounted in the panels. We plan on placing the ring terminals on the studs and securing them with toothed lock washers and nuts. I have a question about the requirements for this Protective Bonding Conductor. According to UL 60950, clause 2.6.5.7, at least two screws must be used for each connection. Does this clause apply in a case such as this? Or is it meant to be used in cases where two metal panels are fastened together with screws to provide Protective Bonding between the panels. I don't really see how we can use two screws for each connection when using a wire for this purpose unless there is a crimp terminal that has two rings on it which I haven't seen. I have copied the clause from UL 60950 below for your reference. Any advice on this matter would be greatly appreciated. Kurt Andrews Compliance Engineer Tracewell Systems, Inc. 567 Enterprise Drive Westerville, Ohio 43081 voice: 614.846.6175 toll free: 800.848.4525 fax: 614.846.7791 http://www.tracewellsystems.com/ <http://www.tracewellsystems.com/> 2.6.5.7 Screws for protective bonding NOTE -The following requirements are additional to those in 3.1.6. Self-tapping (thread-cutting and thread-forming) and spaced thread (sheet metal) screws are permitted to provide protective bonding but it shall not be necessary to disturb the connection during servicing. In any case, the thickness of the metal part at the point where a screw is threaded into it shall be not less than twice the pitch of the screw thread. It is permitted to use local extrusion of a metal part to increase the effective thickness. At least two screws shall be used for each connection. However, it is permitted to use a single self-tapping screw provided that the thickness of the metal part at the point where the screw is threaded into it is a minimum of 0,9 mm for a screw of the thread-forming type and 1,6 mm for a screw of the thread-cutting type. Compliance is checked by inspection. ------------------------------------------- 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: Michael Garretson: [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://www.rcic.com/ click on "Virtual Conference Hall,"
