Hello Joe: Glad to hear from you.
SELV can only be considered zero volts when considering isolation from a PRIMARY CIRCUIT. When considering isolation from a secondary circuit to a secondary circuit, then this is not the case. Therefore, when figuring the working voltage between PRIMARY and SELV, or PRIMARY and TNV, or PRIMARY and SELV, ELV, and TNV would be considered zero. However, between SELV and TNV, or ELV and TNV, one must take into account the difference in voltages between both the SELV and TNV circuits, or ELV and TNV circuits. See clauses 2.2.7.2 and 2.2.7.3. Mel Pedersen Midcom, Inc. Homologations Engineer Phone: (605) 882-8535 [email protected] Fax: (605) 882-8633 ---------- From: [email protected][SMTP:[email protected]] Sent: Monday, October 12, 1998 12:11 PM To: [email protected] Subject: Re: Crepage/Clearance On Telecom Modem design In a message dated 98-10-12, Mel Pederson writes: << If one considers the worst case (DC) voltages that could possibly exist on TNV-3 lines and also SELV circuits, and considers the likelihood that these are at opposite polarity, one would obtain a worst case 180VDC = -120VDC (TNV-3) - 60VDC (SELV). >> Mel: I have a vague recollection that somewhere in EN 60950, it states that for purposes of determining working voltage, SELV circuits can be assumed to be zero volts. Perhaps this explains the slight difference in your calculations of working voltage from the calculations of the labs you reference. Joe Randolph --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators). --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators).
