Peter Thank you so much for the detailed response, I totally agree with your review of CSU/DSU and telecom interfaces.
But I cannot see the COAX interpretation in the same way. I have done dozens of safety approvals with multiple agencies that had T1/E1 or higher data rate interfaces. No one today makes a product with a coax that may be subjected to Overvoltages since they never leave the building. Only A twisted pair may leave the building and maybe defined as TNV1. Regarding Fiber to the curb- If this is true, that a coax is allowed to travel >from the curb to the building, than Overvoltage is required and coax will be defined as TNV1. but I am getting the feeling this will not comply with international standards such as ITU, ETSI , or ACTA/FCC part 68. >From a technical point of view - an unbalanced coax (Non floating) is impossible to implement. The ground loops.. equapotential bonding....There are major safety and EMC problems here?!?! Can anyone point out were this is acceptable by the standards? Ilan ---------------------------------------------------------- Ilan Cohen Technical Director, Safety & Telecom Divisions I.T.L (PRODUCT TESTING) Ltd. 26 Hacharoshet St, POB 211, Or Yehuda, Israel. Tel 972-3-5339022, Fax 972-3-5339019 [email protected], website: http://www.itl.co.il ------------------------------------------------------------ ---------------------------------------------------------------- I-SPEC: The best place on the internet to learn about safety !! http://www.i-spec.com/ (I-Spec is provided free of charge as a service by ITL to the compliance community) ----------------------------------------------------------------- -----Original Message----- From: Peter L. Tarver [ mailto:[email protected]] Sent: Monday, December 01, 2003 7:12 PM To: [email protected] Subject: Overvoltages on coax (was: RE: IEC 60950 Clause 6 questions) Robert & Ilan - I am breaking my response into parts, to maintain separate foci for each. The subject line is modified for each, as appropriate. Since Robert references Figure 6C (he mistakenly stated 5C) and overvoltage testing of coax connected equipment [Customer Premises Equipment (CPE) was not specifically mentioned], it is assumed the applicable standard is CSA/UL60950, rather than the referenced IEC version. > From: Ilan Cohen > Sent: Sunday, November 30, 2003 10:05 PM > > 2) Coax is typically regarded as SELV since it is > not used for TNV type of interfaces in 60950. For > example the E1/T1 coax 75Ohm interface is never > subjected to Overvoltage since it resides with in > the building only. Same applies to DS3 or other > interfaces. OVERVOLTAGES ON COAX: The above is not strictly true, especially if the equipment under consideration is a CSU, DSU or similar piece of equipment. Coax is routinely used for T1/T3 carrier connected to exposed (outside) plant. In the early '90s, perhaps one in 50 telecommunications equipment evaluations at UL's Santa Clara office included coax (or a coax option) for just this purpose. There is no doubt that coax *can* be subject to overvoltages and has been tested as such, particularly under CSA 22.2 No. 225 and UL1459. The practice should still exist under CSA/UL60950. Short of complying with the construction only path in Fig. 6C of CSA/UL60950 or presence of an equivalent mitigating circumstance, there's nothing to allow otherwise. EQUIVALENT MITIGATING CIRCUMSTANCE (for CPE): It is normally the case that a CSU (or similar equipment) will be used with a T1 or T3 carrier line (or 'fractional,' thereof). However, a number of pieces of equipment I've seen and evaluated incorporated the functionality and physical layer isolation normally associated with a CSU/DSU (or similar equipment) into the CPE. In such cases the connecting circuit is most definitely TNV-1. When a CSU/DSU (or similar equipment) is used, there is still not a 100% certainty that the device is an isolating type or that it would provide protection against overvoltages. I say this because when asked the question some number of years ago, no telecommunication carriers would state with certainty that all CSU type equipment in their networks is isolating or that it provides overvoltage protection. This answer probably reflected liability concerns, but told its tale, nonetheless. The only reasonable mechanism to CPE manufacturers is to either: 1) always provide a CSU/DSU (or similar equipment) 2) build the physical layer isolation and overvoltage protection into the CPE 3) specify use of a CSU/DSU (or similar equipment) as a requirement in your installation instructions, including its isolating nature, its overvoltage protection character, and that it must be safety certified As imperfect as Option 3 is, it gets one over the safety certification hump. Regards, Peter L. Tarver, PE [email protected] ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ieee-pses.org/ 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://www.ieeecommunities.org/emc-pstc
