Rick, A -48 volt source is considered a secondary circuit, but that does not necessarily make it a SELV circuit. I also don't believe that even if your equipment installation should be proven to be powered by a SELV circuit that the disconnect requirements of Section 2.6 do not apply. Here is why.
Note that UL 60950 plainly states that connections to - and overcurrent protection for -d.c. powered equipment needs to meet the same sections as for a.c. main powered equipment. It stands to reason, therefore, that disconnection from whatever mains should also meet Section 3.4.3 (UL 60950) or 2.6 (I presume UL1950. 3rd edition). Annex NAB.2 states that "... connections to the centralized d.c. power system are subject to the requirements for PRIMARY CIRCUITS and shall be in accordance with 3.2 (AC MAINS SUPPLIES)". Sections 2.7.1 and 3.2.1 in Annex NAE which address requirements for d.c. powered equipment refer you back to a.c. mains powered equipment. Also, reading the standard as a whole, Permanently connected equipment, whether a.c. or d.c., needs to meet the stated requirements. (All my references are to the UL 60950 edition.) There are many reasons why it can be considered unsafe not to have immediate access to a disconnect device for any equipment;-- a shock hazard being just one of them. Thus, equipment powered from a SELV circuit is still subject to fire and/or an energy hazard. For fixed equipment, the presumption is that the disconnect device is in the building installation accessible to trained service personnel and, therefore, the relaxed requirement to provide this information in the installation manual. However, I have a larger concern. If you don't have a MAIN disconnect device in your d.c. powered equipment, something tells me that you probably don't have a circuit breaker for overcurrent protection. Annex NAE refers you back to Section 2.7.1 which states that "... If...PERMANENTLY CONNECTED EQUIPMENT relies on protective devices in the building installation for protection, the equipment installation instructions shall so state and shall also specify the requirements for short-circuit protection or overcurrent protection, or, where necessary, for both." I strongly recommend a good read of the complete Annex NAE. Note that UL 60950 has renumbered or reassigned the Section references; the content, however, has not changed from UL 1950 that I have noticed. Tania Grant taniagr...@msn.com ----- Original Message ----- From: Richard Meyette Sent: Wednesday, May 16, 2001 4:35 PM To: 'emc-p...@majordomo.ieee.org' Subject: Disconnect Devices for Telecom Equipment Here's a question for you telecom experts: Paragraph 2.6.3 of UL 1950 (Third Edition) requires permanently connected equipment, that is not provided with a disconnect device, to include a statement in the installation instructions that an appropriate disconnect device shall be provided as part of the building installation. Telecom equipment intended for connection to a -48V centralized DC power system located in a central office is considered to be permanently connected equipment. However, the centralized DC power source is considered to be a SELV secondary circuit. I don't think that this requirement would be applicable in this case, since section 2.6 deals with the requirements for primary power isolation. However, I am being told otherwise. It is no big deal to provide the statement. I would just like to get some clarification on how this requirement is applied to telecom equipment. Rick Meyette Terawave Communications <br clear=all><hr>Get your FREE download of MSN Explorer at <a href="http://explorer.msn.com";>http://explorer.msn.com</a><br></p>
