Rick, First off, please have a look to UL 60950 (Dec. 1, 2000) Annex NAE ref. cl. 3.6. It's informative and does provide further guidance. It invokes cl. 2.7 and also refers to NEC 422-11 and 240-22 (don't have my NEC handy so I can't confirm just how these references apply in this case....sorry). "Overcurrent and earth (ground) fault protection for DC powered Equipment" is the header..... In a previous life when working with similar equipment, we looked at these matters as well. We ensured the documentation included instructions for a disconnect device to be included as part of the building installation when a rack mounted disconnect device (for the rack) was not implemented, along with type and ratings recommendations. Typically a Listed breaker would do the trick. We worked on the premise that although the centralized DC source is typically SELV and would likely have some form of s/c or over-current protection, we had no firm view on what exactly was provided in each installation in terms of the supply itself or on the output protection scheme and would not default to making those assumptions. Knowing that the equipment we deployed stood a good chance of being ganged into existing facilities with existing supplies and output protectors of type or rating unknown, we had no firm view on what exactly that was or how closely it might have been matched to the ratings of the equipment deployed by my previous-life employer. Along with instructions on the external protection to be supplied as part of the installation, the type and the ratings recommendations, we also decided to ensure the approval reports specified the use of a SELV supply as opposed to just assuming so....thinking back.....the added text in the report was a worthwhile consideration. Taking the time to ensure your product approvals and documentation specify that the supply used must be SELV (sometimes this is referred to by some agency reports by the simple reference of such-and-such equipment as CLASS III) simply sheds a bit more detail as to the intended and approved use for your equipment. When we used this approach, it was usually for the purpose of approving the whole rack..not just a sub-assembly to fit into some unknown rack. BTW - to meet SELV by definition, you also need to maintain the "safe" voltage levels under single fault as well, not just normal operating conditions. My opinion and not that of my employer. Regards, Kaz Gawrzyjal [email protected] <mailto:[email protected]> -----Original Message----- From: Richard Meyette [mailto:[email protected]] Sent: Thursday, May 17, 2001 12:22 PM To: 'Tania Grant'; '[email protected]' Subject: RE: Disconnect Devices for Telecom Equipment
Tania, The -48V centralized DC power systems used in the central offices of telecommunications companies are limited to 60 VDC or less under normal operating conditions, including during charging of the batteries, and are therefore considered to be SELV by definition. See 3.6 of UL 60950. Annex NAB clearly specifies that the connections to the centralized DC power system must meet the requirements for primary circuits as specified in section 3.2, however there are no requirements for disconnect devices or overcurrent protection specified in this annex. This equipment has operator replaceable fuses for over current protection on the power inputs and has numerous overcurrent devices in the PC assemblies. However, it does not have a disconnect device since it is intended for installation into telecom racks with other equipment that is similarly connected. A disconnect device is required for equipment connected to the AC mains, as specified in 2.7 of UL 60950, however there no requirements for disconnect devices for equipment connected to a secondary SELV source. The centralized DC power systems installed in these restricted access locations are provided with disconnect devices for overcurrect and fire protection. I'm not sure how you would specify the short circuit protection requirements for these systems, since they are not branch circuits covered under Article 240 of the National Electrical Code. Rick Meyette -----Original Message----- From: Tania Grant [mailto:[email protected]] Sent: Wednesday, May 16, 2001 7:18 PM To: Richard Meyette; '[email protected]' Subject: Re: Disconnect Devices for Telecom Equipment 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 [email protected] <mailto:[email protected]> ----- Original Message ----- From: Richard Meyette Sent: Wednesday, May 16, 2001 4:35 PM To: '[email protected]' 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 _____ Get your FREE download of MSN Explorer at http://explorer.msn.com <http://explorer.msn.com>
