I'm trying to make a decision on this issue right at the moment and
here is my thoughts so far.
Going through this old copy of the NEC index and looking for
Disconnecting means I have a couple of interesting options.
Electronic computer/data processing equipment, 645-2(1) and
645-10. The first paragraph literally says go to the second paragraph.
645-10 says that a means shall be provided to disconnect power to all
electronic equipment in the same room....they shall be grouped and
identified and controlled from locations readily accessible at the principal
exit doors.
No reference to voltages, although I would assume that they are
talking branch circuits. - Tania's note says annex NAB.2 says to treat dc
power systems the same. Section 2.3 (UL 1950) allows for a manual statement
that requires disconnect device be provided during installation. This
matches my experiences with Telecom equipment, that has been accepted and
installed. The installations that I have been in have at the top of each
equipment rack some sort of power distribution system, that is a series of
fuses and/or breakers. They do it for some very practical reasons - there is
just a whole lot less chance of the crafts person hitting the wrong switch
and bringing down the entire frame. That obviously, would also mean that the
craftsperson has less chance of having someone else flip the disconnect back
on line while they have their hands inside the equipment. My cynical side
says this is only a secondary concern but....
Further down in this section the NEC refers to sections 705-20 and
705-21 Interconnected electric power production sources. While they don't
reference it in the index the next section 705-22 (1) is, in my opinion,
very pertinent.
705-20 Disconnecting Means, Sources .... disconnect all ungrounded
conductors
705-21 Disconnecting Means, Equipment ....from all ungrounded
conductors of all sources of supply
This would seem to imply having them at both ends !!!??
705-22 Disconnect device ....manually or power-operable swtich(es)
705-22(1) Located where accessible.
Given the other paragraphs this little modifier looks like a pretty
interesting "gottcha". I have a couple of interesting locations, tacked onto
the outside wall of a building, and on top of a power pole. So this little
note convinces me that I will need to provide the disconnect right next to
the equipment, and I won't be allowed to rely on the upstream stuff -
regardless of a warning in the manual.
However, for CO's and NOC's you probably could just put it in the
manual. Just ask yourself - beyond the standards allow are you really
protecting you users?
Gary
Please remember that I am using a very old NEC, but I doubt these areas have
change much. (The copy I have just has ton's of annotations and page/section
markers all carefully put in by the guy I stole this copy from)
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