At 06:52 AM 3/18/2007, Robert G. Brown wrote:
To answer my own question (GIYF, after all:-) there is a white paper
here:
HOWEVER, if you love your local firemen and want them to live (or love
yourselves and the other employees who sit near the data not-a-centers),
the same white paper says that an EPO switch is still a very good idea
for small server rooms and wiring closets that are not "data centers"
but are just "data closets" or "data rooms that aren't quite centers".
Do not underestimate the power of lobby... e.g. The front matter of
the IEEE Emerald Book, (which used to be called "grounding for
sensitive electronic equipment") explains why the "sensitive" was
removed. Mfrs didn't want their particular piece of equipment to be
referred to as sensitive.
There's a huge "small office and retail" sort of market for middling
sized UPSes, and APC and the like would like to sell into that market
as a "plug and play" product. Also, this is where the local AHJ can
play a role. THEY can say, we don't care if your installation isn't
in a room that meets ALL of the requirements of 645, we want you to
provide a disconnecting means anyway.
However, an explanatory note in NEC Article 645 (Information
Technology Equipment) says, in part:
"...For those ITE rooms constructed in accordance with NFPA 75,
Article 645 contains electrical installation requirements that are
*less* stringent than the requirements of the first four chapters of
the Code..." (my emphasis on "less" added)
For instance they let you connect things by flexible cords that
normally wouldn't be allowed, have subfloors and run cables of some
types of insulation that would normally not be allowed, etc. I
haven't checked other parts of the code, but there might well be a
"disconnecting means" requirement in there somewhere.
Another way that folks avoid the code is by putting the UPS into a
rack with the equipment being powered. That puts it on the other
side the "line of demarcation" between that which is subject to code
and that which is "internal to the equipment" and subject to
different rules. And, here, the regulatory environment gets very
tricky, depending on who's doing the racking and stacking. In Los
Angeles county, for instance, an "assembly" like this would require
UL (or other recognized testing lab:RTL) certification or, possibly,
wet-stamped drawings by a PE in order to be legally
sold/transferred/etc. So that means you can't just order it up from
your distributor already configured, unless they get the right certs.
{There are exemptions for R&D equipment, and things destined for the
movie industry, or things destined for a computer room (oops, now
you're in Art. 645 territory again)}
If you do the rack and stack yourself, it's sort of in a gray
area. As always, it's the paper artifacts you maintain that will
determine how it gets handled in the event that it becomes important
(like if a fire happens, and people start to really take a look at
the stuff...).
What I find interesting is that in California, I could not legally
perform electrical installations that I would design as a PE. That
has to be done by a licensed electrician, and I understand some of
the purported logic (there's a world of difference between practical
and theoretical knowledge), but I view it as a fine bit of union
protectionism promulgated by the IBEW. Yes, there is an exemption for
work done by a homeowner on their own property.. at least for a while.
This white paper STILL doesn't help a whole lot with the actual nuts and
bolts of how to wire a room kill -- it makes is seem like every room is
supposed to be a unique piece of engineering where you should expect to
drop thousands of dollars on the process (more than the cost of the
UPS's being controlled by a factor of two or so) rather than a simple
"wire the following standard cat5 rj45 cables from here to here, plug
them into the EPO switch suitably mounted on the wall here, test" type
HOWTO.
Typically, these things are wired in two different ways:
1) The "shunt trip", where all the EPOs are normally open, in
parallel, and go to a special connection on the main breaker feeding
the room. (Which has the effect of shorting the output of the circuit
breaker, tripping it.. although that's not actually how it's done). A
separate set of contacts would be used for the trip on the battery
side of the UPS
2) The "interlock" scheme, where the EPOs are normally closed, wired
in series, and breaking any, removes power from a latching relay.
Most UPSes have (at least as an option) some sort of scheme where an
external contact closure can be used to control the relay.
There are some subtleties in how all these relays get wired up,
because you need to have power to actuate a relay in the first place,
and having UPSes around makes the power switching a bit more complex
(so that once you've turned the relay on, the battery voltage doesn't
keep it on, even if you remove the original power).
Latching relay schemes are very common: Relay coil and NO contacts
are in series with power. "Turn ON" button shorts contacts, applying
power to coil, pulling in relay. NO contacts are now closed, in
parallel with button, so button can be released. The OFF button is
a NC type in series with the whole thing, so that if it is pressed,
the power is removed from the coil, the latching contacts open, and
you need to press "ON" to get the thing repowered up.
Where there are numerous things that need to be controlled, you wind
up having many sets of contacts on the back of the EPO operator. (The
operator is the actual pushbutton part.. for industrial applications,
they are made with layers of contacts that you just attach to the
back.. need 30 sets of contacts, no problem)
For a complex system, there might actually be a separate "emergency
shutdown power" bus to save the need for zillions of contacts. This
would be engineered to "come up first and go down last". For
instanace, your EPO might be a NC contact set that provides power to
a whole raft of contactors (i.e. the UPSes would be configured so
that their internal contactor is "closed on application of external
coil power")
With half a dozen UPSes, several different phases of AC power, and a
dozen racks, this can get pretty complex.
But I will persevere. Google is still there, and the information I seek
MUST be somewhere in webland...;-)
James Lux, P.E.
Spacecraft Radio Frequency Subsystems Group
Flight Communications Systems Section
Jet Propulsion Laboratory, Mail Stop 161-213
4800 Oak Grove Drive
Pasadena CA 91109
tel: (818)354-2075
fax: (818)393-6875
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