> On Tue, 20 May 1997, Muditha Gunatilake wrote:
>
> > > (4) How many watts does a average computer use (bloody computer manuals
> > > don't have anything in them!) ?
>
> How big is the power supply? If its 250 watt, then you KNOW that your
> computer cannot use MORE than that.
>
> Depending on the configuration of the computer, when we are estimating
> office layouts, I think we estimate 2~3 AMPS per workstation. And MORE of
> course for a laser printer which can draw 10+ Amps on startup.
>
> If you REALLY wanna know... heh heh... take a few power strips, plug into
> 1 20 amp circuit, slowly add pm 2's (or 3's) until the circuit breaker
> trips.
>
This is the worst advice I've seen Jake write! Heres a place to start:
http://www.exide.com/sunsize.htm
Then call the 800 number and get some general advice. For small setups
Exide makes a nice rack mounted product:
http://www.exide.com/serack.htm
My personal picks are Exide and Toshiba. If you have an existing setup of
equipment that you need to move to a UPS you can measure it using a power
strip and a current loop attachment around one of the (power carrying)
conductors. However, to get a correct measurement you need to use a
digital multimeter (with current clamp attachment) that supports true RMS
readings. Now if you want to "do it right" buy or rent something like:
http://www.fluke.com/handheld/266.htm
Read the (lightweight) section "Why True-rms?"
With PC and workstations that use switch mode power supplies you are not
measuring a simple (60Hz) sine wave. PC power supplies will take a "bite"
out of the A/C waveform when then turn on - put enough PCs on a single
circuit and this bite will be visible on an oscilloscope. Choosing a UPS
is no longer a simple task.
One other feature to watch for: Static Bypass Switch (SBS). This is an
electronic switch which bypasses the UPS circuitry in the event of a UPS
internal failure and connects the load directly to the incoming power. It
will also bypass the UPS in the event of an overload situation, which
protects the UPS itself and still keeps the load powered up. (Like when
the cleaning crew plugs in a big floor polisher into one of your UPS wall
sockets.) Toshiba also monitors the switching transistors heat sink temp.
So, if you overload your UPS by adding extra computers in little steps
(like gradual system expansion) the UPS may run for several minutes (or
hours) before activating the SBS.
Exide also has some cool software to monitor the UPS and even an ethernet
option for their UPS. This is great if you hate dealing with serial
devices!
Al Hopper Logical Approach Inc, Plano, TX. al@logical-approach.com
(972)-379-2133 or (972)-849-5765. Fax 972-379-2134