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Posted by Floyd L. Davidson on February 11, 2005, 5:55 pm
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wdg@[206.180.145.133] wrote:
>Battery return (unswitched/unfused +48) is bonded to frame/building ground
>(which should be one & the same) at the main distribution panel (only)
>where the batteries and rectifiers come together.
>
>I cannot quote the precise spec, but there should be an infinitesimally
>tiny and insignificant amount of measurable current in the frame/building
>GROUND leg.
>
>Equipment load requires two wires (- 48 supply and return) all the way
>back to the dist panel. Equipment should also be bonded between it's
>frame/chassis and the nearest ground. As above, there should be no current
>in the ground lead. Equipment bonding (ground) lead should -NOT!- be
>connected to the +48 return lead at the equipment end. The ONLY connection
>between +48 return and GROUND should be at the distribution panel in the
>power room.
Let me add a little bit to clarify that excellent description.
Every piece of equipment should have three separate connections.
-48V, +48V, and Ground. The +/-48V are as described.
The ground should have a single common point for the entire
building (or more specifically, for each part of a building that
has a unique AC power distribution system). Each floor should
have a single common point (all of which are individually
connected to the building's common point).
Each row of equipment racks should have an individual connection
to the floor's common point. Typically that is done at the top
of the racks, and each will have a copper bus bar at the top,
and each bus bar is connected to the adjacent rack's bus bar.
Each rack then has, on each side, a ground wire extending from
top to bottom.
Each piece of equipment is connected to one or the other of
those ground wires. Just as the racks are connected to a
common ground bus across the top, sometimes a number of similar
equipment chassis are strapped to ground by a common strap that
goes from one chassis to the next and ends up connecting to all
of the chassis and to ground, exactly once per item.
>Measured at the source, with rectifiers/chargers online (floating) the -48
>will actually be closer to -54.
The exact voltage depends on the type of battery, and that value
should be obtained from the manufacturer's specifications.
To end with, let me pull out one sentence from wdg's
description, and emphasize it even more:
"The *ONLY* connection between +48 return and GROUND
should be at the distribution panel in the power room."
The positive side of the battery and the building ground system
connect exactly one time only.
--
Floyd L. Davidson <http://web.newsguy.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@barrow.com
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