Want to move to all 208V for server racks

Fri Dec 3 16:57:03 UTC 2010

On Fri, Dec 3, 2010 at 04:02, John van Oppen <jvanoppen at spectrumnet.us> wrote:
...
> GFCI breakers are often required on large services, most large (new) 480v services I have seen (1000A and larger) a have Ground fault breakers,

Actually, my recollection is that large new services include arc
suppression rather
than ground fault (480V service may be floating in any case, since it
would depend
on delta-wye distribution).  There has been strong efforts to protect
the low voltage
electricians (in common power distribution speak, 12K+ voltage is high voltage,
less is considered low voltage; yes, this is a different point of
view).  Even with
a 100Cal suit on, you really want arc suppression at those high joule ratings
to protect a life (every master electrician has a story about arc flashes, and
some stories include the outline of the ex-individual on the opposite
wall).  It is
now common when doing work on downstream devices to reduce the arc
limits so that ones life has increased protection.  A protective trip
is better than
the alternative.

> in fact I have seen some bad outages on entire datacenters where the main breakers had a lower ground-fault current setting (for tripping) than a branch circuit that had a phase-to-ground fault resulting in the main breakers tripping instead of the branch circuit.

*Proper* engineering is more than just putting in a breaker with a
high enough rating.  The days of nice resistive (think incandescent
light bulbs) or inductive (motor/transformer) loads are long gone.
Switching power supplies (or large pulse rectifiers) require a more
careful analysis.  I have seen too many upstream breakers being
set at the wrong trip values (the larger breakers have internal
adjustments), and trip first.

Gary