Re: Want to move to all 208V for server racks

[Steven Bellovin <smb () cs columbia edu>]

On Dec 2, 2010, at 3:54 15PM, Jay Ashworth wrote:

> ----- Original Message -----
> > From: "Ingo Flaschberger" <if () xip at>
> >
> > in europe GFIs are always needed for prection and by law.
> > to avoid the cascading effects the GFCIs are better.
> > break current ranges from 10mA (bath) up to 300mA; for servers I use
> > the 30mA with pulse protection (internal delay) to avoid the server
> > powersupply capacitor loading GFCIs "flip".
>
> And that, indeed, is one of the circumstances in which Chris Lewis and
> Steve Bellovin's Wiring FAQ suggests that you should *not* use a GFCI:
> in places where the inevitable "nuisance trip" is troublesome, like
> powering servers. 
>
> That FAQ is a bit dated, of course.

Indeed; it's been unmaintained for quite a number of years at this point.

The major place I personally have trouble with GFCIs is on things with
big motors, and in particular my basement dehumidifier -- a place I really
want a GFCI because we've occasionally had water problems...
> And indeed, I never liked GFCI breakers for the usages for which they're
> mandated in the US, cause the milliamp currents they're supposed to trip
> on are no match for all that copper resistance...
Wire resistance shouldn't matter.  A GFCI is measuring the current in the
hot wire compared to the current in the neutral wire; if they differ by more
than about 5 milliamps, the device trips.  That's why motors cause problems:
the inductance of the windings can cause a brief current imbalance.

Anyway -- in response to the original question: the US electrical code
requires GFCI protection for outlets in kitchens, bathrooms, or unfinished
basements, for outdoor outlets, and for any other outlet near water.  Canada
has slightly different rules, or at least it did when we last updated the
FAQ (Chris Lewis is Canadian): their code requires that every duplex
kitchen outlet be served by two separate circuits, which generally share a
common neutral.  A simple outlet GFCI can't handle that setup, since the
actual current flowing through the neutral will vary depending on the loads
on the two hot wires.  You'd need a specialized outlet or breaker GFCI that
summed the current across all three wires; such devices may exist but I've
never seen them.  (Btw -- the usual reason for using outlet GFCIs is that
they're much cheaper than breaker versions.)


                --Steve Bellovin, http://www.cs.columbia.edu/~smb