Want to move to all 208V for server racks

Date: Fri, 3 Dec 2010 09:42:10 -0800
From: Leo Bicknell <bicknell@ufp.org>
Subject: Re: Want to move to all 208V for server racks

In a message written on Fri, Dec 03, 2010 at 04:57:03PM +0000, Gary Buhrmas=
ter wrote:
> limits so that ones life has increased protection. A protective trip
> is better than
> the alternative.

Not always.

I worked in a data center with something I thought was very, very cool.


The concept, at a high level, is rather than tie the (service, not
signal) ground back to grounding rods directly you run it through a
large resistor. Now when a phase is "grounded" it runs through the
resistor, allowing a small but safe current to flow.

Why is this cool? Well, say you have a power strip running at 10A
with a bunch of servers on it. If you took a paperclip and inserted it
in an empty plug connecting hot to ground with a normal system
(simulating a faulty bit of gear) the breaker would trip, all your
servers would go off.

If you did this with a high resistance setup the paperclip would conduct
about 0.5A, maybe less. An alarm, dectecting current, at the resistor
would go off to say there was a fault. Your circuit would draw 10.5
amps and everything would stay up and running. That faulty bit of gear
didn't take down your entire power strip.

This totally eliminates arc faults, and there isn't enough current to
ground to arc. I think GFCI's are also unnecessary, as the fault can't
conduct enough current to be harmful.

All is "well and good", *UNTIL* "something happens" that introduces _another_
path to 'ground' that bypasses the 'high rresistance' links.

(Reminiscent of the old "Branch on C.E. grounded" programming joke.)