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.
http://www.hilkar.com/highresistance.htm
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.)