> Date: Fri, 3 Dec 2010 09:42:10 -0800 > From: Leo Bicknell <bickn...@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.)
