NANOG Digest, Vol 59, Issue 80

Jakob Heitz jakob.heitz at ericsson.com
Fri Dec 21 21:19:49 UTC 2012


Voltage causes sparks, but...

Maybe you got the spark when you disconneted the wire.
In that case, you likely have a ground loop carrying
current and a long wire.
When you disconnect the wire, the current wants
to keep flowing due to loop inductance.
This causes the voltage spike and hence the spark.

> Date: Fri, 21 Dec 2012 14:14:29 -0600
> From: "Naslund, Steve" <SNaslund at medline.com>
> To: "George Herbert" <george.herbert at gmail.com>, "Matthew Kaufman"
> <matthew at matthew.at> Cc: nanog at nanog.org
> Subject: RE: Fiber only in DataCenters?
> Message-ID:
> 	<2A76E400AC84B845AAC35AA19F8E7A5D0DB3E9E1 at MUNEXBE1.medline.com>
> Content-Type: text/plain;	charset="us-ascii"
> 
> It takes a lot of voltage to cause an arcing spark.  I would suspect
> static buildup along the way and bad grounding.  Even a big facility
> with a good ground should not have enough voltage differential between
> grounding points to cause sparks.  Having the right size rack
> grounding should give you a very low resistance to ground from any
> point.  The most common problem I have seen in large facilities is
> multiple grounds
> that are not tied together or cables that are grounded at multiple
> points causing a loop current.  It is critical that everything have a
> single ground, that includes racks, electrical distribution,
> cable tray,
> etc.  Most Cat X cables are unshielded and do not have a ground
> conductor so you must have equipment at the same potential at
> both ends
> or you will get loop current for sure.
> 
> As far as voltage in Cat X cables, the real factor is the current
> carrying capacity of a particular wire gage. It does not really matter
> whether it is Cat 6 or a coat hanger, current capacity is a
> function of
> cable cross section and what material it is made of.  Copper has a
> specific resistance as do all other metals.  A copper cable needs to
> have enough cross section to dissipate the heat generated by its
> resistance.  A less conductive material requires more cross section to
> dissipate the increased heat.  At extremely high voltages
> things become
> more complex because of the skin affect that causes the power to move
> through the outer parts of the cable more than the inner parts.  These
> levels are not a factor in communications cables.
> 
> The main factor for fiber over copper in data centers is all
> about cost.
> Most servers include copper connections and fiber costs
> something extra.
> For switches, the cost of the optics is significant.  Fiber does help
> prevent damage due to surges or electrical faults but if these are a
> problem in your datacenter you have bigger fish to fry.
> 
> Steven Naslund


-- 
Jakob Heitz.



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