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@medline.com> To: "George Herbert" <george.herbert@gmail.com>, "Matthew Kaufman" <matthew@matthew.at> Cc: nanog@nanog.org Subject: RE: Fiber only in DataCenters? Message-ID: <2A76E400AC84B845AAC35AA19F8E7A5D0DB3E9E1@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.