Thanks everyone for the advise I will pass it on
Marc From: Ted Lienhard Sent: Monday, October 05, 2009 9:43 AM To: RBASE-L Mailing List Subject: [RBASE-L] - Re: Off topic Network Q Hi Marc, It is a good standard practice to avoid running network wires directly over flourescent fixtures and to have as much distance as possible from them as can be arranged...12" is good, the farther the better. The problem is induction of the 60Hz field from the ballasts into the network cable. Here's the good news: all cables with pairs of two conductors (Cat 5 has 4 pairs in the jacket) have a certain amount of what is called "common mode rejection". That's electronic engineer talk for the fact that a pair of wires in a voice or data circuit generally work in a "push-pull" mode (one conductor is at a positive voltage and the other is simultaneously at a negative voltage-they are different voltages...you could say they are "uncommon" in their normal mode). Induction of an electrical current results in the same polarity in both conductors of a 2 conductor circuit, resulting in the rejection of the unwanted inductively coupled voltage being rejected by the push-pull circuitry - hence "common mode" rejection. When the two conductors are twisted together, any inductance from an unwanted source is more likely to be completely equal on both conductors...resulting in no interference due to the fact that the "common mode" voltages on both conductors are the same exact voltage. The push-pull circuitry detects no difference between the conductors, voltage-wise, and does not recognize the interference. This all works fine in "free space"; the case where the cable is some distance away from the flourescent fixture, for example. Where you get into trouble is where the magnetic field from the ballast is induced into the metal light housing, which then has your cable running over it. In this case, it is entirely possible that one conductor of one pair will be in closer physical proximity to the housing (with its magnetic field) than the other conductor of the pair, resulting in induced voltages that are not equal; one conductor of the pair has a higher or lower voltage than the other. Since common mode rejection requires a "common mode"; i.e., the same voltage on both conductors, the push-pull circuit interprets the interfering signal as legitimate voice or data. In a voice circuit, you hear hum, in a data circuit the amplitude of the interference can be larger than the data being sent, resulting in lost packets and other errors. Bottom line, you want to keep your network cables as far from power lines and flourescent lights as possible, but never run a network cable directly in contact with flourescent fixtures or any armored power cable connected to them. That is probably more information than you wanted to know, but there it is! Best Regards, Ted Ted Lienhard CNE NCT Golden Valley Consulting >>> MDRD<[email protected]> 10/5/2009 6:59 AM >>> Hi I have heard you should have your Cat5 cables 4ft away from fluorescent lights but the electrician wiring my new office says it is not necessary. Is there any info from major companies that I can give him or am I being overly concerned? Thanks Marc
