[ale] Low voltage work in GA

Tue Jul 31 14:28:55 EDT 2012

So how much of this applies to installing Ethernet in a residence, outside
of the Metro Atlanta area? Are these code requirements statewide?

--Cameron <http://ghostfreeman.net>

On Tue, Jul 31, 2012 at 2:21 PM, John Pilman <jcpilman at gmail.com> wrote:

> The reason to use plenum cable is not to prevent the cable from starting a
> fire.  The conductors are sized large enough for the current so that the
> cable will not overheat.  The only reason for plenum cable is that it is
> made with fire retardant material and produces less toxic gas in a fire.
> Oh, that's two reasons.  The two reasons to use plenum rated cable are ...
>
> On Tue, Jul 31, 2012 at 2:02 PM, mike at trausch.us <mike at trausch.us> wrote:
>
>> On 07/31/2012 01:28 PM, Jim Kinney wrote:
>> > Note: most of the installation code is good practices. A missing part
>> > is(was?) distance between cable runs and 4'/8' florescent lights.
>> > Ballasts cause _nasty_ interference.
>>
>> So running signal wires inside of grounded shielding is a good idea.
>> Whoda thunk it?  :-P
>>
>> That said, there are ways to do that without mandating the use of EMT as
>> well.  Again, I could be understanding the code incorrectly, but it
>> seems that EMT is required even for Cat3/5/6 and their standard
>> applications.
>>
>> EMT is nice, don't get me wrong.  It'd be perfectly suited to the task
>> of grounded shielding, as well.  But a flexible aluminum tube can do the
>> same thing for runs of extremely low-voltage wire such as Cat6 in a
>> gigabit network.
>>
>> Even category 3 runs -48VDC and a limited amount of AC current on the
>> line when used in telecommunications.  Not that I've ever heard of
>> anyone killing themselves on a simple, analog telephone wire.  But hey,
>> if someone is so talented as to be able to do that, more power to 'em
>> (hah).
>>
>> What _really_ boggles the mind is that it is seemingly impossible to
>> independently bootstrap oneself in Georgia for almost anything.  One
>> seriously doesn't need to be an expert in the NEC to install Ethernet
>> networks.  They're friggin' Ethernet networks!
>>
>> ±2 VDC on any of the 4 pairs at any given time, unless it's 0 VDC.  Not
>> terribly difficult.  And honestly, if your Ethernet is carrying voltage
>> due to some catastrophic, multiple cascade failure of the surge
>> protections in place, you have much bigger problems, like the hundreds
>> or thousands of dollars of equipment you're likely to need to replace in
>> order to bring your network back online.  Besides, the conductors in
>> cat6 are small enough that they'll simply melt if any "real" juice is
>> applied to them; it's not like they can carry lots of power.  The most
>> that can be carried in a configuration which utilizes PoE is about 15.4
>> watts over two pairs (802.3af-2003) or 25.5 watts (802.3at-2009).  Some
>> non-compliant applications will pull up to 51 W by pulling power on all
>> four pairs, but that's prohibited by the standard.
>>
>> By my off-the-cuff calculations, that means that in the original PoE
>> spec, 7.7 W per pair (there are two used).  In PoE+, 12.75 W per pair.
>> In non-standard and non-compliant (e.g., forbidden by 802.3at-2009)
>> applications, it's still 12.75 W per pair, but across all four pairs.
>>
>> According to Wikipedia, this is still within the limitations of the
>> cabling itself; my understanding of the information there is that you
>> can have up to 31.25 V and up to 577 mA (for a total of 18.61 W per pair
>> or 74.43 W across all four pairs) before exceeding the capability of the
>> 24 AWG conductors and causing failure.
>>
>> Given that a standard gigabit Ethernet implementation we'll only have ±2
>> VDC at about 100 mA max, and being that any failure of, say, a switch or
>> a computer to isolate the cable from an overload will simply cause the
>> wire to melt... perhaps I should experiment and see what happens in
>> controlled but extremist conditions.  But I seriously doubt that I could
>> cause a fire in a cat6 cable without introducing some extremely insane
>> operating conditions the likes of which I have never seen on any of
>> several hundred networks.
>>
>> Any of the handful of networks that I've seen and had to rebuild after
>> damage from lightning never even suffered failure catastrophic enough
>> that the Ethernet cables caught fire or anything like that.  The
>> conductors have melted first, essentially acting as a fuse for
>> protecting the cable itself, in every single instance I can recall
>> first-hand.
>>
>> Wanna do forensics?  Work under someone before you can pass a test.  Oh,
>> and get a degree in a criminal justice program or related field. (!)
>> Oh, and work in law enforcement or under a private security firm which
>> is licensed for at least two (?) years. (!!)
>>
>> Wanna install cat6?  Work under someone before you can pass a test.
>>
>> Hey, when will it get to the point where I need an apprenticeship and a
>> Guild membership so that I can flush the toilet and send my waste to the
>> county processing plant?  :-P
>>
>>         --- Mike
>>
>> --
>> A man who reasons deliberately, manages it better after studying Logic
>> than he could before, if he is sincere about it and has common sense.
>>                                    --- Carveth Read, “Logic”
>>
>>
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