[ale] Low voltage work in GA

[John Pilman]

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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