> NESC rule:
> As I understand the rules, ALL facility grounds must be
> tied together with a minimum size conductor back to the
> service entrance. Maybe someone can qualify this, but I
> don't 'think' there is a distance limit. This is a power
> frequency safety issue, NOT a lightning safety design issue.
Failing to bond the power line entrance ground (safety ground),
tower grounds, "entry window", and telephone/satellite/cable
grounds together is a prescription for equipment damage.
USB interface chips are particularly sensitive to reverse
voltage. If the station ground (tower/entry window) moved
even a few volts above the power (safety) ground - the
"ground pin" on the computer power supply - and the shield
connection on the USB cable is questionable you're likely
to lose a USB UART.
The same problem occurs with cable modems, telephone
modems, DSL modems, network switches/hubs/firewalls,
etc. if the telephone and cable grounds are not bonded
to the power ground.
Unless everything enters your home at ONE location, install
a perimeter ground system and bond everything to that.
73,
... Joe, W4TV
> -----Original Message-----
> From: towertalk-bounces@contesting.com
> [mailto:towertalk-bounces@contesting.com] On Behalf Of jimlux
> Sent: Sunday, August 09, 2009 10:39 AM
> To: K1TTT
> Cc: 'Towertalk e-Goups'
> Subject: Re: [TowerTalk] "Faraday Shield" for Coax and Control Lines
>
>
> K1TTT wrote:
> >
> >
> > NESC rule:
> > As I understand the rules, ALL facility grounds must be
> tied together
> > with a minimum size conductor back to the service entrance. Maybe
> > someone can qualify this, but I don't 'think' there is a distance
> > limit. This is a power frequency safety issue, NOT a
> lightning safety
> > design issue.
>
>
> No length limit. All systems must be bonded together. There are some
> really funky schemes you might see in a factory with large motors fed
> from 3phase (where the center point of the Star/Wye is tied to ground
> with a resistor).
>
>
> >
> > Next, on the separation of the ground... there are 2
> important cases
> > here. First, the near miss case:
> >
> > As I stated above from simple physics, the bigger the loop
> the higher
> > the induced current. For conductors above ground the air
> filled loop
> > is easy to see. The current induced in the loop created by raised
> > cable runs and the ground is relatively easy to calculate
> and can be
> > used to predict the peak voltages at gaps or arresters in
> the loop.
> > Simple rules are: the higher the cable is above ground the
> higher the
> > voltage/current, and the longer the loop the higher the
> > voltage/current... so grounding long cable runs at intermediate
> > support posts would reduce the voltages seen at the ends from near
> > misses.
>
>
> There's also an interesting phenomenon where the transient
> propagating
> down the above ground line flashes over at an intermediate
> point (e.g.
> at a protective gap), and the magnetic field from THAT new
> transient (as
> the spark discharges the "charged" transmission line) induces a
> transient in the down stream line (the field is smaller than the
> original lightning strike, but it's a LOT closer to the victim loop).
> This is why series transient protection is recommended in some cases.
>
> _______________________________________________
>
>
>
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