Re: lightning

["Bill" <billnomailnospamx@xxxxxxxxx>]

"Christopher Glaeser"  wrote in message
>
> 1. Judging from the photo, the shielded low-voltage wire appears to be a
> sparse spiral.  Is that sufficient?
>
> 2. The shield from the outdoor low voltage wire should be grounded to the
> metal power box, correct?  What about the Cat5?  Is the shield just cut
> back with the insulation?
>

So long as there is conductive metal all around the wiring, then you have a
"Faraday cage" protecting the wire from outside noise. Some cheaper
shielding does not shield the wire 100% and will let in some outside noise.
This is typical of some cheap coax cable which has a loose wire braid with
"holes" in the braid through which noise can enter the wire.

And you would just peel back the shielding, then strip the wire as normal.

As to peeling back the shielding and having an inch or two of exposed
unshielded wire, this is no big deal for your application. The problem with
outside noise and a nearby lightning strike is via a LONG length of
unshielded wire. This acts as an antenna. And a nearby lighting strike can
"induce voltage" in a long length of wire. This is how a transformer works.
Called inductance...

...I looked for an easy to understand web page explaining inductance, but
only found "Einstein" level pages with tons of math formulas. Anyway it is
sort of like running a magnet under a table to make a metal object move on
top of the table. And the magnet makes the metal object want to move or the
electrons in the metal object to move producing electricity. Like how a
generator works. Place another piece of metal in-between and the magnet no
longer has an effect on the metal object (Faraday).

Then if you place a piece of metal in-between the magnet and the object on
the table and that piece of metal has one small hole in it, that will not be
enough of a "leak" to get the object to move. Thus equal to your couple of
inches of exposed non-shielded wire. Not a big deal in your case.

Then as to grounding the metal shields from around the wires, very good
question! There is a thing called a "ground loop" and this can be a problem
which in itself causes noise in electronic circuits. That is the ground at
an outside box may be at a different "potential" than the ground inside a
house! And if you ground the shield at both ends, this could cause a small
voltage to flow. So best to only ground one end of the shield for your
application.

THEN as to what someone posted above as to how lightning seeks a path to
ground, keep that in mind. You might be better off not grounding any of the
shields inside the house, rather just outside. Thus you would not be giving
the lightning a path to inside the house.

And you might not want to ground the metal boxes or shields on the wiring at
all, but simply connect the shields to the metal boxes outside and leave it
"floating". Then it would have a Faraday shield, but would not "attract"
lightning. Only do this with 100% low voltage stuff. Any house voltage
(120V) metal cases and any shielding connected to those metal cases would
need to be grounded for safety.

BTW, it you get a direct hit, it is going to fry everything. But if nearby,
the shielding and metal boxes will protect the wiring and sensitive
electronics.

Ground loop...
http://www.epanorama.net/documents/groundloop/