TELECOM Digest OnLine - Sorted: Re: Electrical Grounding

Re: Electrical Grounding

w_tom (
18 Nov 2006 08:54:43 -0800

Dan wrote:

> What is the effect of having an electrical ground close to a telecom
> (telephone land line) ground and what is the recommended distance they
> should be apart.

Once they did not even have to be the same earthing electrode. But
transistors did not exist then. Others have cited wire resistance as
a reason for short distance. However that voltage different would be
still near zero if the distance was 20 feet or 200 feet. They are
discussing a common ground for resistance.

However wire impedance is why wire distance must be short. Wire too
long, with sharp bends, inside metallic conduit, or with splices would
have little effect on wire resistance but would adversely affect wire

Code calls for a distance less than 20 feet. You really want a ground
wire to common electrode of 'less than 10 feet'. Furthermore, you
want that wire routed separate from all other non-grounding wires AND
not meet other grounding wires until all meet at the earthing

Furthermore, if telco earth ground electrode is not same as earthing
electrode for cable or AC electric, then consider this recommended
solution. Figures show good, recommended and bad earthing solutions:

Carl Zwanzig wrote:

> That said, the ground that you find in an outlet should not be
> trusted, run a wire directly to a metalic water pipe (but not a
> sprinkler), building steel, or back to the electrical system grounding
> conductor. (IIRC, in some cases, which I'm not going to look up at
> the moment, you're -required- to bond to the electrical system
> ground.)

Do not connect a grounding wire to a water pipe. That connection is
not permitted by code and suffers same problem of a grounding wire
that is too long. Required connection to water pipe is only to remove
electricity from that pipe for human safety.

But again, earthing is also for transistor safety. That means wire
impedance - length of that wire must be short.

Mike Holt article cited by Neal at says:

> Grounding is the intentional connection to earth through
> a connection of sufficiently low impedance.

Impedance -- not resistance as others have posted. With impedance,
transient current can result in voltages too far from zero. A shorter
wire means less impedance - voltage closer to zero. Essential that
all wires entering a building be at same voltage potential - for
transistor safety and for human safety. That means all earthing
(which is only one type of grounding) must be to a single point
earthing electrode.

Impedance is also why grounding to wall receptacle safety ground is
not sufficient for grounding telecom equipment. That telecom ground
must be low impedance. AC electric safety (equipment) ground is low
resistance -- but not low impedance.

Do not ground to water pipes. Once that connection was acceptable.
Today grounding to water pipes is a violation and typically not a good
low impedance ground.

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