RFI: NEC/Safety and RF grounding
 

RFI: A short overview of what a safety ground as required by the NEC
is and what a "good" RF gound
is. They are seldom the same thing.


Please note that while the National Electric Code does allow the use
of 5' grounding rods to be used on
telecommunication systems and CATV systems, and that 10AWG and 14AWG
wire can be used, I
have never been able to find a 5' UL approved grounding rod. I always
use 8' X 5/8" UL rated and NEC
compliant grounding rods and I also always use 6AWG solid copper wire
because first it isn't that much
more expensive and I decided that if a grounding system is required
for safety, then to just standardize
on the largest conductor to provide the best possible safety I could.
Besides it means I only have to
deal with and to stock the same wire size and NEC approved clamps.


The National Electric Code is not designed to make life for hams,
SWLs, Recording Studios, TV production
facilities, IT operations or anyones life complex. It is designed to
provide a reasonable level of safety from
electric shock or electrocution and to prevent fires as a result of
electrical fault conditions.

The short and simple version is that all grounds in a home, or
business, must be bonded in common.
Period.

An isolated ground rod is illegal. And may cause insurance problems
and legal problems should a
lightning event cause dame to the home structure, think fire, or shock
or electrocution. This isn't
idle legal theory but a fact. OSHA requires compliance with NEC
regulations. Injuries in a work place
that involve a violation of the NEC will result in heavy fines. I have
been there and seen the result. Our
local electrical inspector informed me that failure to comply would
result in prosecution if there was an
insurance claim or personal injury. A straight no none sense attitude.


The NEC now requires that the primary service entrance electrical
ground have a resistance of 25
ohms or less if only one grounding rod is used. If two ground rods are
used, then the 25 ohm
requirement is waived. The second ground rod can not be close then 6'
to the one at the base of
the service entrance. [8' would be more effective]. And that all
incoming conductors be bonded with
approved mechanical clamps to this primary grounding system. This
includes the over voltage clamp
in the telephone NID, the grounding block required on all cable or
satellite systems, and for any "ham"
or radio antennas. Coax for hams and SWL must have the outer
conductive braid effectively grounded
prior to entry to the residence, or as close to the entrance inside as
"practicable'. For single or balanced
wires feed lines, they must use approved lightning suppressors, and
the protective ground connection
must be bonded to the service ground.

All grounding conductors should be 6AWG solid copper wire. Grounding
conductors should be as straight
and direct as "practicable" and sharp bends must be avoided.

The NEC allows the use of a 2' square of non ferrous, ie copper sheet,
buried at least 24", or a ground
perimeter made of 6AWG solid copper wire that is at least 20' in
length and buried at least 24" deep.


The grounding system described above is a great safety ground. But
will seldom if ever make a decent
RF "ground'.

In particular, the lengths of grounding conductors can not exceed 1/20
of a wavelength of the frequency
of interest. The higher the frequency, the shorter the grounding
conductor can be.

6AWG is a large copper wire, but still a very poor RF grounding
conductor. The small size with respect
to wavelength creates considerable inductance, and this acts like a
resistor which decreases the
efficiency of a grounding conductor.

A better choice for RF ground conductors is 1/2" or 1" tinned copper
braid.

The best choice for a RF ground conductor is 2" or wider, copper
strapping.
Sadly very few of us can afford this step.



I will address exactly what makes an effective RF ground in the next
post because it is no obvious and
in many ways counter intuitive. And why we need a good "RF ground" is
a little understood issue.

Terry