[TowerTalk] Shack ground

[jimlux]

Mickey Baker wrote:

> 
> The object (and original discussion) was how to provide a better path from
> your communications ground to the SPG than through the household appliances
> - from one point on earth to another.
> 
> Every electrical outlet in every house with single phase service is
> connected to the SPG through the ground lug on the outlet, both of which
> should be bonded to neutral at the SPG.  When you connect your ground wire
> from your ground rod to your rig, you then are connecting your ground rod to
> the SPG through your rig's cabinet.
> 
> The bonding REQUIRED by the NEC provides that multiple ground systems be
> bonded directly for a number of safety reasons as well as a direct lightning
> strike. I'll allow the group to ponder this - I can think of one case
> related to measuring 100 Ohms to ground from an outlet!
> 
> For almost all cases, a #6 solid wire from the SPG ground to your tower
> ground rod system will be fine to keep your home safe. Certainly it is
> compliant with NEC. Compliance with code is my reasonable accommodation of
> risk. It is what is required of a contractor or engineer, legally.
> 

It should be noted that the NEC bonding requirements aren't driven by 
lightning protection (you need to look to NFPA 780 for that). It's more 
to insure that faults have a reliable path back to somewhere useful, so 
that they'll trip the overcurrent protection (OCP) device and prevent fires.

A lot of the details in Art 250 (and Arts 800,810,820) are aimed at 
making the connection mechanically reliable (e.g. using a fairly large 
conductor, requiring compression clamps or exothermic welds) so that the 
protection doesn't degrade over time. And, in making sure that the wire 
is big enough to carry enough fault current to trip the OCP.

The fact that bonding according to NEC helps with lightning protection 
(and induced transients in general) is all the better.  However, it's 
possible to build a NEC compliant bonding system that actually 
aggravates transient problems (imagine having two very long wires to 
bond a receptacle at one end of a building to the service entrance 
bonding point, and then, route the wires on opposite sides of the 
building, making a giant loop antenna). I think the NEC would allow you 
to run your bonding wire suspended 10 feet above the roof, as long as 
that happened to meet all the other requirements.

NEC cares about "don't burn down the building" and "don't kill/injure 
the occupants".  Disasters like the MGM Grand hotel fire and swimming 
pool shocks feature a lot in the discussions of code makers when talking 
about Article 250.

NEC cares a little bit about "protect the contents" but really, for that 
you want to look at things like IEEE-1100 (The Emerald Book)"IEEE 
Recommended Practice for Powering and Grounding Electronic Equipment"
which has the following great statement:

"3.3.1 Grounding for Safety
A lot has been written on grounding for industrial and commercial power 
systems. Proper grounding is essential to safe and satisfactory 
performance of a power system. There are generally three requirements 
for such grounding, as follows:
a) Providing a low-impedance path for the return of fault currents, so 
that an overcurrent protection device can act quickly to clear the circuit
b) Maintaining a low potential difference between exposed metal parts to 
avoid personnel hazards
c) Overvoltage control
"

And then, in the next section
"3.3.2 Referencing for Performance"

So they clearly distinguish between "ground" and "reference potential" needs


A fascinating paragraph comes up much later in the emerald book (since I 
have it open..)

Importantly, and because of the high-frequency components in the 
lightning strike’s current path, special wiring and grounding techniques 
must be used to properly conduct lightning currents on sites where
electronic equipment is installed. The use of appropriate low-inductance 
wiring means with appropriate I2t rating for the conductor(s) and 
multipoint grounding (MPG) *as opposed to* high rms current-carrying
capability techniques and single-point ground (SPG) arrangements are the 
core of this special design requirement.

Note what that says... SPG may be the *wrong* way to deal with lightning.