|
Posted by w_tom on July 14, 2008, 11:10 am
> I don't know. How is the quality of an "earthing" determined or
> measured?
"Measuring" earthing is not practical. Although we can measure
earth resistance, still, that does not determine quality of that
earthing 'system'.
In many cases, a single 10 foot earth ground rod is more than
sufficient if soil is conductive and moist. In simple terms, fine
soil tends to be more conductive. Sandy soil is typically less
conductive. Current habit is to install two ten foot ground rods
separated by more than 6 feet to make that single point ground more
conductive.
We earth to achieve a more conductive connection. But that
connection can never be sufficient. So we single point earth to make
earth beneath the building more equipotential. But we can never
create sufficient equipotential. So we make the earthing electrodes
more conductive.
If in sandy soil, other techniques include a halo (loop) ground
buried around and outside the building. In FL, with more lightning
and sandy soil, also standard is Ufer grounds. Effective because
concrete is an electrical conductor:
http://members.aol.com/gfretwell/ufer.jpg
http://scott-inc.com/html/ufer.htm
http://www.mikeholt.com/mojonewsarchive/GB-HTML/HTML/UferGroundPsi~200309=
30.htm
http://www.psihq.com/iread/ufergrnd.htm
Ufer grounding means surge protection is installed when footings are
poured - not when the electrician arrives to install wires. Meanwhile
a utility offers suggestions on how to fix defectively installed
earthing:
http://www.cinergy.com/surge/ttip08.htm
That buried interconnection wire converts multiple earthing electrodes
into single point earth ground while increasing conductivity.
Unfortunately we don't always know what is in the earth. For
example, one building was adjacent to a vein of graphite. Or a
transcontinental pipeline is buried nearby. Surge instead ignored
service entrance ground, passed through the building, to obtain earth
via more conductive graphite. Solution was to surround that building
with a buried conductor (halo ground) so as to make earth beneath the
building into a big single point ground - create equipotential. Surge
that traveled underground around and outside a building need not enter
the building (see Faraday shielding to appreciate the concept).
In another case, lightning would repeatedly strike an exterior
bathroom wall. They installed lightning rods. Lightning struck that
bathroom wall again. Why? Plumbing inside that wall connected to
deeper and more conductive limestone. Lightning rods were only
earthed 10 feet in sand. The bathroom wall, not lightning rods, made
a better connection to earth borne charges.
Make earthing as best as practicable. Then if damage does occur,
learn why that earthing (or connections to it) was not sufficient. No
good way to lest earthing without an actual lightning strike. Best we
can do is install earthing correctly using what has been well learned
the past 100 years.
Many assume a cold water pipe makes a best earth ground. Well, it
is conductive but it does not necessary provide equipotential. Also
it may be too far away (ie more than 10 feet). Pipe joints (ie
solder) may also compromise protection.
Another problem is a water well. Lightning may pass destructively
through a building to obtain earth via that well pump. Just another
reason why we want the service entrance ground to be a most conductive
earth ground AND why all incoming wires (overhead or buried) enter a
building connected short to the single point earth ground.
Described previously were factors that can increase wire impedance
and compromise surge protection. That earthing connection must be
short ('less than 10 feet'), no sharp bends, separated from all other
non-ground wires, not pass through or inside metallic conduit or
sheets, no splices, and all ground wire routed separately to meet at
the earthing electrode (single point earth ground).
One cannot have too much earthing. However most locations with
conductive soils have massive and probably sufficient earthing with
only one 10 foot ground rod. Every addition to the earthing system
has a diminishing return. But many facilities install massive
earthing system to obtain just a little better earthing. Ham radio
operators who learned this stuff will install better earthing systems:
http://home1.gte.net/res0958z/
Routine is to have direct lightning strikes without damage. But if
damage does occur, then plug-in protectors are not a solution.
Instead the earthing system is reevaluated for defects or
automatically upgraded:
http://www.psihq.com/AllCopper.htm
http://www.copper.org/applications/electrical/pq/casestudy/nebraska.html
> In one memorable instance at KROA, lightning ignored
> the existing grounding system and instead followed the
> coaxial cable directly into the transmitter room. The hit
> destroyed expensive equipment, taking the station off
> the air for several weeks. Luckily, no one was injured
> but the incident was a strong indication that the
> grounding system should be improved.
>
> Based on a belief that "too much" grounding was
> attracting lightning strikes, grounding connections on the
> tower's six sets of guy wires had been disconnected
> sometime in the past (Figure 4). This action may, in fact,
> have helped direct lightning discharge current down the
> antenna tower itself, bringing the strike closer to the
> studio/transmitter building.
>
> CPC has found that when near-surface ground resistance
> is high, it is often better to drive one or more deep
> electrodes than to rely on multiple shallow rods.
>
> First and foremost, the entire electrical system must be
> properly installed according to NEC requirements.
> Equipment and system grounding circuits must be connected
> to the neutral bus only at the primary electrical panel (first
> disconnect), from which point a connection must be made
> directly to the primary grounding electrode. ...
> It is absolutely imperative that all surge suppressors be grounded.
For more information about earthing, see an industry benchmark -
Polyphaser's application notes such as:
http://tinyurl.com/3y747k
http://tinyurl.com/2rsdhj
and other papers at:
http://www.polyphaser.com/technical_notes.aspx
Meet post 1990 National Electrical Code requirements - then exceed
them:
http://www.eham.net/forums/Articles/40885
Welcome to an introduction to earthing - what provides the surge
protection.
http://www.citelprotection.com/citel/grounding.htm
> A protection system with a poor ground is the same as having no protectio=
n at all.
So we do what has been proven effective elsewhere for the past 100
years. Then we learn from the experience. On average, typically
destructive surges occur once every seven years. However that number
can vary significantly (due to geology and other factors) even within
the same town.
| 
Yahoo! 
Google 
Windows Live 
del.icio.us 
digg 
Netscape 
XML Sitemap