On Jan 23, 11:07 am, m...@sushi.com wrote:
> Isn't there an issue with the potential difference between the grounds
> of the two buildings?
Yes. And the technology used even 100 years ago (and often unknown
to computer techs) makes that potential difference completely
irrelevant.
The discussion is about an ethernet cable between two buildings.
Why is this a problem? Lightning striking one building can find earth
ground, destructively, via equipment in the other. IOW one building
becomes a lightning rod connected directly to electronics in the other
building. Damage is created only when current has both in incoming
and outgoing path through that electronics.
Every building in town connects to the telco's Central Office (CO)
$multi-million computer. Why is that computer not damaged? According
to 1950s research, that computer (connected to overhead wires all over
town) is typically threatened by 100 surges during every
thunderstorm. Why no damage? Again the same technology used even 100
years ago. Every wire entering (or leaving) the building must first
connect to earth. Every wire inside every cable.
If every wire connects directly to earth, then no service. If
every wire inside that ethernet cable connects directly to earth, then
no ethernet. So we make that earthing connection via a protector.
The protector is not protection. Earthing is the protection.
Protector is simply a connecting device. Better earthing and a
shorter connection to earth make protection better. Yes, distance to
earth affects how well a protector performs. Therefore that CO puts
each protector (for each wire) at earth ground and up to 50 meters
distant from electronics.
If your ethernet cable makes that earthing connection where it
enters every building, then electronics in the other building are no
longer connected to a 'lightning rod' - that first building. That
earth ground must be same earthing electrode used by AC electric,
telephone, etc. That connection but be short (ie 'less than 10
feet'). Protection for everything in the building made even better
when the earthing system is enhanced. Protection is defined by the
quality of earthing and how every incoming wire connects to earthing.
Two front page articles discuss this in Electrical Engineering Times
on 1 Oct and 8 Oct 2007 entitled "Protecting Electrical Devices from
Lightning Transients" at:
http://www.planetanalog.com/showArti...leID=201807127 http://www.planetanalog.com/showArti...leID=201807830
Notice everything is about earthing and how that earthing connection
is established. It is routine to have direct lightning strikes
without damage. But when one assumed the protector is some kind of
'magic box' protection, then damage becomes acceptable.
Important to inter-building ethernet (both overhead and underground)
is how each end connects to earth where it enters the building. No
earth ground means no effective protection - which scam protector
promoters forget to mention. Why is damage unacceptable in any telco
CO? It connects to every building in town and suffers no damage.
Proper earthing means no destructive potential difference. As Jeff
notes, ethernet already has (the required) internal protection. All
electronics do. But that internal protection can be compromised IF a
human does not properly earth every incoming wire. Effective
protectors mean a surge (potential difference) is earthed before
entering the building - so that protection inside electronics is not
overwhelmed.