Water pipe makes a good low resistance ground. Human safety
demands a low resistance ground for reasons that include, for
example, neutral wire failure.
Transistor safety uses grounding components for a different
purpose. Transistor safety requires a low impedance ground.
If connecting a bonding wire to a water pipe 50 feet away, the
house has a low resistance ground and a high impedance
ground. High impedance means ineffective transistor
protection. Take this fax machine protection as an example:
http://www.epri-peac.com/tutorials/sol01tut.html
Unfortunately AC electric and phone service enter on two
sides of the house - bad construction practice. If 'pictured'
water pipe is a significantly lower impedance connection, then
a destructive transient will find the building's single point
earth ground via NID (surge protector) and cold water pipe.
But in reality, cold water pipes are typically too long, too
many solder joints, too many sharp bends, etc. To connect to
a common point, an additional wire that is not shown. All
increase impedance so much that a transient will also seek a
destructive path to earth via the fax machine. Incoming on
phone line. Outgoing on AC electric wire. Fax machine
damaged because NID was not earthing via a low impedance earth
ground connection.
This demonstrates but one example of how portable phone base
stations, fax machines, and modems are so easily damaged.
Essential to transistor protection is a connection from each
incoming utility, either through a protector or by direct
hardwire, to a common earth ground connection. Single point
earthing most easily accomplished with a ground rod wired
short (ie. less than 10 feet) to AC electric box. For
transistor safety, all incoming utilities (telephone, cable
TV, satellite disk) must make a less than 10 foot connection
to this common earthing point.
Another problem with using a cold water pipe as an earth
ground connection - code wants each utility to be earthed less
than 20 feet to the common point. Pipes (ie outside faucet)
often are just too far - more than 20 feet - to the common
earth ground point.
Labeled 'arrestor' in that picture, same rules apply to that
a 'whole house' protector. Arrestor must make a short
connection to the single point earthing. 30 feet across the
basement to where cold water pipe enters the building is all
but no earth ground to that 'arrestor' - too much impedance.
An earth ground rod adjacent to a breaker box is also for
transistor safety - a low impedance earthing connection.
A most common source of transistor destructive transients to
things such as computer modems is incoming on AC electric.
Wires highest on poles - AC electric - are more often struck;
not lower cable and phone line. If that 'arrestor' does not
connect AC electric wire to earth ground, then a transient
(again in that picture) goes into fax machine (modem) on L
connection, then destructively out on phone wires to NID and
to earth ground. Above demonstrates why fax machines, modems,
and portable phone bases stations are so often damaged by AC
electric wire transients.
Above is pre-WWII technology now made necessary in dwellings
due to something new - transistors. Transistor protection is
often little understood by some electricians who only
understand NEC requirements. NEC does not require transistor
protection. NEC is for human protection. Transistor
protection is defined by the most critical component in a
protection system: earth ground.
Gfretwell has posted how homes are now being built so as to
provide superior transistor safety in pictures at:
http://members.aol.com/gfretwell/ufer.jpg
Another also demonstrates better earthing. None of this
would be required if city water pipe was sufficient as earth
ground for transistor safety:
http://www.knology.net/~res0958z/
Above was earthing for secondary protection. Also necessary
is earthing for primary protection:
http://www.tvtower.com/fpl.html
Oscar_Lives wrote:
"w_tom" wrote in message
...
What you have described as a "grounding electrode system" is
the classic single point earth ground proven even in the 1930s
to 'harden' high reliability facilities from electronics
damage. Yes, separate earth grounds must be interconnected
also for human safety reasons. But that is completely
irrelevant to the original poster's question, is completely
irrelevant to what I have posted, AND is what we want when
earthing for transistor safety.
What is this "transistor safety" that you keep talking about?
Please explain.