Earthing for two separate reasons. First for human safety
which is what the National Electrical Code (NEC) addresses.
All safety grounds long ago have to be bonded together. In
days past, the typically was to city water pipe where it
entered earth or, if using well water, to a dedicated earth
ground rod. In the past decade, earthing via a water pipe is
not acceptable no matter what that water pipe currently is -
copper or plastic.
Code since 1990 demands a dedicated single point ground at
the service entrance which means where AC electric enters
building. Telephone, CATV, satellite dish and even exterior
TV antenna should make a connection to this ground (either by
hardwire or via a protector). Code demands services such as
phone make a less than 20 foot connection.
Ground wires to pipes are for human safety - only to remove
electricity from those pipes. IOW wire once connected to city
water as an earth ground is now only to remove any electricity
that might leak into those pipes. Also required would be a
connection from hot water to cold water pipes at water heater
if pipes are metal; so that heater does not become part of the
grounding circuit. Water meter must also be bypassed so that
when water meter is replaced, then cold water pipes remain
connected to safety ground (meter man is not electrocuted when
changing meter).
Any connection to a pipe for grounding or earthing (ie a
cable connected to exterior water faucet) is not legal. Again
connections made to pipes only to remove electricity.
Ground to gas pipe is really a domain of the local gas
company. Some want gas line bonded to AC electric so that
electricity is removed from gas pipe - so that gas pipe will
not be used as an electrical path to earth. In one location,
neutral wire on transformer failed. AC electric then used gas
line as a path back to transformer via earth. This
unacceptable ground path eventually caused gaskets on gas
meter to break down; house exploded. But again, that safety
ground to gas pipe is according to local gas authorities.
Exterior gas line has a wire that is not an earthing wire.
All pipes have a wire so that radio signals can be transmitted
through pipe. This is how underground utility pipes and wires
are located - radio waves. That wire on incoming gas line is
how the locator service puts a radio wave on that plastic gas
pipe. Wire is not for earthing.
A meter typically cannot report a good ground; meter can
only report a defective ground. A ground that measures good
may simply vaporize - is too small - when it must conduct
larger currents due to an electric power fault. Meter would
not detect an insufficient ground. No replacement for visual
inspection. Furthermore, connections must be intended by the
designer to conduct electricity. A water heater was not
typically intended to connect hot water pipe to cold water
pipe. No matter what meter says, those two pipes still must
be bonded at that water heater (if metallic pipes).
volts500 wrote a good summary of bonding for human safety:
alt.home.repair entitled "Grounding Rod Info" on 12 July
2003:
http://tinyurl.com/hkjq
That covers safety grounds as required by NEC. Now second
reason is to enhance earthing for transistor protection.
Those safety grounds were defined by wire resistance.
Transient protection is about wire impedance. That means
connections to that single point ground must be short, direct,
and independent. No splices, no sharp bends, less than 10
foot connection from incoming utility to that single point
earth ground, and not inside metallic conduits or pipes. Phone
line connects directly to earth ground rod - not to AC power
earthing wire. IOW every earth ground wire must meet at same
single point location and should route separated from other
non-earthing wires. (non-earthing wire can suffer induced
surges if bundled with an earthing wire.) This technical note
demonstrates principles of earthing a tower and building:
http://www.erico.com/public/library/...es/tncr002.pdf
Building and tower are handled as if separate structures.
Each has it own single point earth ground. Any wire entering
each structure first connects to single point ground either by
direct connection or via components even sold by erico.com -
and others. Single point earthing for both structures is
interconnected by buried wire to enhance both earthings and
minimize potential differences.
One technique is to dig a shallow hole before driving
earthing rods. Buried wire connects each ground rod. A 4 or
6 inch plastic pipe placed in hole and backfilled so that
plastic cap can be removed to inspect wire bond to ground rod.
Size of buried interconnecting bare copper wire, depth of
that wire, and spacing between multiple rods are defined by
NEC guidelines. However, effective earthing materials (ie
ground rods) must be deep enough into earth where earthing
will not freeze. Frozen earth is not conductive - not a good
earth ground.
Minnie Bannister wrote:
...
No sign of any wire between the water pipe at any point and the
electrical system ground, and no sign of connection between hot and
cold pipes. Yet I did measure a low resistance ( 1 Ohm) between
the pipes on either side of the water meter (despite the
Teflon-taped joints), between the hot and cold water pipes (after
all, they are both conencted to the water heater), and between the
water pipes and the case of the electric panel.
...
No sign of a connection between the gas line and the electrical
grounding system either. But the gas line is connected to the water
heater, which is connected to the water line, which appears to have
a low-resistance connection (somehow -- see above) to the
electrical system ground.
...
I found where the wire from the telephone network interface box
goes: it is connected to a cold water pipe inside the house, which
is ultimately conected to the incoming water supply -- all copper
piping, except for the sprinkler system.
How recent is the requirement that all grounds must be bonded
together? This house is 30 yrs old.