"Roy Lewallen" wrote in message
...

Ian White, G3SEK wrote:
Roy Lewallen wrote:

My NEC book is getting pretty old now, 1990, but at that time,
multiple grounds were often not only permitted, but required. And a
*minimum* spacing was sometimes specified.

For example, section 250-84, Resistance of Made Electrodes: "A single
electrode consisting of a rod, pipe, or plate which does not have a
resistance to ground of 25 ohms or less shall be augmented by one
additional electrode of any of the types specified in Sections 250-81
or 250-83. Where multiple rod, pipe, or plate electrodes are installed
to meet the requirements of this section, they shall be not less than
6 (1.83m) apart."


Is it something like this? Multiple ground rods are permitted
(encouraged, even) in order to establish one good ground connection; but
the wiring system can have only one ground connection.

Well, yes and no. As I read it, all the ground electrodes have to be
connected to the service box, as they are at my house. I have a
water-pipe ground and a ground rod; they're physically widely separated,
and each has its own conductor going back to the service box. So I guess
you could call the service box a single "ground connection". The neutral
and safety ground conductors of each circuit are individually grounded
at the service box also, giving further support to interpreting its
function as a single ground connection. A number of ground methods and
devices are approved, such as buried rods or plates, concrete grounds,
or (in my old code book anyway), water pipe ground. Some can be used
alone and others need a second ground device as a supplement. When
multiple ground rods are used, the code has requirements for bonding
them to each other.

Roy Lewallen, W7EL
Using a cold water pipe is a bad idea and is not allowed by some local codes
even though it may be allowed by the national code. This is partly because
it should only be connected to the ground system at one point and it is too
easy to connect it at more than one. Also there is the possible interaction
with gas lines and plumbers and home owners sometimes modify plumbing in
ways that make them poor grounds..