That's the part that doesn't do anything for me. What's the real
point in any ground rods, etc at the building. Isn't there a ground
rod of some sort at the transformer 100' away. How much potential
difference could develop between the house and the transformer?
On Tue, 10 Nov 2009 12:21:26 -0800 (PST), jamesgangnc
A bunch. We typically measured 25-35 volt spikes on our dranitz logger
between 2 grounding electrode systems in separate buildings.
That is why I always suggest bonding the GES if you are stringing data
cables between them.
If you have a water pipe or concrete encased electrode you have a much
better earthing electrode than is at the transformer.
If there is a very near lightning strike, like at a tree, the potential
of the earth at the transformer and your house could be many tens of
thousands of volts different. Grounding electrodes at your house pull
the potential of the wiring in your house nearer the earth potential at
your house (which is the potential of concrete basement floors and
walls). Even though your garage is very near your house, there could be
a difference of many thousands of volts between the earth at the house
If lightning hits the utility pole there is an even bigger difference
between the earth potential at the pole (and utility wires) and your house.
Earthing at the house helps protect against surges coming in on power,
cable and phone wires.
Around here I have seen missing copper wire to the earthing electrode at
utility poles where it has been stolen.
You need a grounding electrode system, which consists of
interconnecting _all_ the electrodes present. Then if there aren't
enough present, you need to add more. See NEC article 250 part III,
starting at 250.50.
Common electrodes are: qualifying rebar or a 20' length of bare #4
copper wire in the footer of a foundation (required for new
construction, otherwise only if accessible), any metallic water pipe
that is buried for at least 10', and ground rods. The first one
(called a concrete encased electrode, or CEE) is deemed adequate by
itself; the other two are not adequate by themselves are require a
second electrode (which could be a ground rod).
Hope this helps.
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