10 Sep
2014
10 Sep
'14
1:01 a.m.
On Fri, 5 Sep 2014, Chuck Guzis wrote:
On 09/05/2014 04:32 PM, Eric Smith wrote:
NEC 250.56
If a single grounding electrode (rod, pipe, plate) does not have a
resistance to ground of 25 ohms or less, the NEC requires you to install a
second electrode at least 6ft apart from the first. If the calculated
parallel resistance of the two electrodes is still not 25 ohms or less
(they have to be disconnected from each other and measured separately),
the NEC does not require you to install any additional electrodes or
attempt to lower the resistance to ground.
In common practice, you usually just install two rod electrodes at a
/minimum/ of 6ft apart (12 feet or more is better) and don't even bother
checking the resistance to ground. It is usually just faster and easier to
just install two rod electrodes than to measure the electrode to ground
resistance with a 3-point fall of potential meter.
I forgot to mention that in that case, code
requires that you mark the
GFCI outlet "no equipment ground". It also has to be marked "GFCI
protected", but that marking should be present when you purchase the
outlet.
I seem to recall something odd about NEC requirements for earth ground.
I seem to remember (and this is from long ago, so it's probably wrong)
that you get to drive one ground rod into the ground and if it measures
more than 600 ohms to line neutral, you can drive another one into the
ground and add it. After that, code is silent as to what to do next.
This can be a problem in dry, sandy soils. It just seemed a bit odd to me that code didn't
require more than just
the two attempts to get a good earth ground.
Likely due to diminishing return. Adding yet another electrode isn't going
to help very much. In specialty applications where a low resistance ground
is required (radio communications for example), they make special chemical
type ground rods. It is also common to install large ground rings in these
sorts of applications.