If a small diameter ground rod (1/2" or 5/8") doesn't give the 25 ohms
(or less) to ground.....would a larger (3/4" or 1") diameter rod in the
same soil do better?
Is the soil the culprit or the soil / rod contact resistance? The
larger diameter rod contacts more soil..better grounding behavior?
This is just opinion; I don't have facts to back it up...but..
The two are related. You have two resistors in series, one
represents the rod to ground resistance, the other ground resistance
back to the utility ground. The same factors that increase one (poor
soil conductance) influence the other. But I'm SWAGing that the ground
resistance back to the utility dominates most of the time. If you're
right on the hairy edge, a bigger rod might help enough. But I'd be
inclined to go with a longer rod, not larger diameter, because it
might get you to better soil, where a larger diameter rod just makes
more contact with the same (lousy) soil.
As I said, just conjecture on my part...
The diameter of a rod has very little to do with the resistance of the
ground. The length will have a large effect. Ground rods should be
atleast 6 feet apart. Much closer and there is enough coupling through the
ground that the second rod does not reduce the resistance by very much.
The problem is the soil resistance. Increasing the rod diameter does not
change the resistance by much. I agree with other posts that increasing
the length with a longer rod or multiple rods is effective.
I've found (through discussing with electricians) that soil conditions
have a greater effect on grounding effects, when comparing 1/2 to 1"
Remember, if you are concerned about resistance for code, just drive
the second rod. Then you don't even have to check ohms, since even if
you don't have the min, the code only requires a second rod.
The idea is that everyone grounds the 'neutral' the voltage on teh
grid will be 'stablized' so you are only adding a drop to the bucket
(the grid) and helping your line to your house.
FYI: Ground rods aren't for ground faults.
tom @ www.BlankHelp.com
I know that the code says if you fail the 25 ohm level drive another
rod 6' away.
But I was curious about the variables that effect the performance of
the rod / soil system. I also really liked the technique that someone
posted as to how to test a ground rod; simple & pretty cool :)
Don't everyone jump on me but I'm not a huge fan of codes in
general.....they tend to substitute blind adherence for understanding
of what's rally going on
ie what is the intent behind the code.
also having worked on committees I know that codes tend to minimum
standards that everyone in the room at the time could live with; either
due to lateness or lack of energy
if one has ground rod that "fails" & puts in another.....conceivably
one could still be above the 25 ohm level but still "meet code".
I'd rather know that my installation was good & I'd rather know what
parameters improve my chances of having a good installtion.
that's why I asked if bigger was better.
looks like longer is better.... I gues that's the way things go :)
Codes are designed for safe operation of you electrical system. The
completion of work below codes is dangerious, and the completion of
work 'over' codes is just extra money spent. Also, if you don't do
'standard' work, another electrician following you will have to
'figure out' what you did.
So, depending on who is spending the money, you might be wasting it.
I agree with your post about work "over" or "under" code (& the
non-standard stuff potenttially being hard to figure out later, that
should be avoided)
but I happen to know of examples of work when done per code (UBC /
IBC structural stuff) will yield a poorly performing system but it
satisfies the code
Example....with the new wood preservative for sill material the code
allows galv or stainless fasteners.....the perfromance difference
between glav & SS is miles
another example....block wall rebar requirements vary all over the map
( a factor of ~3 low to high) from city to city in
SoCal.....earthquakes do not stop at the city limits
I actually have more faith in the NEC (maybe 'cause I don't know it as
well as the structual stuff)
but if the code wants 25 ohms max to earth but it lets you do two rods
& not test?????......where is the performance in that? It's an
assumption that it's probably "good enough" but it could be pretty
bad, couldn't it?
the code is a minimum standard (like a doctor that just barely passed
med school) I'd rather go a little over code (cost impact is usually
prety small) than "at code".....there might be very little margin if
things do not go as expected
btw independent of who is spending, unnecessary overkill is wasting
money.......money spent for no performance increase or a performance
increase that is "unnecessary"
I tend to use SS fasteners for outdoor applications; IMO SS fasteners
(esp in trim, fence, deck or patio cover applications) are worth the
extra cost. SS fasteners in dry locations indoors are wasting money.
I seen a lot of really nice gates ruined by rust stains from galv nails
when less than $20 worth of SS fasteners would have prevented
it....over kill & wasting money or money well spent
I have been involved in this a few times. The thinking is that if 2
rods will not get you 25 ohms 20 rods probably won't do much better.
Where we did need performance (radio towers, toll booths etc) we went
to extremes that a normal residential contractor wouldn't be able to
and still have an affordable home.
Early in the construction process the Ufer (concrete encased
electrode) is probably the best. If you are grounded to the foundation
of the building, at least you are safe in reference to the concrete
floor in the garage or the tile in the ground floor bathroom
On Tue, 19 Sep 2006 08:26:20 -0400, firstname.lastname@example.org wrote:
Amen brother, this concept is so hard to get people to understand.
But once they understand, they start crying about GFCI's not working
then, and I have to shake my head and walk away. Unless I'm charging
by the hour, then I'll take all day to explain how GFCI's work, and
normal breaker overcurrent, and short circuit protection work with no
grounding electrode conductor. Oh with drawings too!
I love when people agree to pay me by the hour!
Having worked electronic facilities where the ground resistance has to
be less than 5 ohms, I've tried a lot of tricks to get the resistance
down. There is no sure-fire trick to the process, but rod diameter is
WAY down the list of possibilities. The number of rods (up to 12) and
the depth of the rods (sometimes 40ft) is the usual key, but if that
doesn't work try pouring 25lb of salt in a hole next to the rods and
water it good. Salt is a good conductor & decreases ground resistance.
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