We have a customer requesting FOP. the site already existing and constructed. When doing a FOP test which ground rod is used to start your test. I can understand if the site was a new construction and raw land. the site already has grounds and ground rod in place. which ground rod already 3 foot deep would be the best to use. Or just install new ground rods. Sorry first time doing FOP so im still not comfortable doing test. I think we would everntually sub out to contractor but i still want o know the process. I can figure it out for raw land but the existing construction is my problem.
equipment grounding
- Thread starter MBLES
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K8MHZ
Senior Member
- Occupation
- Electrician
One problem with doing a proper FOP test is that the rods have to be disconnected from the service. The FOP is meant to give earth resistance of either individual electrodes, or an entire system while not connected to the POCO's MGN system.
Isolate the utility gnd
but leave all building connections in place
use the bldg rod/system as the near or tested rod
you may not need to do fop
estimate bed size and space rods per this manual
https://assets.tequipment.net/assets/3/7/GettingDownToEarth.pdf
Pages 23-24
but leave all building connections in place
use the bldg rod/system as the near or tested rod
you may not need to do fop
estimate bed size and space rods per this manual
https://assets.tequipment.net/assets/3/7/GettingDownToEarth.pdf
Pages 23-24
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
And a problem with doing a fall of potential test of a system of ground rods is that the sphere of influence, depending on how seriously you take things, is either well beyond the sphere of influence of the closest rod to your test line or determined by a multiple of the radius of the entire ground system.
Can you find clear land without any other grounds along such a line?
This is one situation where careful use of a clamp-on type ground resistance tester may be the best option. If you do that, it does not really matter which rod you start with as long as you isolate each rod from the system as you test it.
You mean one of these ?
the problems is that everything is caldwelded to everything. I could do add aground rod in area with out any influence and re do test. I went around site and took a few readings from grounded equipment onground ring and i got between .98 to .112 ohms. I took it with a grain of salt and do a seperate test to see what happends
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Sounds like you are reading a Cu loop
use the equip gnd strap
clip on it as the test rod
then run the far out 100' and middle 62' (or further keeping the 62/100 ratio)
I do dozens of these per year
a large sub 100' x 200' with a mat and bored electrodes + rods isblucky to get to 1-2 Ohms
what instrument are you using?
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
That is what I had in mind, yes. Has the advantage, AFAIK, that you do not have to disconnect the rod under test from the rest of the GES. But there are heated arguments as to its validity and accuracy.
if used properly very accurate
We've compared it to our megger and biddle
always within a few tenths
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Good to know. As I understand the theory its accuracy is dependent on having a relatively good ground network on the other side of the clamp, whether it is building GES or POCO MGN. Is that correct? Or does that cancel out of the measurement. At a minimum you require a source of remote ground to determine potential on the rod?
Basic example
drive 2 rods 10' feet about
string a wire between them
clamp on wire
The clamp induces a current and measures v drop
you want to make sure you are measuring an earth loop not Cu
open ckt vs short
I don't find it useful to test equipment gnds
say swgr on a pad
if you clamp the bond cond it is meaningless, might read, might not
if you drive a rod outside the gnd bed and run a wire from bond cond to rod then measure that
you know the swgr to bed bond is good, it's a Cu strap lol
the manual gives a good idea of strengths and short comings
we had people do this
xfmr on pad
2 bonds to gnd bed on opposite sides
clamp on one...<0.1 Ohm, great!
bad...they read a Cu loop: strap to gnd bed mat, thru mat, up other bonding strap, thru xfmr frame
metal loop
brian john
Senior Member
- Location
- Leesburg, VA
Do they know what the driven electrodes are comprised of?
Is there a concrete slab?
How big is this facility?
If you want a relative idea of the ground resistance drive a new 8' rod and do a 3-point test on the single rod, if the single rod has a relatively low resistance, depend on the age of the existing system you will have a fair idea of where you stand.
But as I try to tell our customers worry more about equipment ground in lieu of the resistance to earth of the distribution system.
Is there a concrete slab?
How big is this facility?
If you want a relative idea of the ground resistance drive a new 8' rod and do a 3-point test on the single rod, if the single rod has a relatively low resistance, depend on the age of the existing system you will have a fair idea of where you stand.
But as I try to tell our customers worry more about equipment ground in lieu of the resistance to earth of the distribution system.
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- Location
- Tennessee NEC:2017
- Occupation
- Semi-Retired Electrician
I assume you meant 8' not 8" so I took the liberty and changed it.
- Occupation
- Licensed Electrician
I hear ya, but dirt worshipers are tough bunch to covert, and then you have to find the super minority who are willing to break loose and proclaim they want to be dirt free.
brian john
Senior Member
- Location
- Leesburg, VA
brian john
Senior Member
- Location
- Leesburg, VA
12 story building customer on the 11th floor with a small data room, they wanted a separate ground electrode to EARTH per their engineer to resolve what they thought were ground issues. I said we should do an investigation first and found they did need a ground connection, the transformer XO/neutral was never bonded, they had the old floating neutral, the electrician that did the build out argued with me tooth and nail over the need for this connection. They never had any equipment issues they had weird voltage to ground measurements.
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