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instrument earth impedance

instrument earth impedance

instrument earth impedance

(OP)
dear all
i made a test of instrument earth impedance and i found it 4.6 ohm. i need to know if this is acceptable value or not and the electtrical standards related to that
thanks in advance

RE: instrument earth impedance

Dear Mr ELEC07 (Electrical)(OP)2 Mar 21 12:32

E. " ...i made a test of instrument earth impedance and i found it 4.6 ohm. i need to ..."
C. Please tell us a little more detail:
a) what do you mean by " instrument earth impedance "?.
b) what instrument is being used? Using an AC voltage? Or are you using an insulation (high resistance) instrument which is using DC voltage?
c) how is the measurement being carried out, between which points of the system?
Che Kuan Yau (Singapore)

RE: instrument earth impedance

I do not know what you meant by "INSTRUMENT EARTH". I looks like you are referring to "CLEAN EARTH"
concept we had about 20yrs ago. It is no longer valid.
Therefore, an instrument supply will only have the system earth at the source point and the
safety (exposed & extraneous conductive surfaces) earth.
You have not mentioned whether you have measured the system earth or the safety earth.

RE: instrument earth impedance

(OP)
sorry for the late reply
che12345;
this test is for mesuring earth resistance of the rod for a telecomonication building (data center)
i used earth ground clamp CA6416
the mesure has been made according to stakeless test method
using one existant earth rod by disconecting it from the earth loop and then connet it with this instrument earth rod
the CA6416 displayed 6.2 ohm which is the sum of the resistance of the two earth rods ;we've already known the resistance of the existant earth rod which is 1.6 ohm
so the resistance of the earth rod being mesured is 4.6

RE: instrument earth impedance

(OP)
hello Kiribanda
instrument earth is earthing in telecominication and sensitive local
the safety earth

RE: instrument earth impedance

The resistance of two earth rods will be lower than that of one earth rod. Are you putting the clamp on a wire that is connecting both earth rods to a system multigrounded neutral? Using the stakeless test method depends on the resistance of the measured rod or rods being much greater that the combined resistance of the system that it is being connected to.

What is the governing standard for your system? The US National Electric Code (NEC) requires a single rod, pipe, or plate grounding electrode to have a resistance of 25 ohms or less. If it does not, a supplemental electrode is required, but no particular combined resistance is specified. See NEC 250.53(A)(2).

RE: instrument earth impedance

Dear Mr ELEC07 (Electrical)(OP)4 Mar 21 13:11
E "...this test is for mesuring earth resistance of the rod for a telecomonication building (data center)
i used earth ground clamp CA6416 the mesure has been made according to stakeless test method. using one existant earth rod by disconecting it from the earth loop and then..."
.
C.1. Your existing single rod reads 1.6 ohm. When connected (in parallel) to the second rod reads 6.2 ohm by stakeless method. It seems to be not? right. When the existing is 1.6 ohm , with the new one in parallel, the resistance shell be max equal to 1.6 ohm or lower; but NOT >1.6 ohm!. You read 6.2 ohm, which seems to be NOT right.
C.2. It is NOT right to assumed that the new electrode resistance to be 6.2-1.6 = 4.2 ohm.
C.3. The intension of the stakeless measurement is to measure the paralleled resistance. It shall NOT > 1.6 ohm.
C. 4. Connect two electrodes in parallel and carry out a "drop of potential" method to verify the reading.
C. 5. For telecommunication installations, the recommended earthling resistance is NOT exceeding five (5) ohm.
Note: your existing single electrode 1.6 ohm is good enough. Addition of another is superfluous.
Che Kuan Yau (Singapore)

RE: instrument earth impedance

Quote:

The resistance of two earth rods will be lower than that of one earth rod.
As I understand this test, there is an existing earth grid.
One electrode is disconnected and the resistance between the one rod and the grounding grid is measured.
The indicated resistance will be the sum of the grounding grid and the resistance of the ground rod under test.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: instrument earth impedance

Dear Mr Dear Mr ELEC07
Follow-up No.1. My sincere apology. I wish to make the correction shown in "...italic..."
E "...this test is for mesuring earth resistance of the rod for a telecomonication building (data center)
i used earth ground clamp CA6416 the mesure has been made according to stakeless test method. using one existant earth rod by disconecting it from the earth loop and then...".

C.1. Your existing single rod reads 1.6 ohm. When connected (in parallel) to the second rod reads 6.2 ohm by stakeless method. It seems to be not? right. When the existing is 1.6 ohm , with the new one in parallel, the resistance shell be max equal to 1.6 ohm or lower; but NOT >1.6 ohm!. You read 6.2 ohm, which seems to be NOT right.
"... C.1. Your existing single rod reads 1.6 ohm. When connected (in parallel) to the second rod reads 6.2 ohm by stakeless method. It can be right. When the existing is 1.6 ohm , with the new one in parallel, the resistance can be 6.2 ohm!. You read 6.2 ohm, which is possible ... Very confusing!..."

C.2. It is NOT right to assumed that the new electrode resistance to be 6.2-1.6 = 4.2 ohm.
[i]"...C.2. It is right to assume that the new electrode resistance to be 6.2-1.6 = 4.2 ohm...."


C.3. The intension of the stakeless measurement is to measure the paralleled resistance. It shall NOT > 1.6 ohm.
"... C.3. The intension of the stakeless measurement is to measure the paralleled resistance. Attention: for two electrodes, it is measuring the resistance in Series NOT parallel[l..."

C. 4. Connect two electrodes in parallel and carry out a "drop of potential" method to verify the reading.
C. 5. For telecommunication installations, the recommended earthling resistance is NOT exceeding five (5) ohm.
Note: your existing single electrode 1.6 ohm is good enough. Addition of another is superfluous.
Che Kuan Yau (Singapore)

RE: instrument earth impedance

Read my post, Mr. Che.
The test circuit is as follows:
Test instrument to ground loop.
Ground loop to earth.
Earth to electrode under test.
Electrode under test to test instrument.
The connection is a series connection.
The conductor to the electrode under test may be broken and an Ohmmeter inserted or a ground loop tester may be used without breaking the conductor.

Stakeless Ground Loop Tester.


Stakeless earth ground loop testing
June 12th, 2017, Published in Articles: Vector
Testing the grounding components of equipment in hard-to-reach spaces is challenging. Fluke, represented locally by Comtest, now has the Fluke 1630-2 FC stakeless earth ground clamp, a heavy-duty clamp jaw that stays in alignment and in calibration even in industrial environments. Staying online to identify ground loop resistance without the need to disconnect, then reconnect the earth electrode from the system, is now possible. This clamp measures earth ground loop resistances for multi-grounded systems using the dual-clamp jaw. This test technique eliminates disconnection of parallel grounds, as well as finding suitable locations for auxiliary test stakes. The 1630-2 FC supports the Fluke Connect wireless system, which connects the clamp wirelessly with an app on a smartphone or tablet.
Link
A stakeless test requires at least two ground electrodes and measures the sum of the two resistances to ground.
Preferably there is a ground grid or array of ground electrodes.
The stakeless test always returns the sum of the resistances of the electrode under test and the ground grid or array.
The resistance of the electrode under test will always be less than the total resistance indicated by the test meter.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: instrument earth impedance

Dear Mr waross (Electrical)8 Mar 21 14:50
W. " The test circuit is as follows: Test instrument to ground loop. round loop to earth. Earth to electrode under test....".
C1. Thank you for your learned advice.
C2. Please advise whether the existing electrode A is 1.6 ohm. After connecting to another electrode B, the reading is 6.2 ohm by stakeless method. Isn't it confusing? .
C3. MY first mail was based on the (INCORRECT) assumption that electrode A and B are in parallel. Therefore the reading shall be 1.6 ohm or lower.
C4. After going through various publications on stakeless measurement method, I come to the conclusion that for two electrodes only situation; the measured value is in Series NOT parallel.
C5. Do you agreed that the electrode B is 6.2-1.6 =4.2 ohm? . Please advise.
C6. I admit my mistake and therefore summitted my apology and included the correction in italic.
C7. This would remind readers to be cautious when making stakeless measusrement! There are limitations!
Che Kuan Yau (Singapore)

RE: instrument earth impedance

Quote (Mr. Che)

C5. Do you agreed that the electrode B is 6.2-1.6 =4.2 ohm? . Please advise.
Yes, I agree.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: instrument earth impedance

The earth ground loop testers work by inducing an EMF into the circuit formed by two ground electrodes or groups of electrodes and the conductor between them.
The instrument forms a transformer with a one turn, shorted secondary.
from the current and voltage, the instrument calculates the induced current and voltage in the ground loop and from that calculates the total resistance of the series pair.
I am not a fan of instrument grounding systems.
Any who are familiar with step potentials under fault conditions will understand how a supposed "Clean" isolated instrument ground may pick up a voltage several times as great as a step potential and transfer that voltage to instrument circuits and equipment.
A step potential is a fault current induced voltage gradient over the space of a step or approximately 3 feet.
An independent instrument ground may be much more than 3 feet away from the nearest point on the main grounding grid.
The independent ground electrodes may thus pick up and transfer a fault induced voltage much greater than the step voltage.
But the situation is not as dangerous as it may be.
In Canada there is a code requirement for the industrial group to connect the instrument bus bar to the nearest main system bus bar.
The connection may be with a #4/0 AWG copper conductor.
This renders the independent instrument ground both safe and inconsequential.

Example.
In our area many petro-chemical plants are supported for a large part on screw piles.
A screw pile may be a steel tube about 16 inches or so in diameter, 30 to 50 feet in length with flights welded onto one end.
These are screwed into the ground as foundations for a lot of equipment.
They are connected together with copper cables and form a very large ground grid.
There may be hundreds of these at a large plant.
That is hundreds of tons of steel grounding electrodes, well below the frost line. (Frozen ground tends to be non-conductive.)
In addition, there will probably be tons of copper cables buried as part of the ground grid.
Then the instrument guys drive three ten foot steel rods in a triangle and think that they have a special and superior ground.
The frost level may go down 5 to 8 feet, reducing the effective length of their ground rods considerably.
After they have finished their grounding and have moved on to other tasks, the industrial group installs the 4/0 AWG jumper between the instrument ground bus and the system ground bus in accordance with codes and engineering standards.
It's their culture and you can't change culture.
By the way, I have been watching the instrument and communications grounding cultures develop over about 50 years.
By the 70s a whole industry had grown up around independent grounding, based on misunderstandings, lack of knowledge of past commercial practices and habit.
By that time, regular code grounds were as good if not better than the isolated grounds.
You can't change culture.
Now the instrument guys have a really good and safe ground.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: instrument earth impedance

(OP)
hello again mr Che Kuan Yau
just a simple question if i use the earth ground clamp between tow rods
as showing on the picture
the resistances are in series : resistance of the 1st rod and the 2sd rod plus the resistance of the wire and the earth which are typically very low
and the tester will display the sum of the two rods resistance isn't it

RE: instrument earth impedance

(OP)
thank you mr Che Kuan Yau for your clarification and correction it s in series not on parallel

RE: instrument earth impedance

(OP)
mr waross thank your detailed explication and Example

RE: instrument earth impedance

Thank you ELECO7 for the nice diagram.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: instrument earth impedance

Dear Mr ELEC07
E. " ....the resistances are in series : resistance of the 1st rod and the 2sd rod plus the resistance of the wire and the earth which are typically very low and the tester will display the sum of the two rods resistance isn't it ..."
C. YES , the Sum of two rods in SERIES when the two rods are connected in [parallel !]. Isn't it confusing?.
It is understood that earth resistance and wire are typically very low.
C2.Repeat: This would remind readers to be cautious when making stakeless measurement! There are limitations!
C3. I am considering to raise the " alarm " to the attention of the engineering world to be aware or to BAN is misleading practice/method.
Che Kuan Yau (Singapore)

RE: instrument earth impedance

Mr. Che
Would you care to explain how to measure ground resistance without the effect of the reference ground acting in series?

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: instrument earth impedance

Dear Mr waross (Electrical)10 Mar 21 14:10
W. " Mr. Che....Would you care to explain how to measure ground resistance without the effect of the reference ground acting in series?.."
C1. My apology if I failed to understand/misinterpreted or did not answer your question.
C2. I am of the opinion that with A electrode of say X ohm and the electrode B of Y ohm planted some distance far away from A; when connected in parallel; the paralleled resistance shall be equal to/LOWER than X ohm. But NEVER higher than X ohm..
C2.1. The stakelesss method reads the paralleled resistance to be (X+Y ) ohm!. This is unacceptable.
C3. With stakeless method measurement it can NOT " ...measure ground resistance without the effect of the reference ground acting in series?..".
C3.1 For this reason I am considering to raise the " alarm " to the attention of the engineering world to be aware or to BAN is misleading practice/method.
Che Kuan Yau (Singapore)

RE: instrument earth impedance

Mr. Che;
You misunderstand the common use of the stakeless tests and the stakeless measurements.
How do you measure ground resistance without a reference ground?

In recognition of your contributions to Eng-Tips and of your knowledge, I will try to explain stakeless testing.
The tester forms a transformer with a one turn secondary winding.
Think of a CT in reverse as the CT is a transformer with a one turn primary winding.
The instantaneous current flow is from the tester to the electrode under test, from there through the earth to the reference grid or electrode, up the reference electrode and back to the tester.


Ground resistance specs are generally expressed as "Less than X Ohms".
X may be 5 Ohms, 25 Ohms or some other value selected by the design engineer or required by applicable engineering standards.
The result of a stakeless test may be expressed as "Less than X Ohms".
Often the resistance to ground of the reference grid will be much less than the resistance to ground of the electrode under test.
The stakeless test is widely used and is quick and easy.
The results are accurate within the limitations of the test.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: instrument earth impedance

Dear Mr waross
W. " .... You misunderstand the common use of the stakeless tests and the stakeless measurements. How do you measure ground resistance without a reference ground? ...".
C1. Thank you for your explanation and the sketch which is also described in the documents of the instrument manufacture. Irrespective of the brand, size and shape, the design/operation principle is the same i.e. stakeless method.
C2. My basis opinion is: when electrode A is X ohm and electrode B of Y ohm, when connected in parallel; the value shall be X or lower. But NOT (X+Y) ohm. This is the usual case in practice. When a single electrode exceeds the required value say X ohm ; you plant another electrode B as far apart from A and connect them in parallel. You expect the paralleled A and B to be lower than X ohm. This is the main reason why you plant additional electrode B.
C3. We have different opinion/expectation.
C4. I wish to keep the case closed.
Che Kuan Yau (Singapore)

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