Can a "Ground Resistance Test" be done with a DMM?

[iwire]

I say no, you must use a proper ground resistance tester.

Here is a post from another Forum that I would like to have others comment on.

Let me pass on some information from my past life about ground testing

Off the shelf test equipment is very costly

The cheap way of doing it is beyond the scope of home inspection but is nice to know information

1. Locate all ground points - there must be at least 2 and they should be 6 feet or more apart
2. Kill all power
3. Electrical disconnect the grounds from each other
4. Measure the DC resistance between all points
5. Draw the CKT of the grounding systems
6. Compute the resistance using ohms law
7. Connect all conductor back up
8. Restore all power

If it's that easy why would we bother with clamp on or fall of potential tests?

Thanks, Bob

 

[jtester]

I believe that there are both ground resistance tests and ground impedance tests. IEEE 81 describes "Fall of Potential Method" as an impedance test. Impedance tests are important when the resistance is .5 ohms or less, not the 25 ohms we are used to.

I also agree that a dc test with a dvm is usually inaccurate, because of the probability of galvanic action or other stray dc currents, unless you monitor and control the test current carefully.

Jim T

 

[George Stolz]

Bob, the method described would not calculate the resistance of the grounding electrodes to earth. It would be the resistance of the electrodes to each other, so the resistance of the earth, the first electrode's contact with earth, and the second electrode's resistance to earth would be in series.

I'm (obviously) speaking outside my experience here, but the resistances in series would give an inaccurate calculation, on the high side, I would think.

Instead of two electrodes contributing in parallel to a give a structure connection to earth, this test gives a meaningless resistance of electrodes to each other.

This information would only be useful if they were setting up a SWER, which obviously wouldn't jive with the NEC and would be pretty useless to a home inspector looking at a 120/240 service.

Somebody's got some bad information.

Testing in a meaningful pattern, as opposed to the pattern described, I've heard (and believe) that a DMM doesn't have the current available to accurately measure ground resistance.

 

[rlb]

New to the board

I want to than Bob for posting my posting from another board over here where I might get some additional information. In short HELP

My background includes RF - USN electronics - Sub service - telecom etc. I understand that a DC check between two points has some limitations

Yes this will give the series resistance and should be recomputed to two grounds in parallel which will not mean much unless the two are similar ie. two 8 ft rods in the same soil

Galvanic action must be put in the picture and can be some what compensated for by reversing the meter leads and checking the resistance again

And one must be aware of noise and other currents in the area that would also influence this and other type of tests.

So I am looking for additional information on a 2 point test using DC that would render it incorrect and not able to be duplicated

As an additional note I have been given the name and model # of a ground tester that is about the price of a good DVM. So if one is going to check grounds one can go out and buy a meter that seems to do the job.

Looking forward to some additional knowledge here


Thank all

rlb

 

[George Stolz]

There is a quick, cheap and dirty (and a bit dangerous) method of determining the resistance of the grounding electrodes at a structure, what you're describing isn't it.

Home inspection is mentioned in the post. Why would you care about the resistance of grounding electrodes in a home inspection? There are no real resistance requirements in the NEC, and it could be argued that grounding doesn't accomplish much in a residential setting anyway.

What is your objective, can you clarify that?

 

[iwire]

Home inspection is mentioned in the post. Why would you care about the resistance of grounding electrodes in a home inspection?

Can we forget the HI aspect of this at least until we get to the bottom of the original question?

There are no real resistance requirements in the NEC, and it could be argued that grounding doesn't accomplish much in a residential setting anyway.

:lol:

For the purposes of this discussion lets assume 'earth' connections are needed for proper operation of the service. :wink:

Here is the original thread

http://www.nachi.org/forum/showthread.php?t=3625

 

[George Stolz]

Thanks for the link, Bob.

Looks like a pretty tall case against the DMM has already been built there. The ECM article pretty much covers it.

In Mike Holt's Grounding Versus Bonding book, he details the ground-clamp method, and the fall of potential method, and stresses the details required to ensure an accurate test for either method.

I have sent a PM to Dereck, perhaps he can explain the nuts and bolts of why the DMM is no good for this application in a better way than we can.

 

[iwire]

I have sent a PM to Dereck, perhaps he can explain the nuts and bolts of why the DMM is no good for this application in a better way than we can.

8)

Thanks, that was a good idea and there is no 'perhaps' about his explanation. :lol:

 

[dereckbc]

I?ll take a shot at it. The quick simple answer is a DMM is too low of a current and voltage source to override the residual noise. Also as already stated if you use two existing electrodes you are not measuring the impedance to earth, you are measuring DC resistance between two points. All you are really doing is verifying continuity

The problem is this. We all know there are currents flowing through earth. These currents will interfere with the DMM high impedance input causing false readings. To prove this to yourself take your DMM, set it on voltage, and hold the probes connected to your fingers or an open circuit. You get a voltage don?t you? Ever hear of it called a phantom voltage? A DMM measures resistance by supplying a very small current in micro-amps from a 9-volt transistor battery then reads the voltage that is developed from the current across the UUT or load (that is Unit Under Test for those from Rio Linda). So you can imagine any current flowing between the two points from other sources will greatly influence the readings. It?s like whispering to someone across a room with a lot of background noise.

To measure earth resistance accurately you need to either use a high current source, high voltage source, or both to overcome the residual noise. FWIW using a high current source would be dangerous. Imagine sending something like 10-amps through a 50-ohm circuit. That is something you find in Digital Low Resistance Ohmmeters (Topic for another discussion)

A Three-Point Tester works on high AC voltage at a frequency ranging from 110-Hz up to 200 Hz. Most I have seen use 140Hz or 160Hz so that 60Hz commercial power will not interfere with the test, and have detectors/filters to prevent interference.

Now there is a cheap and dirty way to do the test I think everyone knows about. I will not go into details but I call it a Poor Mans James Bond method using a rod, wire, 20-amp breaker and a secret which I will not discuss on a public forum.

Hope that helps.

 

[iwire]

Yeah that helps. 8)

Thanks a lot, I learned some new things today. 8)

 

[zog]

You cant measure resistance

You cant measure resistance

Well let?s get back to basics and review how an ?Ohmmeter? works.

Your ?Ohmmeter? is actually measuring current. The meter applies a test voltage and measures the current flow through the circuit and uses Ohms law to calculate resistance which is displayed on the meter in Ohms. If the applied voltage is not high enough, the amount of current passed through the resistance is below the sensitivity range of the meter.

Ohm meters are connected to a component which is removed from the circuit. Ro is an adjustable resistor whose purpose is to zero the ohm meter and correct for battery aging. It is also a current-limiting resistor which includes the meter resistance Rm. Zeroing the ohm meter is accomplished by shorting the ohm meter terminals and adjusting Ro to give full-scale deflection; this is done automatically by modern digital ohmmeters.

When the unknown resistance Rx is connected across the ohm meter terminals, the current is measured by calculating the total series resistance. Using the following equation:

I= V/ (RO+RX)

A digital ohmmeter will automatically do this calculation for you while an analog meter reading 0 ohms will cause full meter deflection. The meter face says ohms, but it is actually indicating amps, therefore full deflection (All the way to the right) will indicate 0 ohms while a reading of near 0 amps will cause little reflection of the meter and indicate near infinite resistance. (There is also no such thing as infinite resistance but we will save that argument for future discussion)

For digital displays, if the applied voltage of the circuit under test is not high enough to cause enough current flow through the circuit for the meter to sense, the display will read ?O.L?. Contrary to popular belief, this does not mean overload, it means ?Outside Limits?, in other words, the measured current flow is below the sensitivity capabilities of the meter.

You can explain this to the next customer that tells you they used their ohmmeter to test the insulation resistance of their switchgear before energizing and it was ?infinite? and they just don?t understand why it failed.

 

[dereckbc]

Re: You cant measure resistance

Re: You cant measure resistance

Your ?Ohmmeter? is actually measuring current.

I do not think so. They are a precision current source up to 10-volts, in labatory grade test equipment. They are reading the voltage drop developed by the current flowing through the load in which it is applied.

 

[zog]

That was describing an analog meter movement, I guess I should have clarified. thanks.