Fluke 1625-2, 1623-2 Application Note

Checking ground

/&&

electrode impedance for

commercial, industrial

and residential buildings

Most facilities have grounded electrical systems, so that in the event of a lightning strike or utility overvoltage, current will ﬁnd a safe path to earth. A ground electrode provides the contact between the electrical system and the earth. To ensure a reliable connection to earth, electrical codes, engineering standards, and local standards often specify a minimum impedance for the ground electrode. The International Electrical Testing Association speciﬁes ground electrode testing every three years for a system in good condition with average up-time requirements. This application note explains earth/ground principles and safety in more depth and then describes the principle testing methods: 3 and 4 pole Fall-of-Potential testing, selective testing, stakeless testing and 2 pole testing.

Why Ground?

The US National Electrical Code (NEC) gives two principle reasons for grounding a facility.

• Stabilize the voltage to earth during normal operation.

• Limit the voltage rise created by lightning, line surges or unintentional contact with higher-voltage lines.

Current will always ﬁnd and travel the leastresistance path back to its source, be that a utility transformer, a transformer within the facility or a generator. Lightning, meanwhile, will always ﬁnd a way to get to the earth.

In the event of a lighting strike on utility lines or anywhere in the vicinity of a building, a lowimpedance ground electrode will help carry the energy into the earth. The grounding and bonding

Figure 1: A grounding system combining reinforcing steel and a rod electrode

Application Note

systems connect the earth near the building with the electrical system and building steel. In a lightning strike, the facility will be at approximately the same potential. By keeping the potential gradient low, damage is minimized.

If a medium voltage utility line (over 1000 V) comes in contact with a low voltage line, a drastic overvoltage could occur for nearby facilities. A low impedance electrode will help limit the voltage increase at the facility. A low impedance ground can also provide a return path for utilitygenerated transients. system for a commercial building.

Ground Electrode Impedance

The impedance from the grounding electrode to the earth varies depending on two factors: the resistivity of the surrounding earth and the structure of the electrode.

Resistivity is a property of any material and it deﬁnes the material’s ability to conduct current. The resistivity of earth is complicated, because it:

• Depends on composition of the soil (e.g. clay,

gravel and sand)

• Can vary even over small distances due to the

mix of different materials

• Depends on mineral (e.g. salt) content

• Varies with compression and can vary with time

due to settling

• Changes with temperature, freezing (and

thus time of year). Resistivity increases with decreasing temperature.

• Can be affected by buried metal tanks, pipes,

re-bar, etc.

• Varies with depth

Since resistivity may decrease with depth, one way to reduce earth impedance is to drive an electrode deeper. Using an array of rods, a conductive ring or a grid are other common ways of increasing the effective area of an electrode. Multiple rods

Figure 1 shows a grounding

F r o m t h e F l u k e D i g i t a l L i b r a r y @ w w w . f l u k e . c o m / l i b r a r y

should be outside of each other’s “areas of inﬂuence” to be most effective (see Figure 2). The rule of thumb is to separate the elements by more than their length. For example: 8-foot rods should be spaced more than 8 feet apart to be most effective.

The NEC speciﬁes 25 ohms as an acceptable limit for electrode impedance. The IEEE Standard 142 Recommended Practice for Grounding of Industrial and Commercial Power Systems (“Green Book”) suggests a resistance between the main grounding electrode and earth of 1 to 5 ohms for large commercial or industrial systems.

Local authorities including the authority having jurisdiction (AHJ) and plant managers are responsible for determining acceptable limits for ground electrode impedance.

Note: Power distribution systems deliver alternating current and ground testers use alternating current for testing. So, you’d think we would talk about impedance, not resistance. However, at power line frequencies, the resistive component of the earth impedance is usually much bigger than the reactive component, so you will see the terms impedance and resistance used almost interchangeably.

How do ground impedance testers work?

There are two types of ground impedance testers. Three and four point ground testers and clamp-on ground testers. Both types apply a voltage on the electrode and measure the resulting current.

A three or four-pole ground tester combines a current source and voltage measurement in a “lunch box” or multimeter-style package. They use multiple stakes and/or clamps.

Ground testers have the follwing characteristics:

AC test current. Earth does not conduct dc

•

very well.

Test frequency that is close to, but distinguish-

•

able from the power frequency and its harmonics. This prevents stray currents from interferring with ground impedance measurements. Separate source and measure leads to compen-

•

sate for the long leads used in this measurement.

Input ﬁltering designed to pick up its own signal

•

and screen out all others.

Clamp-on ground testers resemble a large clamp meter. But they are very different because clampon ground testers have both a source transformer and a measurement transformer. The source transformer imposes a voltage on the loop under test and the measurement transformer measures the resulting current. The clamp-on ground tester uses advanced ﬁltering to recognize its own signal and screen out all others.

Figure 2: Ground electrodes have “areas of inﬂuence” that surround them

Ground Testing Safety

Always use insulated gloves, eye protection and other appropriate personal protective equipment when making connections. It is not safe to assume that a ground electrode has zero voltage or zero amps, for reasons given below.

To perform a basic ground test (called Fall-ofPotential) on an electrode, the electrode must be disconnected from the building. New selective methods allow accurate testing with the electrode still connected. See “Selective Measurements.”

A ground fault in the system might cause signiﬁcant current to ﬂow through the ground conductor. You should use a clamp meter to check for current before performing any impedance testing. If you measure above 1 amp you should investigate the source of the current before proceeding.

If you must disconnect an electrode from an electrical system, try to do so during a maintenance shutdown when you can de-energize the system. Otherwise, consider temporarily connecting a backup electrode to the electrical system during your test.

Never disconnect a ground electrode if there is a chance of lightning.

A ground fault in the vicinity can cause voltage rises in the earth. The source of the ground fault may not even be in the facility you are testing, but could cause voltage between the test electrodes. This can be especially dangerous near utility substations or transmission lines where signiﬁcant ground currents can occur. (Testing grounding systems of transmission towers or substations requires the use of special “Live Earth” procedures and is not covered in this app note.)

Ground impedance testers use much higher energy than your standard multimeter. They can output up to 250 mA. Make sure everyone in the area of the test is aware of this and warn them not to touch the probes with the instrument activated.

2 Fluke Corporation Checking ground electrode impedance for commercial, industrial and residental buildings.

Checking Connection Resistance

Measured Resistance

Distance of P2 from E

Current Spike

Potential Spike

Electrode Under test

Electrode/Earth Impedance

d1 d2

P2 C1&P1

Leading Up to the Electrode

Before testing the electrode, start by checking its connection to the facility bonding system. Most Fall-of-Potential testers have the ability to measure 2-pole, low ohms and are perfect for the job. You should see less than 1 ohm:

At the main bonding jumper

•

Between the main bonding jumper and the

•

ground electrode conductor

Between the ground electrode conductor and

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the ground electrode

Along any other intermediate connection

•

between the main bonding jumper and the ground electrode

The Fall-of-Potential Method

The Fall-of-Potential method is the “traditional” method for testing electrode resistance. The procedure is speciﬁed in the IEEE-81 standard “Guide for Measuring Earth Resistivity, Ground Impedance and Earth Surface Potentials of a Ground System.” In it’s basic form, it works well for small electrode systems like one or two ground rods. We will also describe the Tagg Slope Technique which can help you draw accurate conclusions about larger systems.

Remember: for this method, the ground electrode must be disconnected from the building electrical service.

The tricky part comes in determining where to drive the stakes to get a true reading of the resistance between the electrode and the earth. At what point does the dirt surrounding the electrode stop being a contributor of resistance and become the vast earth? Remember that we are not interested in the resistance between the electrode and our stakes. We are trying to measure the resistance that a fault current would see as it passes through the mass of the earth.

The current probe generates a voltage between itself and the electrode under test. Close to the electrode, the voltage is low and becomes zero when the P stake and electrode are in contact.

How it works

The Fall-of-Potential method connects to the

Figure 3: 3-point measurement

earth at three places. It is often called the “threepole method.” You may want to use a fourth lead for precise measurements on low-impedance electrodes, but for our initial discussions we will consider three leads.

The connections are made to:

E/C1 – the ground electrode being tested

•

S/P2 – A voltage (potential) measurement stake

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driven into the earth some distance away from the electrode. Sometimes called the potential auxiliary electrode

H/C2 – A current stake driven into the earth a

•

further distance away. Sometimes called the current auxiliary electrode

Figure 3 shows this schematically and Figure 4 shows the three connections made using a typical ground tester.

The ground tester injects an alternating current into the earth between the electrode under test

3 Fluke Corporation Checking ground electrode impedance for commercial, industrial and residental buildings.

(E) and the current stake (C2). The ground tester measures the voltage drop between the P2 stake and E. It then uses ohms law to calculate the resistance between P2 and E.

some distance from the electrode under test. Then,

To perform the test you position the C2 stake at

keeping the C2 stake ﬁxed, you move the P2 stake along the line between E and C2, measuring the impedance along the way.

Figure 4: A plot of measured impedances versus position of the potential stake allows us to see the earth impedance

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