Application Note

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Application Note

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

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 connec-

tion 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 least-

resistance 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 low-

impedance ground electrode will help carry the

energy into the earth. The grounding and bonding

systems connect the earth near the building with

the electrical system and building steel. In a light-

ning 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 facili-

ties. A low impedance electrode will help limit the

voltage increase at the facility. A low impedance

ground can also provide a return path for utility-

generated transients.

Figure 1 shows a grounding

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 elec-

trode 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: A grounding system combining reinforcing steel and

a rod electrode

/&&

Summary of content (8 pages)

PAGE 1
Checking ground electrode impedance for commercial, industrial and residential buildings Application Note 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.
PAGE 2
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.
PAGE 3
Checking Connection Resistance 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.
PAGE 4
Measurement Tips • Bring a good, long tape measure. • Finding the horizontal part of the curve will require at least 5, but more likely 7 or 9 measurements. • It’s a good idea to take three of your resistance readings with the P2 stake at 20 %, 40 % and 60 % of the distance between E and C2. This will allow you to use the Tagg Slope Technique. • When placing the stakes make sure the current stake, the potential stake and the electrode under test form a straight line.
PAGE 5
The 62 % Rule You may be able to use a shortcut if your test meets the following criteria: • You are testing a simple electrode (not a large grid or plate) • You can place the current stake 100 feet or more from the electrode under test • The soil is uniform Under these conditions you can place the current stake 100 feet or more from the electrode under test. Place the potential stake at 62 % of the distance between the current stake and the electrode under test and take a measurement.
PAGE 6
The Selective Method The Selective Method is a variation of the Fall-of-Potential method, available on high-end ground testers like the Fluke 1625. Testers with this capability can measure the ground impedance of a specific ground electrode without disconnecting it from an array or from a structure’s distribution system. This means you don’t have to wait for a shutdown to test or risk the safety hazards of disconnecting the electrode from a live system.
PAGE 7
This method requires a low-impedance path in parallel with the electrode under test. The ground electrode of most facilities is in parallel with numerous utility ground electrodes. These electrodes can be pole electrodes, pole butt plates or un-insulated neutral conductors. The impedance of the utility ground electrodes usually combines into a very low impedance. Let’s take an example.
PAGE 8
Summary of Ground Electrode Test Methods Advantages Drawbacks Fall-of-Potential • Widely accepted • When you see the characteristic curve you know you’ve got a good measurement. • You have to disconnect the ground • The stakes may not be easy to drive • There may not be space around the ground electrode to drive the stakes Selective Method • Don’t have to disconnect electrode • Widely accepted • When you see the characteristic curve you know you’ve got a good measurement.