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 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 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 1000V) 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 utility-generated transients.
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 (eg. 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 effected by buried metal tanks, pipes, re-bar, etc
- Varies with depth, usually getting lower with depth
Since resistivity tends to 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 should be outside of each other's "areas of influence" to be most effective. 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.
For detailed descriptions of: 3 and 4 pole Fall of Potential testing, selective testing, stakeless testing, and 2 pole testing, view How to Use Cable and Pole Earth Ground Testers (.pdf).