What is Ground Potential Rise (GPR)?
When a large amount of energy is rapidly deposited into the ground by a cloud-to-ground lightning strike or by an electrical fault on a utility power system, the ground potential at this injection point rises to a higher level with respect to the more distant ground.
This has the effect of creating a voltage potential gradient in the earth, which can cause dangerous touch and step potentials to personnel exist. By creating an equi-potential ground plane beneath a facility by electrically bonding all separate “grounds” into a “system” or by burying ground mats and meshes, this danger to personnel and equipment can be reduced. It is also important to note that GPR is not only dangerous to personnel, it can also cause damage to equipment.
I have heard I should not use separate grounding systems?
Separate “grounds” or “ground references” can result in damage to equipment during lightning activity. A cloud-to-ground discharge can deposit extensive charge very quickly into the local ground mass of the earth causing the ground at the injection point to rise up in voltage with respect to more remote grounds. The resultant potential gradient established in the ground means that separate grounds could rise to different potentials resulting in a loop current and possible damage to equipment referenced to these two different points. This phenomenon can present itself in a more subtle way when equipment is connected to multiple services.
An example of this can be a personal computer with modem where connections are made to utility power and telecom line. If these two services are not referenced together to create a common, equi-potential, ground plane, damage can result. In fact, this is one of the more common causes of equipment damage. A well-designed multi-port protector will ensure such equalization between services at the equipment.
What is single point grounding?
It is important to ensure that ground potential differences are not derived across equipment within a facility during ground potential rises. One way to ensure this is to adopt a single point approach to grounding of the equipment and services in the facility. This usually entails referencing all equipment in the facility to a single ground bar (or a number of ground bars that are solidly electrically bonded together), and ensuring that this internal bonded system is connected to the external ground system. “Single point grounding” refers to the single connection between the internal facility ground system and the external ground network. The external ground network can utilize multiple grounding elements such as ground rods and/or counterpoises.
How do I measure ground resistance?
There are a number of techniques for measuring ground resistance, the more popular being the “fall of potential method”. Measurements require a ground resistance testing instrument and qualified personnel. With larger facilities, it is important to take ground resistance readings by placing the injection and reference electrodes in the “far field” – essentially some few hundred feet from the inspection ground point.
This will ensure that false or misleading results are not obtained by having electrodes too close to buried parts of the overall ground system. Clamp-on type instruments are not preferred in such situations due to the possibility of large errors in results.
What ground resistance must I achieve?
This is probably one of the most often asked questions of grounding experts. Again there is no one answer. As a rule of thumb, an effective ground for lightning and surge protection purposes should be somewhere around 10 ohms. Obviously this can be difficult to reach in poor soil conditions and a cost benefit relationship comes into play. It is also important to stress that no definitive applies to grounding values.
As an example, it is pointless insisting that a contractor achieve a ground resistance of precisely 10 ohms or less, when the testing method can be subject to as much as 2 ohms variation depending on how the test rods are laid. It is also worth keeping in mind that, the soil water content can vary as much as 50%, depending on the season of the year. There are “ground enhancing materials” which can be used to improve (decrease) the local ground resistivity.
More important than the absolute value of the ground resistance, is to ensure that all the equipment in the facility is referenced to an equi-potential ground plane through adequate bonding. By ensuring this, all separate pieces of equipment will raise to the same potential during a surge condition. This statement can be illustrated by considering the Space Shuttle, it is not “grounded” however all the equipment onboard will be referenced to an internal equi-potential ground plane. (Don Wilfong -Global Solutions LLC)