Our team has developed the Electrical Resistivity Imaging technique in 1992 by automating a well known method developed originally by the Schlumberger brothers in 1913. At that time a volt meter and electric field generator was used to manually make measurements. We automated the data collection process in 1992 and later developed the 3D visualization software for modeling the collected data as the personal computer became more powerful and was capable of calculating a small amount of data within in minutes rather than days and as Windows 95 made working with computers more friendly.Just what does, "design electrical resistivity equipment" mean?
Grounding is such a broad term and that may be the culprit. The electric utility company's ground is bonded at the service station to the grounding grid of the structure. That bonded electrical connection is to make sure that your local ground is not at a different potential to the reference ground of the utility company. A difference in potential can be measured in volts. If it were at a different potential then current would flow between the two grounds creating a dangerous ground loop. The safety ground (EGC) for equipment is to provide that low resistance pathway.Measuring resistance or conductivity of the soil has very little or maybe even nothing to do with what an effective equipment grounding conductor is all about.
Your local ground is not always the same potential as utility ground and current loops are developed. This happens because many POCO use the multi grounded neutral conductor of their distribution system to carry current. Voltage drop develops on this conductor like it does on any other conductor carrying current, but since it is grounded multiple times there are additional paths for current to flow through the ground. This is number one reason for stray voltage problems. Trying to achieve balance on that grounded conductor so it carries as little current as possible helps minimize the problem but is almost impossible to achieve 100% balance.Grounding is such a broad term and that may be the culprit. The electric utility company's ground is bonded at the service station to the grounding grid of the structure. That bonded electrical connection is to make sure that your local ground is not at a different potential to the reference ground of the utility company. A difference in potential can be measured in volts. If it were at a different potential then current would flow between the two grounds creating a dangerous ground loop. The safety ground (EGC) for equipment is to provide that low resistance pathway.
That very same buried grounding grid is also used for lightning protection. The concept of EGC and lightning protection serve slightly different purposes. EGC is for a low resistance ground and the grounding grid is for a high capacity conduit to the earth. FYI...I refer to earth as the soil, rock, groundwater, etc and Earth as our planet.
In the case of a lightning strike there is a lot of energy that needs to be dissipated quickly. I like to think of it using the water analogy. When the lightning strike occurs a lot of electric charge is transferred very rapidly. We can think of the electric charge as water being poured from one container in the clouds to another container in the earth. The container in the earth is the earth (soil, rock, fluid, etc.) and the grounding grid is the opening to that container.
There are two important factors we need to consider when dealing with the electricity: capacity (the size of our container) and the electrical resistance (the opening of the bucket).
In that case of a building ground dangerous scenarios can happen if there is not an adequate pathway for the lightning strike to travel. A lot of energy is harmless if distributed over a larger volume (i.e. current density) but if that same energy is constrained to a very small pathway for a long period of time the temperature goes up and fires start or in another case where the electric chargers are trapped by a small opening the voltage goes up so much it damages equipment sensitive to large voltages. Either way we do not want either to happen.
If the container in the clouds is a 55 gallon drum it would be futile to try to catch it with a 5 gallon bucket. The capacity is too low.
We can think of the opening of the container as the electrical resistance. In this scenario our container is larger enough however the opening is too small. Consider a bucket in the form of long garden hose capable of containing 55 gallons of water but incapable of transferring the water quickly. Try pouring a 55 gallon drum into a garden hose It would take some time.
Measuring the earths resistivity (resistivity is not the same as resistance). Resistance is proportional to the length and cross-sectional area of the material the electricity is flowing through. Think of a thick conductor versus a thin conductor and a long conductor versus a short conductor. Resistivity is the electrical property of that material regardless of size.
Ohms law Volt=Current*Resistance is a special case where the conductor is uniform in size and uniform in material.
The IEEE Fall of Potential Method for evaluating earth grounds is an attempt to measure both the goodness of the electrical path into the earth from your grounding location and also its capacity to store the charge quickly.
A quick review of your multi-meter. When you measure resistance the multi-meter injects a very small known electrical current between the two probes while at the same time measuring the voltage across the SAME two probes. When you measure resistivity you inject electrical current through two probes (we usually refer to them as electrodes but its all the same) however we measure the voltage at a DIFFERENT set of probes in a different location. Fluke makes some lab multi-meters where the 4 probes are separated from each other.
Next time you see a grounding rod you may now wonder how BIG the container of the earth is.
BTW...resistance is a term used in terms of Direct Current. Impedence is the term used when dealing with Alternating Current. AC is a complex quantity with both frequency and phase to consider.
The following is routinely done.You cannot use earth ground nor the building steel to ground the sub panel. If there is metallic conduit then the conduit may be used as a ground. If it is an existing install then no equipment ground is necessary if there are no other metallic pathways back to the building.
Can he install another wire in the conduit?
Interesting!The following is routinely done.
Multi-story steel structures are connected to the underground grounding system - interconnected ground rods around the perimeter - at grade level. Individual electrical enclosures and transformers throughout the structure then bonded to the nearby steel. That is not the only means as EGC is carried with every feeder and due to the nature of our installations conduit is not used for this purpose. The ground integrity test - performed @ 5 years intervals, minimum - is done on this external connection.