# Building structural frame for grounding of subpanel?

#### liquidtite

##### Senior Member
Maybe a dumb question but if there is running 208V 3-phase to the sub-panel then the EGC is bonded to the neutral back at the service entrance? If so can it bonded again at the sub-panel?

thx

Markus
yes but in a subpanel the nuetral and egc should be seperated

#### markusius

##### Member
Just what does, "design electrical resistivity equipment" mean?
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.

Electrical Resistivity Imaging equipment is an imaging technique used to create a 3D scan of a material (usually into the Earth's subsurface). Electrical resistivity is a bulk electrical property of all material. It is a measure of how well a material resists the flow of electricity. In our application we deal strictly with DC.

Most people are familiar with MRI's that are used by medical doctors to image a patients body. A well known relations ship between magnetic fields and the magnetic moment of the proton in Hydrogen. This is the property determined when using MRI's.

In our case we create a static electric field by injecting (transmitting) DC into the subsurface through two electrodes hammered into the ground. A static electric field, if visible would show the current density of the electricity passing from one electrode to the other electrode. Think of an onion cut into two halves where the shells of the onion represents the electric field.

The electricity will flow everywhere in the material, however it will want to flow more (more current density) through conductive mediums and less in resistive (less current density) mediums. During the time the electricity is injected we measure the potential field. The potential field is like a map of voltage. You could stick the two probes of a volt meter into the ground and measure some voltage. This is actually what our equipment does but it does it automatically and very quickly at many thousands of combinations of current injection locations and voltage measurements.

In a homogeneous material (a material with the same resistivity property) the electric field will be uniform and un-distorted like the onion shells. Now lets think of one of the most intuitive cases our equipment is used, exploring for tunnels and caves. Imagine that we place an air filled cave in the middle of our homogeneous subsurface. Since air is very resistive at low voltages the electricity will have to "go around" the cave. This creates a distortion of the electric field. We measure these distortions at the surface and/or in boreholes. In order to properly define these distortions we have to make many thousands of measurements.

Back to the imaging part...in order to create a 3D model of the ground we use computers to perform all the dirty work for us. This particular step involves solving lots of non-linear equations in a smart way using. The mathematical approach is called Finite Element computation. To make things more difficult, it turns out that the data (volt measurements) we collect can be reproduced with an infinite number of computer models. We then use some statistical norms to narrow down the solution that fits our data but not so well that it incorporates the inherent noise in our hardware, software, computer rounding errors, telluric noise from the Earth's electric storms, Magneto-telluric noise from our Sun's interaction with our Earth's magnetic field, streaming potentials due to moving fluids in the ground, self potential due to acid rain oxidation of minerals, and positional errors we have made hammering the electrodes in the ground at a grid.

Some of the uses for Electrical Resistivity Imaging by industry:
Geotechnical: finding electrical properties of sub-surface, grounding grid evaluation (IEEE Fall of potential and ASTM G57)
Geophysics: exploration for water, mineral deposits, pollution, geologic features
Archeology: determine where to dig, find artifacts, looking for treasure (really), image pyramids, catacombs, burial sites
Civil engineering: locate old mine shafts, find sink holes, locate competent ground to build on, define aggregate deposits for road and infrastructure, dredge channels, determine weakness' in levy's
Mining: locating ore deposits, provide safety for mine workers by monitoring water flow into mine, coal mining, leach pad imaging
Petroleum: locate and re-mediate spills, maintaining drilling collection ponds, some shallow oil exploration has been done but it really isn't useful for this since oil is usually too deep for us to image
Military applications: looking for prison tunnels, mass graves
Environmental: image pollution plumes, locate pockets of pollutants, finding DNAPL (chlorinated solvents like Creosote)

Our website has lots of case histories: www.agiusa.com

#### kwired

##### Electron manager
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.

Soil with a low resistance is still high enough resistance that at low voltages (below 600 volts) that it almost is never low enough to allow high enough current flow to open an overcurrent protection device quickly to eliminate possible other hazards resulting from what ever has happened to cause this current to be flowing in an unintended path in the first place.

If you get soil resistance of 10 ohms which is pretty low for most soil and subject it to 120 volts you will only get 12 amps of current flow - not enough to trip a fuse or breaker for the typical general purpose or lighting branch circuit. Reduce the resistance to 1 ohm and now you have a good enough conductor that a significant amount of current will flow that that overcurrent device will open pretty quickly. A good equipment grounding conductor will have even less than 1 ohm of resistance unless you have an excessively long run - but that is another situation to deal with by using voltage drop calculations.

#### markusius

##### Member
Resistivity vs resistance

Resistivity vs resistance

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.
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.

Resistance=Volt/Current=Resistivity*Length/CrossSectionalArea

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.

#### kwired

##### Electron manager
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.

Resistance=Volt/Current=Resistivity*Length/CrossSectionalArea

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.
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.

The only real reason to "earth" a conductor of a system is for lightning dissipation. If we would stop using grounded conductors as current carrying conductors, or at least only ground each system at the source and separate grounded and grounding conductors beyond the source (like we do for NEC installations) a lot of stray voltage problems would disappear.

It has been mentioned on this forum before - the earth is a very good conductor, but it is difficult to make a low resistance connection to it. You can drive many ground rods and never achieve low enough resistance to earth for what you may be looking for.

Conductivity required to allow current from lightning to flow (at thousands of volts) does not need to be nearly as great of conductivity as it does to just allow a few amps at 120 volts to flow.

Resistance is a term used with AC current. A resistive load is resistance whether it has AC or DC current flowing through it.

Impedance is the effective resistance when you put togther resistance, impedance, and reactance.

#### weressl

##### Esteemed Member
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?
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.

#### markusius

##### Member
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.
Interesting!

Is this a special case for multi storied buildings?

Are there any grandfathered issues to consider with the age of the building?

#### iwire

##### Moderator
Staff member
Interesting!

Is this a special case for multi storied buildings?

Are there any grandfathered issues to consider with the age of the building?
What he describes is typical for industrial and research buildings.