Grounding for containerized building 1

[oscaril]

One of the electricians I work with says that the metal external walls of the container should be grouned to the earth separately (mechanical grounding)from a grounding rod having connection with the grounding bus of the panelboard (electrical grounding)in the building, so that there wouldn't be a single grounding system. Have you ever had such a practice?

 

[davidbeach]

I don't know where you are, but if you are governed by the NEC, you would have a code violation; all grounds shall be connected.

 

[waross]

After the electrician installs his individual grounds, the code in the US and Canada requires that his grounding system be connected to the main grounding system. Grounding is for safety. Under fault conditions the ground system does not alsways keep all protected objects at ground potential, it keeps all protected objects at the same potential. With poor soil conditions for grounding, the voltages of the protected objects may rise considerably above true ground potential.
This may result in dangerous potentials between the unconnected ground systems.

Consider your electrician. Supose he is drilling a hole in a piece of wood near the metal container. He is holding the drill in one hand and resting his other hand on the container.
The soil has poor grounding properties.
There is a fault or event upstream that results in the system neutral and ground assuming a potential that is elevated above true ground.
1> In a properly grounded system, with the grounds interconnected, the grounded drill case will be at the same potential as the metal building.
2> Separate grounds. The grounded drill case will be maintained at the same potential as the system ground.
The metal container will be at true ground potential.
The difference between system ground potential and true ground potential may easily be "terminal".
Remember that a fault (or lightning strike) close by on a primary circuit may elevate the ground potential by several hundred volts or more. This voltage rise may be present on the secondary grounds also.

Now, we left our electrician with one hand on a grounded dril motor and the other hand on a grounded metal container.
A block away, a car hits a power pole and a 13,800 volt primary conductor breaks and falls onto the neutral conductor. (7967 Volts to ground)
With poor grounding conditions in this area the system ground rises several hundred volts above true ground potential.
Now with you right hand connected to the system ground (at several hundred volts above true ground) and your left hand connected to a grounded metal building, do you want the building connected to "True" ground or would you prefer that it was connected to and at the same potential as the system ground?
Another reason that all grounds and ground systems should be connected together.
respectfully

 

[BraJacTech]

I have seen where the equipment within the containerized building is grounded to an isolated bar within the container and then, when installed at site, the ground bar and the structure itself are both connected to the main system ground. The only time I've seen an independent grounding system is for control systems, which require an "isolated" ground system to get away from noise and switching surges. But even such equipment still has an equipment ground which is connected to the main.

 

[davidbeach]

The "isolated" ground would be a code violation and a safety hazard, recommended by someone who doesn't understand the issues.

 

[BraJacTech]

The "Isolated" ground sytem referred to by me is on the controls/instrumentation side where all of the individual sheild drain wires ultimately end up to ground. There is no interconnection between this ground and the equipment "safety" ground.

The control equipment itself is grounded to the main system in the same manner as all other electrical equipment, but also carries the isolated ground for the control signals.

 

[waross]

As davidbeach said. This is a code violation in both Canada and the US.
In a properly designed and installed power system there is typically no noise on the ground, and few switching surges reflected on the ground.
There are several conditions which may lead to elevated voltages on the grounding system. In the event of elevated voltages on the ground system, it is important that all ground systems be interconnected so that an equi-potential zone is maintained in the installation or facility.
I used to have a grounding safety information package issued by a major Utility. It included accident investigation reports from a number of accidents.
Some incidents resulted in the death of workers.
Some incidents resulted in injury to workers.
Some incidents resulted in electric shocks that were severe enough that the incident was investigated.
In every instance, the accident was the result of voltages between two different GROUND SYSTEMS that were NOT connected together.
I echo davidbeaches comment about people who don't understand grounding systems.
respectfully

 

[davidbeach]

Many years ago in one on the high tech facilities in the "Silicon Forest" west of Portland, OR, someone tried to install an "isolated ground" without proper permission or oversight. Said person found a rotohammer and knocked a hole in the concrete slab floor and started to drive in a ground rod. After a while he started felling a tingling, or worse, as he held onto the ground rod as he tried to drive it into the ground. It got bad enough that he stopped and called for assistance to investigate. Upshot was that the room had to be sealed off until the next time the 480V circuit he intercepted could be taken out of service so that the ground rod could be deenergized and safely removed and the circuit repaired. Only one conductor damaged, and so little current to ground that it never tripped a ground fault device. The "isolated ground" if installed would have accomplished what? Nothing! There is no useful "isolated ground" or "quiet ground". If it is truly to be ground, it has to be ground, part of the whole ground system. Sure you can gather up all your signal grounds and connect them to the ground system at a single point, but it has to be to the ground system, not to some isolated point.

 

[waross]

I think that there is a historical reason for the precieved need for isolated grounds.
I do believe that this preceived need follows from from faulty analysis of some problems years ago which resulted from code violations. As sometimes happens, even though the reason for the issues were misunderstood, the solution cured the problem.
The solutions have been passed down for a few generations now like folklore or urban ledgends.
I'm out of time this morning. I expand on this later today.
respectfully

 

[BraJacTech]

I stand (or sit rather) corrected. My experience comes from the offshore where eventually, everything ends back to the vessel hull. The current plant I'm at in Northern Alberta shows details for the "isolated" ground system, and there is indeed another tail from the grounding delta back to the structure.

Waross, if you can continue with your explanation I would love to read more.

Cheers,

Trev

 

[jghrist]

See Demystifying Isolated Grounded Systems — Part 1

http://ecmweb.com/mag/electric_demystifying_isolated_grounding/index.html

and Demystifying Isolated Grounded Systems — Part 2

http://ecmweb.com/mag/electric_demystifying_isolated_grounded/index.html

from EC&M Magazine

 

[BraJacTech]

Cheers jghrist!

 

[waross]

Equipment grounding methods have evolved over the last 40 or 50 years to the point where the original issues are (mostly) no longer present.
The first widespread commercial applications of data transfer comunications that I am aware of were central computers and data terminals integral to cash registers.
Equipment grounding methods in the 40s and 50s were liable to deterioration. Typically the raceways were used for equipment grounding and the both the old style rigid couplings and to an even greater degree the indent style couplings on EMT were prone to developing high resistance paths. Armoured cable in use before the 50s did not have a grounding conductor. The armour provided a spiral, high resistance ground path with a high reactance due to the spiral. (Due to surface corrosion the current tended to flow around the armour rather than crossing all the joints and flowing in a straight path.
EMT was typically coupled with indent style couplings (illegal in Canada for decades). These were prone to developing high resistance.
Rigid couplings up until I think about the 70s were quite loose. They were designed so that the conduit ends could butt together inside the coupling. Again subject to developing high resistance over time.
When computer based cash registers were introduced to grocery stores, many of the buildings dated back to the 40s and 50s.
Now the data techs made a few mistakes.
1> They did not read the codes which specifically stated that no piece of equipment could depend on a ground connection for proper operation. (Any exceptions did not apply to data loops.)
2> 2> They assumed that any points shown on a drawing as connected to a “ground” symbol were at the same potential. Fine for chassis connections and lab tests, but not realistic in the real world.
1> They assumed that all the grounding systems in the real world were perfect.
2> They used ground as a signal return path to save the cost of an extra wire, or for circuit simplicity. By so doing, they introduced all sorts of resistances and impedances into the signal path.
Now over the last several decades, grounding methods and practice have improved to the point that any circuit grounded in compliance with the codes with a copper conductor is as good or better than any extra grounding conductor imagined by the data people.
An isolated ground that is not connected to the main grounding system can be the source of dangerous voltages in signal circuits. See David’s anecdote and my hypothetical example in previous posts.
IMHO collecting all the instrument and/or data grounds on a dedicated ground bar is good engineering.
Further, connecting this dedicated/isolated ground bar to the system grounding electrodes with a copper jumper size according to the codes is good engineering.
An isolated ground rod, quite ground, tripod ground, etc., is urban legend or folk lore which will cause no harm if properly connected to the system ground, but which may be a potentially lethal hazard if left isolated.
Grounding is NOT about providing a signal path. The purpose of grounding as stated by the codes is to avoid, reduce or eliminate dangerous voltages on the surface of electrical equipment. This may be the source of some of the misunderstandings concerning grounding.
respectfully