Isolated ground

[dcv]

If a ground is to be a true isolated ground ,shouldn't the isolated ground have its own ground rod separate from any of the building electrical system grounding,why are these grounding systems kept separated until they get back to the building steel or the system ground at the panel? It seems that I'm missing the purpose and the point.
just thought I would ask
Craig

 

[charlie b]

Re: Isolated ground

Planet Earth is not involved in the current path. The ground rod is not relevant. Current leaves the source, and seeks its way back to the source.

If there is a leak from a hot conductor to case in any item of equipment, current will follow the EGC back to the source. If the leakage current is high enough, it will trip the breaker and terminate the event. If the leakage current is not that high, then there could be a leakage current flowing along the EGC for a long time. That will cause there to be a voltage drop along the length of the EGC. Any other equipment that has its EGC connected to the one with the leak will have the same voltage imposed upon it. Some special items of equipment could be damaged, or at least might not work right, if their metal cases have that voltage imposed upon them.

By installing a separate EGC from the ?special equipment? back to the source panel (not just to any sub-panels along the way), then the leakage current will not impose a voltage on the isolated ground wire. Thus, the leak will not have an adverse impact on the ?special equipment.?

 

[pqtest]

Re: Isolated ground

Hello Craig

Although not addressed by the NEC, one additional role of a buildings grounding system is to act as a zero reference point for technology based equipment such as computers. The concept of the isolated ground wiring method is to reduce the noise on the equipment grounding conductor that might cause digital electronics to mistake a "0" for a "1".

Charley B is correct- this type of system must be installed to safely operate the OC protection first and noise reduction second.

If you are considering installing, or have a customer who wants it installed, the effectiveness of the IG wiring method to reduce noise is debatable- no reports have been published providing any supporting evidence that this system works.


Regards,

Mark

 

[dereckbc]

Re: Isolated ground

The first issue is safety. Earth shall not be used as the sole equipment grounding conductor or effective fault-clearing path (NEC-250.4(B)(4)). So as others have pointed out if you were to have a fault imposed on the isolated ground in the manner which you suggest, earth impedance would not be low enough to even operate a 15-amp breaker.

In addition if you were to install an IG in the manner you suggest, it would likely add considerable noise to the system and cause malfunction. The only use for an IG is a possible means of obtaining common mode noise reduction on the circuit in which it is used. It has no other purpose and its effects are variable. Results from the use of IG range from no observable effect, desired effect, or worse noise. They are easily corrupted by interconnecting I/O cables that use ground or ground reference. If used the circuit should be a dedicated single branch circuit, and the equipment should not be interconnected to anything or make any contact with the building structural or electrical systems. Much better results are obtained by using isolation transformers using SG circuits.

 

[dcv]

Re: Isolated ground

Thanks
I love it when you heavy hitters answer these questions.

Earth shall not be used as the sole equipment grounding conductor or effective fault-clearing path

after passing through the cold water pipe or building steel isn't earth the last stop or maybe its better if I say it this way ,doesn't the earth make the building steel, cold water pipe or ground rod the effective ground source which they become.
Craig

 

[ron]

Re: Isolated ground

Consider that the primary plan for fault clearing is the neutral to ground connection at the service or separately derived source. This allows the fault to go to it's source's neutral without having to be overly concerned about the resistance of the earth and the associated grounding electrode system.

 

[don_resqcapt19]

Re: Isolated ground

dcv,
The main reason for having a grounding electrode for our electrical system doesn't have much to do with the electrical system itself...its main purpose is for lightning protection.
Don

 

[dcv]

Re: Isolated ground

Thanks all,
You guys again have really opened my eyes and mind
Brentp who has been reading along just pointed me to the Mike Holt articles
"Ground rod does not assist in clearing a fault" and "Grounding vs Bonding" 2 very helpful articles
Think maybe I need to go back to just reading and learning from the conversations between you heavy weighs.

Thanks again for making it clear to me.
Craig

 

[612278]

Re: Isolated ground

Hello-

This also may help you; NEC 250-146(d).
Good luck,
STEVE

 

[haskindm]

Re: Isolated ground

The "heavy hitters" are correct. The ground rod is not really part of the electrical system and the earth is not to be used as a conductor or even a "return point". Think of it this way: go out to your car and connect one lead of a 12-volt test lamp to the positive terminal of the battery. Connect the other lead of the 12-volt test lamp to a driven ground rod? Will the bulb light? How many ground rods would I need to drive to lower the resistance to earth enough for the bulb to light? The answer is that the bulb will never light until we establish a connection to the negative terminal of the battery! This circuit couldn't care less about "ground". For the circuit to be complete we must establish a return path to the negative terminal of the battery. The same is true of a premises wiring system. The electrons want to return to their source, not to earth.

 

[izak]

Re: Isolated ground

funny thing is though, if you set your car battery on a cement slab for too long, it will completely drain the battery and eventually ruin it... take an analog ohm meter and set you a car battery on a slab one day.... see how you have continuity to the slab from the negative post of the battery
i know it has nothing to do with the topic, but it Is interesting