Different resistance at the same network

[m sleem]

For the record, I was not bashing, nor was my reply intended to be offensive in any way. I got your meaning from the get go. My response was a play on words stemming from the lack of detail pertinent to providing any semblance of an intelligible answer. 15 replies later, we get the first glimmer of detail...

OK, after you got the first glimmer of detail where is your comment .

 

[m sleem]

The specifications for ground resistance (measured how?) from different engineers and attached to different products may vary. The reason could be serious (as perhaps if the equipment generates electromagnetic interference which needs to be shielded) or spurious (as when the earth ground is mistaken for a fault clearing path.)
And the actual measurement from grounding conductor or auxiliary electrode to remote earth, will consist mainly of the ground electrode resistance, with wire or raceway impedance playing only a small part.
If you see large (ohms) differences in measurement either the measuring technique is flawed or the ground system bonding is poor.

Let me summarize that for convenience:
If you are saying what I think you are saying, then you are right. They should all be almost identical in practice.
Sent from my XT1080 using Tapatalk

Thank you sir, the specs that i'm referring to are an international specs basically based on US codes (NFPA, NEMA, ANSI....etc), the project that we are in is a hospital & the grounding spec indicates the max allowable resistance for each system, e.g. 1 ohm for IT system, 2 ohm for IPS, 3 OHM for building grounding, my first question if all of these systems are connected to one network & typical one resistance accordingly, why we specify a different resistances per system? otherwise we need to achieve 1 ohm at max if the resistance is typical for all.

 

[K8MHZ]

Thank you sir, the specs that i'm referring to are an international specs basically based on US codes (NFPA, NEMA, ANSI....etc), the project that we are in is a hospital & the grounding spec indicates the max allowable resistance for each system, e.g. 1 ohm for IT system, 2 ohm for IPS, 3 OHM for building grounding, my first question if all of these systems are connected to one network & typical one resistance accordingly, why we specify a different resistances per system? otherwise we need to achieve 1 ohm at max if the resistance is typical for all.

Q) Why are there different specs for different systems?

A) The systems have different requirements for bonding and grounding. An IT system needs a near perfect grounding / bonding to keep the system's data clean. The building premises wiring's grounding / bonding is to keep metal parts at near or the same voltage. Intrusion Protection Systems will have a reqirement that satisfies both.

Every system would benefit from a perfect 0 ohm value, but the best we can get is the closest to 0 that will work and is within the scope of reasonable financing. If the IT guys could be assured their system would provide the service it was sold to do using 25 ohms, that's what they would spec it at. But, the IT guys are requiring a 1 ohm connection, that's what they need to assure a quality system and what the other systems require is none of their concern.

It's the same with the other systems. So long as their requirements are met, the requirements of the other systems on the same network are none of their concern.

Q) Since all the systems are going to be bonded together, why not just design the entire network at 1 ohm?

A) The lower the resistance, the more expensive the task will be to achieve it. If this was an IPS only, the extra work and materials to achieve 1 ohm would be unnecessary and a waste of money and effort.

Although a poor analogy, it's like a shop that uses air pressure to run machinery. In this example the pressurized air is from the same source and shares the same pipes. The press room may need a 100 psi 90 cfm supply while shipping may only need 30 psi 10 cfm. The assembly line may need 125 psi at 50 cfm. The requirements are different because different equipment is being used. The staple gun in shipping won't have the same requirement as the 1" air impact wrenches in production.

So, since all the systems are together, their unregulated pressure will be the max value of the system. The only way for each of them to have their own unregulated pressure would be to have their own supply and not be on the 'network'.

Since your issue involves electrical bonding on the same premises, all system and device grounding / bonding must be connected together for safety and compliance to the NEC. AFAIK, the only specific value the NEC requires to be met is the 25 ohm value for a single rod or plate electrode. That is far to high for many applications and is the bare minimum. Design engineers may need a much tighter value, say 1, 2 or 5 ohms as 25 ohms is pretty lousy.

Q) Since the required values of 1, 2 and 3 ohms are very close, why can't the designers come up with a standard for all low impedance systems?

A) One reason is that having to adhere to a standard of 1 ohm when only 3 is needed adds cost and takes away some of the designer's freedom to be creative. Another is that different systems are going to have different standards, anyway. You still have to deal with the different systems on the same network. And, if you use 3 ohms for the standard, a system that needs 1 ohm will be compromised and the designer will just require the 1 ohm anyway.

Am I properly addressing your question?

 

[Smart $]

OK, after you got the first glimmer of detail where is your comment .

By the time I read your posts with pertinent information, another participant has already provided you a response to which a comment from myself will not add any benefit.

 

[m sleem]

................................
Am I properly addressing your question?

That was the best answer i could find, thanks a bunch.

.............................................And, if you use 3 ohms for the standard, a system that needs 1 ohm will be compromised and the designer will just require the 1 ohm anyway.

May be in this part i did not understand you very well, in my case, shall i consider 1 ohm for all systems for the best design? since the 3 ohms will be compromised for the 1 ohm system.

 

[m sleem]

By the time I read your posts with pertinent information, another participant has already provided you a response to which a comment from myself will not add any benefit.

It was just kidding, you already answered me many times.

 

[Sahib]

I think it is the resistance of EGC that is specified in respect of grounding network of IT. If the resistance is any higher, it may change the signal state: from 0 to 1, for example.