I&C Engineers vs NEC 250 and 645

Status
Not open for further replies.
K

kenaslan

Senior Member
Location
Billings MT
Does anyone have a good argument besides "that's what the Code states and that is how we will do it" To convince an I&C engineer that the isolated ground in a DCS/PLC/Instrument cabinet MUST be bonded to the EGC. I even had a PE tell me that as long as you go to a ground rod if there is a short, it will trip the breaker. After a quick refresher on ohms law did he see the error. But alas, how do I convince an Instrument engineer to properly bond their equipment. Even the drawings from a manufacture insist on no neutral to ground bond XO is neutral, and the case goes to a ground rod. Never to be connected. They don't seem to care what the NEC says.
 
Not sure I'm following what you're asking exactly. Why the isolated ground needs to be bonded to the grounding system? The isolated ground shouldn't be bonded to the normal EGC in the instrument panel via the backplane, but does need to be bonded to the grounding system somewhere upstream of the instrument panel. If they're trying to achieve isolation using a local ground rod, then you already mentioned the reason not to do that, which is Ohm's law will show insufficient current flow during a fault via a ground rod alone. Start getting them to think of it as "Insulated Ground" or "Separate Ground" rather than "Isolated Ground" and that may change their perception.
 
EC Dan said:
Not sure I'm following what you're asking exactly. Why the isolated ground needs to be bonded to the grounding system? The isolated ground shouldn't be bonded to the normal EGC in the instrument panel via the backplane, but does need to be bonded to the grounding system somewhere upstream of the instrument panel.
Click to expand...
The insulated EGC would need to start at wherever the N-G bond is made for the SDS, not just "somewhere" upstream.
 
Good article on adding ground rods to CNC machines (or I&C equipment).
 

Attachments

  • Power Quality Considerations for CNC Machine Grounding.pdf
    930.9 KB · Views: 9
645.15 Equipment Grounding and Bonding. All exposed non–current-carrying metal parts of an information technology system shall be bonded to the equipment grounding conductor in accordance with Parts I, V, VI, VII, and VIII of Article 250...

In my case, the Isolated ground goes 800 feet to a IG bus, then to a triad another 100 feet away. That tiad is connected to the buried ground next to it. The panel supplying the 120V to the DCS is 400 feet in the other direction. We now have a ground loop. The Isolated Ground is to "drain" noise away from the electronics. However my limited understanding is that the loop will cause more problems. In addition there will be a difference in impedance between the two grounds and a nearby lighting strike will cause problems due to the difference in grounds. I believe that the IG should bond to the EGC at the panel, AND also be bonded to a ground rod driven at the location that is also tied to the grounding pad. This would be just like the requirements that a ground rod be driven when you bring power to a separate building, even though you have a EGC going back to the panel. NEC 250.32(A)
I ask as this is such a muddled part of the code.
 
kenaslan said:
645.15 Equipment Grounding and Bonding. All exposed non–current-carrying metal parts of an information technology system shall be bonded to the equipment grounding conductor in accordance with Parts I, V, VI, VII, and VIII of Article 250...

In my case, the Isolated ground goes 800 feet to a IG bus, then to a triad another 100 feet away. That tiad is connected to the buried ground next to it. The panel supplying the 120V to the DCS is 400 feet in the other direction. We now have a ground loop. The Isolated Ground is to "drain" noise away from the electronics. However my limited understanding is that the loop will cause more problems. In addition there will be a difference in impedance between the two grounds and a nearby lighting strike will cause problems due to the difference in grounds. I believe that the IG should bond to the EGC at the panel, AND also be bonded to a ground rod driven at the location that is also tied to the grounding pad. This would be just like the requirements that a ground rod be driven when you bring power to a separate building, even though you have a EGC going back to the panel. NEC 250.32(A)
I ask as this is such a muddled part of the code.
Click to expand...
Read the research paper I posted. Its not muddled if you understand grounding and bonding and that electrons return to their source in a line to case fault.
There is a lot of electrical myth information that seems to get passed on.
 
That is
tom baker said:
Good article on adding ground rods to CNC machines (or I&C equipment).
Click to expand...
That is from 1997 WAY out of date as far as the NEC is concerned. I am only concerned with Code compliance and safety. Please don't think I am flippant, however I dont care about anything other than being in code compliance. If WE violate the code we are held responsible. However if we design per code, and something happens or if the client does not install per code, not my problem.
 
kenaslan said:
645.15 Equipment Grounding and Bonding. All exposed non–current-carrying metal parts of an information technology system shall be bonded to the equipment grounding conductor in accordance with Parts I, V, VI, VII, and VIII of Article 250...
Click to expand...
IMO, a DCS is not an "information technology system".
 
I'm having a hard time picturing your facility. Where does your isolated ground originate from? Are you actually going into separate structures? A rough sketch would help to clarify. As it is, your suggestion of bonding to EGC at a panel and to a local ground rod does not sound correct.
 
EC Dan said:
I'm having a hard time picturing your facility. Where does your isolated ground originate from? Are you actually going into separate structures? A rough sketch would help to clarify. As it is, your suggestion of bonding to EGC at a panel and to a local ground rod does not sound correct.
Click to expand...
It might not be "correct" but it might be code legal, although I thought the rule was you had to use the closest grounding electrode, not add one.
 
This is a refinery, maybe 100+ buildings and structures. the power panel is in substation #2, The IG triad network is next to substation #5, the new DCS is in a new cabinet on a new gas pad that has a new gas chronograph. The new DCS has aprox 120 control cables going to different structures and pressure vessels. A DCS is part of a very large Information Technology System with tens of thousands of points and multiple redundant network communications (safety in case of failure of a single line) There is an underground bonding network to bond everything as this is a typical C1D1 and C1D2 area. This is the size of a small country town. The heat trace just for this small part is 38kVA.

So we always add/extend existing grounding network to new areas, and add more ground rods
 
I am still not convinced a DCS is an information technology system, although the rules are not substantially different whether it is or is not.

The code requires a grounding electrode system at every structure usually, regardless of its area classification or whether there is a DCS there or not, or for that matter whether there is an information technology system there.

There was once a widespread belief that intrinsically safe barrier grounds should be run back to the service, something about lightning, but I never quite got what they were trying to accomplish. They also used to connect instrument signal cable shields to this ground bar as well. I don't think connecting the shields to the bar would be a code violation as they are not required by code to be grounded at all.
 
Last edited:
kenaslan said:
That is

That is from 1997 WAY out of date as far as the NEC is concerned. I am only concerned with Code compliance and safety. Please don't think I am flippant, however I dont care about anything other than being in code compliance. If WE violate the code we are held responsible. However if we design per code, and something happens or if the client does not install per code, not my problem.
Click to expand...
It is still correct per the NEC. Later I will attach a graphic Mike Holt still uses based on this article.
 
Lets get back to the original question,

645.15 Equipment Grounding and Bonding. All exposed non–current-carrying metal parts of an information technology system shall be bonded to the equipment grounding conductor in accordance with Parts I, V, VI, VII, and VIII of Article 250...

As this is a separate structure, we are all in agreement that a grounding electrode must be added. The question is, should the Isolated ground in the new enclosure be bonded to the new/closest ground rod. or remain isolated for 100's of feet first to a isolated bus in a IT only building then to the isolated triad.

It is my belief that the code requires the initial IG to GEC/EGC bond to be at the new enclosure. If I am wrong, I would like to know and to learn more.

Here is a link to an article in Electrical Contractor https://www.ecmag.com/section/codes-standards/grounding-and-bonding-requirements-nec

Now lets say the hot, for whatever reason, comes in contact with the Isolated Ground. There is no "Low Impendence Path" back to the source. It will travel thru the IG network, to the IG triad. How many ohms between the triad, and the GEC at the transformer feeding the power panel? I am willing to bet it is a lot, MUCH more than 25 ohms. Lets do some simple math I stole from one of Mike Holts books. E/IR 120V 25 ohms = 5A. The OCPD will NEVER trip. So Sad Too Bad Your Dead. Hope he had good life insurance. That is why I believe that a bond is required at the new RIO DCS panel. To trip the feeder breaker.

The purpose statement of the NEC, section 90.1(A) states, “The purpose of this Code is the practical safeguarding of persons and property from hazards arising from the use of electricity.” The NEC DOES NOT CARE about noise on electronic equipment. It is all about life safety.
 
Re: Information Technology
Per the definition, this DCS system, extending to many buildings, is probably not an IT System
645.1 Scope. This article covers equipment, power-supply wiring, equipment interconnecting wiring, and grounding of information technology equipment and systems in an information technology equipment room.
Information Technology Equipment Room. A room within the information technology equipment area that contains the information technology equipment.
If all the wiring in the IT complies with 645.4 Special Requirements for Information Technology Equipment Room. The alternative wiring methods to Chap‐ ter 3 and Parts I and III of Article 725 for signaling wiring and Parts I and V of Article 770 for optical fiber cabling shall be permitted where all of the following conditions are met:
 
Status
Not open for further replies.
Top