"Isolated Ground" for DCS server room - NEC 250.54

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trallio

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Our firm is doing the engineering for the main power to a maintenance shop complex on an industrial site. The rest of the building is being engineered by an A&E. The Owner recently contacted me saying that he is challenging the Isolated Ground design by the other Electrical Engineer. They ran a ground wire from an "instrument ground bus" (a copper ground bus bar mounted to the wall) to outside of the building and to a ground rod or possibly a triad. There is no connection to the main building ground loop. When the Owner challenged this, their response was that a Supplementary Grounding Electrode is allowed to not be connected to the main ground loop per NEC 250.54.

250.54 appears to me to be for lightning rod down-comer ground rods and not for actual equipment or building grounds. I guess the ground rods for portable generators would also fall under this section.

I am sure that the "isolated" or "instrument" ground must be connected to building ground. Ground must be Ground.

I would like to explain to the Owner why 250.54 does not apply. He is, I'm sure, going to require that they are connected but he would like a little code back up so he doesn't get hit with an "extra" since NEC compliance is part of the contract requirement.
 
trallio said:
Our firm is doing the engineering for the main power to a maintenance shop complex on an industrial site. The rest of the building is being engineered by an A&E. The Owner recently contacted me saying that he is challenging the Isolated Ground design by the other Electrical Engineer. They ran a ground wire from an "instrument ground bus" (a copper ground bus bar mounted to the wall) to outside of the building and to a ground rod or possibly a triad. There is no connection to the main building ground loop. When the Owner challenged this, their response was that a Supplementary Grounding Electrode is allowed to not be connected to the main ground loop per NEC 250.54.

250.54 appears to me to be for lightning rod down-comer ground rods and not for actual equipment or building grounds. I guess the ground rods for portable generators would also fall under this section.

I am sure that the "isolated" or "instrument" ground must be connected to building ground. Ground must be Ground.

I would like to explain to the Owner why 250.54 does not apply. He is, I'm sure, going to require that they are connected but he would like a little code back up so he doesn't get hit with an "extra" since NEC compliance is part of the contract requirement.
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250.54 applies to aux grounding electrodes, such as the ones that serve no useful purpose that CNC manufacturers seem to advocate. You can have as many of these connected to an EGC where ever you want. For the most part they serve no useful purpose but are allowed.

Hard to know what "instrument" ground means. It is generally considered to be an EGC that starts at the service and is brought through the plant without being connected to any other EGCs or metal along the way and is mostly used to tie instrument shield wiring to. I am unconvinced it serves any real useful purpose.

As for isolated grounds, I am not sure you are allowed to "isolate" anything that is not a receptacle. All you are allowed to do is run an insulated EGC that does not have to be connected to the box the receptacle is located in (assuming it is a metal box). The insulated EGC does have to go with the branch circuit back to an EGC at some point upstream. So there is a connection at some point. It is just not required to be at the box the receptacle is located in.
 
trallio said:
a Supplementary Grounding Electrode is allowed to not be connected to the main ground loop per NEC 250.54.
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Incidentally, if I understand it correctly, the supplementary grounding electrode is the electrode added to supplement a single rod/pipe/plate type electrode or an underground metal water pipe type electrode. They are required to be bonded together as part of the GES. See 250.53 (A)(2) and 250.53(D)(2).
 
If the "instrument ground" connects to the equipment chassis or other exposed metal surfaces it must have a metallic connection back to the supply grounded conductor to serve as a fault current path (see zbang's citation). Without that a fault can energize those surfaces without tripping any OCPD.

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Instrument and Isolated ground are used interchangeably by controls/server/instrument guys to describe a ground system that is not tied to the main building ground so that there are no harmonics or "noise" on the ground. As an electrical guy, I take their requirements with a grain of salt and we usually run a ground wire from the server equipment room to a grounding triad or similar and then connect that triad to the main building ground. We minimize the connections to the main building ground but it can not be eliminated.

I had never seen anyone try to justify a fully independent ground system before using 250.54. It just doesn't apply. I was just looking for a good understanding of where 250.54 is intended to apply.



By the way, 250.54 applied to "supplementary" ground rods in 2005. As of 2011, it applies to "auxiliary" ground rods. I am trying to grasp exactly what it is intended for...
 
I think in 2008 they basically renamed what they had been calling supplementary electrodes in 250.54 to be auxiliary electrodes. Possibly because they also called supplementary electrodes supplementary and it was confusing.
 
GoldDigger said:
If the "instrument ground" connects to the equipment chassis or other exposed metal surfaces it must have a metallic connection back to the supply grounded conductor to serve as a fault current path (see zbang's citation). Without that a fault can energize those surfaces without tripping any OCPD.

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the instrument ground is not supposed to connect to anything other than the shields of instrument signals. How this can ever be enforced in a large plant escapes me. My guess is most instrument ground buses have multiple unintentional cross connections to the normal EGC bus.
 
petersonra said:
the instrument ground is not supposed to connect to anything other than the shields of instrument signals.
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And the shields are usually only supposed to connect at one end....

OTOH a bit of 22g drain wire makes a nice fuse :D.

That brings up an interesting point- using an instrument ground is completely wrong for an IT room anyway; it may be isolated, but it isn't an isolated-ground system as generally understood for power distribution.

Oh, and I just tripped over this in 645.15-
Where signal reference structures are installed, they shall be bonded to the equipment grounding conductor provided for the information technology equipment.

Is this room being designed to Art. 645?
 
zbang said:
And the shields are usually only supposed to connect at one end....

OTOH a bit of 22g drain wire makes a nice fuse :D.

That brings up an interesting point- using an instrument ground is completely wrong for an IT room anyway; it may be isolated, but it isn't an isolated-ground system as generally understood for power distribution.

Oh, and I just tripped over this in 645.15-
Where signal reference structures are installed, they shall be bonded to the equipment grounding conductor provided for the information technology equipment.

Is this room being designed to Art. 645?
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the shield is not generally a signal reference at all. it is just a shield that has no reference to the signal at all, although there are a few cases where the shield is used as a signal lead.
 
"Oh, and I just tripped over this in 645.15-
Where signal reference structures are installed, they shall be bonded to the equipment grounding conductor provided for the information technology equipment.

Is this room being designed to Art. 645? "


I think that pretty much nails it Zbang. As I am not actually working on the design for the building, only the substation and was just helping out the Owner on this question, I can only guess that the A&E has never heard of NEC 645.

Thanks!
 
Although...

645.15 does reference 250.54 auxiliary ground rods.

I still believe that the Ground needs to be Ground and you should tie the IT ground to the main ground in at least one place. But, I guess I couldn't blame someone for arguing it doesn't based on 645.15 and 250.54.
 
Remember that 645 is not mandatory for a data center. It is an alternative to simple Chapter 3 installation and it relaxes some general rules in exchange for additional specific requirements.

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petersonra said:
the shield is not generally a signal reference at all. it is just a shield that has no reference to the signal at all, although there are a few cases where the shield is used as a signal lead.
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Agreed.

No matter how one looks at it, there must be a metallic fault-clearing path.

(I love this sort of question- I always learn something, even if that's what I already knew was wrong.)
 
In the early 90s that company that I worked for did an isolated ground for the instrument ground of a DCS control system. It was insulated wire and run to a set of 3 ground rods about 50' away from the building. This was per the requirements of the DCS manufacturer. That summer they had a number of input and output card failures. They came back to us and asked us to prove that their ground was isolated. We told than that the isolation was the cause of the problem. They did not agree and had us prove that it was isolated. The next summer the same thing happened and the DCS manufacturer's response was the same. The third summer they let us bond their isolated ground to the plant grounding system and there were no more issues.
 
petersonra said:
the shield is not generally a signal reference at all. it is just a shield that has no reference to the signal at all, although there are a few cases where the shield is used as a signal lead.
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No, but the "common" of grounded DC instrumentation power supplies are typically bonded to the instrument ground, making the ground bus, and coincidentally the shields, the 0V reference.
 
trallio said:
Although...

645.15 does reference 250.54 auxiliary ground rods.

I still believe that the Ground needs to be Ground and you should tie the IT ground to the main ground in at least one place. But, I guess I couldn't blame someone for arguing it doesn't based on 645.15 and 250.54.
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Even per 250.54, these auxiliary grounding electrodes must be connected to the equipment grounding conductor... which in effect connects them to the building and system GES.
 
Smart $ said:
No, but the "common" of grounded DC instrumentation power supplies are typically bonded to the instrument ground, making the ground bus, and coincidentally the shields, the 0V reference.
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while this is true The Shield is no more a signal conductor than the EGC. Although it's not unusual to have people use it that way just as it's not that unusual for people to use the EGC as a signal path just because they can cheat and skip a wire and it works.
 
petersonra said:
...The Shield is no more a signal conductor than the EGC. Although it's not unusual to have people use it that way just as it's not that unusual for people to use the EGC as a signal path just because they can cheat and skip a wire and it works.
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And I'm in agreement. I wasn't trying to say otherwise.
 
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