New Isolated Ground

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I work as a maintenance electrician in a metal foundry. A new piece of equipment in the lab requires an isolated ground. The panels here are from the 70's and there is no ground system at all beyond the neutral. I am adding an isolated ground bar and pulling a #8 to a ground rod outside. I am installing another ground rod 6' from the other to ensure the resistance is low enough for the sensitive piece of equipment. Can I keep the 8 wire continuous through the 2 ground rods? Does the wire between the 2 rods need to be bare?
 

ron

Senior Member
Your method of establishing an IG is not compliant. You cant have separate ground rods from the grounding electrode system.

The IG typically is derived at the last N-G bond before the panel you want it at (first upstream transformer or SDS), and gets extended from the N-G bond equipment through the distribution to the one you want it at.
 

infinity

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Location
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Journeyman Electrician
I work as a maintenance electrician in a metal foundry. A new piece of equipment in the lab requires an isolated ground. The panels here are from the 70's and there is no ground system at all beyond the neutral. I am adding an isolated ground bar and pulling a #8 to a ground rod outside. I am installing another ground rod 6' from the other to ensure the resistance is low enough for the sensitive piece of equipment. Can I keep the 8 wire continuous through the 2 ground rods? Does the wire between the 2 rods need to be bare?

You're method is not code compliant and is downright dangerous.
 

don_resqcapt19

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Location
Illinois
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retired electrician
Can you please explain to me why this is dangerous?
You will have no fault clearing path and in the event of a fault, the non-current carrying parts will be energized and there will be a serious shock hazard if you touch the equipment and are in contact with any grounded item.
 
The equipment grounding conductor (EGC) needs to, somehow, tie back to the grounded conductor (aka, "the neutral") as a fault current path; that's the only way to trip the breaker on a short-to-ground. IG systems are only isolated in that they use a separate insulated EGC from where the neutral is bonded to the grounding system (SBJ/SSBJ/etc) out to the outlet/equipment. And if that equipment, say, has a water pipe connection, or has metal-to-metal contact with some other grounded thing, all your isolation goes away anyway.

For most purposes, IG systems are too much hassle and of no real use.

Oh, and the presence of what looks like a EGC that doesn't actually give fault path is what makes it dangerous.
 
Thanks for the help everyone. To make sure I'm on the right track, here's what it looks like:
-New dedicated 220 outlet
-MC Cable into junction box above ceiling.
-Ground from MC cable ties into ground going to panel, but is not bonded to junction box.
-Ground into panel goes to isolated ground bar.
-Ground from isolated ground bar ties into NG Bond in upstream transformer.
 

Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
Can you please explain to me why this is dangerous?

I'll do you one better, it won't accomplish what an isolated ground is specified for and will likely be worse than just using the regular ground. Equipment that needs an isolated ground needs it as a reference point to zero volts. Zero volts is a relative quantity not an absolute one. The reference is the bond point of the system. It needs to measure the potential between the hot wire and the zero point. When the path back to the zero point is through the ground rod and the ground it can be far more voltage than if it is merely travelling through the various solid grounds of the normal grounding system.
 

kwired

Electron manager
Location
NE Nebraska
Is this equipment requiring an isolated ground or a supplemental grounding electrode?

Either way you must still be bonded to the equipment grounding conductor of the supply system.
 

Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
This link may be helpful

https://www.quora.com/What-is-isolated-ground-in-an-electrical-circuit



Sent from my LG-D415 using Tapatalk

While the safety aspect of the link seems right, I don't agree about IG primarily being used in patient care areas. In fact, I am pretty sure it is the exception not the norm and creates issues that are not well dealt with in the code. An IG and a conduit ground, don't create parallel grounds in compliance with NEC 514 Another green wire will still need to be run. But the receptacle would have no access to either of those redundant grounds.
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
Thanks for the help everyone. To make sure I'm on the right track, here's what it looks like:
-New dedicated 220 outlet
-MC Cable into junction box above ceiling.
-Ground from MC cable ties into ground going to panel, but is not bonded to junction box.
-Ground into panel goes to isolated ground bar.
-Ground from isolated ground bar ties into NG Bond in upstream transformer.

Not compliant. The sheath of standard MC does not qualify as an EGC on it's own. Since you have used the grounding conductor in the MC for the isolated ground you now have lost the ability to bond all the metallic items in the circuit to the EGC.
 
A new piece of equipment in the lab requires an isolated ground. I am adding an isolated ground bar and pulling a #8 to a ground rod outside. I am installing another ground rod 6' from the other to ensure the resistance is low enough for the sensitive piece of equipment. Can I keep the 8 wire continuous through the 2 ground rods? Does the wire between the 2 rods need to be bare?

Isolated Ground? I think it is not really isolated but rather it is connected to the system ground in a star configuration and not in series or daisy chain (box A bonded to grounded neutral, then box B bonded to A, then box C bonded to B, etc.) The OP wishes to minimize the ground noise (isolated) at box D from A,B, and C. In this case D should be bonded directly to the grounded (neutral) service entrance conductor. By doing this, it is code compliant and any leakage ground current (ground loop current or inductive generated current from high power electric motors and such) from A,B,C will not cause the ground reference voltage on D to jump up and down (which might be bad (signal and data wise) for sensitive equipment powered from D); therefore this star connection provides D with an "isolated" or "island" ground.

For this purpose, then why D panel needs to be connected to grounding electrode? Is D in a different building away form A,B,C that it needs to be connected to grounding electrode?
 
The OP wishes to minimize the ground noise (isolated) at box D from A,B, and C. In this case D should be bonded directly to the grounded (neutral) service entrance conductor.

But only if box D is the actual service disconnect or the first disconnect of a Separately Derived System (or one of the few other places where it's allowed). Everywhere else the Grounded conductor must be isolated from the Equipment Grounding Conductor. (There are a few exceptions, but they're not relevant.)
 
I think I might have confused you. No, I didn't say bonding D enclosure directly to neutral conductor in D, but rather bonding D to A, where A is the service disconnect with grounded neutral.
 
I think I might have confused you. No, I didn't say bonding D enclosure directly to neutral conductor in D, but rather bonding D to A, where A is the service disconnect with grounded neutral.

If I was confused, that's probably because your writing was not clear at all. Assuming metallic boxes, bonding D to A is accomplished by normal installation practice, although it will touch B and C along the way. That is, unless you mean adding a separate conductor directly from D to A but that won't add much of anything.

What you did not say is "run an insulated conductor from the receptacle or utilization equipment all the way back to the SSBJ", but even that wouldn't do much good unless the equipment itself is completely isolated from all other "grounds", which can be hard to do for anything bigger than a toaster. (Search around these forums for more about "isolated grounds".)

And running to the actual service disconnect would not be a good idea if the equipment is supplied by an SDS. Heck, it might not be possible at all.
 

Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
If I was confused, that's probably because your writing was not clear at all. Assuming metallic boxes, bonding D to A is accomplished by normal installation practice, although it will touch B and C along the way. That is, unless you mean adding a separate conductor directly from D to A but that won't add much of anything.

What you did not say is "run an insulated conductor from the receptacle or utilization equipment all the way back to the SSBJ", but even that wouldn't do much good unless the equipment itself is completely isolated from all other "grounds", which can be hard to do for anything bigger than a toaster. (Search around these forums for more about "isolated grounds".)

And running to the actual service disconnect would not be a good idea if the equipment is supplied by an SDS. Heck, it might not be possible at all.

You are still confused. Since I know what an isolated ground is, I fully understand what Brian was saying, both times. When he said run back he meant electrically. If I have a single 20A IG circuit I can legally take the green wire (with yellow stripe allowed) and run it to and through the branch panel, through the conduit containing its feeder and continue until I get to the physical location where the neutral and the ground are bonded together as required in NEC 250. That is what Brian clearly meant. Or I can run an isolated ground wire from that service bond to each subpanel terminating on a separate isolated ground bus that is physically just like a neutral bus, which I can then land the incoming wires from the receptacle feeding the piece of equipment. In any case an isolated ground extends from the receptacle to the bonding point of the service or separately derived system without touching ground along the way, period.

You supposition about isolating the equipment itself is not relevant. What happens when the equipment is plugged in to the outlet is for them to design. If they are concerned about exposed metal parts being unintentionally grounded they can design it so they don't have any.
 

cuba_pete

Senior Member
Location
Washington State
...but even that wouldn't do much good unless the equipment itself is completely isolated from all other "grounds", which can be hard to do for anything bigger than a toaster. (Search around these forums for more about "isolated grounds".)

I have hundreds of pieces of equipment, most mounted in standard 19" racks (some larger), which all utilized a single-point ground. Mine is just one of hundreds of facilities that use this method.

It can, and is, done.

You must only have one ground. How your equipment gets there is up to you.
 

iwire

Moderator
Staff member
Location
Massachusetts
I have hundreds of pieces of equipment, most mounted in standard 19" racks (some larger), which all utilized a single-point ground. Mine is just one of hundreds of facilities that use this method.

It can, and is, done.

You must only have one ground. How your equipment gets there is up to you.

Help me out here.

How does the IG stay isolated when it is connected to multiple pieces of equipment on a metal rack?
 
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