Isolated Grounding for Equipment

[Machiavelli999]

I have a three phase piece of equipment whose vendor is requesting an "Isolated Ground". Old story...I know.

We provided an isolation transformer upstream of the equipment. We thought this would suffice, but the grounding conductor is still bonded to the secondary breaker right near the transformer and the disconnect near the equipment. The vendor does not like that and claims that the bonds at the breaker and disconnect box invalidate the isolation.

We suggested to bring another ground wire from the X0 terminal of the isolation transformer which he can use as the isolated ground for his electronics inside his equipment. He is fine with this, but also wants to disconnect the EGC and use the isolated ground to ground the enclosure.

I am 90% sure this is illegal, but I can't find anything in the code that would disallow it. Also, this would return all the fault current back to the source so I am not sure as to the reason why it would be illegal. Any ideas?

 

[John120/240]

For machine tools-CNC equipment you can drive a ground rod right next to the tool. This will make the CNC manufacturers happy, but provides limited benifits

 

[ActionDave]

I have a three phase piece of equipment whose vendor is requesting an "Isolated Ground". Old story...I know.

We provided an isolation transformer upstream of the equipment. We thought this would suffice, but the grounding conductor is still bonded to the secondary breaker right near the transformer and the disconnect near the equipment. The vendor does not like that and claims that the bonds at the breaker and disconnect box invalidate the isolation.

We suggested to bring another ground wire from the X0 terminal of the isolation transformer which he can use as the isolated ground for his electronics inside his equipment. He is fine with this, but also wants to disconnect the EGC and use the isolated ground to ground the enclosure.

I am 90% sure this is illegal, but I can't find anything in the code that would disallow it. Also, this would return all the fault current back to the source so I am not sure as to the reason why it would be illegal. Any ideas?

You are correct you need an EGC and is code required. The isolated ground is redundant and is optional.

What is the wiring method between the transformer and the equipment? If it qualifies as an equipment ground you can give the vendor what he wants.

 

[GoldDigger]

I have a three phase piece of equipment whose vendor is requesting an "Isolated Ground". Old story...I know.

We provided an isolation transformer upstream of the equipment. We thought this would suffice, but the grounding conductor is still bonded to the secondary breaker right near the transformer and the disconnect near the equipment. The vendor does not like that and claims that the bonds at the breaker and disconnect box invalidate the isolation.

We suggested to bring another ground wire from the X0 terminal of the isolation transformer which he can use as the isolated ground for his electronics inside his equipment. He is fine with this, but also wants to disconnect the EGC and use the isolated ground to ground the enclosure.

I am 90% sure this is illegal, but I can't find anything in the code that would disallow it. Also, this would return all the fault current back to the source so I am not sure as to the reason why it would be illegal. Any ideas?

The isolation transformer could (must?) be treated as a separately-derived system. In that case there must be an X0 to ground bond if the circuit in question uses an X0-connected neutral.

The NEC allows you to run the isolated ground wire back to a panel or panels back to the main service bond or to the ground-neutral bond point of a separately derived system. It does not generally allow you to omit the EGC, connect the EGC only to an ungrounded/unbonded X0 or even connect it directly to a separate ground electrode. (with a few specific detailed exceptions, IMHO.)

See 250.146(D) [2011]

...so as to terminate within the same building or structure directly at an equipment grounding conductor terminal of the applicable derived system or service.

If this is done, the metal of the equipment (or the box containing the isolated ground receptacle) does not have to be connected to the raceway supplying although that raceway still has to be bonded/grounded somewhere too. This allows the raceway (if metallic) to serve as a shield without enabling it to carry ground loop current, while eliminating any offset of the equipment ground that might result from stray currents from other equipment in the area.

 

[GoldDigger]

For machine tools-CNC equipment you can drive a ground rod right next to the tool. This will make the CNC manufacturers happy, but provides limited benifits

And although you can drive and bond a local rod, you still cannot avoid bonding the equipment to the equipment grounding terminal of the service or system that supplies, IMO.
If the isolation transformer is right at the equipment, the two points may be one and the same.

 

[Machiavelli999]

Thanks for the responses. The isolation transformer is not right at the equipment. Hence we have a disconnect at the equipment and an enclosed circuit breaker to protect the secondary feed at the transformer. The problem is we do have a ground from the X0 terminal of the transformer that goes all the way to the tool. But this ground is also bonded to the circuit breaker and disconnect enclosures. This is what the tool vendor doesn't like.

Can I have one EGC for the tool disconnect and another EGC for the tool itself? Keep in mind that both would bond back to the transformer X0 terminal.

 

[GoldDigger]

Thanks for the responses. The isolation transformer is not right at the equipment. Hence we have a disconnect at the equipment and an enclosed circuit breaker to protect the secondary feed at the transformer. The problem is we do have a ground from the X0 terminal of the transformer that goes all the way to the tool. But this ground is also bonded to the circuit breaker and disconnect enclosures. This is what the tool vendor doesn't like.

Can I have one EGC for the tool disconnect and another EGC for the tool itself? Keep in mind that both would bond back to the transformer X0 terminal.

Congratulations! You have figured out exactly how it has to be done. :thumbsup:
All of the raceways and equipment enclosures (including disconnects) will need a continuous EGC that stops just short of the tool. The only EGC connection to the tool will be the insulated isolated ground wire which runs directly with no other connections all the way back to the X0.
This means that you will have to use PVC, NM, or metallic sheathed cable with the ground/sheath deliberately kept insulated at the tool end to make the last step connection to the tool.

 

[ActionDave]

Can I have one EGC for the tool disconnect and another EGC for the tool itself? Keep in mind that both would bond back to the transformer X0 terminal.

Yes. You can have two of each or more if you like. As far as the NEC and safe practices are concerned you only need one; and it has to do what it is supposed to do. Add more and as long as they do what they are supposed to do no worries.

 

[GoldDigger]

Yes. You can have two of each or more if you like. As far as the NEC and safe practices are concerned you only need one; and it has to do what it is supposed to do. Add more and as long as they do what they are supposed to do no worries.

The key point for the OP is that if they are connected together at the tool end, or anywhere in between closer to the tool than the X0, they will be multiple redundant grounds (which some hospital installations require) not an isolated ground.
The tool manufacturer, out of theory, experience or just superstition feels that the ground isolation is important, and the NEC specifically addresses that situation and permits it.

 

[petersonra]

you can run as many EGCs back to the transformer as they can afford.

 

[GoldDigger]

you can run as many EGCs back to the transformer as they can afford.

Since it is three phase equipment, maybe you should run 3 insulated EGCs.
That is also why whenever you have three ground rods they are in a triangle.