NEC 250.86 Exception 1(3)

[erickench]

The above NEC exception states that Metal equipment added to existing knob and tube wiring shall not be required to be connected to the EGC if the metal enclosure is free from contact with ground(or earth). What would happen if the enclosure was in contact with the earth? Does this mean that the earth becomes a fault current path with it's high resistance?

 

[LarryFine]

The idea is to prevent energization of nearby surfaces or equipment due to an inetrnal fault. It's the same reason you mustn't use EGC's between grounding-type receptacles when added to similarly non-EGC-equipped circuits, like with GFCI's.

The exception requirements are inclusinve, by the way, meaning that all four must be met.

 

[erickench]

Lets assume that the enclosure was in contact with the soil. I mean "Ground" as the 2008 NEC defines it means "earth". What would happen if you had an EGC? I guess you would have parallel fault paths. One through the earth and one through the EGC.

Sorry but I just purchased a new 2008 NEC. We're two years behind the cycle here in NYC.

 

[don_resqcapt19]

Lets assume that the enclosure was in contact with the soil. I mean "Ground" as the 2008 NEC defines it means "earth". What would happen if you had an EGC? I guess you would have parallel fault paths. One through the earth and one through the EGC.

Sorry but I just purchased a new 2008 NEC. We're two years behind the cycle here in NYC.

Parallel paths with the EGC almost always exist and do not cause any type of problem. In fact since they are in parallel with the EGC, they reduce the impedance of the fault return path and decrease the time it takes for the OCPD to open the circuit.

In the exception you cited, I think the only reason it has to be free from connection to the earth or other conductive paths is because in the event of a fault to the metal enclosure, that fault would energize any conductive object that the enclosure is in contact with. Given that these are not solid fault return paths, it is likely that the OCPD would never see enough current to open the circuit and the shock hazard would be extended beyond the metal enclosure.

 

[erickench]

Don, what kind of shock hazard would exist if a metal enclosure with a fault came in contact with the earth's soil and not any other metal? The NEC does not allow an EGC for this case.

 

[don_resqcapt19]

Don, what kind of shock hazard would exist if a metal enclosure with a fault came in contact with the earth's soil and not any other metal? The NEC does not allow an EGC for this case.

There will be a voltage gradient in the earth around the faulted equipment creating a step potential hazard. The NEC also does not allow contact with the earth in this case either and requires the metal parts that are not connected to an EGC to be guarded against contact by people.

 

[LarryFine]

Don, what kind of shock hazard would exist if a metal enclosure with a fault came in contact with the earth's soil and not any other metal? The NEC does not allow an EGC for this case.

You're looking at it wrong. This isn't a case of "if it's in contact with the earth, you cannot instal an EGC", it's a case of "if you want to omit having to add an EGC to an existing circuit, these are the conditions you must meet."

 

[Pierre C Belarge]

This exception is not for new work, it is for existing installations that have not been provided with an effective ground fault current path. The concern it to keep the potential contact of the different surfaces from occurring, as it may energize those surfaces.

Notice the type of wiring methods they consider in this exception.

 

[erickench]

You're looking at it wrong. This isn't a case of "if it's in contact with the earth, you cannot instal an EGC", it's a case of "if you want to omit having to add an EGC to an existing circuit, these are the conditions you must meet."

Yes, I think you're right. An EGC would not be needed if a metal enclosure was in contact with the earth. The voltage cause by a fault would be brought down to ground(earth) potential. I guess only the voltage gradients would cause injury.

 

[Pierre C Belarge]

Yes, I think you're right. An EGC would not be needed if a metal enclosure was in contact with the earth. The voltage cause by a fault would be brought down to ground(earth) potential. I guess only the voltage gradients would cause injury.

I am not sure I understand what you are saying.
Earth and the earth path are not part of the effective ground fault current path (not where earth would actually make a difference for under 600V).

 

[erickench]

True. Earth is not a good fault current path because of the high resistance. But the NEC rule states that an EGC is not required if a metal enclosure is in contact with the earth. The OCPD device may not trip but at least the voltage is brought down to that of the earth. But you still have the problem of voltage gradients.

 

[don_resqcapt19]

Yes, I think you're right. An EGC would not be needed if a metal enclosure was in contact with the earth. The voltage cause by a fault would be brought down to ground(earth) potential. I guess only the voltage gradients would cause injury.

An EGC is most certainly needed to make the installation safe under fault conditions.
Contact with the earth will not bring the voltage down to earth potential unless the OCPD opens. The metal object will remain at line voltage and be a serious hazard. If the connection to earth brought the voltage down to eatth potential there would be no voltage gradients. What really happens is the connection to earth raises the voltage of the earth, with respect to remote earth, around the point of connection. As you move away from this point of connection there is a voltage drop. This voltage drop across the earth is what creates the voltage gradients.

 

[don_resqcapt19]

T... But the NEC rule states that an EGC is not required if a metal enclosure is in contact with the earth. ...

The NEC rule does not say that an EGC is not required if the metal enclosure is in contact with the earth. The section in question in this thread actually says the opposite.

 

[erickench]

Okay. The EGC will shunt out any voltage gradient and cause the OCPD to trip.

 

[LarryFine]

Okay. The EGC will shunt out any voltage gradient and cause the OCPD to trip.

"Ah, Daniel-san, Miyagi have hope for you!"

 

[erickench]

You know we have discussed this many times before on this forum and I always came up with the same answer. I ask questions when I should already know the answer.:grin:

 

[LarryFine]

You know we have discussed this many times before on this forum and I always came up with the same answer.

Insanity: doing the same thing and expecting a different outcome.

 

[erickench]

I need to be out in the field again inspecting contractors like I used to. As a young construction inspector I knew very little about the NEC except some sizing rules. All I did was look at drawings and find discrepancies and then squashed their hopes of sending me to designs. I hope my next job is in construction and this way I don't have to keep refreshing my memory.