250.188(E)

[gregRooster]

Understanding 250.188(E)

Can anyone explain what the basis for 250.188(E) is?

Why would you not connect the grounding electrode of the portable system to the rest of the system? If you connect the portable device so that the equipment grounding conductor bonds to existing systems, then any lightning will find a path through the ECG from one electrode to the other. This seems totally counter to the rest of the code.

250.188(E) Isolation. The grounding electrode to which the portable or mobile equipment system neutral impedance is connected shall be isolated from and separated in the ground by at least 6.0 m (20 ft) from any other system or equipment grounding electrode, and there shall be no direct connection between the grounding electrodes, such as buried pipe and fence, and so forth.

 

[LarryFine]

Because of the neutral/ground connection. The isolation minimizes parallel neutral current.

 

[jaggedben]

... If you connect the portable device so that the equipment grounding conductor bonds to existing systems,

Why would you do that? By accident?

then any lightning will find a path through the ECG from one electrode to the other. This seems totally counter to the rest of the code.
...

FWIW I disagree that the rest of the code is written to avoid that problem with lightning or prohibits such problematic installations. I agree that what you describe is bad design, but the code allows it. In fact a few cycles back the code even required the bad design for solar arrays before cooler heads prevailed.

 

[gregRooster]

Because of the neutral/ground connection. The isolation minimizes parallel

Because of the neutral/ground connection. The isolation minimizes parallel neutral current.

Neutral ground bond is handled elsewhere in the code. We are just talking about grounding electrodes.

 

[gregRooster]

Why would you do that? By accident?



FWIW I disagree that the rest of the code is written to avoid that problem with lightning or prohibits such problematic installations. I agree that what you describe is bad design, but the code allows it. In fact a few cycles back the code even required the bad design for solar arrays before cooler heads prevailed.

Regarding the 2nd paragraph, sure. There have been junk things. But, what is the purpose of 250.188(E)? How is this improving safety/ avoiding fires?

 

[jaggedben]

I was curious so I looked up the archival revision info at https://www.nfpa.org/codes-and-standards/nfpa-70-standard-development/70. But all I can tell you is that this language was added to the 1975 NEC, and that it hasn't changed since. It was originally 250-154(e). The posted report on proposals for that year is apparently the proposed draft (presumeably the same as the text of the adopted code) and does not include the details of individual proposals and their substantiations like for most other years.

 

[tortuga]

I think its because they are impedance grounded systems, not solidly grounded, and required to have the ground fault detection / monitoring relays, and if a bolted fault were to occur they dont want a difference in potential near the ground resistor.

 

[TX+ MASTER#4544]

Understanding 250.188(E)

Can anyone explain what the basis for 250.188(E) is?

Why would you not connect the grounding electrode of the portable system to the rest of the system? If you connect the portable device so that the equipment grounding conductor bonds to existing systems, then any lightning will find a path through the ECG from one electrode to the other. This seems totally counter to the rest of the code.

250.188(E) Isolation. The grounding electrode to which the portable or mobile equipment system neutral impedance is connected shall be isolated from and separated in the ground by at least 6.0 m (20 ft) from any other system or equipment grounding electrode, and there shall be no direct connection between the grounding electrodes, such as buried pipe and fence, and so forth.

2023 NEC

It's not the function of the EGC and it is not a grounded conductor, Article 100 ,Grounding Conductor, Equipment (EGC).

See Article 100 for Grounded Solidly, Grounded Conductor, Grounded System, Impedance.

Yeah, just do what 250.88(E) Isolation.... It Says......

You're in Part X Grounding of Systems and Circuits of over 1000 Volts
250.186 (A) (4)

Review these sections for additional information.

TX+MASTER#4544