Isolated ground

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iwire

iwire

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melanconelectric said:
What is the most effective manner by which to achieve an isolated ground? I've used bldg steel in the past but hasn't worked well.
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If the only fault path you installed for equipment is through building steel that is a violation and potentially dangerous although I think it is better then just running it to a ground rod.

There really is no such thing as an truly isolated ground, all grounding conductors have to eventualy connect to the neutral of the transformer or service disconnecting means.
 
chris kennedy

chris kennedy

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nakulak said:
250.146 d .
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Also 250.96(B).

Here they are.


250.96 Bonding Other Enclosures.

(B) Isolated Grounding Circuits. Where installed for the reduction of electrical noise (electromagnetic interference) on the grounding circuit, an equipment enclosure supplied by a branch circuit shall be permitted to be isolated from a raceway containing circuits supplying only that equipment by one or more listed nonmetallic raceway fittings located at the point of attachment of the raceway to the equipment enclosure. The metal raceway shall comply with provisions of this article and shall be supplemented by an internal insulated equipment grounding conductor installed in accordance with 250.146(D) to ground the equipment enclosure.
FPN: Use of an isolated equipment grounding conductor does not relieve the requirement for grounding the raceway system.

250.146 Connecting Receptacle Grounding Terminal to Box.

(D) Isolated Receptacles. Where installed for the reduction of electrical noise (electromagnetic interference) on the grounding circuit, a receptacle in which the grounding terminal is purposely insulated from the receptacle mounting means shall be permitted. The receptacle grounding terminal shall be connected to an insulated equipment grounding conductor run with the circuit conductors. This equipment grounding conductor shall be permitted to pass through one or more panelboards without a connection to the panelboard grounding terminal bar as permitted in 408.40, Exception, so as to terminate within the same building or structure directly at an equipment grounding conductor terminal of the applicable derived system or service. Where installed in accordance with the provisions of this section, this equipment grounding conductor shall also be permitted to pass through boxes, wireways, or other enclosures without being connected to such enclosures.
FPN: Use of an isolated equipment grounding conductor does not relieve the requirement for grounding the raceway system and outlet box.
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infinity

infinity

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Isolated grounds to building steel were the norm around here for years. Seems that the people designing them had little idea about what the NEC requires. Here's an example of an isolated ground conductor that is supposed to be connected to building steel. When the building steel is encased in concrete use the next best thing, a piece of rebar in the ceiling of the electric closet. And this was in a federal office building. :rolleyes:

Isolated%20Ground%20connection.jpg
 
infinity

infinity

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iwire said:
A lot of problems for such a simple picture.

  1. Isolated ground to rebar
  2. Use of a beam clamp as a grounding means
  3. Lack of a bonding bushing
  4. Ugly
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I totally agree. It's likely that an IG receptacle connected to this mess with a ground fault imposed upon it would never trip the OCPD given that this was the only ground connection for the device.

FWIW we just installed a distribution panel with an #4 IG to building steel just as the engineer had it on the print. Not sure what it was to be used for but it was in a theater so maybe that had something to do with it.
 
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