Short Ferrous Metal Sleeves and GEC's

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infinity

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Been having a discussion with the field engineer about using short pieces of EMT (about 30") to protect a common grounding electrode conductor riser as it runs up through the floors in a high rise building. He's of the opinion that no bonding at both ends is required because the short section of raceway is merely a sleeve and is not a raceway as outlined in 250.64(E). That code section does mention raceways but says nothing about sleeves and calling a sleeve an enclosure seems to be a stretch to me.

250.64 Grounding Electrode Conductor Installation.
(E) Enclosures for Grounding Electrode Conductors. Ferrous metal enclosures for grounding electrode conductors shall be electrically continuous from the point of attachment to cabinets or equipment to the grounding electrode and shall be securely fastened to the ground clamp or fitting. Nonferrous metal enclosures shall not be required to be electrically continuous. Ferrous metal enclosures that are not physically continuous from cabinets or equipment to the grounding electrode shall be made electrically continuous by bonding each end of the raceway or enclosure to the grounding electrode conductor. Bonding shall apply at each end and to all intervening ferrous raceways, boxes, and enclosures between the cabinets or equipment and the grounding electrode. The bonding jumper for a grounding electrode conductor raceway or cable armor shall be the same size as, or larger than, the enclosed grounding electrode conductor. Where a raceway is used as protection for a grounding electrode conductor, the installation shall comply with the requirements of the appropriate raceway article.
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Dennis Alwon

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I take it back. As I re-read 250.64(E) I think he is incorrect as it states all ferrous raceways between. It does not say they must be attached to a cabinet.
 
infinity

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chris kennedy said:
The sleeve appears to meet the 100 def of enclosure, IMO bonding is required.
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Where does it say that in Article 100?

Enclosure. The case or housing of apparatus, or the fence or walls surrounding an installation to prevent personnel from accidentally contacting energized parts or to protect the equipment from physical damage.
FPN: See Table 110.20 for examples of enclosure types.
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Dennis Alwon said:
I take it back. As I re-read 250.64(E) I think he is incorrect as it states all ferrous raceways between. It does not say they must be attached to a cabinet.
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When is a short piece of EMT a raceway and when is it not?
 
Dennis Alwon

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Well Rob it states ferrous metal raceways shall be electrically continuous from the point of attachment to the cabinet or equip to the gec. If a sleeve is not a raceway, at least in this case, then there would be no need to say it needs to be continuous. The definition, as Chris pointed out, does not require a raceway to be a complete system to be a raceway. It is still a raceway.
 
tom baker

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Ask if your engineer has a copy of the IEEE green book.It has a good explanation of why metallic raceways need to be bonded at each end.
The simple and best solution is to use PVC Sch 40 or 80 conduit
 
rbalex

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I suspect the field engineer ia attempting to make a coorelation between GECs and EGCs. See 280.86, Exception 2.

A sleeve is both a raceway and an enclosure by Art 100 definitions.

Having said that, I agree bonding the GEC at both ends is a Code requirement. However, I don't believe that it is always necessary, there is just no appropriate Exception. The purpose of the bond is to reduce the "choke" effect a ferrous enclosure may impose during a fault. However, when we realize there should be no current in the GEC (even during most ground faults) if it is otherwise installed properly, an unbonded "short section" is no big deal. Lightning strikes may be another story. And I might be concerned about how "short" it really is in a high rise application.
 
infinity

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tom baker said:
Ask if your engineer has a copy of the IEEE green book.It has a good explanation of why metallic raceways need to be bonded at each end.
The simple and best solution is to use PVC Sch 40 or 80 conduit
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I agree but the building doesn't allow PVC to be used for fire reasons and Aluminum is NG because it will be patched with concrete.

I agree that it should be bonded but the engineer thinks that there is a point, based on the length of the piece of ferrous metal, that it's nothing more than a sleeve. I've asked he can determine that specific length but so far he can't explain how. :blink:
 
chris kennedy

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chris kennedy said:
The sleeve appears to meet the 100 def of enclosure, IMO bonding is required.
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infinity said:
Where does it say that in Article 100?
Click to expand...


Enclosure. The case or housing of apparatus, or the fence or walls surrounding an installation to prevent personnel from accidentally contacting energized parts or to protect the equipment from physical damage.
FPN: See Table 110.20 for examples of enclosure types.
Click to expand...
Sure its a stretch Rob, but one could argue.

Equipment. A general term, including material, fittings, devices, appliances, luminaires, apparatus, machinery, and the like used as a part of, or in connection with, an electrical installation.
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I would consider the wire used for the GEC as material.
 
infinity

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chris kennedy said:
Sure its a stretch Rob, but one could argue.

I would consider the wire used for the GEC as material.
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So you're saying that a short piece of EMT is either a the case or housing of apparatus?

Still quite a stretch in my book.

Not trying to be a pain here but I need to have sufficient ammo when this argument comes up again on the job site. I still say that bonding is required but this guy will have many alternative arguments.
 
R

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At what point does the size of sleeve through the floor not become a choke?

Thinking about it more, a sleeve is usually the depth of the floor. A 30" piece of raceway is not a sleeve. Its a short section of raceway and all intervening sections of raceways are required to be bonded both ends.
Rick
 
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rbalex

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Technically, the choke effect exists for any length of sleeve. The total effect is proportional to the combined total length of the sleeves that are common to a single GEC. The effect may range from trivial (most likely) for a few feet to exceptional. The method of grounding; e.g., solid or high impedance and the groundfault detection method would also play a part.

I failed to mention that triplins may also be a consideration since they are effectively zero sequence AND are higher frequency (180Hz minimum). That would increase the choke effect under "normal" conditions and possibly cause the sleeve to heat up a bit; but it would still probably be of little effect under fault if the total length were short.

Whether it would be more cost effective to do a full engineering analysis or simply bond as already required is a exercise left up to the student;)
 
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