Short Ferrous Metal Sleeves and GEC's

[John120/240]

It only needs to be nonferrous; aluminum would be fine. I have been operating under the assumption that this has already been installed.

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.
. :blink:

If I remember correctly, there are issues when aluminum is in direct contact

with concrete.

 

[rbalex]

If I remember correctly, there are issues when aluminum is in direct contact

with concrete.

There are; but a copper "sleeve" can have similar problems in direct contact. In either case, direct contact (galvanic) corrosion occurs as more of a concrete mix issue.

This is another paper from the Copper Development Association. Remember, they are trying to sell the stuff and still can't claim there are no problems with copper.

 

[Jon456]

But the Copper Dev Assc are discussing the integrity of copper pipe containing pressurized water. Obviously, that requires more concern than a protective sleeve for a GEC penetrating floors. If steel EMT can survive the concrete mix, then I wouldn't be too concerned about copper, even though the corrosive factors may not be identical.

But I think this discussion is academic since I believe the copper pipe would have to be L&L for use as a raceway.

 

[glene77is]

There have been several threads in the past on the subject of GEC / Lightning.

See this URL, to gain further insight on the confusing aspects of the GEC:
http://forums.mikeholt.com/archive/index.php/t-112578.html
http://forums.mikeholt.com/showthread.php/112578-Purpose-of-GEC?

Please note there is a difference between Grounding and Bonding.
Please note there is a difference in frequency between 60Hz power
and 100 KHz lightning.
Reading a Wiki on Faraday Sheilds is some help, if you are into Electronics.

Here I will quote a post from me, responding to a fine engineer Weressl:
http://forums.mikeholt.com/showthread.php/133020-Shielded-Cable?highlight=faraday+sheild

glene77is Senior Member

Join Date
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Posts
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Originally Posted by Weressl

Two different animals.
One is shielded to prevent 'noise' generated by the ciruit itself from escaping
and affecting other equipment - ASD cables -
and the other is shielded to prevent external interference
to change the low signal value carried within the cable.

Weressl,
Check me on this please.

These are two types of Faraday sheilds.
Grounded at One End only, produces an Inductive Faraday shield.
Grounded at Both Ends, produces a Capacitive Faraday shield.

NEC requires the Capacitive Faraday shield on GEC conductors
to the ground rod, if protective EMT conduit is used at all.
This conducts the high frequency lightning pulse
towards the real Earth reference for lightning discharges.
As you might have said it, "allows 'noise' to escape".

And the corollary,
If only one end were grounded, then the lightning pulse would see a 'choke'.

Wiki has this:
"The shield of a screened cable, such as the coaxial cable
used for cable television, protects the internal conductors from electrical noise."
Which says that the Capacitive Faraday shield
allows noise to escape to the grounding points.

Comments please {^.^}

The Comment pointed to this URL on forms/reasons of cable sheilding:
http://ntrs.nasa.gov/archive/nasa/ca...2009031335.pdf

So, I suggest the following from the National Lightning Safety Institute:
http://www.lightningsafety.com/nlsi_lhm/lpts.html
Where this quote introduces an interesting topic:

"A spectral study of lightning's typical impulse reveals
both a high and a low frequency content.
The high frequency is associated with an extremely fast rising "front"
on the order of 10 microseconds to peak current.
. . .
The grounding system appears to the lightning impulse
as a transmission line where wave propagation theory applies."

Note, this high frequency component is in the order of 100 KiloHz frequency,
which is considered a radio frequency.

Relevant to this thread is this quote from
http://www.lightningsafety.com/nlsi_lhm/lpts.html#bonding :

"
... Under fast rise time conditions, cable inductance becomes important ..."

Which re-inforces the high frequency characteristics of lightning voltage.

Which leads me back to the GEC , the Ground Rod, the #4/#6 connection between the "earth ground" and the POCO Service line.

The GEC contained inside an EMT conduit is a form of Faraday Sheild.

These are two types of Faraday sheilds.
Grounded at One End only, produces an Inductive Faraday shield.
Grounded at Both Ends, produces a Capacitive Faraday shield.

NEC requires the Capacitive Faraday shield on GEC conductors
to the ground rod, if protective EMT conduit is used at all.
This conducts the high frequency lightning pulse
towards the real Earth reference for lightning discharges.

I present the above in order to broaden our perspective on the GEC system.
It is NOT to Prove a Point, just stimulate/broaden discussion.
I admit there are few NEC stipulations, requirements, relating to the above,
BUT, the GEC system starts with High Voltage, High Frequency Transients
in the Physical world.

The NEC tries to describe safe methods of dealing with these Transients.

Comments are always welcome,
that is how we all learn. :slaphead:

 

[volt101]

358.12 (6) Where practicable, dissimilar metals in contact anywhere in the system shall be avoided to eliminate the possibility of galvanic action.

Does the copper GEC belong in the EMT to begin with?

 

[infinity]

Does the copper GEC belong in the EMT to begin with?

Well in this case it's insulated but bare copper conductors are permitted in EMT.

Also in this case IMO lightning isn't really relevant. This is a common grounding electrode conductor riser running through the electric closet in the interior of a 40 story building. It's purpose will be to provide a place to land the transformer GEC's on each floor. From a practical standpoint bond or don't bond the sleeve shouldn't matter much but the code compliance is what's important here.

 

[rbalex]

As with my original statement, a bond at both ends is a requirement for ferrous EMT. In reality, it may not always be necessary; but it always works - just like a second ground rod when the first electrode doesn't measure less than 25 ohms to ground. (In some cases, many of us would just drive the second rod and not even take the first measurement)

Total combined length of EMT, lightning, harmonics, grounding scheme, ground-fault detection scheme, concrete mix and possibly other things would need to be reviewed to justify eliminating the bonds - honestly, it would most likely work just fine, but bonding at both ends will always work.

 

[Twoskinsoneman]

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) ....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. .....

This has been a very interesting thread to read. It seems like this sentence kinda links enclosures to raceways in the way it is written.