Tinned Ground Conductor why?

[kellytshort]

I am doing a raised floor installation and the drawing calls out for a 2/0 tinned stranded copper ground conductor. My question lies in the chapter 9 table 8 of the NEC which has the dc resistance of the conductor with .096 ohms/kft and .3 ohms/kft. My understanding is the coated column refers to tinned conductors which protect from corrosion I assume but the resistance is just so much higher. For a solidly grounded system especially with sensitive electronics wouldn't you want the lowest resistance to ground? Is there something/advantage I am missing with the tinned ground conductor.

 

[rt66electric]

designer called for it

designer called for it

I am doing a raised floor installation and the drawing calls out for a 2/0 tinned stranded copper ground conductor. My question lies in the chapter 9 table 8 of the NEC which has the dc resistance of the conductor with .096 ohms/kft and .3 ohms/kft. My understanding is the coated column refers to tinned conductors which protect from corrosion I assume but the resistance is just so much higher. For a solidly grounded system especially with sensitive electronics wouldn't you want the lowest resistance to ground? Is there something/advantage I am missing with the tinned ground conductor.

Its probably because "its what the engineer speced out" mikey likes it

It is ususlly easier to do it the 'hard way' than to change the plans

 

[SG-1]

The only reason I know of would be corrosion resistance. Tin is the most inert coating possible. The conductor may be subjected to moisture somewhere along the length of it's run or the engineer is worried something may spill through the floor & cause corrosion.

 

[rcwilson]

The difference in resistance will have very little effect. The total impedance of the grounding or equipotential bonding system is what matters. The #2/0 impedance is not affected much by the difference in resistance of tinned vs. bare. Connection and joint resistance is probably a lot higher than that difference.

Some computer room designs use a large flat plate for equipotential ground bus because it has a much better impedance than wires, especially at higher frequencies.

 

[gar]

100402-0857 EST

There is something very wrong with 0.3 per 1000 ft.

See
http://www.alanwire.com/awscripts/makspecs.cgi?ntype=TINNED
Note: this is the 20 deg C value.

.

 

[winnie]

... dc resistance of the conductor with .096 ohms/kft and .3 ohms/kft. ... I assume but the resistance is just so much higher.

Are you confusing the ohms per kFt (1000 feet) and the ohms per km (1000 meters) entries?

Looking at table 8, for 2/0 copper at 75C, the values are 0.3170 ohm/km _uncoated_, 0.0967 ohm/kFt _uncoated_, 0.329 ohm/km _coated_ and .101 ohm/kFt _coated.

-Jon

 

[ohmhead]

The difference in resistance will have very little effect. The total impedance of the grounding or equipotential bonding system is what matters. The #2/0 impedance is not affected much by the difference in resistance of tinned vs. bare. Connection and joint resistance is probably a lot higher than that difference.

Some computer room designs use a large flat plate for equipotential ground bus because it has a much better impedance than wires, especially at higher frequencies.

Well iam interested in learning more about the flat plate at high freq & impedance to a round conductor can you help me understand this better?

 

[rcwilson]

Computer Room Grounding

Computer Room Grounding

Ohmhead - I'm trying to locate the IEEE articles on recommendations for computer room grounding that recommended the flat grounding plate. The articles discussed the effectiveness of regular wire versus braided wire, flat cable, bus bar or even steel I-beams for bonding. The main concern was high frequency noise, similar to lightning protection ground connections.

I'll post it when I find it.