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Author Topic: Galvanic Isolation for Tower grounds  (Read 10321 times)
W2RWJ
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« on: October 23, 2012, 06:25:56 PM »

We recently put up a short Rohn 25 tower at the N2MO station.  The plan is to CadWeld the 1/0 ground leads to the tips of the three ground rods using a GCR162C mold.

On the tower end, we are using a T&B/Blackburn 3902 ground clamps.  This is a heavy duty cast bronze clamp with a substantial u-bolt to wrap around the tower leg.

Question is do we need a stainless steel galvanic isolator between the clamp and leg to protect the tower galvanizing?

73 Martin
W2RWJ
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KH6AQ
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« Reply #1 on: October 23, 2012, 06:44:22 PM »

http://en.wikipedia.org/wiki/Galvanic_corrosion

Galvanic series:

Zinc    = -1.25   
SS      = -0.60
Bronze = -0.40

The closer they are the less electrolysis there will be.
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NK7Z
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« Reply #2 on: October 23, 2012, 08:49:13 PM »

I will be facing the same question in a few months...  I will follow this thread with much interest...  If I understand what is happening, why not use an aluminum contact point to the tower?
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Dave
For reviews and setups see: http://www.nk7z.net
K4SAV
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« Reply #3 on: October 23, 2012, 11:28:30 PM »

If I understand what is happening, why not use an aluminum contact point to the tower?

Well someone is thinking.  Almost everyone will say put a stainless shim between zinc and copper, and even some manufacturers of clamps, intended for tower grounding, make them that way.  I have asked the "why" question several times on several forums, and so far no one has answered. 

In the galvanic series, in flowing sea water, in order from least noble to more noble:
zinc is -.98 to -1.03
aluminum alloys are -.76 to -1.0
copper is -.3 to -.37
tin is -.3 to -.33 and
stainless steel varies a lot depending upon the alloy and may be -.57 to 0.0.

The least noble metal will corrode.  The higher the potential difference, the faster it will corrode.  So looking at the table, why is stainless appropriate?  It seems to me that aluminum alloy is more appropriate.   

Since my tower requires a clamp that can be mated to a flat surface I chose a large standard aluminum alloy clamp (available from Home depot, Lowes, or any electrical house).  They sell two versions of the same clamp.  One is tin plated.  I wouldn't recommend that one.

I used an oversized clamp because I liked the robust clamping screw it had. I bent the end of my #4 ground wire into a tight J loop, which I placed under the screw to gain more surface contact area. When you tighten the clamping screw, the copper wire actually squashes into the aluminum a little.  These clamps work well under a tower bolt after you file off the little ridge on the bottom of the clamp, and you will probably have to drill the mounting hole larger.  I coated the aluminum alloy to copper junction with an anti-oxidant and sealed it against water.  I used duct seal to encapsulate the whole thing.

I have not done a survey of what is available for a clamp which has to be mounted to a round tower leg.

Jerry, K4SAV
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W2RWJ
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« Reply #4 on: October 24, 2012, 03:17:07 AM »

I will be facing the same question in a few months...  I will follow this thread with much interest...  If I understand what is happening, why not use an aluminum contact point to the tower?

Take a look at the Motorola site standard: http://www.radioandtrunking.com/downloads/motorola/R56_2005_manual.pdf In chapter 4 it states aluminum shall not be used in grounding systems.
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K3GM
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« Reply #5 on: October 24, 2012, 04:28:04 AM »

......Almost everyone will say put a stainless shim between zinc and copper.....

I wrap the leg with a strip of 0.015" thick stainless sheet, and then clamp over it.
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AB4D
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« Reply #6 on: October 24, 2012, 05:07:23 AM »

......Almost everyone will say put a stainless shim between zinc and copper.....

I wrap the leg with a strip of 0.015" thick stainless sheet, and then clamp over it.

I also do the same.  I previously installed the bronze clamp directly on the leg, but it did not take long for corrosion to form.  A broadcast engineer recommended to me the SS sleeve between the clamp and tower leg.  I understand that Aluminum is not recommended because it would melt if the tower took a direct lightining strike.  But I am not sure.

73
« Last Edit: October 24, 2012, 05:16:15 AM by AB4D » Logged
NK7Z
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« Reply #7 on: October 24, 2012, 07:29:18 AM »


Take a look at the Motorola site standard: http://www.radioandtrunking.com/downloads/motorola/R56_2005_manual.pdf In chapter 4 it states aluminum shall not be used in grounding systems.


That is a very interesting paper on grounding, thank you!
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Dave
For reviews and setups see: http://www.nk7z.net
W8JI
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« Reply #8 on: October 24, 2012, 09:10:44 AM »

This is a classic case of people looking at a single factor and using that single factor to reach a conclusion when the actual problem is complex.

Someone on a local SE USA DX club reflector said stainless hardware interacts with aluminum elements. He actually suggested galvanized steel hardware, based on a similar narrow view of galvanic tables only. Anyone who has used stainless and used typical galvanized hardware knows the stainless (as a clamp or fastener) actually holds up MUCH better than normal galvanized hardware.

There are multiple things at work, such as the presence of electrolytes, passivity of the metals, and the area or size of the sacrificial part. Stainless clamps are great for mechanical connection to aluminum because the aluminum area is large and aluminum will quickly self-insulate even in air. Aluminum is very passive, in that it forms an insulating layer quickly.

The problem is aluminum self-insulates even by itself in air, so it makes a lousy ground clamp or ground connection. Any electrical contact area of aluminum is likely to be very small after a period of time, except where air has been excluded.

I would use what experienced manufacturers in the industry recommend, or consult an expert, and ignore opinions based solely on the galvanic table. Some materials further apart on the table are better than materials close together for corrosion just because when they do break down, they break down much slower. Other materials that don't corrode easily, like aluminum, quickly build a protective insulating layer or melt easily with an arc.  

73 Tom
« Last Edit: October 24, 2012, 09:13:55 AM by W8JI » Logged
K4SAV
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« Reply #9 on: October 24, 2012, 10:32:14 AM »

Take a look at the Motorola site standard: http://www.radioandtrunking.com/downloads/motorola/R56_2005_manual.pdf In chapter 4 it states aluminum shall not be used in grounding systems.

I think everyone knows that buried grounding conductors or buried joints cannot be aluminum.  Some soil properties can turn aluminum into aluminum oxide very quickly.  The R56 spec prohibits this use.  For a tower to ground wire clamp at a tower, you don't have to worry about the aluminum corroding in a zinc to aluminum junction.  The aluminum is not the least noble metal.  That would be the tower zinc coating and it will corrode at a rate determined by the galvanic difference between the metals. When that coating is gone, the steel underneath will corrode very quickly.  

Here is what R56 says about tying the ground conductor to the tower:

"All above grade grounding electrode system connections (such as grounding electrode conductor
connection to ground bus bars and tower legs) shall be joined using exothermic welding, or listed
irreversible high-compression fittings compressed to a minimum of 13.3 tonnes (12 tons) of pressure, or
as otherwise required by the specific component manufacturer (ANSI T1.313-2003, figure 3(a))."

"NOTE: In some instances, exothermic welding may not be possible or may be prohibited by the specific
component manufacturer (such as towers or fences); in these cases, other suitable means for bonding is
allowed."

Exothermic welding a tower leg may be OK for large commercial towers, but the little towers we use could be weakened by welding to a tower leg.  That's a bad idea.  Also irreversible connections don't work well for a tower that periodically needs to be tilted over.  

Notice that R56 doesn't say to use a stainless shim either.

I guess I'm a little different from most.  When someone says "do it this way", I want to know why.  Thanks to Tom for some of the "why" information on this question.   I understand those comments and I can see where misuse of an aluminum clamp can be a problem.  With the clamp I used, the copper to aluminum junction is gas tight, and sealed from water.  The zinc to aluminum junction (surface area 2.5 x 3 inches) is coated with an anti-oxidant and sealed against water.   I can see where achieving this type connection to a round tower leg might be difficult.

Jerry, K4SAV
« Last Edit: October 24, 2012, 10:34:22 AM by K4SAV » Logged
NK7Z
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« Reply #10 on: October 24, 2012, 10:54:55 AM »


I would use what experienced manufacturers in the industry recommend, or consult an expert, and ignore opinions based solely on the galvanic table. Some materials further apart on the table are better than materials close together for corrosion just because when they do break down, they break down much slower. Other materials that don't corrode easily, like aluminum, quickly build a protective insulating layer or melt easily with an arc.  

73 Tom

Hi Tom,
Can you state what is the recommended method for connecting a ground to a tower is, or at least point me at one you trust?  As you said, there are many ideas, and I tend to agree, the best way will be what has worked over time...  I have a Galvanized tower leg, ending in an open tube, and a bolt plate.  I will have a copper ground setup, and am still unsure what is the best way to connect to the tower.  I would like to have both a good ground, and not eat the galvanizing off the tower in a few years...
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Dave
For reviews and setups see: http://www.nk7z.net
AA4PB
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« Reply #11 on: October 24, 2012, 12:27:06 PM »

Here's a clamp made by Rohn for 25G and 45G tower legs:

http://www.3starinc.com/rohn_r-cpc11_25_tower_base_ground_clamp.html

It's made of tinned bronze.
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Bob  AA4PB
Garrisonville, VA
W2RWJ
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Posts: 330




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« Reply #12 on: October 25, 2012, 04:31:20 AM »

Here's a clamp made by Rohn for 25G and 45G tower legs:

http://www.3starinc.com/rohn_r-cpc11_25_tower_base_ground_clamp.html

It's made of tinned bronze.


Actually, that part is made by CPC/Harger:  http://www.harger.com/library/brochures/CPC-GAR%20Cross%20Reference.pdf
 
That being said, Harger and Thomas & Betts are both well respected names in the industry.   Either would work just fine.

We will use the shim stock.  From what I can see, it will not hurt anything, and may offer additional protection to the galvanizing.

73 Martin


« Last Edit: October 25, 2012, 04:34:07 AM by W2RWJ » Logged
W8JI
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« Reply #13 on: October 25, 2012, 05:52:11 AM »


I would use what experienced manufacturers in the industry recommend, or consult an expert, and ignore opinions based solely on the galvanic table. Some materials further apart on the table are better than materials close together for corrosion just because when they do break down, they break down much slower. Other materials that don't corrode easily, like aluminum, quickly build a protective insulating layer or melt easily with an arc.  

73 Tom

Hi Tom,
Can you state what is the recommended method for connecting a ground to a tower is, or at least point me at one you trust?  As you said, there are many ideas, and I tend to agree, the best way will be what has worked over time...  I have a Galvanized tower leg, ending in an open tube, and a bolt plate.  I will have a copper ground setup, and am still unsure what is the best way to connect to the tower.  I would like to have both a good ground, and not eat the galvanizing off the tower in a few years...

Use a heavy galvanized clamp. They are available. Just search the internet for tinned pipe clamps.

http://www.mcmaster.com/#electrical-system-grounding-clamps/=jvgeys


If you can't manage that, use stainless but grease it. I don't know the number off hand, but get a grade of stainless that is compatable with zinc.

Or do what I do, heavily tin copper flashing and form it to the leg, and clamp over it.

Anything you do, grease it. 

 
 
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