Cad -welds and grounding grids .

[drg]

While being involved in the construction of new sub station I have a habit of hitting all the cad weld connections ( field testing ) with my hammer before buring them with machinery,
problem is these connections have all been tested and approved but I still feel like they need to be physically tested before being buried with heavy equipment !!!!!

There have been a couple that broke loose to my suprise after inspection, they are then repaired and tested again but one would think the cad welds need to take the action of machinery that is placing them into the ground .

If these welds come apart with a little hammer tapping what is going on when a D4 dozer and vibratory roller cover them with 24" of earth and rock?

Anyone with grounding grid systems and cad welding experience your thoughts and ideas will be appreciated.

For what it is worth the welds that I have found as failures have a great looking apperance . have passed the inspectors testing , but the failures have indicated that the cables were not placed centered ( butted against each other) into the cad- weld mold.

These faliures were also on "T" connections only.

 

[roger]

Re: Cad -welds and grounding grids .

DRG, excellent topic. I have also "pecked on" thermo welds that are beautifull in appearance yet have broke loose. Like yourself I have never in my 29 years in the trade seen an inspector ask for a strike test. Of course I don't know if breaking the weld by striking would be indicative of a bad connection in it's intended function although a heavy surge could cause the same percussion effect I would think.

Roger

 

[drg]

Re: Cad -welds and grounding grids .

Roger, believe me the inspector was not the one asking for a strike test.
at least not at first,
just out of habit I do this because I am the guy burying them and know that we are putting a lot of force on them connections .
The inspector is a good guy and has been asking me to hammer test connections even though he has good readings!!!
One came apart when stepped on going into the trench, let me state that these are not "cold welds" but just appear to be be placed in the mold inaccurate.
These cad welds are a lot more involved that what they first appear to be.
I would like to learn more about them from other people with experience .

 

[bennie]

Re: Cad -welds and grounding grids .

drg: There is a lot of information by using key word "exothermic welding".

 

[Guest]

Re: Cad -welds and grounding grids .

I don't know Jack about "exothermic welding" but it sounds a lot like soldering or brazing. When items are cad welded together is there any other mechanical connection (like a keyway or interlock)?

In soldering or brazing it is usually wise to have the joint mechanically strong without the solder or brazing-- the solder or brazing just finishes off the continuity of the connection-- but the solder or braze is not the sole mechanical connection.

../Wayne C.

For instance a joint like this would never make it in electronics:

There is no mechanical connection outside of the cad weld itself.

[ September 12, 2003, 05:04 PM: Message edited by: awwt ]

 

[bennie]

Re: Cad -welds and grounding grids .

I first encountered exothermic welding while I was in Korea with the Marine Corps. Only it was called thermite.

Thermite charges were installed on the chambers of all field artillery pieces. The purpose was to make the weapon inoperative should it have to be left behind.

We had to destroy a lot of the weapons during a (bug out) or strategic withdrawal, for a better sounding phrase for retreat.

Exothermic welds have been x-rayed and break tested many times. They are the best connection in the trade. The trade secret is to use good dry and clean molds, correct mold for the wire size, correct thermite load, clean copper, and apply all required safety issues.

Generally if the weld looks good, it is good. If in doubt try redundancy.

 

[dereckbc]

Re: Cad -welds and grounding grids .

Originally posted by bennie:
Exothermic welds have been x-rayed and break tested many times. They are the best connection in the trade. The trade secret is to use good dry and clean molds, correct mold for the wire size, correct thermite load, clean copper, and apply all required safety issues.

Generally if the weld looks good, it is good. If in doubt try redundancy.

Bennie you are right on target. The problem with exothermic is when you install the bare conductors out on the dirt, they get contaminated. So what do you clean them with, water. Water, dirt, contaminants, and improper mold size will destroy the weld. The only thing I would change to what you is said is "Generally if the weld looks good, measures good, don't trust it, use a hammer!"

 

[don_resqcapt19]

Re: Cad -welds and grounding grids .

Wayne,
This is an actual welding process, and the weld provides the mechanical connection. Look at how steel structures are put together. There is no mechanical connection before the weld is made.
Don

 

[roger willis]

Re: Cad -welds and grounding grids .

Dereck,
Beware of any water around cad welding. If there is a drop of water in the mold and you fire it the water turns to steam and it blows up. So if you a cad welding and it starts to sprinkle rain, stop cad welding.
Roger

 

[Guest]

Re: Cad -welds and grounding grids .

Originally posted by don_resqcapt19:
Wayne,
This is an actual welding process, and the weld provides the mechanical connection. Look at how steel structures are put together. There is no mechanical connection before the weld is made.
Don

Point taken. My soldering/brazing analogy was not the best. Those are not welding processes.

I can tell you this about welded I-Beams say where a post meets a beam: If the inside-right-angle joint is not GUSSETED properly the joint is prone to failure. In this homily the rod will be the post, and the grounding conductor will be the beam.

After my posting above I more closely analyzed the graphic that I posted. The joint does offer a limited amount of added mechanical strength in that the mold allows the weld to increase the diameter of the copper conductor, and it increases the diameter of the rod at the tip.

When the Cypress Structure (freeway/highway) collapsed in Oakland, California it was found that there was no gusseting between the columns and the beams at the inside corner. They met at a sharp 90-degree angle. These columns and posts were made of concrete but the same engineering principle applies: Without gussets any cracks at the joint will concentrate INFINITE force on the end of the crack. I say infinite because whatever force is applied to the defect will all go to the defect. The crack will continue to spread until it finally makes the assembly fail. Your nomenclature may vary.

To that end, if I was an engineer, and if I was designing cad weld molds I would make 45 degree gussets at the inside corners where the cable meets the rod at 90 degrees. In the graphic above I would basically gusset from point-5 to point-6, and from point-5 to point-7. This would increase the mechanical strength of the system, and it would avoid crack failure at the inside corner.

Welded inside corners need to be gusseted to prevent stress crack failure. That's not the whole story about cad weld failures but it's a start, and it's an obvious defect IMHO.

../Wayne C.

[ September 12, 2003, 12:52 PM: Message edited by: awwt ]

 

[don_resqcapt19]

Re: Cad -welds and grounding grids .

Wayne,
I've never seen a noncontaminated cadweld connection fail. Every one that I have ever seen that broke with the "hammer test" showed evidence of contamination at the point where the weld broke, or there was excessive leakage of weld material as the result of a worn mold. This connection does not require the strength of a structural steel connection because copper can bend and move outside of the weld area without causing any problems.
Many installers do not take the time to make sure every thing is clean before making the weld and they will often use molds that have excessive leakage. The normal life of a mold is less than 50 shots and that assumes that the mold is treated with care. I've seen installers clean the mold with a wire brush and then wonder why it leaked. As with most everything in our trade, proper training and workmanship is required to produce the desired end result.
Don

 

[dereckbc]

Re: Cad -welds and grounding grids .

Originally posted by roger willis:
Dereck,
Beware of any water around cad welding. If there is a drop of water in the mold and you fire it the water turns to steam and it blows up. So if you a cad welding and it starts to sprinkle rain, stop cad welding.
Roger

Roger that is what I was eluding too, a contaminant.

I had several grounding teams once upon a time, and they went through a process of cleaning each joint by first using water on the joint and mold, then alcohol, finished up with a torch to dry. Keep it clean and dry.

 

[brian john]

Re: Cad -welds and grounding grids .

When I first did cad welding 25-30 years ago the engineer that taught me to do cadwelding told me ALWAYS;

1. Use good clean molds
2. Dry molds
3. Clean connections

And test every weld with a 2 hammer's, one to support the weld and one to smack the weld.

Cause 1,2 and 3 are hard to come by in aa ditch.

 

[drg]

Re: Cad -welds and grounding grids .

The responses and pointers are appreciated,
sounds like what we are doing on our project is normal and the hammer testing will continue.
No bad welds found today.

Thanks everyone .

 

[Guest]

Re: Cad -welds and grounding grids .

Grandpa always said if a small hammer won't break the weld try a bigger hammer until it does.

../Wayne

Posted with a smiley face hoping it makes you smile.

[ September 12, 2003, 10:42 PM: Message edited by: awwt ]

 

[roger]

Re: Cad -welds and grounding grids .

Are you giving us a physics lesson now?

Roger

 

[Guest]

Re: Cad -welds and grounding grids .

NO.

I am here to learn. Every time I open my mouth here I learn something. I learn a lot here without opening my mouth too. Either way it's learn, learn, learn. Thanks for the opportunity to learn. The Internet and this forum is a beautiful thing. Kudos.

Peace.

[ September 12, 2003, 10:48 PM: Message edited by: awwt ]