Monday, July 18, 2011

Electrical grounding

I believe I have uploaded to this blog quite a number of pictures on electrical grounding. However, there is one work of the grounding system that I have always wanted to show the readers especially true beginners (i.e. students and young engineers). That is the process of exothermic welding.

Picture 1 – 25mm x 3mm grounding copper tape permanently bonded to ground using exothermic welding

This ground rod and the inspection chamber are in place. The grounding copper tape has been permanently bonded to the ground rod.

The inspection chamber has been placed tentatively at the approximate finished ground level.

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Picture 2 – A closer view of the exothermic welding.

Personally I would prefer all grounding connections inside inspection chambers to be of the compression type (eg. using brass clamps). That is the purpose of the chamber, so that the maintenance people can inspect the grounding connections and do some work to improve the grounding resistance if and when necessary.

If the grounding resistance has deteriorated over time (i.e. the resistance to earth gets higher), then maybe we need to add one or more grounding electrodes and loop them to the existing grounding network.

The “looping” of the new electrodes to the existing ones would have been done inside this inspection chamber.

If the existing connections inside the chamber use exothermic welding such as that in Picture 2 above, connecting new earthing conductors here can be difficult.

Having said that, it should be acknowledged that under certain circumstances it might be better to have a permanent bonding. Exothermic bonding is a form of permanent bonding and it is maintenance-free.

In fact, with a properly carried out exothermic welding, the inspection chamber may not even be necessary. Some engineers may disagree with that, but that is how I think.

The above connection at the inspection chamber was already completed when I inspected it. Just for the purpose of showing it to readers of this blog, I have asked the electricians to make another joint so I can take some pictures. They are shown below.

Picture 3 – The grounding conductor and electrode before exothermic welding

This electrode was just another electrode not far from the one in Picture 1. There was no particular reason that I chose this one for the demonstration.

Maybe it was just because the top of the electrode was protruding quite a few inches above the expected finished ground level.

I guess I wanted to show that the driven electrode needed to be cut first before the exothermic welding process was carried out.

Picture 4 – A close-up view of the ground rod and copper tape conductor

Picture 5 – Cutting the excess top part of the electrode.

Keep in mind that later the top of the rod and joint between the rod and the copper tape should be inside the inspection chamber.

The concrete inspection chamber itself would have a removable concrete cover.

Therefore the top of the ground rod should be just below the concrete cover when the cover is in place.

The whole of the chamber and the cover should be flushed to the finished ground level, or flushed to the finished road level if it is installed under road.

Picture 6 – Preparing the top of electrode to accept the copper tape.

If you look at Picture 2 again, you can observe that the copper tape is like “standing” or “slicing” the electrode. Some electricians prefer to put the tape flat on top of electrode.

I think it makes no difference either way. It’s just that the opening at the mould (you will see the mould soon) should be cut accordingly. Electricians do not normally make the mould themselves. They order them from electrical shops.

In the above picture, the two workers were making a shallow slit at the top of the electrode. It was cut small enough to just “park” the copper tape into it.

That would give the joint a stronger mechanical strength, they said. I doubt that, but then I didn’t think it would much difference either way.

Picture 7 – A closer view

Picture 8 – The workers trying to park the copper tape onto the ground rod.

Picture 9 – A closer view

Picture 10 – Now the worker places the mould to the joint and encloses it.

The work you see here requires is not difficult, but it requires at least two or three persons. Now the workers place the mould in such a way so that it encloses the joint between the copper tape and the copper-jacketed steel earth rod.

Picture 11 – Now the mould is in place.

Observe how the mould is constructed with a handle that can grip both conductors to be jointed.

Picture 12 – A closer view of the mould enclosing the joint.

By now even a first time viewer should be able to make a conclusion that a different mould would be necessary if two other different types of conductors were to be jointed.

Picture 13 – Tying the mould with a metal wire for extra strength.

Here the workers tried to give the grip of the mould over the joint an extra strength by tying it with a metal wire.

Picture 14 – Filling the mould with an explosive powder mixture.

Now the mould is being filled with a type of explosive powder mixture. Contained in the mixture also is a form of copper material so that during the quick combustion the copper elements melted onto the joint and forms a permanent joint.

It is similar to jointing two different pieces of concrete blocks with liquid concrete. After the liquid concrete has hardened, the two concrete blocks would become one larger block. The difference is that the liquid concrete takes much longer to harden. Whereas here the copper element in the powder mixture melts during the explosive combustion and then hardened. So the process here is very much quicker.

Picture 15 – A closer view of the powder mixture.

Picture 16 – Preparing a gas torch to ignite the powder.

Picture 17 – The gas torch

Picture 18 – BOMB! Take cover!!

I do not remember exactly what I was doing, but I did not have my camera ready when the worker ignited the explosive powder in the mould. So I was not able to catch the bid smoke during the hard combustion.

Actually during the whole process, I was have a visitor to the site who wanted to see the exothermic welding process. So while I was taking pictures for this blog, I was also sort of “entertaining” the visitor, and missed the big smoke.

If you click on the picture to make it larger, you may still be able to “feel” the remaining smoke there. I am so sorry about that. I will try to catch the big smoke some other time.

Picture 19 – The explosive powder has been spent.

Now the mould seems to be empty. After the combustion, the mould had to be left there for a few minutes so it can get cooled enough before anyone can try to pry it open.

Picture 20 – Untying the metal wire around the mould.

Picture 21 – Now the mould has been taken off.

Picture 22 – A closer view of the joint after the mould has been taken off.

I heard someone actually chuckled and said “ Wow! It’s like a cup cake!”.

I don’t think it is in any way resembling a cup cake.

However, after so many years seeing it done, I always have that little excitement inside whenever a new mould and powder mixture is used to make exothermic joint.

Because when the mould is cracked open, the resulting joint piece is like a new artwork.

Picture 23 – Someone knocked off the still hot copper flakes off the joint.

Picture 24 – A beautiful exothermic joint

Do I need to say more?

See you guys around.

Copyright Electrical grounding


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Jack Daniel said...

Dear Sir,

Is it necessary to install the rod spike and the driving rod?,i saw some installation without rod spike and driving rod.

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