Electric cable bridge

Continuing the tradition of sharing engineering knowledge through pictures, today I am sharing with you some pictures of cable bridges for electrical cables.

Picture 1 – An existing electric cable bridge

(Click on the picture to enlarge it)

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When the idea of this post popped up in my head, I typed in the Google search for the word "cable bridge".

The results were not what I expected. Instead of bridges for electrical cables, the results were mostly on real bridges employing suspension cables to transfer the bridge loads to the bridge columns and the foundation

Then I tried "electric cable bridge", the results were a little better. Google was still not giving me much information electric cable bridge that I need.

Lastly I keyed in "electrical cable bridge". The results were just almost the same as the electric cable bridge.

I was disappointed. I guess not many people need information on bridges for electrical cables.

So the title of this new post was confirmed by the lack of websites providing information on this topic.

Well, I am not going to write that much either. I am just going to share with you a few photos on this matter.

Picture 2 – Location of the new cable bridge

(Click on the picture to enlarge it)

The M&E manager of the main contractor in one of my earlier projects came to my office one day and said, "I cannot complete the underground cable containment for the authority by the end of the month."

I said, "Then you have a 200,000 dollar per day of penalty on your head, my friend. That is one million per week. It's in your contract."

Then he explained to me that there was a very small stream crossing the route of the 33kV underground cable containment where he was going to lay the underground ductbank.

The location of the stream crossing was outside this contract package boundary. Not many of us (including me obviously) have actually done a thorough inspection of the area yet since the construction of that part of work has not yet started.

To make it worse, this contract has a peculiar schedule which requires that the authority's 33kV substation should be handed over to them at the beginning of the project instead of at the end as was the usual practice.

To make the story short, this cable bridge was overlooked in the design drawing and now the design consultant need to quickly come up with a complete design for the electric cable bridge.

I assisted with whatever I could to provide the consultant's design office with information so that they can complete the design quickly.

Part of the assistance was to go around taking pictures of existing electrical cable bridges. This could help us design a similar one that would be acceptable to electricity supply authority.

Since the cable bridge was to be part of the 33kV cable containment for the authority's incoming supply cable to the project, then the cable bridge was also to be handed over to them after completion.

I am showing to you here a few of the pictures that I took, including the new cable bridge that we built.

Picture 2 above shows the location of the small stream along the route of the 33KV authority supply cables.

Observe the existing water supply pipe over the stream.

After some investigation, we found out that the pipe was installed by the water supply authority five to six years earlier as a provisional infrastructure for future development because there was a huge empty land area in the vicinity.

Picture 1 at the top of this post was one of the existing electric cable bridges that I visited.

It was carrying 33kV supply cables for a large residential are there.

Picture 3 - A view of the cable bridge from below

(Click on the picture to enlarge it)

Note the large residential development behind the bridge.

When I said "large residential development" it does not mean that the area only has residential building such as high rise apartment, bungalows and terrace houses.

Nowadays large residential developments are developed complete with their own commercial centers at strategic locations throughout the development.

So from electricity supply point of view, the supply infrastructure must be adequate for the KW demand of not just the planned development, but also for any unanticipated further growth as a spin-off from this development.

Picture 4 - A view of one end of the electric cable bridge

(Click on the picture to enlarge it)

This is one of the cable bridges.

Observe how the red PVC ducts leave the bridge and enter the large above-ground cable trough.

Well, this is not an above-ground cable trough, but it needs to come up above ground as it approaches the bridge end in order to protect the cable ducts leaving the bridge.

Note that this is a 33kV cable ducts as is obvious in the following picture.

Picture 5 - Front end view of the cable bridge

(Click on the picture to enlarge it)

Observe the protection measures employed in order to prevent the public from climbing onto the bridge or access the PVC cable ducts.

I know that some readers think that I made a mistake by typing "PVC" cable ducts.

Many people have the opinions that normal PVC ducts is a bad choice in this type of applications because the ultra-violet (UV) ray from the sun will turn the PVC ducts into "crispy biscuits" relatively quick.

Actually that was my opinions too.

But the electricity supply authority have a very different opinion and they have thousands of bridges like this installed throughout the country.

I think their experience outweighs our opinions on this matter for this particular application.

In any case, the new cable bridge in my project was to be handed over to them.

The design of the new electric cable bridge itself had to be submitted to their office for approval prior to construction of the bridge or fabrication of the steel structure at the factory.

Picture 6 - Another view

(Click on the picture to enlarge it)

I think this is the best single view of the existing bridge because it the whole complete system that is viewable.

Picture 7 - The new electric cable bridge has been completed

(Click on the picture to enlarge it)

Picture 8 - This give an overall view of the cable bridge "system"

(Click on the picture to enlarge it)

Here you can see the overall "system" of the cable bridge.

Beginners please note the partially above-ground cable trough connecting one end of the bridge to the underground cable-pulling manhole.

The other side of the cable bridge was implemented similarly.

Observe at the far background in the picture that there was a long red-and-white "thing".

This was new jersey barrier units lined up along that side of the main road.

It was along this main road that the 33kV incoming supply of the authority would be taken from via a loop-in loop-out method.

Picture 9 - Cable bridge construction in progress

(Click on the picture to enlarge it)

This is the last picture for today. I show this one for those who are interested in the bridge column and foundation.

You cannot see the foundation, but it was not a pile-type. The foundation was constructed as a simple large plate without piling.

Okey guys. That is all I can give today.

See you again in the next post.

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Underground electrical manhole

The picture below shows an electrical manhole intended for underground installation being unloaded from the transport truck.

Picture 1 – The electrical manhole being unloaded


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Most building works require at least one or two underground electrical manholes. That is because a building of significant size usually require a few hundred amperes of electric current at low voltage (i.e. 240 volt, 3 phase current).

Above a few hundred KVA (kilo-volt-amperes), the electricity supply authority usually delivers the electrical power to the consumer loads at higher than 240 volt, usually at 11,000 volts.

(Note: 100 amperes x 240 volt x 3 phases = 24,000 VA x 3 phases = 72,000 VA = 72 KVA. For readers who are intimidated by the KVA term, this is what KVA is. That is measure of electrical power delivered to a building. It is also the most common unit used in specifying the size and rating of electrical equipment and switchgears.)

Okay, back to the electrical manhole.

When the supply is at 11,000 volts (i.e. 11 KV), high voltage cables installed below ground level (i.e. underground) is the most popular method of electricity distribution unless the building is in remote areas such as the countryside.

So, in building works, we need at least one or two of these manholes to bring in the authority cables from outside the boundary of the building works to the electrical substation inside the building compound or the inside the building itself.

Picture 2 – The electrical manhole at a closer look



Notice the note I put in the picture saying “precast conduit sleeves”.

These sleeves were made in the factory to facilitate the connection of underground electrical conduits carrying the cables to the manhole.

If these openings on the concrete walls of the manhole are not made in the factory, then the openings have to be manually made at site using electric hammers etc.

Most of the times, some modifications are still needed because the high voltage cables are usually large and they are difficult to turn and bend.

The underground conduits may also not arrive at the manhole at the same levels of the precast sleeves. If they do, they may not all be at exactly 90-degree angles to the manhole walls.

This means some hacking still need to be done to the precast sleeves.

I forgot to tell you that the precast sleeves are made to accept 150 mm diameter of electrical conduits. It is a common practice to use 6 inch diameter underground conduits for electrical distribution cables.

Smaller sized conduits are also used, but they are generally for underground street lighting cables and compound lighting cables inside the building compound.

In these cases, 4 inch diameter conduits are used and they are installed when the compound lighting cables need to cross under internal roads.

I did not mean this post to be discussing underground cabling works. I just wanted to show some pictures of electrical underground manholes so that I can just refer to this post when talking about underground electrical manholes.

However, the above brief issues on the manhole are necessary to give some meaning to the pictures here.

So for the readers with more advanced knowledge on these things, please be patient with me. This blog is for beginners.

Picture 3 – An installed underground electrical manhole



This is how it looks after the manhole has been installed. Even though it is called “underground”, the manhole is not really totally “buried” below ground.

The exposed part of it is still visible and accessible at ground level.

Picture 4 – Manhole cover



This is the top of the electrical manhole, which is leveled to the finished ground level, exposed and accessible for access.

Observe carefully that there are 4 pieces of the manhole cover. These covers are made of reinforced concrete. So they are very heavy. Breaking it into 4 pieces make it easier to be opened by manually hand-lifting it.

Even at that smaller-sized, it usually takes at least 2 normal-sized persons to lift open a single piece after a few years. Yeah. I know. Hulk Hogan may only need two fingers to do it).

Picture 5 – The base of the manhole pit



You cannot just dig a hole of sufficient size in the ground and plant in the concrete electrical manhole.

If you do that, sooner or later one of the manholes would sink in deeper into the ground, or get tilted enough to break the underground electrical conduits and possibly damaging the underground cables.

When that happens, you would then need to carry out excavations when one of the cables need repairs or when additional cables need to be installed along the same underground route.

In fact this is the very reason the underground electrical conduits and manholes are used: to facilitate maintenance, repair and upgrading of the underground electric cables in future, long after the building is completed and occupied.

At the base of the opening in Picture 5 is a layer of sand. It is a practice to put some river sand at the base and compact it to give about 4 inch thick after compaction.

Of course, before that sand is poured in, the ground at the bottom should be firm and solid. If the soil at the bottom of the pit has been spoilt because of water accumulating there, then pit bottom must be excavated further to remove the spoilt earth. This also means more sand may be needed as the volume to be refilled would then be larger.

There is one very important I would like readers to note, especially those directly involved in construction.

You must NEVER allow the contractor to just install the manhole without first being inspected by someone responsible.

If the preparation of the base of the pit is not good enough, the manhole may sink in sooner than you would hope for. To repair it would require re-excavation works which are usually messy.

Someone may need to take the blame. It is not always easy to cover up mistakes like these.

Picture 6 – Excavated pit for an electrical manhole



I just include this picture to make my point above.

This particular opening in the ground was made in front of an electrical substation. It was supposed to be for the manhole of the same type and size as shown in the pictures above.

However, for some reasons the delivery of the factory-manufactured manhole did not arrive. So the opening was just left there. Sooner or later it would collect rainwater, which it did as can be seen in the picture.

Suppose one day the contractor calls you and say that he has finished installing the underground electrical manhole in front of the substation. With that the electrical authority can start pulling in the high voltage electrical cables.

In two weeks, the substation would be energized and the new building would have permanent electrical power that would also facilitate the testing and commissioning of all the electrical and mechanical services in the building, which has been delayed due to the delayed energization of the electrical substation.

This was a good news, and a reason for celebration. Of course, the real celebration should be AFTER the actual energization of the substation.

Being the one giving the good news to you, the contractor has made a dinner reservation at a nearby six-star hotel. It is a celebration and YOU are the man.

What would you do?

What would I do? I would accept the invitation to the dinner. After having a very nice meal and laughs, I would excuse myself earlier than normal. On the way out, I would tell the contractor to immediately start the arrangement to remove the installed manhole and prepare again the manhole pit in front of the substation.

Only after I say okay he would be allowed to re-install the manhole.

What would you do?

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