The following pictures show electrical busduct risers in actual installation.
Picture 1 – Typical busduct rising main at individual building floors
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The above picture shows a typical installation of busduct rising mains in the riser room at individual floor of a high rise building.
This picture was taken in the riser room at one of the upper floors of a high rise building.
From the picture you can see most of the major components of a busduct riser system.
(NOTE: for readers who are not from electrical disciplines, a “riser” is a feeding cable or pipe giving supply to upper floors of a multi-storey building.
In electrical works, a riser is a set of cables the supply the upper floors. The electrical distribution diagram in Diagram 3 may make you understand this better.
The feeding cables “rise” up straight to the top floor. Then at each floor a tap-off unit is connected so electricity can be supplied to that floor.
An alternative is to run one individual set of cables to each floor. Then there would be many cables that need to be installed the number of which is directly proportional to the number of upper floors.
One single set of bigger cables is always cheaper to install and much easier to handle.)
Observe that there are three tap off units there: two units of 60A three phase, and one unit of 100A three phase. The one at the highest position of the three is the 100A tap off.
Why three units?
There are actually three risers here: one for the “normal” main electricity supply. This supply is just the normal authority supply like the one you have in your house. The 100A tap off unit is for this riser.
The second tap off, which is one of the 60A units, is what is called “essential supply”, or “emergency supply”. It is a normal authority supply like the one you have from the 100A tap off unit above.
However, it is also backed by a standby diesel generator. This means that if the electricity supply from the authority distribution network fails due to problem with their underground distribution cables or whatever, the standby electric generator would kick in and switch in the locally generated electricity to this electrical riser.
Large capacity electric generators are expensive, however.
Therefore, it is not economical to supply all electricity needs in the building from this generator. That is why separate electrical risers are used, and the “normal” riser is not backed by the generator supply.
The third tap off unit (the second 60A unit) is for the air conditioning system.
It is quite common (and is considered a better design) to have a separate feeder cable for the air conditioning and mechanical ventilation (ACMV) system in a large building. That is the reason for the third electrical riser here.
One more point to note here is that some office buildings use the generator-backed supply (i.e. ESSENTIAL supply) to feed the electrical riser for the air-conditioning system.
With this arrangement, ACMV equipment that need to continue operating even during the mains failure do not need to be connected to “ESSENTIAL” supply riser, which is why it is given a separate riser in the first place.
Observe the large flexible conduits coming out from the bottom of the tap off units and connect to the orange-colored electrical trunking. Some installations use rigid metal trunking for this purpose.
Cables are run from a tap off unit into the flexible conduit to go to the orange metal trunking.
They run inside the trunking to connect to the respective sub-switchboard.
You can see only two sub-switchboards here. These are for the “NORMAL” supply and the “ESSENTIAL” supply.
The switchboard for the air conditioning system is normally located inside the AHU room of that particular floor.
From the sub-switchboards, separate outgoing cables are run inside the trunking to connect to separate distribution boards (DB) on that particular floor.
That is all that I wish to elaborate on Picture 1 above.
My intention is so that beginners in electrical installations can understand the major components that make up a busduct riser system.
Now let us look at a few electrical diagrams. The diagrams can give a more complete overall view of a simple electrical system in which a similar busduct riser forms a part.
While the busduct risers in Picture 1 are for a high rise office building, the following diagrams are for much simpler installations. They are for the nurses hostels at a hospital complex.
Even though electrical installations at hospitals are relatively much more complex than the office building above, the installation at their hostel and staff quarters buildings are usually very simple.
That is why I choose to use them here. My objective here is to give casual readers and beginners in electrical works an overview and general understanding of the basic functions and installation of a busduct riser system.
The actual electrical diagram used to construct the installation in Picture 1 might be too complicated for this category of readers. I do not wish to scare them away.
Diagram 2 – Part single line diagram for a multi-storey nurses hostel
This diagram is part of a larger diagram which is in Diagram 3.
Diagram 3 – The full layout of the single-line of Diagram 2
As you can see, you cannot read much of the labels and notes in Diagram 3.
I produced this JPEG graphic from an AutoCAD drawing files (i.e. DWG file format) which is probably the most widely used drawing programs in the construction industry.
I used the program’s BMPOUT command to produce a BMP file format form the DWG file format. Then I converted the BMP format into the JPEG file format using the Microsoft Paint program.
However, this method does not seem to be effective enough for AutoCAD drawings that have been produced for A1 or A0 printing as the original sizes.
If anybody here knows a better method, please let me know.
Okay. Let’s get back to the electrical single-line diagrams.
In Diagram 1, you can see that the hostel building is six-storey, which Ground Floor, then Level 1 to Level 5.
Usually for a building of this height, designers do not use busducts. However, they do in this case. Maybe that was because the project already used so many busducts at the main hospital complex and other (much higher) staff quarters buildings, it made sense to also use busducts here.
In any case, the components of the busduct system that you see in Picture 1 were used at each floor of Level 1 to Level 5.
However, at Level 1 you have one or two more components. Picture 4 below shows a view of the electrical riser room at Level 1 (the first higher level). It is the same building as that in Picture 1.
Picture 4 – A view of the electrical riser room at Level 1 (the first level above ground level
As you can see, there are definitely more components here that in Picture 1.
However, actually there are basically only two new components at each riser, which is the TERMINATION BOX (some people call it “FEED IN BOX”), and the incoming cables that terminate into the termination box.
Apparently there is an extra tap off unit here, but it is the same component type.
Observe the cables coming into the termination boxes.
The biggest termination box has cables with black insulation terminated to it, while the other two termination box has red cables.
The red cables are red-colored because they are fire rated cables. Remember the “NORMAL” and “ESSENTIAL” supplies that I explained above.
Why the generator-backed “ESSENTIAL” supply cables need to have red colored insulation?
It is because this supply is part of the fire emergency system of the building. Which means this part of the electrical installation system must also comply with the Fire Requirements of the Building Bylaws and the Fire Department.
A fire-rated cable must be able to continue operating for a certain number of hours during fire before it fails. This is the requirement. It used to be MICC (mineral insulated copper cables) cables that play this role, but now people use mostly the “fire-rated cables” for this purpose except in very special installation condition.
The fire-rated cables are cheaper, easy to install and maintain.
(See more pictures of fire-rated FR cables at this post, FR electric cable installation pictures.)
The black-colored cables are from the “NORMAL” supply. That means the other ends of the cables are connected to the “NORMAL SUPPLY” main switchboard.
While the red cables are connected to the “ESSENTIAL SUPPLY” main switchboard.
Which means the “ESSENTIAL SUPPLY” main switchboard is the one that is backed by the standby electric generator.
Is it too complicated? I hope not.
Even if it is, have no worry. I will spend a post or two on the overall system of electrical installation for large multi-storey buildings soon. Now I am actually building up section by section.
I will end this post by attaching a single-line diagram for a building similar to that of Diagram 4. However, this one used the normal cable riser instead of busduct riser. I hope the all readers of this post can locate the difference in the two diagrams.
There are also a few more pictures for the risers of Picture 1 and Picture 4. These pictures provide more information and better views for a few of the busduct components.
Diagram 5 – A distribution system using a cable riser instead of the busduct riser
Picture 6 – Another view of the busduct risers at riser room
Picture 7 – The front view of one of the 60A TPN tap off units
Picture 8 – A closer view of the vertical busduct showing the arrangement of the conductors inside
Notice that there are five conductors inside the busducts: the three phases, the NEUTRAL conductor and the EARTH conductor. The arrangements of the conductors are as shown on the busduct casing.
Picture 9 – A view of the cable entry into the feed in box and the connection of the equi-potential earth conductor
Observe that the copper tape was connected to a wrong place.
The metal plate where the cables are connected to is a removable plate. The copper earth tape should stay clear of this removable plate.
The tape should be connected to the main body of the termination box at a location where it is least likely to be disturbed.
This installation work was actually still in progress when I took these pictures. The copper tape connection was later rectified.
You can see more pictures at this post, Electrical installation pictures.
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