Thursday, December 31, 2009

Electrical DB pictures

I have uploaded below a few pictures of electrical DB being installed at a high-rise office-building project. I only have enough time for these few pictures today, but I will be sending a more detailed post on the DB installation soon. Plus a few more of the DB pictures maybe.

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Picture 1 – Mounting position and location


These electrical boards are permanent boards, not a temporary electrical installation. I have been sending so many pictures of temporary electrical installations that I feel the need to make that clear. Otherwise, some readers may get confused because in these pictures you can see that the boards and the cable trunking are solidly mounted on the concrete wall.

Actually there are three DB’s in this picture. The two big ones on the left (your left side) are the electrical distribution boards (DB) and the one at the far right is the telephone distribution panel (some people call it “DP panel”).

These DB’s are located along one of the corridors on the office floor. Therefore, they will need to be hidden from view and protected from unauthorized access. Therefore, a cabinet will be built around these boards, and a lockable door will be provided.

The installation work is in progress and the building is still under construction. However, if you see closely to the doors of the distribution boards, you may notice that the DB doors are not provided with lockable door handles. They seem to be provided only with door screws.

With this type of doors, then they are not protected from access to the live parts of the boards. So, if there is no provision to build such electrical cabinet or rooms as I have mentioned and secure it with lockable keys, then the construction supervisor in charge of the electrical installation must insist for it.

There is no problem in installing the distribution boards exposed like that. However, they must be protected from unauthorized access. In this case, you have three choices:

Provide lockable doors and raise the mounting height of the DB’s above reach by hands.
Raise the mounting heights to unreachable heights without providing the lockable keys to the doors. This a little bit of a compromise already, but some may say that is already okay. In many scenarios this sort of issues are quite subjective.
Build a cabinet around the boards, enclosing all three electrical and telephone boards. A lockable door should be installed.
Build wall around them so it will become an electrical rooms, plus a lockable door.

Picture 2 – Dropper trunking from above ceiling


This second picture shows the metal trunking coming down from above the ceiling and into the distribution boards.

Some of you may be wondering why these trunking are like that… with the big trunking branching into the smaller ones above the ceiling. The reason is to make the space bigger for the wiring cables to crisscross each other while coming out of the MCB’s (miniature circuit breakers) inside the DB’s and into its intended trunking route (either one or the three branch trunking above the ceiling.

Actually, the three branches of trunking above the ceiling is for the following:

One for lighting wiring final circuits. All wiring cables supplying all lights and maybe ceiling fans and toilet exhaust fans. This is an air-conditioned office building, and ceiling fans are not used at all. However, wiring for the air-conditioning equipment is not allowed to run in trunking for lighting wires. Emergency light and the lighted exit signs also have their wiring installed inside this trunking, but the one from the ESSENTIAL SUPPLY board.

I should have mentioned this earlier. We have two electrical DB’s there because one is supplied from the standby electrical generator. This is the smaller DB. The bigger DB is supplied from the authority’s mains supply. The one supplied form the standby electrical generator is usually called “essential DB” or “essential distribution board”, while the one supplied from the mains is normally called “normal DB” or normal distribution boards.

The Essential DB is usually smaller because the number of electrical loads it need to supply is smaller. In ordinary office buildings, the number of light fittings that need to be turned on during a power supply failure is usually only one out of three lights or about thirty percent of the general lighting. So the Essential DB will have only half the number of final circuit wiring as the Normal DB. That is why they are generally half the size, which is what you can see in this picture.

The second branch trunking is for the small power final circuit wiring, the wiring supplying all the switched socket outlets and other small power points like window air conditioners, etc. Cables supplying power to other mechanical equipment such as water pumps, etc are not allowed to share this trunking. The need to be installed in a separate trunking, which is the third branch trunking.
This carries the cables and wiring for the mechanical equipment like the building’s water pumps, etc.

So the above describes what those three branches of the metal trunking are for. The same applies for both DB.

Picture 3 – Bottom cable entry


This picture shows the bottom entry for cables. Both DB’s has bottom cable entry. Some of the readers may be wondering why cables need to enter the DB cabinet from both to and bottom. The reason being this is an office building with an underfloor trunking system.

The electrical socket outlets, the telephone socket and the computer data sockets for all office tables are provided from outlet boxes of this underfloor trunking system. So there are three separate high impact PVC trunking running inside the concrete floor throughout all office areas of this multi-storey office building. At each worktable, an outlet service of approximately 12 inch x 12-inch box is provided and all the power, telephone and data sockets are provided there.

You can see that the orange electrical trunking under the DB’s going downward to the floor. The green telephone DB also runs downward to the same location. This is where they are connected to the 3-way underfloor trunking system. The piece near the floor is called the “vertical access box” of the trunking system.

That is all on the bottom trunking for now. In future, I will talk more on the underfloor trunking system and maybe show you some real construction photos.

The top entry trunking are for all other cables and wiring. The wiring to the window air-conditioning units run above ceiling, never inside the underfloor trunking. The nature of underfloor trunking installation make it very difficult and messy to extend. You need to hack the concrete floor to install additional runs and that can disrupts the operation of the whole floor. So only the wiring for the final circuits to the office work desks are run inside them. They are dedicated only for this purpose most of the time.

Usually some general-purpose power sockets are still provided on the walls even where underfloor trunking system is used. These sockets are usually run in the above ceiling trunking and a dropper conduit is used to make the connection to the sockets on the walls.

The lighting wiring are always run inside the above ceiling trunking. A system of trunking and conduit is the dominant method for this wiring purpose in almost all office buildings.

From the above it is quite clear that a much larger number of wiring runs above tha ceiling than under the floor. So that is the reason the bottom entry trunking is much smaller than the top entry ones. In this case the bottom trunking from both DB’s can be joined and still use the same size to connect to the vertical access box.

Telephone DB
As for the telephone DB, the trunking does not need to run above ceiling to go to the sockets at the worktables. The top entry trunking is for the incoming multi-core telephone cable from the telephone riser shaft. All telephone final wiring cables run inside the underfloor trunking. Since this blog is about electrical matters only I will not talk too much on telephone works. That may be a topic for one of my new blogs.

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1 comment:

Anonymous said...

How is it that your field wirings
trunking from different DB (ess and normal ) are electrically & mechanically connected ?
I thought field wiring from different DB must not be connected
but that is essentially what you did in the picture ??