Friday, February 12, 2010

Switchboard earthing pictures

This post provides a few pictures of switchboard earthing. They are pictures of actual switchboards and electrical DB’s that were installed in one of the projects I was involved in.


Picture 1 – LV sub-switchboard



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Picture 2 – Three-pole 160 A Terasaki MCCB and the neutral link



Picture 1 above shows a sub-switchboard at one of the upper floor of the multi-storey building.

Picture 2 shows the main isolation switch at the switchboard. I show the enlarged version of this component because I want to show the readers the neutral link component on the right of the MCCB. The neutral link is a very important part of the safety aspects of an electrical installation. I may want to refer to component later in this post or maybe when I send an update.

In Picture 1, you can see at the bottom left corner of the sub-switchboard the earthing connection to earth the switchboard front door. Picture 3 below gives an enlarged view.

Picture 3 – Earthing conductor to earth the sub-switchboard front door




Observe that the conductor is made of braided aluminium conductor. The braided type of conductor is a very good material for earthing door switchboards because the door is a frequently moved part of the board.

If an unsuitable material is used, the frequent movement of the door will either weaken and damage the conductor, or loosen the contacts at the door and the main panel cubicle. A loosened contact would increase the contact electrical resistance and defeats the purpose of having a properly sized earth conductor in the first place.

A loosened earth conductor is usually seldom noticed during the operation of the equipment until something happens. The problem is that “something” may actually be a serious electrical shock.

It is for this reason the earthing of these grounding connections should be properly checked by visual inspection during the installation works.

Notice also the termination to the switchboard door. The braided aluminium conductor is terminated using a compression type cable termination lug. It is fixed to the door using appropriately sized earthing bolt and nut with spring washers.

Picture 4 – Lighting and small power distribution board (DB)




Picture 4 above shows a floor distribution board for the lighting final circuits and also for the small power needs at the floor level.

Both the sub-switchboard in Picture 1 and this DB were not actually installed yet at the time I took these pictures. They were still on the floor at the storage area of the construction site. That is why you see these pictures were taken at angles looking downward to the boards.

The earthing connection to the front door of the board was also using the braided aluminium similar to Picture 1, as shown in the enlarged view below.

Picture 5 – DB door earthing conductor



There also is another type of panel door earthing conductor in common practice, as you can see in Picture 6 and Picture 7 below.

Picture 6 – Another electrical DB




Picture 7 – DB door earthing using green insulated PVC cable



Here the conductor is just the normal PVC insulated wiring cable with green colored insulation. The PVC cable is not very flexible. It is therefore is installed in the inverted U-shape as in Picture 7.

Sometimes the PVC cable is formed into coils between the two mounting bolts to give similar flexibility. I do not have the picture of this method now, but I will send an update as soon as I can get one.

I personally prefer the braided aluminium conductor as in Picture 1. However, contractors would normally prefer the PVC cable type because it is simply cheaper.

Manufactures would always prefer the Picture 1 method because it make the their board looks more high-quality. However, they would have to follow contractor’s choice unless the design consultant specifically stated their requirements clearly in the contract specifications.

Main Earthing Cable

In all the above Pictures, I have only highlighted the internal branch earth connections. I wish to highlight the main earthing conductors of the switchboards.

Look at the bottom of the distribution board in Picture 6. You can see at the bottom of the DB cubicle a length of green PVC insulated wire just laid there and stops just below the three-pole MCB at the bottom right corner of the DB. You can see it better in the enlarged view (Picture 8) below.

Picture 8 – Distribution board main cables



Unlike the first two electrical panels, this distribution board has already been wall-mounted in its final position. However, it is not yet wired and the cables not terminated. The electrical contractor just wanted to get my approval of the DB mounting method before they proceed with the cable termination works.

You can see the sub-main supply cables coming down from the top trunking. The outer sheath and wire armor of the 4-core armored PVC cables have been terminated with a cable gland. That is why you can see the individual cable cores coming down along the right side of the cubicle wall.

An extra slack has been provided during the sub-main cable installation to ensure there is enough length provided in case adjustment need to be made to the final position of the DB. The actual route inside the DB also may require some extra slack to the incoming cable cores.

Back to the earthing cable. The specifications for this project requires the use of 3 mm by 75 mm copper tapes as the earthing conductors for all submain circuit.

However due to installation difficulties I have allowed the use of PVC copper cables as an alternative to earth the sub-switchboards and distribution boards on case by case basis. This DB is one of the cases.

Therefore, here the main earth conductor is the PVC cable. This cable is connected to the main earthing conductor, which is the 3 mm x 75 mm copper tape, at the multi-storey building’s riser room.

You can see in Picture 8 that the earthing cable is not terminated yet. It is a common practice to provide a main earthing busbar in side an electrical board. The earthing busbar is usually pre-installed at the factory just like the rest of the components.

I do not see the earth busbar in the picture. When I went for the inspection and took these pictures, I was only paying attention to the mounting of the board. I only realized the missing earth busbar when I started to write this article. In any case, this matter would be picked up during the inspection of the internal wiring of the distribution board.

If the main earth busbar is actually missing, the electrical contractor will need to install the busbar himself at a suitable location inside the DB. The green PVC cable will then be terminated to the busbar using compression type cable lug.

The fixing to the busbar will also be similar to the door earthing method explained above: bolt and nut with spring washer.

Usually the busbar would be pre-drilled with sufficient number of termination points plus a few spare points. This will eliminate the need for disassembly of the busbar and all existing connections when additional earthing connections are needed during the life of the switchboard.

There is one more picture that I would like you to see on the earthing of LV electrical panels. See Picture 9 below.

Picture 9 – Switchboard earthing conductor



Where is the switchboard??

They have not been installed yet at the time I took this picture. These “dropper trunkings” have been installed but they stop just a few feet above the location of the distribution board and switchboard.

Observe the two lengths 3 x 75mm copper tape coming down from the electro-galvanized steel trunking. This is the standard earthing conductor used for this project. The PVC cables used in Picture xx was a replacement to this copper tape.

Earthing of 11 kV switchboards

The switchboards and the distribution boards shown in all the above pictures are low voltage boards. They are rated 240V/415V at 50 Hz supply.

Now I will show you a few pictures of high voltage (HV) switchboards. These boards are designed for 11 kV incoming supply.

Picture 10 – 11 kV switchboard rear view



Picture 11 – Closer view of the earthing copper conductor



Why do I show the rear view of these HV panels?

Because these panels are floor mounted with rear access. The LV panels above have been designed for mall-mounted installation. Therefore, they do not have rear access.

Observe the long strip in brown color at the lower edge of the switchboard. This is the earthing copper conductor for the high voltage switchgear panels.

Note: Some readers may notice that I use the terms “board” and “panel” interchangeably. For LV small electrical panels the two terms do not really have any significant difference.

However, for large LV switchboards and high voltage switchboards, the two terms cannot be interchangeable. Look at Picture 12 below.

Picture 12 – HV panel



This is one HV panel. The other big object at the left of the panels is actually another HV panel, which is still inside its waterproofed wrapping.

A high voltage switchboard consists of one or more of these panels. In other words, the panel is a section of the switchboard.

Notice the brown strip at the bottom edge of the panel in Picture 11. This is the earthing copper conductor that you see in Picture 10.

From this picture it is obvious that the long copper strip in Picture 10 is actually jointed lengths of short individual copper strip that are part of the individual HV panels.

This short copper strip at each panel has actually been installed at the manufacturer’s factory. Short lengths of the conductors are then provided complete with pre-made jointing holes and shipped together with the rest of the panels.

The electrical contractor then only needs to re-assemble all the pieces together into a complete HV switchboard.

Picture 13 – Another view of the earthing conductor




This photograph also shows the HV cables installed in the cable trench. The one going into the end panel is the feeder cable to one of the local transformers. While the other one is the incoming supply cable from the authority HV switchroom just adjacent to this HV switchroom.

Picture 14 – Substation earthing



Picture 14 shows the substation main earthing conductors. All substation should be installed with these copper tape conductor along the walls of the substation. It is usually called the substation main earthing conductors. These conductors are terminated to a main earthing busbar.

In this case, the earth busbar has not been installed yet. However the location is as indicated by my comment on the picture.

The main conductor running horizontally on the walls will be cut at the location shown. The busbar will be installed there. The cut horizontal conductors (now it has become 2 lengths after the cutting) plus the three vertically mounted conductors already there will be terminated to the earth busbar using properly sized bolts, nut and spring washers.

Note: You can also see more pictures of electrical wiring by visiting this post, Pictures of electrical wiring.


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4 comments:

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