Quick transformer question.

Ok I had a little walk up to the other end of the overhead cables.

You can see in the photo's the cable goes underground and into an enclosure.

Within the enclosure are three metal "boxes" for want of a better word.

Does anyone know what they are and what they do, what's inside the boxes?



Each box has a different label, you can just about read the names on the labels from my pictures.

They are called "Tee's"

Merlin Gerin, 630A fused load break switches. Obviously the fuse rating can be lowered. If one fuse blows the switch operates, breaking all three phases. They can be used as ring main units or to split a feed in to two.
The little white boxes are earth fault direction indicators.

Thanks Tony.

Right we have three boxes and three underground cables.

Those three underground cables are three phase cables right, I mean the three cables are not single phase cables they are three phase cables in their own right.

In this enclosure we have three separate underground three phase cables coming up and entering three separate enclosures.

The little white boxes you mention are only apparent on two of the boxes, the third box has no earth fault direction indicator.

Would it be correct to say that one of those cables coming up from the ground is the supply cable which then splits into two separate supply cables that go off in different directions or have I got this totally wrong?

One thing I'm having trouble understanding is the earthing on a delta delta transformer.

I mean big network transformers are delta delta right?
The star formation transformer only comes into play at the lower end of the voltage spectrum and the star type of transformer has its neutral or centre tap directly connected to earth through a cable so that if you get an earth fault at a consumers installation there is a direct path for the earth fault current to flow as in from the consumers installation, through the ground to the transformers earth rod/mat and then back to the star point.

Now if you have a network transformer that is wired delta delta there is no where that a cable is connected directly to earth so how can you have earth fault currents?
I mean to have an earth fault current you need to make a circuit, you need a place where the current is injected into the earth as in the consumers premises and a place where the earth fault current is picked up and returned to the network circuit as in through the earth cable that is connected to the star point of the supply transformer, a circuit has been made and so an earth fault current will flow through the ground.

With a delta delta transformer non of the windings are directly connected to the ground so if for instance one of the phases touched the ground or indeed you touch one of the phases how can you get an electric shock, it isn't possible for you to become part of the circuit because there is no return path for the fault current to rejoin the three phase supply.

I can understand if you were to touch between two phases, you would instantly become part of the circuit but if non of the delta delta windings are directly connected to ground how can you get a shock if you were to touch one of the cables whilst standing on the ground?
There would be no flow and return of current.

Anyone got any suggestions?

Thanks.

Right I did a little more investigation and took some pictures with the help of a good telephoto lens.



So am I right in saying this:

These metal enclosures are the end of the line, they are at the end of the three phase supply and are a type of fusing.
So starting from the furthest point from the transformer, one of these boxes is our supply cable or tee, the supply cable starts/ends here then goes overhead until it reaches the transformer in the picture in post 1 of this thread.

If you look at the transformer in post 1 you will see that the overhead lines not only supply the transformer but also become an underground cable that disappears somewhere off under the field.

The underground cable will eventually join a transformer possibly a 33KV/11KV transformer somewhere in our local vicinity.

Is this correct, are these fusing boxes always placed at the end of the three phase supply line, farthest away from the transformer or am I barking up the wrong tree?

The three boxes have labels on them like the above and each label is a different geographical location but still local so either all the supply cables from the three geographical locations end here or this is some sort of splitter where a supply cable comes in and is then split off in different directions.

The supply seems to go from a transformer somewhere, crosses over meadows and often goes underground, 230V/415V three phase star transformers are placed at various stages along the route to supply houses etc and eventually the 11KV supply cables are terminated in the fused units that you see in this post.

Does that make sense, any thoughts?
I'm just trying to get the bigger picture.

Thanks.