# Looped Distribution System



## ZatDevil27 (Mar 12, 2016)

Hello, sirs. Hopefully this is the correct location for said question(s).

I am currently on a project that has, per a foreman that I spoke to earlier, a looped distribution system. Physically looking out at the road, at some odd feet, you will see a green rectangular box. Behind that box you will see something that resembles a MCC (I was later told that this is called a Current Transformer).

Now, to add to this post, I had asked earlier today if I could view a one-line blueprint (which I was able to, for a quick minute). Upon viewing the blueprint I noticed:

- The Service listed/symbolized at the top of the blueprint

- No Current Transformer behind the Service

- Line power, from the Service, splitting out 2 different ways, and then hitting individual circuit breakers

- After passing through the circuit breakers, line power then " daisy chaining " through 4 larger transformers

- Etc.

I am attempting to understand how this all works. Am I correct in assuming that the " individual circuit breakers " that I see on the bp are actually the circuit breakers that are enclosed inside the buckets of the CT ? And that whatever voltage comes off the lines (in this case, I believe it was 13.6 kv) stays that way throughout the CT until it hits the " other " transformers ? And that those transformers will then, in turn, step down the 13.6 kv into 480 v ?

Am I correct in assuming that there will be 2 sets of line power coming in ? One that will backup the other, in case of power failure ? Probably through the use of tie-breakers ?

I did not see a Generator listed on the bp ... would that be because of the looped system (i.e - 2 lines of power coming in) ?

So many questions haha.

Now I am aware that I could Google/YouTube this information. But I feel that this forum, due to the overwhelming knowledge that many of you possess, could better explain it to my apprentice mind. 

Please and thank you.

By the way, the foreman had mentioned that this system is an open loop. I don't 100 % understand what that means.

Again, thanks, guys.


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## kb1jb1 (Nov 11, 2017)

It sounds like the OP is asking about the utility primary power distribution for an industrial area. Am I correct? But what is an open loop? Does that mean one of the high voltage isolation switches is open? Or is one of the pad transformers is taken out of service? Anyway the electricians where I am do not get involved with those voltages or power distributions. That is Utility work.


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## ZatDevil27 (Mar 12, 2016)

kb1jb1 said:


> It sounds like the OP is asking about the utility primary power distribution for an industrial area. Am I correct? But what is an open loop? Does that mean one of the high voltage isolation switches is open? Or is one of the pad transformers is taken out of service? Anyway the electricians where I am do not get involved with those voltages or power distributions. That is Utility work.


Yes sir, you're correct.


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## gpop (May 14, 2018)

If you loop the transformers then you have a option if you need to power down one of them with out powering down the whole chain. 
Some come with A-B switches that give you a option to feed the transformer from side A, side B, or both A&B others are simply done with load break elbows that can be pulled live.

If you have 
incoming power side A --- trans 1 ---- trans 2 ---- trans 3 ----- trans 4 ---- Incoming power side B, then you have the option of all 4 transformers running off both A and B or any combination so lets say trans 3 has a problem then trans 2 switch to A (or pull B and place in a parking spot) and trans 4 switch to B (or pull A and place in parking spot) 3 is now disconnected and after testing can be grounded. 

The CT are probably done on the low voltage side for billing. Main breaker will be probably be on the incoming and each transformer may have fuses. TBH there are so many options that nothing is real standard. If you can sketch the prints we can tell you what options they are using.


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## ZatDevil27 (Mar 12, 2016)

Thank you for taking the time to explain, gpop. I definitely understand it a little more clearer now.

And I will see what I can do, regarding the sketch. Though I won't be able to view it again until Monday.


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## kb1jb1 (Nov 11, 2017)

gpop said:


> If you loop the transformers then you have a option if you need to power down one of them with out powering down the whole chain.
> Some come with A-B switches that give you a option to feed the transformer from side A, side B, or both A&B others are simply done with load break elbows that can be pulled live.
> 
> If you have
> ...


But what is an open loop? 

By me they sometimes have these large green metal boxes that are high voltage by pass and isolation switches so that you can take a building transformer out of the loop and by pass the transformer.


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## gpop (May 14, 2018)

kb1jb1 said:


> But what is an open loop?
> 
> By me they sometimes have these large green metal boxes that are high voltage by pass and isolation switches so that you can take a building transformer out of the loop and by pass the transformer.


I believe open loop is the same as closed except one transformer is always open between A and B. 

Last place i worked we had 2 - 128k stepped down to 25k on separate sides (also had a tie breaker at 25k and generator tie). We then had 3 open loops with 14 x 480v transformers. There were many options to tie the sides together following set switching orders. 
The idea was any cable, transformer including the sub station 128k's could be taken off line if there was a problem. We could also balance the sides based on amps by moving one transformer from one side to the other. Its a little bit more complicated then it sounds as you also have PF correction, voltage regulators and cable/transformer loads that have to be taken into consideration especially when you are switching live.


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## ZatDevil27 (Mar 12, 2016)

gpop said:


> I believe open loop is the same as closed except one transformer is always open between A and B.


Always open between A and B ? Not energized ?


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## gpop (May 14, 2018)

ZatDevil27 said:


> Always open between A and B ? Not energized ?



Lets says its a transformer with switches and its set to A. Transformer is energized by the cables attached to A. 
Now if the switch was set to B then the transformer is powered by the cables attached to B. If you set it A/B then the transformer is powered by either and the cables are now daisy chained together. There is a off switch setting but i have never really used it as its safer to de-energize the cables at other transformers rather than having them live but off at the transformer im working on.


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## ZatDevil27 (Mar 12, 2016)

gpop said:


> Lets says its a transformer with switches and its set to A. Transformer is energized by the cables attached to A.
> Now if the switch was set to B then the transformer is powered by the cables attached to B. If you set it A/B then the transformer is powered by either and the cables are now daisy chained together. There is a off switch setting but i have never really used it as its safer to de-energize the cables at other transformers rather than having them live but off at the transformer im working on.


Understood. 

Thank you, gpop.


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## paulengr (Oct 8, 2017)

Impedance and voltage drop is half with a closed loop but also short circuits are doubled. As mentioned this is common in European systems. Often they use what is called an RMU (ring main unit) which is three disconnects, fused or infused or breakers. Two of them are in series on the loop for isolation. The third is for a branch out to say a transformer. I’ve also seen double transformers with looped feeds (the primary connection is essentially a terminal block with a pass through connection) set up with the RMU so that you have a redundant backup transformer or with double RMUs so that you can run two feeds from one transformer with two ties so you can isolate an entire RMU for service. The RMUs themselves are fairly cheap and mass produced since they are all identical.


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