Tie Breaker Size

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eanton22

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Hello All,

I have a 480V switchgear. The main and reserve are 4000A breakers. I have (2) 1600A breakers and (6) 800A breakers. Now I want to basically split the switchgear with a Tie Breaker. What is the size of the tie breaker? 4000A or 2000A? I have seen it done with a 4000A but my lead explains to me that I want a 2000A breaker. For some reason if the tie breaker closes, its not going to see the full 4000A, becuase the loads are going to draw current. So i just wanted to see what you guys think.

Thanks
 
eanton22 said:
Hello All,

I have a 480V switchgear. The main and reserve are 4000A breakers. I have (2) 1600A breakers and (6) 800A breakers. Now I want to basically split the switchgear with a Tie Breaker. What is the size of the tie breaker? 4000A or 2000A? I have seen it done with a 4000A but my lead explains to me that I want a 2000A breaker. For some reason if the tie breaker closes, its not going to see the full 4000A, becuase the loads are going to draw current. So i just wanted to see what you guys think.

Thanks
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Seems like a lot of misinformation here. Do you have a Seconday selective substation (M-T-M)? Never heard "reserve" term used before. What do you mean "Split" the switchgear? The size of the tie breaker is what it is, not sure what you are asking, typically the Tie breaker is the same size as the mains, because if you lose 1 side the tie breaker will be carring the same load as the main was on the side you lost. Plus having the same size tie gives you a spare to use in a main cell if the need ever arises.

The tie breaker should close on its own, unless you have electronic key interlocks and a auto transfer scheme, which is pretty rare due to the cost and rarity of use.

What type of gear is this? What is your current configuration? Any pictures? What are you trying to acheive?
 
The breaker joining the two bus sections in a Main-Tie-Main Switchgear needs to be sized the same as the incoming feeders. This allows the breaker to accomidate the full current load of the bus. Since it is essentially splicing the bus together, this makes sense intuitively, but the reasoning becomes numerically evident if you perform a power coordination study.
 
gosh i wish i can show my single line.

To clarify switchgear has not been purchased. Still in the design aspect.

For design purposes, we have two 4kV-480 transformers feeding the switchgear. Both breakers (4000A) cannot be closed because you only can power from one transformer. Thats why I am calling the other a reserve. But now they want to split the swithgear which it will be a (M-T-M). What will be the size of the tie breaker? if you are saying that the Tie breaker will be 4000A, than one side of the tie breaker is drawing no current? I was explained that the tie breaker should be 2000A because one side will draw 2000A and the other would draw 2000A.
 
eanton22 said:
Hello All,

I have a 480V switchgear. The main and reserve are 4000A breakers. I have (2) 1600A breakers and (6) 800A breakers. Now I want to basically split the switchgear with a Tie Breaker. What is the size of the tie breaker? 4000A or 2000A? I have seen it done with a 4000A but my lead explains to me that I want a 2000A breaker. For some reason if the tie breaker closes, its not going to see the full 4000A, becuase the loads are going to draw current. So i just wanted to see what you guys think.

Thanks
Click to expand...

Here is the old argument, back again.

You may install a 2000A tie-breaker, BUT install it with a protective relay. If your load is greater than 2000A the breaker will trip, won't it?

If you install a 4000A breaker then you have a spare-in-place, should one of your main bites the dust. In that case the tie would need not to have a protective device, but it is more of a hustle to change the sensors and relay from the faulty breaker, plus those can go bad also.

In real life it is hard to assure - and only engineering supervision can guatantee-it - that one side does not get overloaded. Often M-T-M gears operate A/B equipment that allows for balanced operation and the connected load usually exceeds the actual running load. The combination of these could result in that one side would be more than 50% loaded. Since you would have 4000A buss regardless, there is really little benefit of having a smaller tie breaker.
 
eanton22 said:
gosh i wish i can show my single line.

To clarify switchgear has not been purchased. Still in the design aspect.

For design purposes, we have two 4kV-480 transformers feeding the switchgear. Both breakers (4000A) cannot be closed because you only can power from one transformer. Thats why I am calling the other a reserve. But now they want to split the swithgear which it will be a (M-T-M). What will be the size of the tie breaker? if you are saying that the Tie breaker will be 4000A, than one side of the tie breaker is drawing no current? I was explained that the tie breaker should be 2000A because one side will draw 2000A and the other would draw 2000A.
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If I understand you correctly, each 4160/480V transformer can supply the entire load but the switchboard is split into two parts with a buss-coupler (tie breaker) between them.
If that's the case then, unless you know with absolute certainty that each side needs exactly half the supply capacity, a 2000A tie breaker isn't an option.
 
Besoeker said:
...If that's the case then, unless you know with absolute certainty that each side needs exactly half the supply capacity, a 2000A tie breaker isn't an option.
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And even if you know that the loads are split evenly now they won't be for long because someone will add a new piece of equipment that doesn't have a spare so you will put the load on only one side of the bus. There's no way I could justify a tie smaller than the main - I just have to walk in to any of our substations that are more than 10 years old to see all of the added loads that would mess up the perfect symmetry.
 
The tie breaker is not normally closed (Unless you have fused breakers with 200kA AIC's and arc flash boundarys half way to the moon), so it dosent matter how even the loads are. You should have key interlocks there to prevent both mains and the tie from being closed at the some time.

The tie is only used to single end the substation when you lose one side do to a failure or maintenence, then the tie will have the same load as the main did before you did the switching.
 
zog said:
The tie breaker is not normally closed (Unless you have fused breakers with 200kA AIC's and arc flash boundarys half way to the moon), so it dosent matter how even the loads are. You should have key interlocks there to prevent both mains and the tie from being closed at the some time.

The tie is only used to single end the substation when you lose one side do to a failure or maintenence, then the tie will have the same load as the main did before you did the switching.
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From eanton22's post #4 it would seem that, in this particular case, the board is fed from a single supply in normal operation.
Both breakers (4000A) cannot be closed because you only can power from one transformer.
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That would require the tie to be closed for normal operation.
 
Besoeker said:
From eanton22's post #4 it would seem that, in this particular case, the board is fed from a single supply in normal operation.
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Oops, missed that part, just saw the 2 transformers discussed befire that. I wonder why both mains cant be closed?

Besoeker said:
That would require the tie to be closed for normal operation.
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Yep, but not both mains also. Just 1 main and the tie, thats OK, you are not paralleling the 2 transformers in that lineup.
 
zog said:
Oops, missed that part, just saw the 2 transformers discussed befire that. I wonder why both mains cant be closed?
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It does appear to be an unusual configuration.
 
Besoeker said:
It does appear to be an unusual configuration.
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im wondering if the OP was saying both mains cant be closed since their switchgear wasnt mtm, and thats why they want to change it to mtm so both mains can be closed......
 
wireguru said:
im wondering if the OP was saying both mains cant be closed since their switchgear wasnt mtm, and thats why they want to change it to mtm so both mains can be closed......
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There is no switchgear as I understand, this is all in the planning stage which is why I am confused about the limitations.
 
I just designed and had installed something similar. The client wanted to go cheap on the tie. I convinced him otherwise. Made the tie identical to the two mains. The bus is 4000A, why fight the derating bit. Also, you have a spare in case a main goes..

RC
 
Ragin Cajun said:
I just designed and had installed something similar. The client wanted to go cheap on the tie. I convinced him otherwise. Made the tie identical to the two mains. The bus is 4000A, why fight the derating bit. Also, you have a spare in case a main goes..

RC
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I have seen the regret in many an EE's eyes when the time came they needed the spare and had one too small (Cause they saved a few bucks going smaller on the tie). Sad thing to see, breaks my heart.
 
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